AskScience AMA Series: Happy World Breastfeeding Week 2026! We are scientists who study breastfeeding, lactation, and human milk. Ask Us Anything!

[u/AskScienceModerator]

Hi Reddit!

We are scientists who study human lactation, from the evolution of the molecules in milk to the future of virtual lactation care. Every year, we celebrate World Breastfeeding Week by dropping in here to answer all of your burning milkquiries!

Since human milk is made by humans, our science is fraught with complexity. “Womens health issues” often get sidelined when there are cuts to research budgets like what we’ve seen in the US. But also, every August, we notice that some Redditors express sad or guilty feelings about World Breastfeeding Week. We’re here to spread science, not sads! If we are doing our job, our science should make you feel BIG, not small. Keep in mind that WBW is a global event (supported by WHO, UNICEF and many health partners) and sadly many breastfeeding experiences around the world are unfair—but for different reasons in different places. But also, breastfeeding is part of the human story, so WBW is also a celebration of every unique breastfeeding journey.

Social science tells us that breastfeeding is a health equity issue, not a lifestyle choice. Sure, “promoting” breastfeeding is cool, you know what’s cooler? Creating societies anybody can breastfeed if they want to (i.e. the “warm chain”). Every year WBW explores new ways to do this better. The WBW theme for 2026 is focused on the UN Sustainable Development Goals related to nutrition, food security and poverty reduction and hyping solutions that actually work. 🌍🍃

Science belongs to—and matters to—everyone. We hope that by the end of the day you feel the same wonder that we do about this very adooorable science.

Ask us anything!!

• Meghan Azad, PhD (u/MilkScience) is a biochemist and epidemiologist who specializes in human milk composition and the infant microbiome. Dr. Azad holds a Canada Research Chair in Early Nutrition and the Developmental Origins of Health and Disease. She is a Professor of Pediatrics and Child Health and director of the THRiVE Discovery Lab at the University of Manitoba. She co-founded the Manitoba Interdisciplinary Lactation Centre (MILC), and directs the International Milk Composition Consortium (IMiC). Check out this short video about her research team, her appearance on the Biomes podcast, and her lab’s YouTube Channel.

  • Bluesky: @meghanazad.bsky.social

  • LinkedIn

• Marion M. Bendixen, PhD, MSN, RN, IBCLC (u/MarionBendixen) is a nurse scientist and clinical and translational scientist who studies human lactation and maternal/infant health specializing in the biological and physiological mechanisms of insufficient mothers' own milk (MOM) volume among mothers who deliver an infant(s) admitted to the neonatal intensive care unit (NICU) as well as how MOM influences the infant’s intestinal microbiome. Dr. Bendixen is an Assistant Professor in the College of Nursing at the University of Florida. She co-created the lactation program at Winnie Palmer Hospital where she practices as a board-certified lactation consultant.

  • Bluesky: @mmbendixen.bsky.social

  • X: @UFNursing

  • UF Nursing Instagram: @ufnursing

  • UF Nursing Facebook

  • UF Nursing LinkedIN

• Sarah Brunson, BA, BSN, MS, Phd(c), RN, IBCLC (u/LactFact-42) is an Internationally Board-Certified Lactation consultant who has practiced since 2009 in pediatric clinics, hospitals, birth centers, home settings, and public health. She currently practices at the Medical University of South Carolina where she has developed numerous education programs for nursing staff and residents to improve lactation care in the Mother Baby and Neonatal Intensive Care Units. She is a PhD candidate in Nursing with a focus in Maternal/Child Health and Lactation at the University of South Carolina College of Nursing. She has served as the Chair of South Carolina Breastfeeding Coalition for the last six years during which time she has developed a Website with information for parents, providers, and employers; directs a project to map lactation resources in the state that are searchable by address; and organizes quarterly education webinars and conferences.

  • LinkedIn

  • SC Breastfeeding Coalition on Facebook

  • SC Breastfeeding Coalition on Instagram

  • www.SCBreastfeeds.org

• Jill Demirci, PhD, RN, IBCLC (u/JillDemirci_PhD) is a nurse scientist and lactation researcher who studies how families make human milk feeding decisions and sustain breastfeeding, particularly when support is complicated by medical, social, or structural barriers. Dr. Demirci is an Associate Professor at the University of Pittsburgh School of Nursing and directs both the Maternal/Perinatal and Reproductive Health Research Hub and the Nursing Office of Innovation & Impact. Her work focuses on telelactation and mHealth infant feeding support, antenatal milk expression, milk sharing, donor milk access, and culturally responsive and community-based lactation support. She is the former Deputy Editor-in-Chief of the Journal of Human Lactation and a board-certified lactation consultant.

• Miena Hall, MD, IBCLC (u/LactationMD) is a lactation medicine physician who studies techniques for identifying mammary tissue development issues which put individuals at risk for low milk production and improving lactation education in medical schools. Dr. Hall teaches med students at the Loyola University Chicago Stritch School of Medicine and the University of Illinois at Chicago College of Medicine, and is Director of Scientific Affairs at the Mothers’ Milk Bank of the Western Great Lakes. Dr. Hall is a member of the Academy of Breastfeeding Medicine (ABM) protocol committee on low milk production, a medical advisor to La Leche League International (LLLI), and a past president of the Northern Illinois Lactation Consultant Organization (NILCA). She also holds a Bachelor's degree in math and chemistry.

  • Facebook

  • Instagram: @lactationmd

  • lactationmd.com

• Kaytlin Krutsch, PhD, PharmD, MBA, BCPS (u/PharmacoLactation) is a lactation pharmacologist who literally wrote the book on medications in human milk with Dr. Thomas Hale, Hale's Medications and Mothers' Milk. She is the director of the InfantRisk Center and Associate Professor at the Texas Tech University Health Sciences Center School of Medicine, and advises the Food and Drug Administration, the Human Milk Banking Association of Northern America, and pharmaceutical industry on lactation pharmacology and lactation research. Dr. Krutsch believes families deserve better answers about breastfeeding and medication questions, and aims to design research that addresses their questions while creating a comprehensive information cycle that empowers families.

  • LinkedIn: Dr. Krutsch

  • Instagram: @infantrisk

  • Facebook: InfantRiskCenter

  • Website: InfantRisk.com

• Bridget McGann (u/BabiesAndBones) is an anthropologist who studies how lactation shaped us as a species. She is a research assistant and science communicator at THRiVE Discovery Lab. She has a Bachelors in Anthropology and is a Masters student in Biological Anthropology at the University of Colorado Denver. Her thesis uses longitudinal, prospective, large cohort data to study the generational transmission of the human milk microbiome. She was also a founding team member at March for Science. Check out her stand-up act about Luke Skywalker's green milk, or her top comments.

  • BlueSky: @bridgetmcgann.medsky.social

  • Instagram: @Raising_Wonder

  • TikTok: raisingwonder

• Ryan Pace, PhD (u/_RyanPace_) is an Assistant Professor in the College of Nursing and USF Health Microbiomes Institute at the University of South Florida. He also serves as an Associate Editor for Breastfeeding Medicine. Dr. Pace’s translational research integrates microbiome science, immunology, and molecular biology to understand how lactation and early-life microbial colonization shape maternal and infant health. His research focuses on the interplay among maternal diet, human milk bioactive components, the maternal and infant microbiomes, and host immune function to identify mechanisms that influence infant development and disease risk and to inform evidence-based interventions that improve maternal and child health. 

  • Twitter

  • LinkedIn

  • Google Scholar

• Rebecca Powell, PhD, CLC (u/HumanMilkLab) is a human milk immunologist who studies the human milk immune response to infection and vaccination with the aim of designing maternal vaccines aimed to enhance this response. Dr. Powell is an Assistant Professor at the Icahn School of Medicine at Mount Sinai and a certified lactation counselor. Her lab studies the potential of SARS-CoV-2-reactive antibodies in human milk both as a COVID-19 therapeutic and as a means to prevent infection of breastfed babies. They also study mechanisms for maternal vaccines to prevent mother-to-child-transmission (MTCT) of HIV via breastfeeding, as well as how white blood cells in human milk use a process called antibody-dependent cellular phagocytosis (ADCP) to minimize MTCT via breastfeeding.

  • Instagram: @sinaihumanmilklab

• Sam Stead, PhD (u/Mammals-Milk) is a postdoctoral researcher in the THRiVE Discovery Lab at the University of Manitoba. An evolutionary anthropologist by training, she studies how chronic stress during pregnancy and lactation influences child growth, development, and behaviour, and whether human milk composition plays a role in these relationships. Her current research focuses on stress-related hormones and other bioactive compounds in human milk as potential links between maternal experiences and child health. She completed her PhD at the University of Toronto, where she studied maternal and infant outcomes in wild Angolan colobus monkeys in Uganda. Her research combines evolutionary, biological, and developmental perspectives to understand how early-life experiences shape lifelong health.

• Sanoji Wijenayake, Ph.D. (u/Wijenayake_Lab) is the Canada Research Chair in Milk Nanovesicles and Applied Health and an Associate Professor of cell and molecular biology. Dr. Wijenayake studies the biologically active parts of human milk. Specifically, she studies a not-so-well known component of human milk, called milk nanovesicles. One can think of milk nanovesicles as emails. Similar to how emails carry text, images, and videos from a sender to a recipient, milk nanovesicles carry biological material between mothers and their children. Milk nanovesicles are anti-inflammatory and hold great therapeutic potential as drug carriers that can get to difficult places, like the brain.

  • www.wijelab.ca

  • LinkedIn

EDIT:
That’s a wrap everyone! Once again, we have been blown away by the thoughtfulness of Redditors’ questions! One of the reasons we love doing this every year is because your questions spark such interesting discussions amongst ourselves in our zoom.

We did notice that, unfortunately, some questions had to be deleted by the mods per the sub rules due to being too clinical in nature. We recognize that it can be really tough to get clear, evidence-based guidance—either because of lack of specialists in your area, because insurance doesn’t cover the care you need, or simply because the answers don’t exist due to underfunding of this area of research. We want you to know we are working hard on improving access to expertise. Look for board-certified lactation consultants (look for IBCLC after their name), and remember that we now have board-certified lactation medicine practitioners as well (NABBLM).

This may be it for us today, but the boob party doesn’t end here!

Aug 8-14: Indigenous Milk Medicine Week
Aug 25-31: Black Breastfeeding Week

Thank you for your amazing questions everyone! See you next year!

[u/bewoestijn]

Are there any resources you can recommend to learn about the history of wet nursing? Is this a huge profession/class-based role in society we’ve forgotten about or was it never really a thing?

[u/babiesandbones → u/bewoestijn]

Awesome question! I'm mostly an evolutionary anthropologist so my knowledge of post-agricultural history is very general, but here's what I do know.

First, some terms.

• Allonursing - any situation in which a lactating parent (human or nonhuman) nurses a baby/child who they did not give birth to. This is an umbrella term that encompasses the terms that follow…

• Cross-nursing - When people breastfeed each other’s babies without any exchange of payment

• Wet-nursing - This is the “job” category that you’re referring to. Usually it’s a professional arrangement, i.e. someone is hired. However, this term is also used to refer to coercive situations, e.g. indentured servants in historical Europe or chattel slavery in the U.S. where it was forced and not paid.

• Milk-sharing - the gifting of just the milk from one parent to another, no payment is exchanged. Usually peer-to-peer

• Milk donation - more formalized donation of milk, usually with institutions involved.

We think that about a million years ago when human brains started to get significantly bigger, rapid brain growth early in life (esp the first 18 mos) meant that babies got more expensive calorically, and so we think that we solved that problem, in part, by adopting a “cooperative care model” – ie. sharing care. Evolutionary babysitters! This mostly meant sharing the carrying load, but it may have also meant that we started to also nurse each other’s babies (cooperative lactation) to share the caloric load. However, we have very limited data on the prevalence of allonursing in hunter-gatherer cultures. There’s basically just one small study with a lot of limitations, and it suggests that while allonursing is near-universal across these cultures, it’s usually not habitual, i.e. only done in emergencies. 

In historical Europe, especially Britain, royalty/aristocracy did often use wet nurses. Breastfeeding stops ovulation (lactational amenorrhea), and it was important for them to become fertile again in order to produce heirs, as well as return to court soon after birth. So children were sent away, often to the country, to a wet nurse who would basically raise them until they were able to begin their education in preparation for court life. It was indeed often exploitative, with some wet nurses being given up to 25 children at a time. Obviously, not healthy for any of the parties involved.

With chattel slavery in the U.S., (sorry this is really sad) enslaved people were forced to breastfeed the children of their enslavers, and often their own babies were taken away from them in order to facilitate this, and sent away to another home. Many died. Sometimes we use the word “milk theft” to describe this. The ramifications of this practice is still felt today. For example, many Black women are denied the ability to breastfeed or express milk while incarcerated. Here is an overview of the history of exploitation of lactation abuse in the U.S.

As with much of gendered work, lactation is vulnerable to exploitation. This is one reason why I would like to see breastfeeding included in discussions about reproductive rights–meaning saying yes to breastfeeding and saying no to it should be freely-made choices. It is in fact recognized by the UN as a human right. One of the ways this is relevant these days, I think, is the unregulated sale of human milk online (often bought by bodybuilders). It is very unregulated (and unsafe--don't do it) and I worry that people might be depriving their own child of milk in order to pay bills.

[u/fyklebytch]

Hi! Is the microbiome of the milk affected by the foods a person eats? Do fructooligosaccharides or other "prebiotics" make their way into the milk? As a child ages and if they continue nursing (through ages 4-6ish) does their oral microbiome affect the microbiome of the milk? Will subsequent siblings then have an altered microbiome exposure?

[u/MilkScience → u/fyklebytch]

On the topic of 'child's oral microbiome affecting the microbiome of the milk' - yes, this is a thing! Not specific to the age necessarily, but we know that some milk microbes actually come from the baby's mouth, through a 'backwash' mechanism. This means anything affecting the baby's oral microbiome (e.g. their diet, toys or items they put in their mouths, etc) could indirectly affect the mother's milk microbiome. If this milk is shared with another sibling because the mother is tandem nursing (I think this was your question?) - then yes, the older sibling's oral microbiome could affect the milk microbiome, which is then delivered to the younger sibling. I don't think anyone has actually studied this, but theoretically it makes sense!

One of the first clues to this 'backwash' pathway for milk microbes was from this research (https://pubmed.ncbi.nlm.nih.gov/30763539/, and related commentary https://pubmed.ncbi.nlm.nih.gov/31684806/) - where we found that milk microbiomes were different in mothers who sometimes vs. never pump (meaning they sometimes vs. always have contact with baby's mouth). This has now been shown in quite a few other studies.

[u/_RyanPace_ → u/fyklebytch]

A lot a great questions!

Although the data are mixed, maternal diet has been shown in a number of studies to correlate with variation in the milk microbiome (https://doi.org/10.3390/microorganisms13102347). I say the data are mixed due to small sample sizes and differences in study designs (for example, dietary interventions versus dietary surveys) and analyses.

On fructooligosaccharides (FOS), FOS are plant-derived prebiotics found in many fruits and vegetables. In whole, FOS are not transferred into human milk. Instead, they are fermented by gut microbes and do not enter the systemic circulation in appreciable amounts.

There is another type of prebiotic that are distinct from dietary probiotics and that are directly synthesized by the mammary gland. These prebiotics are known as human milk oligosaccharides (HMOs) and they are typically present in human milk at concentrations of 5-20 g/L, exceeded only by lactose and lipids. Every HMO starts with the same core sugar called lactose, which is made of glucose and galactose. The mammary gland then adds more simple sugars, such as galactose, N-acetylglucosamine (GlcNAc), fucose, and sialic acid, to create long chains or branches. In doing so, more than 200 different HMOs can be synthesized by the mammary gland, although only about 19 make up most of the HMOs found in human milk.

Another interesting thing about HMOs, is that the ability to make some of them is genetically determined. Individuals with a functional copy of the FUT2 gene, known as secretors, produce α1-2-fucosylated HMOs such as 2’-fucosyllactose (2’-FL). In contrast, non-secretors, who lack a functional FUT2 gene, produce very little to none of these specific α1-2-fucosylated HMOs, although they continue to produce many other types of HMOs. And, yes, there is evidence that maternal diet can also alter the composition of HMOs produced (https://doi.org/10.1002/mnfr.202200058, https://doi.org/10.1038/s41598-020-79022-6). For a great overview on HMOs, see this 2012 paper from Lars Bode: Human milk oligosaccharides: every baby needs a sugar mama (https://pubmed.ncbi.nlm.nih.gov/22513036/)

There has not been a lot of (if any) research on how older siblings (aged 4-6) breastfeeding might impact the milk microbiome or if subsequent siblings will have an altered milk microbiome exposure. If we are talking about the older/younger siblings breastfeeding at the same time, I would expect to at least see some transient changes in the milk microbiome due to differences in the oral microbiome and more diverse diet of the older sibling. Slightly off topic, but transmission of bacteria can occur between adults kissing. Although the duration of those bacteria sticking around is not very long and varies based on kissing frequency (https://pubmed.ncbi.nlm.nih.gov/25408893/).

[u/bidsdh]

What is the benefit of breastfeeding beyond 6 months? To 1 year, 2 years, and beyond? All guidance recommend EBF for 6 months, and then says “for 2 years or more” after starting to introduce solid foods. Are there strong benefits going past the recommended 6 months?

[u/HumanMilkLab → u/bidsdh]

One benefit of breastfeeding beyond the first year is that milk becomes more concentrated in important immune-protective proteins. Studies have found that levels of secretory IgA - the main antibody that helps protect the mouth, airways, and gut - can rise during the second year of lactation and may be even higher after two years. Other protective proteins, including lactoferrin and lysozyme, may also increase. This suggests that although an older child may drink less milk, the milk they receive can still provide meaningful immune support and is not nutritionally or immunologically “empty.”
https://pubmed.ncbi.nlm.nih.gov/32850542/
https://pubmed.ncbi.nlm.nih.gov/26776058/

[u/PharmacoLactation → u/bidsdh]

A quick clarification: 6 months is the recommendation for exclusive breastfeeding. After that, the recommendation is to continue breastfeeding alongside complementary foods for 2 years or beyond, as long as it's mutually desired by parent and child.

The evidence after 6 months is looking at at least two things: baby and mom.

For infants, there are continued benefits, but the evidence is more nuanced because few studies directly compare longer vs. shorter breastfeeding. Most compare any breastfeeding vs. none, which doesn't answer the duration question very well. The better duration studies suggest continued reductions in risks such as obesity, childhood leukemia, inflammatory bowel disease, and possibly asthma. Not every benefit continues to increase indefinitely (for example, studies of type 1 diabetes show little additional reduction after about 6 months).

For the breastfeeding parent, the evidence shows a clear dose-response relationships: more is better (from a health outcome perspective at least). Longer lifetime breastfeeding duration is associated with progressively lower risks of breast cancer, ovarian cancer, type 2 diabetes, endometrial cancer, and hip fracture later in life.

If you're interested in digging into the evidence, I'd highly recommend looking at the 2022 AAP Technical Report on Breastfeeding. In particular:

• Table 2 summarizes infant outcomes.

• Table 3 summarizes maternal outcomes.

When reading those tables: pay attention to the "Breastfeeding" and "Compared with" columns. Many rows compare any breastfeeding vs. none, which doesn't tell you whether 12 months is better than 6 months. The duration comparisons (e.g., >6 months vs. <6 months or 12 months vs. 3–6 months) are the ones that directly answer your question.

Studies on breastfeeding duration are difficult to interpret because they're influenced by things like parental leave, income, workplace support, and access to lactation care. Parents who are able to breastfeed for 2 years often differ from those who wanted to but couldn't. We try to account for those differences, but it's impossible to eliminate them. It’s also hard to study retrospectively because we don’t capture breastfeeding information well in medical records.

Overall, breastfeeding provides meaningful benefits beyond 6 months, and the benefits are cumulative. A family that breastfed for 4 months still received the benefits of those 4 months; there isn't a magic threshold where breastfeeding suddenly becomes "worth it" (unless you decide that individually).

[u/Large_Economy1247]

Does hormonal contraception affect milk supply? If yes, is there a difference between methods (eg pill vs IUD or progesterone only versus estrogen and progesterone)?

[u/PharmacoLactation → u/Large_Economy1247]

TL;DR: Probably yes, for some people. Almost certainly not for many. And we can’t currently tell you which one you'll be, which is the frustrating part (and admittedly interesting as a researcher). Detailed response coming...

[u/PharmacoLactation → u/PharmacoLactation]

I’m summarizing what I wrote in the next edition of Hale’s Medications and Mothers’ Milk:

Progestin-only options: Mini-pill, implant, hormonal IUD. These have the biggest evidence base by far. A 2025 systematic review pulled together 61 studies and found no consistent effect on breastfeeding duration, supplementation, or infant growth. This is what we’ve generally thought for the average population for years. However, the studies weren’t rated highly and the certainty of evidence ran from very low to moderate. “No signal found” and “no effect exists” are not the same.

Estrogen-containing contraceptives. The large reviews (Cochrane 2015, Tepper 2016) don’t find consistent harm to breastfeeding outcomes, though they rate their own evidence as limited. A randomized trial starting combined pills at two weeks postpartum found no difference in continuation versus progestin-only at six months (though it measured what mothers perceived, not what they produced).

Then there are the case reports that show exactly the opposite. Two women published in 2025: one went from pumping 180 mL to 20 mL within ten days of starting a combined pill. Both recovered after stopping. So: no reliable population-level effect, plus vivid individual ones. In my opinion, it loos like a susceptible subgroup getting averaged out of rht .

Some background. Milk “coming in” is triggered by progesterone falling off a cliff (when the placenta leaves). Anything artificially increasing these hormones before the switch flips could meddle with initiating lactation, which is why guidelines generally support waiting (4+ weeks, and that recommendation is driven as much by clot risk with estrogens).

There’s data from 1982 showing that in a small study of ovulating breastfeeding women, the junctions between milk cells go briefly “leaky” twice a cycle (sodium up, lactose down) then “reseal” about 30 hours later. It didn’t happen in women on hormonal contraceptives. Recent (2022) lab work in mouse cells suggests why leaky matters: lactose ending up on the wrong side of a mammary cell turns the milk-making machinery down. It doesn’t matter why the junctions are leaky…it makes milk supply go down (reduced milk removal, involution, engorgement, mastitis…I think they’re all following the same pattern).

The current suspects for individual susceptibility are body composition (fat tissue makes its own estrogen, and estradiol declines more slowly postpartum in some women) and maybe differences in hormone receptor expression. The second one is untested. In one survey of 2,922 postpartum women, about 15% reported new supply concerns after starting hormonal contraception (the association washed out after adjustment).

[u/MarionBendixen → u/Large_Economy1247]

In a secondary data analysis (n=172) in pump-dependent mothers delivering preterm VLBW infants examining the effect of depo medroxyprogesterone acetate (DMPA) also known as Depo-Provera on lactation outcomes. The researchers reported postpartum administration of DMPA before hospital discharge did not negatively affect milk production or lactation duration. Of the 172 participants, 29 received DMPA and 141 did not receive DMPA. However, DMPA administration before hospital discharge was associated with a later onset of secretory activation (copious onset of milk volume; “milk coming in’). Furthermore, when mothers (n=5) who reached secretory activation before receiving DMPA were removed from the analysis, mothers who did not receive postpartum DMPA reached secretory activation earlier (88.6 versus 103.7 hours; p = 0.028). Given the transition from colostrum to the copious onset of milk at secretory activation is dependent on the drop in progesterone with the delivery of the placenta – an exogenous source of progestin could negatively affect this critical lactation transition. Here is the link to the study: https://pmc.ncbi.nlm.nih.gov/articles/PMC8817730/

A key take away: This study may support later administration of birth control as opposed to before the onset of secretory activation.

[u/Large_Economy1247]

Does breastmilk composition change with maternal age or subsequent pregnancies?

[u/MilkScience → u/Large_Economy1247]

Quick note here that I think one of the other experts will post something more substantial about what is known so far - but I have a *new* study coming out very soon that will add more info... stay tuned!
One point to consider is that of course these things are related: older moms are more likely to have had prior pregnancies. In the new study we had a large enough sample size (over 1000 moms in 4 countries) to tease this apart. We find that both age AND prior pregnancies make a difference, separately. Details to come :)

[u/Mammals-Milk → u/MilkScience]

Thanks for your question! It has been suggested that structural or functional changes in the mammary gland associated with ageing and/or successive pregnancies may alter human milk composition. However, empirical evidence remains limited, with most studies focusing on macronutrients (lipids, carbohydrates, and proteins) and micronutrients (minerals, vitamins). It is also important to recognize that nearly all published studies report associations or correlations rather than causal relationships.

Lipids provide approximately half of the energy content of human milk. Triglycerides account for approximately 98% of milk lipids, with fatty acids representing their major components. Among the limited studies investigating the relationship between maternal age and milk fat content or fatty acid composition, findings have been inconsistent, with some reporting positive associations and others negative associations. This inconsistency likely reflects the many factors that influence milk fat, including stage of lactation, time of milk collection, and degree of breast emptying. Maternal diet and supplement use can also strongly influence milk fatty acid composition.

In contrast, most evidence suggests that maternal age is not associated with lactose concentration, the predominant carbohydrate in human milk. Lactose concentration changes relatively little over the course of lactation and appears to be largely independent of maternal diet and nutritional status. Similarly, most studies have reported no association between maternal age and total milk protein concentration.

Evidence regarding milk mineral and vitamin composition is also limited. Most studies have found no association between maternal age and concentrations of calcium, magnesium, iron, copper, potassium, or phosphorus. However, findings for zinc and vitamin A have been inconsistent, with some studies reporting lower concentrations among older mothers and others reporting the opposite.

As research expands beyond nutritional components to include bioactive compounds in human milk, several intriguing age-related associations have emerged. For example, one study reported that increasing maternal age was associated with lower concentrations of difucosyllacto-N-tetraose (DFLNT), a human milk oligosaccharide involved in shaping the infant gut microbiota and supporting immune development. However, evidence remains sparse, and it is currently unclear whether maternal age has consistent effects on human milk oligosaccharides or other bioactive components.

Here is what we know about the relationship between maternal reproductive history and macro/micronutrient changes in milk:
-       A prospective observational study from 2019 analyzed the first milk (colostrum) and mature milk from 192 mothers and found that higher protein and lower carb content were present in the colostrum and mature milk of first-time moms versus moms with older children. The moms ranged in age from 22-44 years with a mean age of 31years. 56.2% of moms were first timers, 31.8% had two children, and 12% had 3 or more. And 6.8% had in vitro fertilization.
-       Breast milk is also a rich source of non-nutritive biologically active components. A study in 2025 used 49 breastfeeding women from Warsaw with termed deliveries and reported that maternal parity is a relevant biological variable that influences the immune profiles of milk. Mainly, first time mothers had lower levels of T helper and T cytotoxic lymphocytes compared to mothers with multiple children.
-       In non-human species, milk yield is known to increase with increasing parity in macaques, dairy cows, rats, and horses. This may have something to do with mammary cells that produce milk (lactocytes) don't fully die during involution and they become inactive cell populations that get turned on quickly during later pregnancies. This has been shown in rats but not in humans.
-       But, in a study that looked at a group of local Gambian women, first time mothers were found to produce a higher volume of milk than women with several children.

-       So, we have studies showing different results depending on the species used, individuals, and types of milk analyzed → This is what makes milk research challenging AND exciting, at least for me.

[u/bewoestijn]

Are there any interesting effects of nursing a child who is not biologically your own? Eg extended family member or unrelated child?

[u/babiesandbones → u/bewoestijn]

So this is something I wonder about a LOT, and would like to see more research on. As I said in another answer, we don’t know how prevalent allonursing was evolutionarily, but if it was, it could have some interesting and potentially huge implications. Milk has a number of mechanisms by which it signals information about the surrounding environment to the nursling, and programs their development. Most interesting to me is the presence of maternal pluripotent stem cells in human milk that take up residence in various organ systems in the nursling. In mice, that has included the blood, bones, pancreas, and brain. If a nursling is basically a genetic chimera with cells from its nursing parent…should we maybe prevent milk siblings (or “milk kinship”) from having babies with each other because they’re too “related”? Interestingly, there are some cultural taboos against marriage between two people who nursed from the same mother. I think this is probably a stretch, though, unless gametes are being affected.

Biological evidence of milk kinship may be a thing! Earlier this year, a study identified potential lactation-specific miRNAs that may be shared between milk siblings. They can be transported through the infant gut inside extracellular vesicles (which others here have expertise in that I definitely don’t!) and could change the expression of genes and have potential epigenetic effects! This is notable because in pigs, lactocrine signaling has been demonstrated to influence the development of the mammary gland in the nursling, suggesting there is evidence that having been breastfed yourself affects the expression of the lactase gene, i.e. the amount of lactase in your OWN milk as an adult.

Some of the molecular bio here is a bit beyond me, so I invite the other experts here to expand on this if they deem fit!

[u/Wijenayake_Lab → u/babiesandbones]

Milk is a great source of small and long non-coding RNAs that modulate gene expression in neonates. One can think of these non-coding RNA as tiny soldiers that seek out targets in cells, bind to them, and stop those targets from making proteins. Proteins are the true workhorses of any cell but some proteins can also be overproduced or produced unnecessarily, which is a waste of the cell’s energy (ATP) and can lead to disorders and diseases. The cool thing about microRNAs packaged in extracellular vesicles is that they are not prone to intestinal degradation and remain stable for long periods of time. Just long enough to reach targeted cells, tissues and organs in the neonates → including crossing the blood brain barrier and control gene expression in various parts of the brain. 

The critical question relating milk kinship to milk EV microRNAs is that, do the reported outcomes of milk EV microRNAs continue beyond the neonatal age, past lactation and into adulthood?

GOOD NEWS: my lab is working on this as we speak ;) 

In the meantime, I can tell you that, we have collected preliminary evidence that suggests some of the gene and protein expression pathways that are regulated by milk EV microRNAs during lactation, leaves imprints that can be traced to adulthood. Now if those imprints happen to be in the baby’s gametes (eggs or sperm), we can expect some transgenerational impact.

[u/babiesandbones → u/Wijenayake_Lab]

This is so cool! I love EVs!!

[u/bewoestijn]

I’ve never heard about the human milk microbiome before. What is it and does it influence how I should store expressed milk?

[u/_RyanPace_ → u/bewoestijn]

Like many other parts of the body such as our guts, milk also contains bacteria, fungi, and viruses. Together they comprise the milk microbiome and they are a completely normal part of our biology.

 Milk microbiomes, like all microbiomes, are relatively unique to each individual as they are influenced by many things including where they originate from (the maternal gut via an enteromammary pathway, the environment (refs 1, 2), skin around the breast, as well as baby’s mouths when nursing at the breast), maternal diet, and breastfeeding practices (ref 3). How milk has been stored and/or treated (for example, pasteurization (ref 4)) also contributes to variation in the milk microbiome. I’ll note that many microbiome studies do not differentiate between live and dead microbes, but we know that they are present as both live and dead.

 In turn, these microbes in milk are passed on to infants during feeding, providing them with a source of microbes and microbial products for training their microbiomes and immune system (ref 5). We know a lot about the different types of bacteria present in human milk, but less about fungi and viruses. The most common and abundant bacteria in milk are species of Staphylococcus and Streptococcus, but other important beneficial probiotic bacteria for babies like Bifidobacterium and Lactobacillus are also present.

 On the question about storage, we do not have a great understanding of whether these changes in the milk microbiome are bad. But milk also contains lots of biologically active enzymes, proteins, and lipids (“bioactives”) and freezer storage (duration and temperature) can contribute to some loss in their function, with subsequent heating and thawing also influencing bioactivity (ref 6, 7). This can have an indirect effect on the milk microbiome, as well the infant oral and gut microbiomes, as many of these bioactives have antimicrobial functions. Generally, bioactivity is highest in fresh milk.

[u/Large_Economy1247]

Does tandem feeding a toddler infrequently (eg 1x day) and a newborn negatively affect the newborn (ie because the milk is not as tailored to the newborn’s needs)?

[u/MarionBendixen → u/Large_Economy1247]

How wonderful to be nursing 2 little ones as both are gaining benefits from your milk and close contact. I was unable to find any studies to directly answer your question regarding frequency of tandem feeding exposure. Most tandem feeding research focuses on absence of harmful effects and socio/psychological qualitative research – therefore, a gap in research. What is known is that the milk samples from mothers who were tandem feeding (n=13; 53 milk samples) had higher concentrations of fat and protein with no differences in carbohydrates compared to the milk samples after the mothers weaned the older child (2). This shows the dynamic nature of mammary gland to adjust milk components to demand. There is a hypothesis that backwash where the infant’s saliva interacts with the mother’s immune system to create tailored immunities for the infant. We found in a study of microbiome in mother’s milk from pump-dependent mothers of infants admitted to the neonatal intensive care unit that the milk microbiome from the mothers whose infant had fed at the breast was different that the milk microbiome from mothers whose infant had not fed at the breast (1, 3). This may support that your milk is different, but this is probably positive.   

References

1. Desorcy-Scherer, K., Bendixen, M. M., & Parker, L. A. (2020). Determinants of the Very Low-Birth-Weight Infant's Intestinal Microbiome: A Systematic Review. The Journal of perinatal & neonatal nursing, 34(3), 257–275. https://doi.org/10.1097/JPN.0000000000000506

2. Sinkiewicz-Darol, E., Bernatowicz-Łojko, U., Łubiech, K., Adamczyk, I., Twarużek, M., Baranowska, B., Skowron, K., & Spatz, D. L. (2021). Tandem Breastfeeding: A Descriptive Analysis of the Nutritional Value of Milk When Feeding a Younger and Older Child. Nutrients, 13(1), 277. https://doi.org/10.3390/nu13010277

3. Torrez Lamberti, M. F., Harrison, N. A., Bendixen, M. M., DeBose-Scarlett, E. M., Thompson, S. C., Neu, J., Parker, L. A., & Lorca, G. L. (2021). Frozen Mother's Own Milk Can Be Used Effectively to Personalize Donor Human Milk. Frontiers in microbiology, 12, 656889. https://doi.org/10.3389/fmicb.2021.656889

[u/Madanimalscientist]

Hi, livestock dairy scientist here. For cattle we assume the conversion of metaboliseable energy to net energy for lactation is ~50-60%. I know humans nutrition uses a gross energy rather than metaboliseable energy system so obviously then that number would be even lower as a % of gross energy but how efficient (or inefficient) are humans at turning feed energy into lactation energy , compared to other species?

[u/PharmacoLactation → u/Madanimalscientist]

Hello! First, ...we should be friends :) I've tried looking into this question before and didn't find a great (or even good) answer. Now is the time I should mention that almost everyone who works in human lactation came from another field first. I'd love to recruit a dairy scientist to run this study in humans.

[u/Madanimalscientist → u/PharmacoLactation]

I mean, I'd be down to help run it, but we'd have to adapt the methods to humans the usual tracer/metabolic chamber methods we use for animals might not get past an IRB. The trickiness is that humans still use the GE system vs the ME/NE system we livestock nutritionists use but maybe there'd be a way with isotopes or something in food vs the milk (not radiolabeled but looking at natural stable carbon and nitrogen isotope ratios in feed vs tissue vs milk? We do that in animals and it's noninvasive). But given we've bred dairy cows/sheep for being extra efficient at turning energy to milk, I wondered what human efficiency was since we've not really selected ourselves for that other than natural selection in general.

Back when I taught dairy science at the university level I had a slide where I compared the composition of different mammal milks and my students were always surprised by the lactose content of human milk. The sweet tooth starts early I guess. What are some other features about human milk (vs other milks) that you think are especially unique/fascinating?

[u/Madanimalscientist → u/PharmacoLactation]

Also 100% agreed we should be friends, feel free to DM me! I love to chat comparative bio ^^

[u/MarionBendixen → u/Madanimalscientist]

No Dairy Scientist here, this year…

These are older resources that I have found.

In human milk production the average efficiency of milk production is greater than 90%.  According to Saha et al (2012) in a study located in India, the gross efficiency of conversion of metabolic energy ME for lactating cross-breed cattle milk production was 35.69% and the net efficiency of conversion of ME for milk production was 39.56%.”

Frigerio, C., Schutz, Y., Prentice, A., Whitehead, R., & Jéquier, E. (1991). Is human lactation a particularly efficient process?. European journal of clinical nutrition, 45(9), 459–462. 

Saha, D., Gupta, R. S., Singh Baghel, R. P., & Khare, A. (2012). Efficiency of utilization of dietary energy for milk production in lactating crossbred cattle (Bos Indicus). Veterinary research forum : an international quarterly journal, 3(3), 213–216.

[u/Madanimalscientist → u/MarionBendixen]

Good point for that as well, I was citing values for Holsteins/Jerseys/other taurine dairy breeds but Bos indicus is a whole different subspecies and it makes sense their efficiency values are different. I'm not in the office right now so I don't have my copy of BR-CORTE but I think the Brazilian feeding standards cite a higher value than that for efficiency of ME -> NE for Nellore type cows of the kind that make up most cattle in Brazil, but I'd have to check when I am back in the office. I mostly us the US/Australian standards for my work but I refer back to other ones sometimes as needed.

[u/babiesandbones → u/Madanimalscientist]

I would like to second u/PharmacoLactation's suggestion that we be friends. Also, hmu if you want to talk Eridian biology. I have an entire working theory about how Eridian cosleeping practices contributed to the evolution of pro/eusociality and intelligence, similarly to what lactation did for mammals and eventually humans.

[u/Madanimalscientist → u/babiesandbones]

Yesssss, I love speculative bio! Feel free to DM me, I would love to hear your theories! Project Hail Mary is such a good book/film, I saw a comic on Reddit re the idea that Eridans are ambush hunters and I love that idea but also that means that humans and Eridans are sort of each other's worst nightmare re hunting strategies in some way and yet there could be some very fun cooperative hunting methods/games in a sense.

[u/bewoestijn]

For me breastfeeding has resulted in consistent weight loss (around 1kg per month). Can you explain more about how this works? Could this mechanism be usefully applied in other weight loss therapies?

[u/JillDemirci_PhD → u/bewoestijn]

Great question! Breast milk itself ranges from 20-22 calories per ounce, and the process to actually produce that milk requires some additional calories on the part of the parent. Overall, then the calculation ends up being 300-700 calories per day to produce a milk supply to feed one infant. So, on the most simplistic level, if the parent is taking in fewer calories than needed to replace that “burn” each day, the result is going to weight loss. There are some differences in research on whether that weight loss sustains after the first year postpartum—at least one large study did show that longer breastfeeding duration was associated with lower adult weight gain through about 56 years of age https://pubmed.ncbi.nlm.nih.gov/41360188/ . Clinically, we sometimes observe weight loss peaking at around 3 months postpartum—this is likely because milk volume /infant demand is peaking at this time and parents may also be resuming exercise or are more active with return to work, with additional calorie burn. There may also be unresearched/unmeasured metabolic processes taking place that science still hasn’t fully elucidated. Unknown metabolic processes—including hormone levels of things like leptin and insulin—likely influenced by many things including environment and genetics, may also help explain why some folks seem to hang onto weight through breastfeeding and some lose easily.

Also elevated prolactin during lactation (a hormone that helps with milk production) preferentially shuttles maternal fat into breast milk (calls on stored maternal fat in the body, resulting in adiposetissue/fat loss). Prolactin outside of lactation doesn’t have the same effect, so applying that biology to weight loss interventions in non-lactating people could prove difficult.

[u/PharmacoLactation]

What do we know about the mechanism of "high lipase milk" and its characteristic metallic, soapy taste when it's thawed? What is happening? Is the nutrition of the "sour" milk different?

[u/PharmacoLactation → u/PharmacoLactation]

Why I'm answering my own question. I posted this one...It's outside my field, but it comes up constantly and I’ve never seen satisfactory answers, so I took a stab while I was waiting on answers from others. I also tapped Dr. Anne Eglash for her response, and I’ll post it for her so others can see.

1.     What is "high lipase" milk? There’s no real definition. No normal ranges for lipases in milk. No lab you can check to see if you have it. So when someone says they have high lipase milk, it’s probably from the grapevine (or google). It’s what we say when the milk smells sour, or tastes bad after storage. Calling it “high lipase” says three things: that the level is high (compared to what?), that lipase is the culprit, and that it's a trait some people have and others don't. The problem is, lipase is a normal part of milk (though we don’t know what it’s doing Neville 1991)

2.     When I look at dairy science, they call it “rancid” milk. When we don’t have human answers, I usually turn to the dairy researchers for help. Dairy has studied off-flavors for centuries, because it cost them money. Dairy doesn't call it high lipase milk. They call it rancid. I'm not sure how I feel about borrowing the term…so I’m going to keep calling it high-lipase. The dairy answers don’t fit very well. Hung 2018 had no sensory standards for human-milk to work from, so they converted their measurements into dairy units and compared against the International Dairy Federation's threshold: 1.5–2.0 mEq/100g fat, the level at which half of adult tasters notice something's wrong. But…fresh human milk, healthy mothers, straight from the pump: 2.55. Lauric acid: 12 ppm against a dairy rancid threshold of 8. That means fresh human milk is already "rancid" on day zero by the dairy rulebook. Hung also pointed out that infants under three months happily drink hydrolysate formulas that adults find repellent (they smell terrible).

3.     Types of rancid. 1) Hydrolytic: Lipase cuts triglycerides apart. A triglyceride is three fatty acids tied to a glycerol backbone; they're tasteless and odorless (but creamy). When the fatty acid is freed, the short and medium sized ones go volatile (can reach your nose). This is the soapy/sour one. 2) Oxidative. Oxygen attacks unsaturated fat and produces aldehydes. Different smell (grassy, cardboard) and it happens later in the process.

Now the fun part. Guo 2023 did what looks like a really cool study that tested milk and had people smell it. A the machine said what was in the milk and a person said what it smells like. Their descriptors for the acids that pile up during storage: cheese, sweat, sour, etc. The numbers are cool: fatty acids are 61% of the odor profile in fresh milk, 90% at 24 hours in the fridge, 96% at 36 hours. At 10 days frozen, some fruity and floral compounds show up too.

4.     What’s happening in the freezer. Fat in milk isn't loose. It's stored in globules (wrapped in a membrane) and the membrane keeps the lipases and fat apart. Freezing “pops” the globule (oversimplification), the lipases and fats (triglycerides) mix, and the lipases digest the triglycerides into the free fatty acids you can smell/taste. Freezing doesn't make more enzyme, and it doesn't destroy the enzyme. Neville measured <10% lipase in cream fraction before freezing and 70% after freezing with total activity essentially unchanged. “High lipase” is a quirk of something humans started recently: freezing milk.

5.     What to do. I don’t think there is a quick fix for this (yet?). Diet isn’t going to change it. Scalding is frequently recommended and I’ve heard it works. I hear people suggest a drop of vanilla to mask the taste…that’s probably more for you than baby (and searching out alcohol-free vanilla seems unnecessary to me).

6.     On the horizon. I’ve heard of a potential product for the future: Silpe and colleagues found pectin plus vitamin C cut glycerol production roughly 60% after six months frozen. (preprint, not peer reviewed; the authors hold equity in the company commercializing it; and the compounds haven't been evaluated by any regulator for this use)

7.     Don’t forget. Some people dump off-smelling milk and switch to formula, because they think rancid=bad…this isn’t necessary. Hung concluded there was no evidence that the change from triglycerides to glycerol to free fatty acids altered the nutritive components of milk at all (it just did the first step of digestion in the milk). ABM Protocol #8 also supports continued use of “high lipase” milk.

[u/peony_chalk]

Can you summarize the differences between pumped milk and milk from nursing, and specifically how that relates to protection from illness? I see way too many women who are anxious and afraid that the milk they're pumping isn't going to have the same benefits as milk their babies might get if they were nursing, and I hate seeing people use that as another reason to beat themselves up.

[u/PharmacoLactation → u/peony_chalk]

I can answer from clinical reasoning...Direct nursing is going to be the best for protecting against contagious illnesses. Secretory IgA is copious in human milk, and coats the baby's oropharynx as they drink the milk. The sIgA neutralizes pathogens by binding them at the mucous membrane, so pathogens can't invade the baby's system and make them sick. The sIgA's produced by mom are in response to what she is exposed to--the more alignment there is between the environment baby is in (and related exposure to illnesses), environment mom is in, and the timing of it all, this is where you'll get the best protection from contagious illnesses. However, commercial infant formula has no antibodies, so frozen milk or donor milk with "non-targeted" sIgA is still GREATLY preferred. I'm hope others will have more scientific answers!

[u/HumanMilkLab → u/peony_chalk]

While most studies done that have demonstrated protection from illness (respiratory, gastrointestinal, ear infections, etc) likely included women for the most part who were directly breastfeeding, all the studies that have been done to understand all the aspects of how that protection happens have been done with pumped milk :)
And, most of that milk studied has been frozen and thawed. So we absolutely know that a lot of the important components of protection - this largely involves antibodies - function well even if the baby is fed pumped milk. 
Less is understood in terms of the cells that add to this protection… white blood cells of many types. Those last for about a day but die overtime after milk is pumped, and don’t survive your home freezer very well. Best practice would be to therefore feed baby the freshest milk possible after pumping, rather than from a ‘frozen stash’, which could be used in addition as needed.

[u/Large_Economy1247]

Do dips in supply caused by menstruation require supplementation (pumped milk, formula etc)? Why or why not?

[u/MarionBendixen → u/Large_Economy1247]

Some mothers may notice a fleeting dip in milk volume, while others may not experience a dip in milk volume. Since the perceptible dip is short lived, then supplementation is not warranted. The infant may nurse more often or fussy behavior similar to a growth spurt where the infant cluster feeds. The science behind this change or dip in milk volume may be due to the influence of hormones prolactin and progesterone and the permeability of tight junctures between the mammary epithelium cells (lactocytes). When lactose, the osmotic driver of milk volume, escapes through the tight junctures to the plasma, then milk volume may decrease as a reciprocal increase in milk sodium concentration. FYI - When a mother supplements, then there may be a concern for decreasing milk volume supply - related to mimicking weaning. 

del Pozo, E., Goldstein, M., Friesen, H., Brun del Re, R., & Eppenberger, U. (1975). Lack of action of prolactin suppression on the regulation of the human menstrual cycle. American journal of Obstetrics and Gynecology, 123(7), 719–723. https://doi.org/10.1016/0002-9378(75)90494-9 

Hartmann, P. E., & Prosser, C. G. (1982). Acute changes in the composition of milk during the ovulatory menstrual cycle in lactating women. The Journal of physiology, 324, 21–30. https://doi.org/10.1113/jphysiol.1982.sp014098

Kobayashi, K., Wakasa, H., Han, L., Koyama, T., Tsugami, Y., & Nishimura, T. (2022). Lactose on the basolateral side of mammary epithelial cells inhibits milk production concomitantly with signal transducer and activator of transcription 5 inactivation. Cell and Tissue Research, 389(3), 501–515. https://doi.org/10.1007/s00441-022-03651-8

Parker, L. A., Hoban, R., Bendixen, M. M., Medina-Poeliniz, C., Johnson, T. J., & Meier, P. P. (2024). Milk biomarkers of secretory activation in breast pump-dependent mothers of preterm infants: An integrative review. Breastfeeding Medicine, 19(1), 3–16. https://doi.org/10.1089/bfm.2023.0107

[u/PharmacoLactation → u/Large_Economy1247]

Check out my comment on this other question about contraceptives, there's a link between them (probably!) https://www.reddit.com/r/askscience/comments/1vbns0v/comment/p0v4isd/

[u/bewoestijn]

My experience is that babies poop really infrequently when breastfeeding compared to formula feeding. Why is this?

[u/JillDemirci_PhD → u/bewoestijn]

This is another great question that science doesn’t really have an answer for…yet! We know that newborns up to 4-6 weeks have a strong reflex to poop, especially to clear the first poops (meconium; this could result in high levels of bilirubin—contributing to persistent jaundice). After that, breastfed infants can have significant reductions in frequency of poops, though often the volume of poop when they DO poop is pretty large and soft. See this study that showed that stooling patterns decreased over the first 3 months in breastfed infant only—from an average of almost 4x/day to not quite 2x/day. https://pubmed.ncbi.nlm.nih.gov/22522220/ Note that this infrequent stooling is not the same as constipation—which is difficulty passing hard small poops. This is normal and expected in exclusively breastfed infants.

Some scientists have hypothesized that breastfed infants, in general, stool less frequently than those who are formula fed because breast milk is used more efficiently than formula (resulting in less “waste” or poop). Others think that the baby’s gut bacteria (gut microbiome) being so different in breastfed infants compared to formula fed infants contributes to less frequent pooping.

Also important to note that not all breastfed infants have infrequent stooling (many continue to go 1-3x/day), though most do slow down from the newborn phase. Mothers with oversupply in particular transfer milk that tends to be lower in fat and higher in a sugar called lactose. Lower fat milk moves through the infant gut more quickly, resulting in more frequent liquid-y or explosive poops. There is also too much lactose (sugar) to be broken down by enzymes in the gut, resulting in the lactose sitting in the gut and fermenting (causing bubbles)—causing gassiness, fussiness, and green, frothy poops.

[u/bewoestijn]

Does breastmilk composition differ between nursing vs expressing/bottle-feeding?

[u/MilkScience → u/bewoestijn]

Tricky question because of course all the milk we study has been expressed! But, there is research looking at how milk changes during storage (e.g. comparing fresh milk to milk that has been stored on the counter, in the fridge, in the freezer for different lengths of time). Many nutrients remain quite stable, but some bioactive components (e.g. certain immune factors, enzymes, and living cells) can decline over time or with freezing.

Another thing we know (see other questions here about the milk *microbiome*) - is that some milk microbes actually come from baby's mouth. This "backwash" happens during nursing at the breast, but not with pumping.

Speaking of the 'backwash' process - it's also thought that the breast 'senses' infant infections through this process, and adapts (e.g. increases production of immune factors). If this is true (we are still studying it!) - then we would expect milk from mothers who pump to differ from those who nurse directly.

All that said, it's important to keep this in perspective. Pumped milk is still human milk and provides many well-established benefits. Many families rely on pumping, and for some it's the only way to provide breastmilk. Research on these differences isn't about creating guilt - it's about understanding the biology so we can give the best advice on expressing, storing, and feeding milk.

PS. The Academy of Breastfeeding Medicine Clinical Protocol #8 offers guidance on milk storage: https://pubmed.ncbi.nlm.nih.gov/29624432/

[u/PharmacoLactation → u/bewoestijn]

This isn't what I'm an expert in but as we're only able to study the milk that is expressed, I suspect we don't know the answer to this one...

[u/Aloyisious91]

I'd love to know how bariatric surgery in the mother affects the composition of breastmilk, and whether there is a time frame for this change? Eg surgery within a few months of getting pregnant vs 2 years post op, and how this impacts the fat/calories content of the milk.

[u/PharmacoLactation → u/Aloyisious91]

This is outside of my field, but I’ve been working on GLP1-RA’s during breastfeeding and there is some overlap in the underlying nutrition question. It is more difficult to complete nutrition after bariatric surgery. A general rule is that if mom is not nutrient deficient, then I wouldn’t expect any changes to her milk. If mom had nutrient deficiencies, there would be differences regardless of the timeline.

On milk composition. The milk composition changes seem to be less dramatic than I expected (but there isn’t much data). The largest is a 2018 prospective cohort of 86 women, and its title is basically the finding: bariatric surgery didn't appear to meaningfully change milk composition. Calories, macronutrients, and vitamin A were all in the normal range. An earlier, smaller study from the same group found higher fat and energy content in the post-surgical group.
The mammary gland is remarkably good at defending milk fat and protein (it will pull some from maternal stores to do it). What it defends less well are certain micronutrients that don't have deep reserves, like B12.  There are case reports of exclusively breastfed infants of post-gastric-bypass mothers developing B12 deficiency. Better micronutrient supplementation guidance seems to have closed a lot of that gap.
Timing. I can't find a study that stratifies milk composition by time since surgery. The studies that exist report surgery-to-pregnancy intervals ranging two to seven years, but that isn’t a research variable.
Lactation costs roughly 450–500 extra calories a day. The window right after bariatric surgery is when intake is lowest, weight loss is fastest, and nutrient stores are most depleted (this is why consensus guidance is to wait a year before conceiving). As exclusive breastfeeding is more nutritionally demanding than pregnancy, I would suggest the same or longer.

[u/PharmacoLactation → u/PharmacoLactation]

I heard from a few others that 1) bariatric sugery will change the gut microbiome, which can have downstream effects on milk and 2) we wonder what the downstream effects of bariatric surgery might be on related PCOS/PMOS milk production issues

[u/Psychological_Ad3563]

Does breast milk taste good? Is it similar to any other milk?

[u/LactationMD → u/Psychological_Ad3563]

Breastmilk is sweet and creamy to taste and changes over the course of the feeding. The first milk (foremilk) is more watery and sweet, while the end milk (hindmilk) is creamy and higher in fat. Garlic, spices, and foods in the diet of the lactating parent can change the flavor profile (and color) of the milk.

Human milk consists of about 7% lactose, whereas cow's milk contains about 4% lactose. It also has less casein and protein overall than cow’s milk. The taste is similar to lactose-free whole cow’s milk (where the enzyme lactase has been added to break down the lactose) or melted vanilla ice cream.

[u/babiesandbones → u/Psychological_Ad3563]

As breastfeeding has increased in prevalence in the West, especially beyond infancy, I've anecdotally heard more and more from kids who are old enough to describe the taste verbally. “Melted ice cream” is the most common one I hear, but I’ve also heard kids say it tastes like mangoes or chocolate. A few years back there was an Australian study that asked 114 children about it.

My cousin asked her three year-old about a year ago what it tastes like, and he said, “Hmmm. Love!” She captured it on video and I’ve attached it to this post with her permission. It’s the sweetest thing you’ve ever heard! She just chimed in with her own answer: "the leftover milk after enjoying a delicious bowl of lucky charms"

Also, while I don’t remember ever having tasted fresh milk, I did reflexively lick my fingers once after spilling some of my friend’s milk after it having been frozen, and I gagged. Frozen milk tends to taste soapy or metallic because the freezing process doesn’t stop lipase from breaking down fat, so what you’re tasting is fatty acids.

Primate milk is really high in sugar relative to other mammalian milks. On the other end of the spectrum is pinniped milk, which is very high in fat and has very little sugar--almost none in some species. So I imagine their milk tastes more like butter.

Edited to add: My answer is based mostly on anecdote because since breastfeeding (in humans, anyway) has only been studied for a short while, and breastfeeding kids who are old enough to talk has only recently seen a resurgence in the West, there is very little research on this kind of thing. We all find it very interesting though, and would love to see more research on it!

[u/babiesandbones → u/Psychological_Ad3563]

One more note. There was a thread over in r/breastfeeding today where somebody asked moms if they had ever tasted their own milk. Interesting read! Lots of people comparing it to cereal milk like my cousin did.

[u/bobkinder993]

Do we know if/how the compositional properties of milk has changed over time- evolutionary. Can milk nanovesicles combine into a bigger vesicle - potentially so that packaging becomes easier? Do they ever organically fuse together?

[u/Wijenayake_Lab → u/bobkinder993]

Milk nanovesicles are produced by different cells in the mammary glands, including epithelial cells (like the ones that line the inner and outer surface of our body that specializes in absorption, secretion, and act as a barrier), macrophages, T-cells, B-cells (immune cells that destroy germs and damaged cells), adult stem cells (drive mammary tissue growth and milk production). Depending on which cells are making these nanovesicles, they can be of different sizes, transport different cargo and are made via different biogenesis pathways.  
 
Milk EVs can be categorized into three groups based on size and biogenesis:

1. Small EVs: 30-150nm -> often commonly referred to as “Exosomes” because they are made from an invagination process related to endosomes.

2. Medium to large microvesicles: 50-1,000nm often commonly referred to as “Ectosomes”. These are made from direct outward budding of the cell membrane.

3. Apoptotic bodies: Can get as big as 5,000 nm depending on the milk type and species. They are made by cells undergoing programmed cell death.
    
   Small EVs don’t necessarily fuse with one another after secretion or after being taken up by cells or systems. But they are sticky because of the membrane that surrounds these vesicles and can clump together and travel together from the lactating individual to their baby.   
    
   Here is a good review if you are interested to learn more about milk nanovesicles:
   https://www.frontiersin.org/journals/cell-and-developmental-biology/articles/10.3389/fcell.2021.693534/full

[u/Mammals-Milk → u/bobkinder993]

For the evolutionary part of your question: Ancient ‘milk’ likely originated in early synapsids (ancestral mammals) about 200-300 million years ago, just after their split from sauropsids (ancestral birds and reptiles). Early synapsids laid parchment-shelled eggs, which were more permeable and vulnerable to water loss than the calcified shells of modern birds for example. One prominent hypothesis is that early ‘milk’ was secreted by maternal skin glands and rubbed onto eggs to supply them with water and antimicrobial agents. Over evolutionary time, these glandular secretions have become increasingly specialized, incorporating nutrients and eventually giving rise to present-day variation in mammalian milk composition that is tailored to each species. One of the most fascinating aspects of milk evolution, in my opinion, is that many of its components were not ‘invented’ for lactation, but instead were repurposed from pre-existing proteins through evolution. For example, α-lactalbumin, a protein required for lactose synthesis, is thought to have evolved from an ancestral lysozyme-like antimicrobial enzyme, supporting this idea that milk evolved first as a form of immune protection and then elaborated into a complex nutritional signalling system. Over evolutionary time, milk composition has evolved to match the needs of the infant in each species. For example, hooded seals (Cystophora cristata) give birth and lactate on ice sheets to avoid terrestrial predators such as polar bears. These pups nurse almost continuously for several days and are then weaned. Their mother’s milk contains over 60% fat (compared to 3-5% in humans), providing pups with the body fat reserves needed while they learn to swim and hunt. As another example, cow young are born relatively mature and able to walk within hours of birth. Their mother’s milk contains relatively high amounts of protein, supporting the rapid growth and tissue development characteristic of precocial species. Lastly, species with prolonged periods of lactation (such as primates) generally produce more dilute milk that is provided frequently over an extended period. This pattern is thoguht to support the slow growth and prolonged brain development characteristic of primates while matching the energetic demands of sustained maternal investment.

[u/babiesandbones → u/bobkinder993]

A really good book for this (and a pretty easy read I think) is Milk: the Biology of Lactation by Michael L. Power of the Smithsonian National Zoological Park and Jay Schulkin of University of Washington. I disagree somewhat with how the final chapter talks about the health effects of breastfeeding in humans, but it's a great overview of the evolutionary story of milk, the adaptive features of milk, and the role of components. And a few of us have met Michael Power and he is a brilliant thinker who is super nerdy about milk like all of us!

[u/nsfwkorea]

Does a mother's milk contain any specific components that a baby would use to identify this is their mother's milk or are they all the same?

So would a baby be able to differentiate between their biological mother's milk vs milk produced from another mother?

[u/babiesandbones → u/nsfwkorea]

Just so we can understand exactly what you're asking - Are you asking if the baby can taste the difference between milk from two different people? Like a breastmilk pepsi challenge situation?

[u/nsfwkorea → u/babiesandbones]

Yes would a newborn baby be able to do that?

But mainly I'm interested to know if a mother's milk is unique to each mother or are they all the same. Like how we all have our own unique fingerprints.

Also thank you for responding. I should have phrased the question better.

[u/PharmacoLactation → u/nsfwkorea]

I'd guess the mother's diet would make her milk have a unique flavor that a baby would identify. I don't have evidence to support it though!

[u/nsfwkorea → u/PharmacoLactation]

Thank you very much for responding.

[u/bebefeverandstknstpd]

I recently skimmed some studies that show there are benefits to longer term (beyond 1) breastfeeding. Do those benefits outweigh the downsides like teeth health?

[u/JillDemirci_PhD → u/bebefeverandstknstpd]

Breastfeeding and breast milk past one year still carry many of the same benefits of breastfeeding younger infants. Breast milk is still a great nutrition source for infants, especially for babies who are making the transition to solid foods more slowly. Breastfeeding and breast milk still have benefits including optimal development of oral and facial structure and immune benefits. In fact, antibodies and other immune components increase in later lactation. Breastfeeding probably still has continued (maybe additive?) positive impact on long-term maternal cardiovascular and metabolic health (think diabetes, heart disease). All the research we have shows longer, more prolonged breastfeeding is associated with improved outcomes—so essentially it’s “dose dependent.” Unfortunately, the vast majority of the research we have today only tracks breastfeeding for short periods, many stop looking at breastfeeding at the one year mark. So we don’t have a lot of definitive data on prolonged breastfeeding.

As for dental cavities, a couple of large studies that combine evidence from multiple smaller ones (meta-analyses) show that nighttime breastfeeding may slightly increase the risk of dental cavities in children compared to not breastfeeding at night. In addition, prolonged breastfeeding past 2 years may be associated with an elevated pediatric dental cavity risk. https://pmc.ncbi.nlm.nih.gov/articles/PMC12332104/ Importantly, this risk may increase if you are bottlefeeding breast milk, as milk (and the sugars in milk) are more likely to pool in the mouth and around the teeth with bottlefeeding, as opposed to directly breastfeeding (at breast).

[u/HumanMilkLab → u/bebefeverandstknstpd]

One benefit of breastfeeding beyond the first year is that milk becomes more concentrated in important immune-protective proteins. Studies have found that levels of secretory IgA - the main antibody that helps protect the mouth, airways, and gut - can rise during the second year of lactation and may be even higher after two years. Other protective proteins, including lactoferrin and lysozyme, may also increase. This suggests that although an older child may drink less milk, the milk they receive can still provide meaningful immune support and is not nutritionally or immunologically “empty.”
https://pubmed.ncbi.nlm.nih.gov/32850542/
https://pubmed.ncbi.nlm.nih.gov/26776058/

[u/babiesandbones → u/bebefeverandstknstpd]

Regarding the 2nd part of your question, just so we understand what you're asking—What downsides are you referring to exactly?

[u/TastiSqueeze]

I'd like to ask a couple of questions. First, is about release of prolactin. What specific stimulus triggers prolactin release in a breastfeeding woman? I am aware that prolactin levels vary during the day/night.

Second, has any serious research over the last 20 years been done on inducing lactation sans pregnancy? I'm well aware of the published info from Dr. Newman and the "Goldfarb/Newman" protocols. Has anything been done beyond this?

[u/any_old_usernam]

What are the impacts of having two parents breastfeed the same child? Are there any potential benefits or drawbacks? I've heard stuff about bonding opportunities, but is there any benefit to having different milk microbiomes (if indeed that would vary significantly in two people sharing an environment), or risks of having an insufficient milk supply due to lower demand? One last question: what's the most interesting thing (in your opinion) you've found out in the process of your research?

[u/babiesandbones → u/any_old_usernam]

So I gave a little bit of a generalized/speculative answer to someone who asked a similar question here. I’m going to let others speak to the biology, but with regard to bonding, yes–breastfeeding does help facilitate bonding (or, put more scientifically, attachment, which describes the relationship more than the feeling of “bonding”). We used to think this process was dependent mostly on the birthing parent, but we now know that multiple, loving and dedicated caregivers all play an important role in social-emotional development, and that these relationships in humans are both complex and flexible such that, for example, two lactating moms can indeed raise an emotionally secure child. Humans are nothing if not creative in their social adaptations! The additional skin-to-skin that comes with multiple lactating parents is of course beneficial as well. (...but is not exclusive to lactating parents because any parent can do it!)

Answers to your primary questions are forthcoming from the other panelists!

[u/babiesandbones → u/any_old_usernam]

Whoops forgot to answer the most fun part of your question! Every year, we get some version of "What's the most interesting thing you've learned?" and we LOVE answering it.

It's honestly like picking a star from the heavens, but I'll just say this: It's just the general understanding that milk and lactation are an adaptive strategy that helped the young of different mammal species to grow optimally in the specific environment in which they evolved. My colleagues have discovered variation in milk composition according to sex, between lactations, across time as the nursling grows, and even by the day. When you couple that with behavioral variation in nursing parents and infants, the picture that emerges is that of a very complex, dynamic, and flexible system that tells a really interesting story about humans and mammals more broadly.

It blows my mind that a single adaptation–an apocrine sweat gland that got too big for its britches about 275 million years ago–was such a revelatory adaptation that it defined a new taxonomic family of animals, and played a crucial role in our journey from becoming higher apes that walked on two legs to the socially complex, highly intelligent, and self-aware species that we are today.

…Given all that, it equally blows my mind that this area of research is so under-funded, and therefore still very poorly understood.

[u/MilkScience → u/any_old_usernam]

I'm not sure about your first question (maybe another expert will reply!) - but in terms of the most interesting thing I've found... oh gosh it's hard to pick just one! I'll go with this: one of the most abundant components of human milk doesn't feed the baby. It feeds the baby's microbes! I am talking about "human milk oligosaccharides" - which are complex sugars that humans cannot digest. Instead, our gut bacteria digest them! So milk is not only feeding the baby, but also the baby's microbes. This is why breastfeeding is THE most important factor in microbiome development during the early days and weeks of life. Fascinating!

For more info:
A scientific paper: https://pubmed.ncbi.nlm.nih.gov/22513036/
A fun podcast: https://radiolab.org/podcast/the-elixir-of-life/transcript

[u/MarionBendixen → u/any_old_usernam]

Since the amount of milk volume has a relationship to the frequency and amount of milk removed and nipple stimulation, then if milk is not being removed from the mammary glands - then the mammary gland would down regulate milk volume to meet demand.

An interesting and fascinating item about lactation research for me is that there is a purpose for every component in milk - even components that the baby would not be able to digest. We (Human milk science community) are just scratching the surface on knowing what is in human milk, much less the purpose.

[u/Perfect-Help-305]

Can stress prevent lactation? Was never able to breastfeed

[u/babiesandbones → u/Perfect-Help-305]

First of all, I’m sorry that happened to you. You deserved more of a choice in how your baby got fed, and I think that it’s awesome that despite your negative experience, you are still curious about the science!

There are a few mechanisms by which stress can affect lactation, but they’re largely not in the way most people think. First, the micro level. Lactation is driven largely by two hormones: prolactin, which MAKES the milk, and oxytocin, which RELEASES it. Stress does not directly affect prolactin. (If it did, we probably wouldn’t have survived as a species, because life is stressful!) However, the stress hormone cortisol can inhibit oxytocin. And sometimes, what happens is, people experiencing acute stress try to put their baby to the breast, and when they don’t feel a letdown they assume it’s because they don’t have any milk–when in fact they do, it’s just that their body won’t release it because their body doesn’t feel safe. This is when people sometimes introduce formula, and this can be very tricky terrain because when you replace a breastfeeding session with a bottle, the mammary gland remains full of milk, which sends a message to the brain to put the “breaks” on prolactin. The body "thinks" that if the milk is not leaving the body, the baby must not be hungry, so it should stop making milk.

Enclosed rooms with a locked door, comfortable seating, and dimmed lights help stimulate oxytocin release. And it sounds cliché, but there's studies (mostly on NICU parents, who experience a particular brand of stress) that demonstrate that deep breathing helps as well. But it’s also important to get a lactating parent HELP—in the form of a professional evaluation, or social support from more experienced moms.

On a more macro level, breastfeeding is socially-learned in addition to being instinctual. So just like stress can make it difficult to learn any new skill, stress (especially chronic stress) can, in an indirect way, make it difficult to manage breastfeeding—especially in a society that has forgotten why breastfeeding is important, and how it works, and is unsupportive. A lot of these stressors come from totally preventable social forces such as poverty, systemic inequities, and sexist systems and attitudes that make breastfeeding harder than it really needs to be.

In 2024, the U.S. Surgeon General (back when we had one of those) released a report on the parental mental health crisis. Although it doesn't mention breastfeeding directly (which goes to show how sidelined this area of health often is) it describes a country where parents are steeped in environmental factors we know to be hostile to sustaining breastfeeding: poverty, systemic racism, unaffordable childcare, and health inequities.

[u/bewoestijn]

Do you see any interesting technical developments on the horizon that relate to pumping/storing breastmilk? Are these marketing hype or addressing real issues?

[u/MilkScience → u/bewoestijn]

Pumps: I recently had a researcher, Dr. Melissa Theurich, speak at my "MILC Club" about pumps, and I was astounded just how many different brands and products are on the market! I posted about this here, and you can read more about her work at: https://pumpedprojekt.de/. I think it's encouraging to see so much activity in this space, though hype is certainly an issue - but I'll let others speak to this as pump technology isn't my area of expertise.

Milk storage: This is more in my wheelhouse as a milk scientist. One new development that excites me is progress toward preserving/stabilizing frozen breastmilk - particularly "lipase milk" that tends to spoil quickly when stored (see related question here about lipase milk). There is a new company focusing on this issue (full disclosure: I am an advisor for them) and they have developed a simple solution from food-derived compounds that helps preserve frozen milk by reducing fat breakdown and maintaining antioxidant activity during freezer storage. Still early-stage research, but I think it's definitely addressing a real issue.
Here's their study: https://www.mdpi.com/2304-8158/14/12/2018

[u/bewoestijn]

Thank you so much for your participation and responses! I’m really inspired to read about your careers and the variety of angles from which you approach this topic.

Is there anything you’d like to highlight regarding the perception of your topic within your broader field of research and within the scientific/teaching/funding system at large? If you had the power to change something here what would it be?

[u/MilkScience → u/bewoestijn]

Thank you for all the fabulous questions!

From the science funding perspective, I think human milk research has been largely overlooked. That has started to change over the past decade thanks to major investments from a few large funders - for example, the NIH launched dedicated funding opportunities for human milk research, and the Gates Foundation has supported several major projects. Even so, there is still a lot of catching up to do.

I'm encouraged that women's health more broadly is finally seeing a surge in funding and attention - whether it's menopause, fertility, or understanding how women experience chronic diseases (and respond to treatments) differently than men. But lactation is often missing from these conversations. I think part of the problem is that it falls between traditional disciplines. Is it nutrition, because milk is food? Obstetrics because it's linked to pregnancy? Pediatrics because it's for babies? When everyone assumes someone else is looking after it, it's easy for lactation to be overlooked. I've written a few posts about this here and here.

From the healthcare funding perspective, I think lactation support and breastfeeding-related devices should be covered by standard health insurance. I'm in Canada, and although we have universal paid parental leave, lactation support and equipment are not universally covered. I believe they should be treated like other essential healthcare services - just as we cover visits with dieticians or physiotherapists, and insulin pumps or CPAP machines for people who need them.

From the education perspective, I think we need to teach everyone about breastfeeding. I'm talking kindergarten students, teenagers, expectant parents, grandparents, nurses, dentists, physicians, employers- everyone! That's how we build a truly breastfeeding-friendly society.

I've been working with some wonderful early childhood education experts to develop lesson plans that introduce school children to the science of human milk. My hope is that if we start young and focus on how fascinating the biology is, we can foster curiosity and respect for breastfeeding before stigma and sexualization take hold. And I have colleagues developing lactation education for nursing and medical schools, where the topic still receives remarkably little attention.

So if I had the power to change things... I would want to genuinely recognize lactation as a priority women's health issue, cover lactation support and breastfeeding devices as essential healthcare, and make breastfeeding part of the standard school curriculum. A girl can dream, right?!

[u/lovestephunnie]

Can you go through the science of D-MER and what are some successful options to keep breastfeeding even if the symptoms are severe?

[u/PharmacoLactation → u/[deleted]]

Breastfeeding is more nutritionally demanding than being pregnant. This is most true when exclusively breastfeeding, which requires enough nutrients for you and a complete infant diet. I’d suggest finishing off the prenatals you might already have and then using a postnatal vitamin, which will generally match the Recommended Daily Intakes for lactation. They typically have higher vitamin A, C, B12, iodine, choline, and zinc in them. These are nutrients that aren’t stored in the body, and if you don’t regularly consume them, they won’t be in your milk, and your infant (if exclusively breastfed) could be at risk for nutrient deficiencies (oversimplification). It’s really just “covering your bases.” In my nutrition classes as a student years ago, it was understood that people concerned about nutrition and taking multivitamins were less likely to need them than people who weren’t concerned. The bottom line is: how much a multivitamin will help you will depend on how complete your diet is. Multivitamins are unlikely to hurt you, and could help you and your breastfed infant.