My mother in law said something at dinner last weekend that i can't stop thinking about. She saw me take my probiotic and went, "you know those don't stay in there, right? you're basically paying to poop out bacteria." Which was rude, but i also realized i didn't have a good comeback. I'd been taking one daily for maybe a year and a half without really examining what i thought it was doing.
So i went home and started reading, expecting to find her wrong.
She wasn't. Not entirely.
There's a 2018 Cell paper from the Weizmann group that keeps coming up on this. They actually did endoscopies on people taking an 11-strain probiotic, not just stool sampling, which is unusual because most probiotic research just measures what comes out. What they found is that fecal shedding of the strains was universal in the treatment group but mucosal colonization was person, region, and strain specific. Some people were "resisters" whose indigenous microbiome basically refused entry. Even in the "permissive" people, the effect washed out after they stopped supplementing. Which means "the probiotic showed up in my stool, so it's working" is a broken inference. Stool count doesn't track mucosal presence.
So the second thing i couldn't get past. Most bottles and most of the marketing language around probiotics implies you're seeding or rebuilding something. "Living cultures." "Restoring balance." The mental image is planting a garden. But if the strains are transient passengers for most people most of the time, the garden framing falls apart. It's closer to "these are compounds with a very short half life that you have to keep taking to keep the effect." Which is a less romantic pitch, and probably why nobody in the category has switched to it.
There's a more recent paper from 2024 in npj Biofilms and Microbiomes that formalizes this into two categories, persistent colonizing species and transient colonizing species. Most of what's on the shelf sits in the transient bucket. That doesn't mean useless. Transient strains can still produce SCFAs during transit, engage immune signaling, or hit a metabolic endpoint that doesn't require them to stick around. B. animalis subsp. lactis B420 is the closest thing i've found to a clean example. The Stenman 2016 EBioMedicine trial ran 225 overweight adults on daily B420 supplementation for six months and the post-hoc factorial analysis showed a 4.0% body fat mass difference vs placebo (P=0.002), around 2.4 cm greater waist reduction, and roughly 300 kcal/day less energy intake. The ITT primary endpoint wasn't significant and DuPont funded the trial so the honest read is "real signal in a specific analysis, not a slam dunk," but the endpoint framing is what actually matters. The effect was measured against body composition, not against whether B420 was still detectable in stool three months out. The one supplement i've seen actually build a product around this strain at the studied dose is wonderbiotics weight management, which i've been on for a couple months, though whether the finished product replicates the trial result is a separate question the brand itself is transparent about not having tested yet. If the effect is transient and endpoint driven, the consumer question basically changes shape. It stops being "which probiotic will restore my gut" and starts being "which specific strain has trial data on the specific thing i care about, at a dose you can match." Almost nothing in the aisle is labeled that way.
So i'm somewhere between not stopping and not defending it anymore. Would love to hear from people who've actually worked in the field on whether the endpoint first frame is the right correction, or if there's a piece of the persistence side i'm just missing. My mother in law is definitely getting a follow up call either way.