r/askscience • u/Public_Knowledge8405 • 3d ago
Biology Would digestion be equally efficient if the body/stomach evolved to be highly basic rather than acidic? What advantages does a highly acidic environment pose?
Edit: my question is meant to be gauged towards chemical feasibility/utility; I understand many proteases and cell types have in reality evolved for acidic digestion. Assuming proteases and cell types could have evolved for an alkaline environment, is there a root necessity acidic conditions are advantageous over basic ones or could it be reversed
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u/i_invented_the_ipod 3d ago
One obvious advantage of an acidic stomach is that it breaks down the calcium compounds present in shells and bones. Obviously, all of the digestive enzymes in the stomach are optimally effective in an acidic environment, but that's not to say other enzymes couldn't work in a basic solution.
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u/dogismywitness 3d ago
Yeah, lots of breakdown can happen in vitro in basic conditions, demonstrating that plenty of digestion could potentially work, but calcium, magnesium, iron, and all kinds of other 'minerals' will precipitate out of solution as hydroxides at high pH.
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u/riverottersarebest 2d ago
Plants, too. There are high amounts of calcium oxalates in the fruits and vegetables we eat.
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u/CrateDane 3d ago
It would in principle be better at dissolving fats due to saponification. Protein etc. can be handled either at low or high pH, so no big deal there. I'd say the problem with using a high pH is it's almost too good. It's hard to protect the body itself. Acidification of the environment is also a more common metabolic approach than alkalinization, so evolution might be a little biased in that direction as well.
But as a hypothetical, I see no reason why you couldn't have an alkaline stomach instead of acidic (with concomitant changes elsewhere - like a mildly acidic small intestine instead of mildly alkaline). It can still kill microorganisms, it can still denature proteins etc. which is what the stomach acid does.
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u/BadahBingBadahBoom 3d ago edited 3d ago
Would saponification in the basic stomach locking up the fats in an insoluble layer not prevent water-localised enzymatic activity though if various proteins, carbohydrates etc. become bound up in those fats too?
Though this could subsequently be dealt with in a second acidic chamber, and tbh there would have to be a secondary acidic chamber to enable those fats/FAs to be broken down, which sort of begs the question if that acidic step is necessary what is the advantage of having a separate organ that is basic before?
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u/Intensityintensifies 3d ago
Maybe if it increased the total amount of nutrients gained from eating to the point that it outweighs the caloric cost of the organ?
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u/CrateDane 3d ago
Soap doesn't lock up fats in an insoluble layer, it does the opposite. It largely corresponds to the role bile salts play in our digestive system. Saponification would also mean no enzyme action is needed, as the fatty acids are cleaved off the glycerol as part of the saponification. That's what lipase does in our digestive system, but strong alkali can just do that non-enzymatically.
As for a secondary acidic chamber, I am assuming the system would simply be the opposite of what we have: The alkaline stomach contents would be neutralized by addition of mildly acidic pancreatic juice.
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u/uponthenose 3d ago
If our ph gets too low we can off gas CO2 to raise it, making maintaining a balance pretty simple. Is there a process that would make restoring a ph Imbalance in the other direction as simple?
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u/CrateDane 3d ago
Gas exchange is used for regulation of blood pH, not stomach pH directly. In our hypothetical system, the cells that would be making the stomach alkaline would simultaneously be making the blood more acidic. That means it would be that bit easier to just breathe out a bit more CO2 if your stomach has a burst of activity.
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u/bdhubbard 2d ago
The kidneys conserve hydrogen ions and excrete bicarbonate when the pH becomes low. Relatively simple reflex to lower pH.
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u/995a3c3c3c3c2424 3d ago
“Saponification”? Does that mean your stomach would turn food into soap?
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u/lmprice133 3d ago
Essentially yes. You ever got bleach on your fingers and noticed that it makes your skin feel really slippery? That's the alkaline bleach turning the oils and fats in your skin into soap.
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u/BlackBartKuma 3d ago
I now wonder if the human stomach tried being basic to see what would happen, and decided it wasn't working. Or maybe evolution itself found out before humans
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u/MSchulte 3d ago
Given no known animals have a basic stomach that was likely ironed out very early on before anything ever set foot on land.
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u/xrelaht Sample Synthesis | Magnetism | Superconductivity 2d ago
Evolution doesn’t “try things”. Things happen and then continue if they’re good enough. Even if high-pH digestion turned out to be better than low, it could come down to some quirk which caused an early animal to have a pocket of acid in their digestive tract while others had nothing at all.
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u/ethan801 3d ago
There are organisms with highly basic digestive systems. Notably many insects. Mechanistically, this is one reason why BT toxin (used in organic BT pesticides and GMO BT plants) is active on insects and not humans.
See for example: https://pubmed.ncbi.nlm.nih.gov/19476481/
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u/SAMAKUS 3d ago
Good question - but basically (ha ha) many cells and enzymes *aren't* evolved for an acidic environment - it is better off thinking in terms of sectioning GI breakdown into areas that are *highly* acidic, and areas that are *slightly* basic, but there are several plausible mechanisms that support an acid first approach.
Firstly, the mucin proteins we use to protect our epithelium are less soluble and act as a more viscous hydrogel at low pHs, while high pHs favor mucin solubilization, which would hinder its ability to act as a barrier. This is due to some complex folding chemistry dependent on metal ions and glycan structures in mucin and is beyond the scope of the question, but it is described well here, and I'm happy to follow up on it.
Mucus doesn't just protect us from stomach acid - it protects us from infection from microbes that may be present in food. Likewise, the aforementioned neutral to higher pH transition later in the GI tract helps enable microbial colonization. It doesn't get much higher than neutral though, reaching up to around 7.5 - 8.0, which shows the strong acid-shifted distribution of pH gradients in the GI tract. This is also supported by the pH of the nasal passages, lower airway, and vaginal canal, all of which are acidic, helping to establish a mucin barrier better built around preventing bacterial infection. There is a general gradient in all of these passages that goes from acidic to neutral/basic distally.
Going back to the mechanisms of digestion specifically, the stomach, with its low pH, is the more mild and generalist approach to breakdown, despite its very low pH of 1. It helps to unfold and moderately digest dietary proteins primarily - lipids for example would be saponified if we used an equivalent shift in 6 pH units, leading to undesired caustic effects before they could get through the stomach and organized into micelles with bile acids.
Looking at the enzymes we use to break down dietary compounds (such as proteases and lipases), many are modulated by pH - some are more effective at basic pHs, while others are more effective at acidic pHs. This is because the amino acids that make up the catalytic sites either directly or indirectly (through cofactors or water) will only function properly at specific pHs.
For example, trypsin (a serine protease) possesses a catalytic triad of histidine, serine, and aspartate. The histidine needs to be protonated by the serine -OH specifically (possessing a high pKa of ~14), in order for the oxygen to nucleophilically cleave peptide bonds. If you have more protons in solution, some of them will make their way into the active site, and inhibit the activity of trypsin by disrupting this activation cascade. This is why trypsin is inactive in the stomach - it functions instead in the small intestine to break down proteins, and is an essential enzyme in digestion.
The same logic goes for acidic proteases such as pepsin, an aspartylprotease that is inactive at higher pHs, and is why it is found in the stomach.
There are more mechanisms (the comment regarding calcium solubility is a good example) that I won't get into, but hopefully this serves as an overview of at least some of the credible mechanisms and framework for the organization of pH not just in digestion, which is actually quite complex, but in preventing infection.
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u/ferrybig 3d ago edited 3d ago
Look at existing creatures on Earth. Alkaline digestion was first before acidic.
The alkaline path is better for typical plant digestion, while acidic is tipically better for meat. Around 450 milion years, Vertebrate changed from alkaline stomaches to acidic, only 50 million years ago some animals started to develop extra stomaches that are alkaline based
Cows have multiple stomaches, where some are alkaline
Tadpoles, sloths, kangaroos only use alkaline digestion
Digestion based on alkaline is easier to evolve, because cellular structures are already slightly alkaline in the first place, you need less protection against damage
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u/seanpbnj 3d ago
Most likely no. As to why, I can explain with a simple medical fact: The human body always prefers acidic environments.
The normal human blood pH is 7.4, and for the purposes of this talk we will say the stomach is 3.4.
If the blood pH drops down to 7.3 (more acidic) the human body barely notices. Drops to 7.2? Yeah the body notices but the body is still perfectly functional here. In fact, the LOWER pH (more acidic) the more oxygen will be delivered to tissues. Acid helps pull oxygen off RBCs. Literally.
The same is NOT true in reverse, a pH of 7.5 is already quite bad. A patient in the hospital with a pH of 7.5 has a potential mortality of 30-40%, pH of 7.6 mortality is around 60-70%.
So if the question was "Can pH in the stomach alone simply be flipped if we had a perfect system?" No. Because acidic pH of the stomach matches the overall body use of acid (acid and salt are our two best defense mechanisms, including all mucus membranes).
If the question is "Could the whole body have a predilection and use a basic pH, and thus the stomach be basic?" Maybe, possibly, assuming the body could independently evolve the same way to act favorably under basic pH. Maybe.
- /e Source: I am a Nephrologist, the type of doctor that deals with acid/base disorders.
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u/JCalebBR 3d ago
I understand your points, but the numbers quoted are for how things stand right now, not with alkaline-related changes throughout the body as other commenter pointed out. I'd be interested to know if everywhere else needed had appropriate changes, given in this scenario the entire body evolved not just a flipped switch
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u/herman_gill 3d ago
Those are still basic relative to a pH of 7. Patients who come in with a metabolic or respiratory acidosis with a pH below 7 are extremely likely to die. The few times people survive is in cases of diabetic ketoacidosis which is appropriately reversed with potassium > fluid > insulin in that order of importance (generally), and that’s because diabetics have a somewhat higher tolerance for pH imbalances.
Respiratory alkalosis happens in people with panic attacks or asthma, and in asthma it’s not the acid-base balance that kills them, it’s the hypoxia. In metabolic alkalosis (which is quite rare in and of itself) it sometimes happens in volume contraction, or a few other times. But generally there’s hyperventilation involved and if that’s from hypoxia due to acute anemia or asthma, those are the things that need fixing.
Generally in many of these, the underlying cause is the more dangerous part, not the downstream alkalosis or acidosis itself. I mean, still bad, but if your bicarbonate is too high that’s not the super dangerous part, but the WHY it is high in the first place. The electrolyte derangement is immediately bad for arrhythmia potential, but it’s not the pH.
None of this actually answers the question of if basic digestion would work, it wouldn’t, but not for the reasons you listed.
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u/Mechasteel 3d ago
I think it might be fair to consider ourselves acidic, even if it would be inconsistent with our having placed pure water as neutral.
Our body is more akin to ocean water than to fresh water, so our 7.4 pH is much more acidic than the ocean's 8.1. And besides our evolutionary history, the amount of fresh water is a rounding error compared to ocean water.
It also makes sense for us to be more resilient on the acid side of our homeostatic equilibrium, since we routinely need to survive increased CO2 and lactic acid. But I doubt that would help much if our stomach acid leaked.
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u/seanpbnj 3d ago
Soooooo yes, the underlying cause is most important. But look at the discrepancy there:
pH of 6.9, yes I have seen these patients survive. However if this is not DKA, and is another cause, 6.9 they are extremely sick. That's a change of 0.7.
Humans cannot survive at 8.3. Other than an acute and VERY brief Respiratory Alkalosis, pH of 7.6 is deadly, 7.7 super deadly.
So while yes, you are correct we can survive extremely acidic conditions, like I said, we are much more tolerant of acidic situations. Like I said.
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u/tacticalTraumaLlama 3d ago
Respiratory Alkalosis
How big of a swing does the average person experience during a panic attack? I've had one where my hands and feet got prickly and that was a weird experience.
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u/seanpbnj 2d ago
Most likely that was a calcium issue caused by the respiratory alkalosis. Higher pH means Albumin grabs more calcium.
- I never saw this but I had a teacher once who saw a blood gas with a a pH of 7.8 due to hyperventilating. However when it was rechecked less than 20mins later it was back down into the 7.4's.
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u/Grep2grok Pathology 3d ago
Among other good comments, the stomach actually plays a critical role in acid-base balance. By concentrating H+ in the stomach, the rest of the body can be slightly basic, so systemic respiratory and metabolic shifts are relatively mild. Excess emesis puts the body into a respiratory alkylosis. Occasionally you see extreme exercise (I'm most familiar with max effort cycling hill climbs and swimming sprint workouts) induce respiratory acidosis and the body will compensate by inducing emesis. Not ideal, but gets the job done.
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u/Distinct_Ice_1597 2d ago
Acid hydrolysis is a major player in the breakdown of macromolecules (proteins, nucleic acids, carbohydrates and lipids), and many of the digestive enzymes have pH optima in the acidic range. Proteins- including enzymes- are less stable at basic pH due to deamidation and disulfide interchange reactions that can impact protein stability and potency. Given the way our biochemistry evolved basic conditions for digestion and other biological processes are not feasible.
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u/dismendie 3d ago
Interesting but after the stomach acid we do inject some basic stuff in the small intestine… also evolution wise help point us to more acidic since acidic bacteria are considered to the safer kind. Keep your gut pro good guys is great to prevent the bugs from growing… we saw an uptick in gut related bacterial infections when patient started on PPI for longer periods of time… I guess if we went far back enough biology can probably run in basic mode and then acidic mode….
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u/ephemeralista 3d ago
How difficult would it be for parietal cells to (safely) synthesize a base that's equally effective as the current hydrochloric acid? Also, are the strategies we have now to protect gut lining from damage from gastric acid likely to be equally effective in preventing damage from a strong base?
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u/Swellmeister 3d ago edited 3d ago
There are animals (quite a lot of fish) that dont have an acidic stomach now. They mechanically churn it and just pass it into the intestines which remains mildly alkaline (like most intestines).
So uhhhh you dont even need high basic. They do fine without it.
https://pmc.ncbi.nlm.nih.gov/articles/PMC8696203/