Human Stomach pH Compared to Other Animals - What Are Humans Specialized to Eat?

[Qondrar_The_Redeemer]

I found this to be quite interesting, most particularly the part about:  It is interesting to note that humans, uniquely among the primates so far considered, appear to have stomach pH values more akin to those of carrion feeders than to those of most carnivores and omnivores.

Definition of carrion feeder: Any animal that feeds on dead and rotting flesh.

Gastric acidity is likely a key factor shaping the diversity and composition of microbial communities found in the vertebrate gut. The study conducted a systematic review to test the hypothesis that a key role of the vertebrate stomach is to maintain the gut microbial community by filtering out novel microbial taxa before they pass into the intestines. The study proposes that species feeding either on carrion or on organisms that are close phylogenetic relatives should require the most restrictive filter (measured as high stomach acidity) as protection from foreign microbes. Conversely, species feeding on a lower trophic level or on food that is distantly related to them (e.g. herbivores) should require the least restrictive filter, as the risk of pathogen exposure is lower. Comparisons of stomach acidity across trophic groups in mammal and bird taxa show that scavengers and carnivores have significantly higher stomach acidities compared to herbivores or carnivores feeding on phylogenetically distant prey such as insects or fish. In addition, the study found when stomach acidity varies within species either naturally (with age) or in treatments such as bariatric surgery, the effects on gut bacterial pathogens and communities are in line with our hypothesis that the stomach acts as an ecological filter. Together these results highlight the importance of including measurements of gastric pH when investigating gut microbial dynamics within and across species.

Common Name	Trophic Group	pH
Common Buzzard	Obligate Scavenger	1.1
White Backed Vulture	Obligate Scavenger	1.2
Common Pied Oystercatcher	Generalist Carnivore	1.2
Bald Eagle	Facultative Scavenger	1.3
Barn Owl	Facultative Scavenger	1.3
Little Owl	Facultative Scavenger	1.3
Common Crow	Obligate Scavenger	1.3
Common Moorhen	Omnivore	1.4
Humans	Omnivore	1.5 (Can go down to 1)
Ferret	Generalist Carnivore	1.5
Wandering Albatross	Obligate Scavenger	1.5
Possum	Facultative Scavenger	1.5
Black-Headed Gull	Facultative Scavenger	1.5
Common Kestrel	Generalist Carnivore	1.5
Swainson's Hawk	Facultative Scavenger	1.6
Beaver	Herbivore/Hindgut	1.7
American Bittern	Facultative Scavenger	1.7
Grey Falcon	Facultative Scavenger	1.8
Peregrine Falcon	Facultative Scavenger	1.8
Red Tailed Hawk	Facultative Scavenger	1.8
Rabbit	Herbivore/Foregut	1.9
Common Starling	Specialist Carnivore/Insect	2.0
Cynomolgus Monkey	Omnivore	2.1
Mallard Duck	Omnivore	2.2
Magellanic Penguin	Specialist Carnivore/Fish	2.3
Bottlenose Dolphins	Specialist Carnivore/Fish	2.3
Gentoo Penguin	Specialist Carnivore/Fish	2.5
Snowy Owl	Generalist Carnivore	2.5
Domesticated Pig	Omnivore	2.6
Woylie Brush Tailed Bettong	Herbivore/Hindgut	2.8
King Penguins	Specialist Carnivore/Fish	2.9
Great Cormorant	Specialist Carnivore/Fish	3.0
Great Horned Owl	Generalist Carnivore	3.1
Rhino	Herbivore/Hindgut	3.3
Elephant	Herbivore/Hindgut	3.3
Southern Hairy Nosed Wombat	Herbivore/Hindgut	3.3
Skyes Monkey	Omnivore	3.4
Crab-Eating Macaque	Omnivore	3.6
Cat	Generalist Carnivore	3.6
Baboon	Omnivore	3.7
	Specialist Carnivore/Insect	3.7
Mouse	Omnivore	3.8
Ox	Herbivore/Foregut	4.2
Guinea Pig	Herbivore/Foregut	4.3
	Herbivore/Hindgut	4.4
Rat	Omnivore	4.4
Horse	Herbivore/Foregut	4.4
Howler Monkey	Herbivore/Hindgut	4.5
Dog	Facultative Scavenger	4.5
Porcupine	Herbivore/Foregut	4.5
Sheep	Herbivore/Foregut	4.7
Gerbil	Herbivore/Foregut	4.7
Hamster	Herbivore/Foregut	4.9
Common Pipistrelle Bat	Specialist Carnivore/Insect	5.1
Minke Whale	Specialist Carnivore/Fish	5.3
Brocket Deer	Herbivore/Foregut	5.5
Collared Peccary	Herbivore/Foregut	5.8
Langur Monkey	Herbivore/Foregut	5.9
Silver Leafed Monkey	Herbivore/Foregut	5.9
Shetland Ponies	Herbivore/Hindgut	5.9
Colobus Monkey	Herbivore/Foregut	6.3
Camel	Herbivore/Foregut	6.4
Echidna	Specialist Carnivore/Insect	6.8
Macropodid	Herbivore/Foregut	6.9
Llama	Herbivore/Foregut	7.0
Guanaco	Herbivore/Foregut	7.3

Obligate - Animals that depend solely on that diet.

Generalist - Is able to thrive in a wide variety of environmental conditions and can make use of a variety of different resources.

Specialist - Can thrive only in a narrow range of environmental conditions or has a limited diet. Eats only insects or fish as a carnivore, for example.

Facultative - Does best on a said diet, but can survive-but-not-thrive on a different one.


Results

In total, the studies' literature search yielded data on 68 species (25 birds and 43 mammals) from seven trophic groups (Table 1). A general linear model based on diet explained much of the variation in the stomach pH (R2 = 0.63, F1,6 = 17.63, p < 0.01). The trophic groups that were most variable in terms of their stomach pH were omnivores and carnivores that specialize in eating insects or fish.

The studies' hypothesis was that foregut-fermenting herbivores and animals that feed on prey more phylogenetically–distant from them would have the least acidic stomachs. Tukey-Kramer comparisons indicated that scavengers (both obligate and facultative) had significantly higher stomach acidities compared to herbivores (both foregut and hindgut) and specialist carnivores feeding on phylogenetically distant prey. Specifically, foregut-fermenting herbivores had the least acidic stomachs of all trophic groups while omnivores and generalist carnivores, with more intermediate pH levels, were not distinguishable from any other group (Fig 1).

The special case of herbivory

Carrion feeding imposes one sort of constrain on the ecology of the gut, an increase in the potential for pathogens. Herbivory imposes another, the need to digest plant material refractory to enzymatic digestion (cellulose and lignin). In order to digest these compounds, herbivores rely disproportionately on microbial processes. Different regions of the gastrointestinal tract (either rumen, caecum or in the case of the hoatzin a folded crop) function primarily as fermentation chambers. Thus, a challenge with fermentative guts is favoring those microbes that are useful for digestion while reducing the risk of pathogen entry into the gut. The study suggests that because the threat of microbial pathogens is relatively low on live leaves , herbivores can afford to maintain a chamber that is modestly acidic and therefore less restrictive to microbial entry. However, it finds several interesting exceptions to this generality. Beavers, which are known to store food caches underwater where there is a high risk of exposure to a protozoan parasite Giardia lamblia, have very acidic stomachs. The high stomach acidity may have evolved to manage this prevalent environmental pathogen. The other herbivore in our dataset with a very acidic stomach is the rabbit, which provides an interesting example of a behavioral modification of the stomach environment. Rabbits are known to engage in frequent coprophagy which allows them re-inoculate themselves with microbes. The specialized soft pellets that house microbes also reduce the stomach acidity creating an environment suitable for fermentation.

Human evolution and stomach pH

It is interesting to note that humans, uniquely among the primates so far considered, appear to have stomach pH values more akin to those of carrion feeders than to those of most carnivores and omnivores. In the absence of good data on the pH of other hominoids, it is difficult to predict when such an acidic environment evolved. Baboons (Papio spp) have been argued to exhibit the most human–like of feeding and foraging strategies in terms of eclectic omnivory, but their stomachs–while considered generally acidic (pH = 3.7)–do not exhibit the extremely low pH seen in modern humans (pH = 1.5). One explanation for such acidity may be that carrion feeding was more important in humans (and more generally hominin) evolution than currently considered to be the case. Alternatively, in light of the number of fecal-oral pathogens that infect and kill humans, selection may have favored high stomach acidity, independent of diet, because of its role in pathogen prevention.

The human stomach and the loss of mutualistic microbes

In general, stomach acidity will tend to filter microbes without adaptations to an acidic environment. Such adaptations include resistant cell walls, spore-forming capabilities or other traits that confer tolerance to high acidities and rapid changes in pH conditions. The study considered the role of the stomach as a pathogen barrier within the context of human evolution. Another potential consequence of high stomach acidity, when considered in light of other primates and mammals, is the difficulty of recolonization by beneficial microbes. A large body of literature now suggests that a variety of human medical problems relate to the loss of mutualistic gut microbes, whether because those mutualists failed to colonize during hyper-clean C-section births or were lost through use of antibiotics, or other circumstances. The pH of the human stomach may make humans uniquely prone to such problems. In turn, it might be expected that, among domesticated animals, that similar problems should be most common in those animals that, like humans, have very acidic stomachs.

The special risk to juvenile and elderly humans

If, in carnivores and carrion-feeders, the stomach’s role is to act as an ecological filter then it would also be expected to see higher microbial diversity and pathogen loads in cases where stomach pH is higher. We see evidence of this in age-related changes in the stomach. Baseline stomach lumen pH in humans is approximately 1.5 (it can go down to 1) (Table 1). However, premature infants have less acidic stomachs (pH > 4) and are susceptibility to enteric infections. Similarly, the elderly show relatively low stomach acidity ( pH 6.6 in 80% of study participants) and are prone to bacterial infections in the stomach and gut. It is important to note that these differences may be related to differences in the strength of the immune system however it is argued here that the stomach needs more consideration when studying these patterns.

Conclusion

The study demonstrates that stomach acidity increases with the risk of food-borne pathogen exposure and propose that the stomach plays a significant role as an ecological filter and thus a strong selection factor in gut microbial community structure and primate evolution in particular. In light of modern lifestyle changes in diet, hygiene and medical interventions that alter stomach pH, we suggest that stomach acidity in humans is a double-edged sword. On one hand, the high acidity of the human stomach prevents pathogen exposure but it also decreases the likelihood of recolonization by beneficial microbes if and when they go missing. However, in those cases where acidity is reduced, the gut is more likely to be colonized by pathogens. Though it is widely discussed in both the medical and ecological literature, data on pH are actually very scarce. Thus, to fully understand the patterns highlighted here more detailed studies on the gut microbiota across stomach acidities and diet are required.

Personal Opinion: I find it interesting how the only other omnivore other than humans on the list with a pH below 2, is the Common Moorhen. Others with a pH below 2, were all facultative scavengers, obligate scavengers and generalist carnivores (as stated in the study: It is interesting to note that humans, uniquely among the primates so far considered, appear to have stomach pH values more akin to those of carrion feeders than to those of most carnivores and omnivores). Additionally, in the case of the elderly, I do indeed suspect that is it ill health which resulted in highest stomach pH, not in fact increased age. The study also shows the harmful effects of antibiotics and hyper-clean C-section births, among others.

The science section is the right place to post this, am I correct?

Source:

- [The Evolution of Stomach Acidity and Its Relevance to the Human Microbiome](http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0134116)

[a_real_man]

Wonderful find.

[TylerDurden]

Excellent find! I'm putting this in a permanent sticky of some sort. Thanks for making a bit of an effort re research.

[Qondrar_The_Redeemer]

I've actually uploaded a table now myself, and sorted it by pH so it is easier to read. Glad you're putting in a sticky, Tyler.

[Iguana]

Thanks for that! I didn't read the whole article, but we've always found that we like aged, matured meat better. Presently, I have still in my fridge some of a mutton leg and other chunks that have been stored 4 and half months! Very tasty!

Quote from INSTINCTOTHERAPY - Instinctive Raw Paleolithic Nutrition
Translated from the French book "Instinctothérapie, Manger Vrai" by Guy-Claude Burger, pages 90-91.

Given results like that, I can hardly cast aspersions on meat as do some vegetarians, and as I myself did at one time. Overly strict prohibitions that have no basis in science are always suspicious; and one should guard against giving in to them or any other form of fanaticism.
With our method, we’ve been afforded further insight: especially meat left out in the open for a while is sometimes appealing to our instinct, exactly as any natural food. It would be surprising if this instinctive appeal went wrong. Considering the good results, there’s little room for hesitation.
_According to you, then, meat left in the open was part of man’s initial diet?
o According to archaeologists who have studied old bones whose flesh our ancestors ate, meat was eaten in substantial amounts some five million years ago.
_Why did you mention meat left out in the open?
o There are two schools of thought: one, involving the theory connected to hunting, and two, the theory relating to scavengers. If our forbears were hunters, they possibly ate meat fresh. If they gathered the remains of carcasses left over by predators, they had to eat them more or less matured and gamy. In fact, one can tell apart several groups of animals. First of all, the fresh meat-eaters_i.e. carnivorous animals who instinctively catch their prey and eat it live. Most of the time, they only eat part of it, beginning with the guts; they then leave the body that begins predigesting itself through the effect of its own enzymes and yeasts that develop subsequently. When it becomes rather stale, it gives off another smell, that is felt to be appealing to a second group of animals including warthogs, rodents, monkeys, etc. Finally, the body, if anything is left of it, turns into carrion. Then real scavengers step into the picture, i.e. jackals, vultures, etc. who find the smell of carcass, which we find repugnant, most certainly very pleasant otherwise they wouldn’t approach it. The only thing left after that is the final clean-up performed by maggots, cockroaches and other forms of life that disgust us because they very much conjure up a feeling of danger associated with rotten meat, which is toxic for us, or our own death which is another form of rot.
By comparing rib steak to carrion as a matter of course as your friend does, he’s jumping the gun as far as the natural process of things is concerned and he forces disgust in where there is none. Raw meat seems wonderfully enjoyable and fragrant if one needs it, when it has matured just enough. Man probably belongs to the intermediary category of carnivorous, somewhere between carnivorous animals and scavengers. It’s not by chance if butchers allow meat to stand for a few weeks before selling it.

A fully revised and corrected English translation is available on pdf for the forum members who ask me and provide me their e-mail address — we can't attach documents on PMs.

[TylerDurden]

You know, Iguana, I am deeply impressed with you and the Instincto movement. Time and again, we RPFers have found that Instincto ideas generally work better than other rawist notions(well, when they apply to our genuine natural instincts, let's forget Burger's past warnings against too much raw meat consumption and over-emphasis on sweet, raw fruits etc.)

One obvious point:- as humans age, their  digestive systems tend to suffer more and more. While this is mostly, obviously, due to decades of past consumption of cooked foods, I do think that some of the deterioration is also due to effects of old age. In other words, older RPDers would be wiser to consume mostly aged, raw meats past the age of 50 or so.

By the way, Iguana, is your "refrigerator" just a normal refrigerator, or is it a chill-room? I know several RPDers who use a chill-room instead of a refrigerator. A chill-room  is a place where one can cool things down to between 2 to 4 degrees Celsius.

[Iguana]

Thank you!

I started to eat "instincto" at 41 years old and I've always been much more attracted by aged meat, finding it more tasty. The only meat I really liked fresh was kangaroo, when I could buy some - unfortunately not anymore.

Burger's warning is about eating mammals meat too regularly for long periods (i.e. every day during several months and years) and vary instead with poultry, fish, shellfish, eggs and if possible, insects since their proteins are more different than ours and less likely to trigger auto-immune reactions in the long run.

I have 2 fridges, an old and small one that doesn't automatically defrost and a bigger one which does. I hang the meat first in the small one so that it dries on the surface and then I put it in the big one before it becomes too dry and hard. I've now also put a small fan from a PC in the big one, so that the meat can be matured in it also. I keep the temperature around 5°C without worrying much about it.

Below is the big mutton leg after 4 months in those fridges. Not much is left now!

[Qondrar_The_Redeemer]

Thanks for that! I didn't read the whole article, but we've always found that we like aged, matured meat better. Presently, I have still in my fridge some of a mutton leg and other chunks that have been stored 4 and half months! Very tasty!

I would say I like aged/fermented meat, but I don't think anything is as invigorating/tasty as fresh organs/blood for me, while muscle and heart I find both aged/fermented equally as good.

Glad you like the study I posted.

[a_real_man]

My personal experience, would suggest that, sun-baked meat, is the cream-of-the-crop of food. I've noticed that when I consume meat left in the sun for several hours, my stomach comes to life; I have a nice pleasant feeling in my gut, my bowel movements are smooth, and I'm generally more mellow and joyful. Another data point, that might be informative, is that my favorite food, by far, is sun-baked chicken breast... I haven't had it in some time, mostly because of logistic challenges, but I do crave it, and when I had it, it was so amazing, I almost felt like crying.

[TylerDurden]

Given my recent experiences with photobiomodulation(aka near-infrared light therapy devices), it does seem logical that exposing raw meat to the sun might enhance things, energy-wise.

[Iguana]

Doesn't it become too hot in the sun, specially if on concrete or on stones?

[Qondrar_The_Redeemer]

My personal experience, would suggest that, sun-baked meat, is the cream-of-the-crop of food. I've noticed that when I consume meat left in the sun for several hours, my stomach comes to life; I have a nice pleasant feeling in my gut, my bowel movements are smooth, and I'm generally more mellow and joyful. Another data point, that might be informative, is that my favorite food, by far, is sun-baked chicken breast... I haven't had it in some time, mostly because of logistic challenges, but I do crave it, and when I had it, it was so amazing, I almost felt like crying.

Interesting... I might try that sometime.

[a_real_man]

Doesn't it become too hot in the sun, specially if on concrete or on stones?

I haven't encountered that problem. Perhaps removing it from the sun after a few hours would solve this issue.

[Projectile Vomit]

The article's conclusions make a lot of sense. While humans can be hunters, more distantly in our past we were certainly primarily scavengers, at least as far as acquiring big game animals is concerned.

[TylerDurden]

Even hunters would have left carcasses out to age. I note that wild animals often leave animal carcasses they have previously killed in various hiding-places to age before slowly eating the whole thing.

[Projectile Vomit]

Yes, that seems to be typical. And the process of eating a carcass can take a while anyway. If a group of people killed a deer on a Monday, it might take them a couple weeks to eat all of it.

[a_real_man]

Interesting... I might try that sometime.

Here's a before picture. It's an egg, a piece of chicken breast, lots of pieces of pork, and a piece of liver:


Here's a picture a few hours later:


Chicken is missing... 

[Grey-Cup]

Thanks all for sharing, I have learned much from this forum.

I'm curious, to those of you who eat very aged meat - do you find you need to drink more water? I have been eating exclusively raw zero carb for a few months, and discovered my natural thirst has greatly diminished to 1-2 cups maximum per day unless I am very active or it is very hot out.

I'm assuming that this very old meat like the 4 month lamb shown above has lost much of its water weight.  Does that create more thirst when you eat food of this age?

[FRANCIS HOWARD BOND]

If you keep it covered most of the time and air it regularly it should retain moisture, but will tend to dry eventually.    Airing in a refrigerator will lead to rapid drying if it is not covered.    Thirst results from consuming any dry matter.    I have known high meat to change gradually to liquid if enclosed.

[thehadezb]

I think we were not scavengers but carcass stealers. We have watched and analyzed carnivores since evolution. We have stealen his teeth to make knifes and tools. A knife is one of the most important things for a human. A human without tools is not a human. All this is just theory and arbitrary, but that is my imagination.
I think we attacked as mass to steal carcass from predators and ate it kind of ferment it. And, think about the mega fauna. Do you think a tribe could eat an entire mammut still fresh? It clearly was fermented, dried or prepared in different ways.
Our ancestors are still humans, they had intelligence and knowledge. They probably did a lot of things with food we don't know or think of.
Trying to theorize what they are or did is impossible, actually.

[surfsteve]

I used to think that  we were scavengers but after reading your post I realize that carcass stealing and scavenging aren't exactly the same thing. When you think about it there would be much more available from stealing than scavenging. You just might be right! Though early humans probably did both. Good analysis! It really made me think!

[van]

You can easily find on Utube tribes running off lions off a kill,,

[surfsteve]

I imagine it's a whole lot easier to run off a lion that's had his fill than trying to run one off that has just killed an animal and hasn't yet had a chance to eat any of it!

[van]

the one I saw there were five or six big Masais with spears and blades running to a lion with fresh kill. 

[van]

And don't forget that large human hunting parties had little problem taking down large big game like elephants.  Again, many thousands of years ago the animals were much bigger and more plentiful.  Meat and fat was easy to have.  More fat on bigger animals. More brain, and more marrow, and more liver...