How Human Digestion Work and What is Our Evolutionary Diet

Humans have long debated what our optimal diet is from an evolutionary perspective. Are we herbivores, carnivores or omnivores? Proponents on all sides make seemingly convincing arguments about the anatomical and physiological evidence. However, much of this evidence is circumstantial or based on flawed assumptions. When we look at the core facts around human digestion and nutritional absorption, it becomes clear that humans are evolutionarily adapted to be carnivores.

We Have Primate, Not Ruminant Digestion

In comparing human digestion to that of other animals, people often claim things like “humans have the same digestion as cows, so we are herbivores.” This glosses over the nuances of our evolutionary ancestry. Humans are primates. Our closest genetic relatives are chimpanzees and bonobos. So while we may have some similar traits with cows, our core digestion was evolved as primates.

Any modifications from our primate digestive system are likely to be based on our early transition to meat eating, not evidence that we are adapted to eat like cows.

Low Stomach Acidity Indicates Meat Eating Origins

One cardinal digestive trait that sets humans apart from herbivorous primates is our highly acidic stomach pH. Human stomach acidity averages around 1.5 to 2.5. This is similar to scavengers like vultures.

Such extremely low pH is needed to kill pathogens from rotting meat. Researchers believe early human ancestors like australopithecines and habilines ate mostly scavenged carrion. They lacked the large teeth, claws and hunting skills to take down large game. Instead, they used simple stone tools to crack bones and consume brains and marrow left behind by apex predators.

Eating meat from scavenging is likely what gave us the calorie surplus to develop larger, energy-hungry brains over time. Low pH stomach acid was essential to make eating carrion less hazardous.

Long Small Intestine and Short Large Intestine

Another divergence between human and herbivorous primate digestion is the relative length of our small versus large intestine. In hindgut fermenting herbivores like gorillas, 60-70% of digestion occurs in an enlarged cecum and long large intestine.

In contrast, humans have a much shorter large intestine, but a long small intestine. The small intestine is where most absorption of proteins, fats and micronutrients occurs. This matches what we see in carnivores like dogs and cats.

Having a smaller large intestine reduces the time meat and fat spends in transit, minimizing risk from putrefaction. It also reduces the amount of water reclaimed from feces, an acceptable tradeoff on a meat-heavy diet.

Inability to Digest Fiber and Ferment Cellulose

All vertebrate animals lack enzymes to directly digest fiber and liberate nutrients from cellulose in plant matter. Herbivorous mammals get around this via fermenting microbes in chambers like the rumen (cows) or hindgut (gorillas).

The microbes breakdown cellulose into short chain fatty acids that the host animal can absorb. Humans lack any ability to digest fiber/cellulose. We completely lost adaptation like the rumen and large fermentation chambers.

This demonstrates that we are not well adapted to extract nutrients from fibrous foods the way herbivores are. Meat and animal fat contain no indigestible fiber, so they avoid this limitation.

Vestigial Cecum and Appendix

Related to fiber digestion is the human cecum and appendix. In plant eating primates like gorillas, the cecum is a large pouch where cellulose fermentation occurs.

The human cecum is extremely small and non-functional. Likewise, our appendix is a shrunken vestigial version of the primate cecum, serving no digestive purpose.

This is additional evidence we lost adaptations for digesting plant matter long ago and switched to easier to digest meat and fat.

Bile and Fat Absorption

Some claim that eating meat and fat causes digestive issues like gallstones. In reality, efficient fat absorption shows we are well adapted for high fat carnivorous diets.

Humans secrete up to a liter of bile daily to emulsify and absorb fat. Bile is stored in the gallbladder and concentrated up to 20x when needed to digest meals high in fat. Without bile, we could not properly absorb fat or fat-soluble nutrients.

Having multiple organs devoted to bile production demonstrates the evolutionary priority of fat digestion. Claims that fat intake itself leads to gallstones are unfounded. Gallstones form when bile sits too long in the gallbladder between meals and precipitates into sludge. Eating more fat ensures complete emptying of bile from the gallbladder so stones cannot form.

Fiber Worsens Diverticulosis, But Low Residue Diets Help

Diverticulosis is outpouching and weakness of the colon wall from excessive pressure. It can become infected (diverticulitis) and require surgery.

More frequent bowel movements and high fiber diets correlate with diverticulosis. Just like overusing any muscle makes it fail sooner from fatigue, high fiber overworks the colon. Yet when treating diverticulitis, doctors put patients on low residue diets lacking fiber, like the carnivore diet, to rest the colon. If fiber were good for intestinal health, it makes no sense to remove it during recovery.

Meat is Easily Digested, Fiber Poorly Digested

Some claim meat rots in the gut if not digested properly. In reality, the entire digestive tract is a one-way tube — anything not digested gets excreted. Poorly digested matter like fiber is what we excrete in feces, not meat.

Studies show even infants digest and absorb 98% of meat and fat consumed, indicating we have innate ability to digest animal products. Meat leaves little indigestible residue compared to plant matter like fiber that we cannot break down.

Conclusion: Humans are Evolutionarily Carnivores

When we set aside circumstantial arguments and look at the core evidence around human digestion, it becomes clear we are evolutionarily adapted for a carnivorous diet, not as herbivores or omnivores.

Key points like our inability to break down fiber, short large intestine, long small intestine, ample bile production, and low stomach acidity all align with adaptations found in carnivorous mammals. Comparisons of our digestive anatomy and physiology to that of herbivorous primates shows we diverged from plant dependence long ago.

Understanding humans as carnivores explains why foods like meat and fat are digested and absorbed so efficiently compared to plant matter. It also explains why fiber-rich diets cause many common “diseases of civilization”. We are simply not built for digesting copious plant matter. Realizing that meat and animal fat are our species appropriate foods offers a simple plan for optimal health.

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