Parts of the following article were originally written by Frank Dixon. Some of the content and information is based on his book Sustainable Food Production and Diet. It has been edited for consistency with CNS editorial guidelines.

We are told by many people, including doctors and other apparent experts, that animal products are essential to a healthy diet. This advice reflects the majority view that humans are fundamentally omnivorous. Still, the debate over what is normal or natural for our species will not die. On the one side, you have people who believe we evolved to eat a diet with substantial proportions of meat, including paleo dieters and devoted carnivores, but also many individuals consuming a standard American diet who implicitly buy into the importance of meat; on the other hand, many vegans insist that humans are not even natural omnivores.

What do our bodies indicate, and does it matter?

The Anatomy of Carnivores versus Herbivores

Many people oversimplify the herbivore-carnivore divide and incorrectly assume that all herbivores are unable to digest animal foods and vice versa. Although carnivores mostly and ideally subsist on animal foods and herbivores likewise on plant foods, countless examples illustrate that animals frequently cross the divide.^[1] Relatively few animals are obligate carnivores incapable of extracting nutrients from plants.^[2] Like nearly everything in nature, our diets exist on a spectrum; just because animals can and do occasionally eat something different does not mean that particular something is ideal.

Anatomy can better help us identify natural preferences. When we look at the anatomy of carnivores, we see animals that have evolved to capture, kill, flay, dismember, and digest other animals. Omnivores, which evolve to also eat plants, must retain their ability to capture, kill, flay, dismember, and digest animals. Even hypocarnivore omnivores like bears, which meet at least 70 percent of their dietary needs from plants, must maintain anatomical features largely consistent with carnivores, plus a few key adaptations to allow for digesting plant foods.

Carnivores tend to have wider mouth openings relative to head size and jaw joints on the same plane as their teeth. The lower jaw cannot move forward or backward and has limited side-to-side motion. This strong, stable jaw helps carnivores capture, kill, and dismember prey. Herbivores, on the other hand, have relatively small mouth openings and jaw joints above the plane of the teeth. This means the herbivore’s jaw is much weaker, but it provides for more front-to-back and side-to-side mobility needed to crush and grind fibrous plants.

Carnivores tend to have short, sharply pointed front teeth for tearing flesh; herbivores have shovel-like front teeth that are long and flat, ideal for cutting plants. Carnivores tend to have long curved canines sharp enough for capturing, killing, and tearing prey; canines vary in herbivores. Some have no canine teeth at all. Others, such as hippos and some primates, have longer canines. Carnivores have triangular molars for cutting flesh; flat molars are more useful for crushing plants.

Obligate carnivores produce no salivary amylase, which is required to digest starch.^[3] Thanks to their wide esophagi, carnivores often swallow large chunks of meat without choking. Herbivores require much more extensive chewing and have digestive enzymes in their saliva better suited to breaking down plant matter. Herbivores also have narrower esophagi and risk choking if they attempt to swallow large chunks of meat.

Carnivores often have larger stomach capacities, which allows them to load up on food from intermittent hunting.^[4] Herbivores tend to have smaller stomach capacities but longer small intestines (10–12 times body length, compared to 3–6 times body length for carnivores) ideal for slowly digesting plant material.

Because meat often has abundant pathogens, especially as it decays, carnivores and omnivores tend to have more acidic stomachs to kill those pathogens; apart from a few exceptions, herbivores tend to have less acidic stomachs.^[5] Humans are the only animals that cook, an innovation that also helps kill pathogens in meat.

Carnivores can eat as many foods high in animal protein and fat as they want without developing atherosclerosis; herbivores cannot.^[6]

What about Humans?

Anatomically, we have far more in common with herbivores than carnivores. Our mouth openings are smaller and our jaws allow for side-to-side and front-to-back movements ideal for grinding. Our canines are short and blunt, and our front teeth are flat. Our flat molars, like the rest of our teeth and jaw structure, are ideal for eating plants, not tearing flesh. Our salivary amylase activity is high, reflecting an ability to digest starchy foods that many carnivores completely lack.^[3] Our small intestines are longer than carnivores’ small intestines, and our stomach capacities are smaller than carnivores’ stomach capacities. Our closest primate relatives consume overwhelmingly herbivorous diets.

Our empty stomachs are surprisingly acidic, but research has shown that the pH spikes (becoming less acidic) when we eat.^[7] Some other herbivores also exhibit unexpectedly acidic stomachs, including beavers and rabbits.^[5] “One explanation for such acidity [in the human],” write researchers, “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.” As mentioned above, humans have effectively learned to kill the pathogens in meat by cooking.

This final point illustrates something critical: it is our intellectual capacity, rather than our anatomy, that enables us to capture, cook, and eat meat. One might say the evolution of our intelligence is just as important of an indicator of what is natural as the evolution of our bodies, and that argument may be worth considering more closely; however, if we are going to follow that way of thinking to its logical conclusion, conferring special significance to our intelligence, the least we can do is also make sure to exercise that impressive intelligence by critically assessing whether our current food choices continue to serve our needs.

Dietary Range Could Once Prevent Extinction; Not So Much Anymore

We know that humans, like most animals, evolved with the capacity to eat all kinds of things, including many of the parts of animals. This makes us behavioral omnivores by definition, but it has little relevance to the question of what is optimal. Evolving to eat animal products might have been very useful for survival, but it is not necessarily optimal for longevity.

Consider early humans migrating to colder climates where plants did not grow as readily. They would have likely perished without the energy from meat. Because the nutrient composition of animal foods generally includes more fat, animal foods are more calorie dense; in the face of short-term starvation, such foods could have been the difference between life and death. The same could be hypothesized about humans trying to outlast climate events that would have disrupted the supply of plant foods. Adaptability under such circumstances surely aided survivability.

We no longer face those harsh conditions. Today, the drivers of meat consumption have nothing to do with human survival. They have much more to do with cultural pressure, preference for animal products, traditions, the influence of the animal foods industry, and even the lack of nutrition training that doctors receive.

Considering how the threats to our species have shifted, now would be an especially great time to stop conflating survival and healthy longevity. Today, more than four times as many people worldwide are overweight or obese compared to those who are undernourished. Chronic diseases comprise the majority of deaths in the US and the majority of healthcare spending (about 70 percent and 75 percent, respectively), and a fourth of Americans have multiple chronic diseases.^[8] Our ancestors during the Paleolithic era, when life expectancy was only 33 years, would have rightly seen it as a great victory to live long enough to die of Alzheimer’s disease.^[9]

We’ve moved so far beyond that world. The challenges are different. The same intelligence that allowed us to kill pathogens in meat by cooking has supported our continually evolving scientific understanding of the world. We now know from numerous studies that diets with little or no animal foods can substantially minimize our risk of dying from the deadliest chronic diseases while promoting health, longevity, and vitality. (Learn from the decades of research studies that show numerous associations between nutrition and better health outcomes.)

Finally, we also face the current threat of environmental destruction caused by agriculture, which owes disproportionately to animal-based diets. Animal-based diets require more land (and therefore contribute to more deforestation and species loss), drain more finite resources, and have a substantially larger carbon footprint than even the least sustainable plant-based options.^[10] They are at the heart of the Earth’s sixth mass extinction. This existential threat would have been inconceivable to humans thousands of years ago as they scraped by on whatever food they could find. Estimates for the global population in the year 10,000 BC range from one to ten million; the upper end of that range is about the same as present-day Los Angeles County.^[11][12] (Learn more about the alternative: sustainable agriculture.)

Although our capacity to eat animals might once have played a role in ensuring our survival, it now plays the opposite role, quickening our extinction. Our anatomy indicates a natural preference for plant-based diets, and our intelligence—if we want to go there—surely backs up that choice.

References

1. Landers J. Bambi ate thumper: why herbivores sometimes eat meat. Slate. November 16, 2012. https://slate.com/technology/2012/11/deer-eat-meat-herbivores-and-carnivores-are-not-so-clearly-divided.html
2. Encyclopedic entry: Carnivore. National Geographic. Accessed June 10, 2024. https://education.nationalgeographic.org/resource/carnivore/
3. Peyrot des Gachons C, Breslin PA. Salivary Amylase: Digestion and Metabolic Syndrome. Curr Diab Rep. 2016;16(10):102. doi:10.1007/s11892-016-0794-7
4. De Cuyper A, Clauss M, Carbone C, Codron D, Cools A, Hesta M, Janssens GPJ. Predator size and prey size–gut capacity ratios determine kill frequency and carcass production in terrestrial carnivorous mammals. OIKOS. 2018;128(1):13–22. doi:10.1111/oik.05488
5. Beasley DE, Koltz AM, Lambert JE, Fierer N, Dunn RR. The Evolution of Stomach Acidity and Its Relevance to the Human Microbiome. PLoS One. 2015;10(7):e0134116. Published 2015 Jul 29. doi:10.1371/journal.pone.0134116
6. Roberts WC. Twenty questions on atherosclerosis. Proc (Bayl Univ Med Cent). 2000;13(2):139-143. doi:10.1080/08998280.2000.11927657
7. Gardner JD, Ciociola AA, Robinson M. Measurement of meal-stimulated gastric acid secretion by in vivo gastric autotitration. J Appl Physiol (1985). 2002;92(2):427-434. doi:10.1152/japplphysiol.00956.2001
8. Raghupathi W, Raghupathi V. An Empirical Study of Chronic Diseases in the United States: A Visual Analytics Approach. Int J Environ Res Public Health. 2018;15(3):431. Published 2018 Mar 1. doi:10.3390/ijerph15030431
9. Goldman L. Three Stages of Health Encounters Over 8000 Human Generations and How They Inform Future Public Health. Am J Public Health. 2018;108(1):60-62. doi:10.2105/AJPH.2017.304164
10. Ritchie H. Less meat is nearly always better than sustainable meat, to reduce your carbon footprint. OurWorldInData.org. February 4, 2020. https://ourworldindata.org/less-meat-or-sustainable-meat#article-citation
11. United States Census Bureau. Historical estimates of world population. Accessed June 10, 2024. https://www.census.gov/data/tables/time-series/demo/international-programs/historical-est-worldpop.html
12. United States Census Bureau. QuickFacts: Los Angeles County, California. Accessed June 10, 2024. https://www.census.gov/quickfacts/fact/table/losangelescountycalifornia,CA,US/POP645218