Much discussion has occurred over many years regarding just exactly what diet is the best for human beings. Simple observation suggests that we can survive on pretty much anything we put into our mouths. The operative word here though is “survive”. Humans have shown themselves to be adaptable enough to tolerate a wide variety of substances from which to gain energy to stay alive but necessity is not necessarily the best incentive for food choice. Is there any way we can discern the optimum diet for human beings, one that will provide energy in the most efficient and anatomically suitable way so that a human being not only survives but thrives in the healthiest way possible?
A good starting point is an examination of anatomy and physiology, attributes that evolve from environmental pressures over millions of years. Food is a strong contact with environment and so the present state of the digestive system of the human body should accurately reflect eating habits during pre-human and human existence. First, let’s look at anatomy.
Animal food sources put a high priority on running away or fighting back when they find themselves in the sights of a carnivorous animal, making dinner for a carnivore not a particularly easy goal to achieve. Research shows that individual lions make kills in only about 17% of their hunts while hunting in groups can increase this kill rate to around 30% (12,13). The anatomy of a carnivore reflects these facts (3).
Carnivores do not waste time chewing their food but swallow large chunks of meat as fast as they can get them down.
Carnivores have a very large mouth relative to the size of their head. Their jaw is a hinge joint that lies on the same plane as the teeth, a very strong and stable joint and necessary to prevent possible dislocation of the jaw during a carnivore’s efforts to catch, kill and tear apart their prey. On the other hand, herbivores have a small mouth relative to their head size and their jaws are attached above the plane of the teeth, allowing side-to-side and forward-to-back movements that are necessary to break down the cell walls of the plants that they are eating. The jaws of carnivores do not allow these extra movements. Also the facial muscles of herbivores are much more developed than those of carnivores and their tongues and fleshy lips aid in getting food into the mouth and sending it back in between the grinding teeth to be chewed until fibrous plant material releases its nutrients.
Carnivores do not have digestive enzymes in their saliva. There is no need when their food is swallowed whole without chewing. The saliva of herbivores however contains amylases, enzymes that break down carbohydrates so that the digestion of the food of an herbivore begins in the mouth.
Teeth also show striking differences. The teeth of carnivores are designed specifically for removing meat from bone. Their molars have jagged edges that move past each other in a slicing motion. Their incisors are short and pointed and their canines are long and dagger-like. Herbivores have blunt and small canines that are flattened and not serrated. The molars of herbivores are square and flat for crushing fibrous material and they come together along the length of the jaw, forming grinding platforms.
Carnivores have very large stomachs, about 60 to 70% of the total capacity of their digestive system. This allows them to hold as much meat as possible when meat is available. The stomach of herbivores make up only about 25% of their gastrointestinal tract. Carnivores also secrete much more hydrochloric acid than herbivores. The pH of their stomach with food present is very low, around 1 or less, a high level of acidity capable of killing harmful bacteria that may be lurking in decaying flesh. The acidity of the stomach of an herbivore when it contains food is much lower with a pH of around 4 to 5. (pH expresses the acidity or alkalinity of a solution on a logarithmic scale on which 7 is neutral, lower values are more acid, and higher values more alkaline.)
The relationship of the length of the small intestine to total body length is also quite different between carnivores and herbivores. Body length is measured from the top of the head to the end of the spine. In carnivores, the small intestine is short, about three to six times the body length. This significantly decreases the time that it takes for meat to travel the length of the digestive system, limiting the amount of time decaying meat is inside the animal. Herbivores have a small intestine that is greater than ten times their body length. This allows much more time to ferment the fibrous portions of their diet.
Herbivores have a highly specialized colon that is involved in water and electrolyte absorption, vitamin production and the fermentation and absorption of fibrous plant materials. The colon of a carnivore is simple and short. Its only function is to absorb salt and water.
How about omnivores? Some examples are bears, pigs, badgers, hedgehogs, opossums, skunks, squirrels, raccoons, chipmunks, mice and rats. Omnivores are a little more difficult to pigeon-hole. Some omnivores show anatomical features of both carnivores and herbivores. For example most bear species are omnivorous but individual diets can range from almost exclusively herbivorous to almost exclusively carnivorous, depending on what food sources are available locally and seasonally. Bears have many carnivorous attributes such as claws, straight jaw plane, short intestines and a simple short colon. Still, their mouth contains molars for grinding to assist them in eating plants and their diet is often 80% plant foods. It is actually quite common to find carnivores that consume plants and herbivores that eat parts of animals (14,15).
So where do human beings fit in? Humans display all the above anatomical modifications consistent with herbivores. More specifically, the human stomach volume is 21 to 27% of the total human gastrointestinal tract volume, the human intestine is ten to eleven times body length and the human colon allows extensive bacterial fermentation of fibrous plant materials and the production of significant amounts of food energy in the form of short-chain fatty acids. In addition the human esophagus is narrow and not able to swallow large amounts of food at once. In fact, poorly chewed meat is one of the most common causes of choking deaths in adults (17).
Furthermore there are some physiological processes to consider.
Body cooling of carnivores is achieved by panting and sweating through the pads in their feet; herbivores sweat through their skin (10).
Carnivores drink fluids by lapping; herbivores sip (10).
Carnivores make their own Vitamin C. Herbivores cannot produce Vitamin C and require a food source. Human beings who do not eat enough Vitamin C develop a disease called scurvy and are unable to make collagen, a building block of many body structures.
Vitamin A can be toxic to human beings in large doses. Vitamin A can be produced in human beings from pro-Vitamin A (carotenoids) found in plant foods. Vitamin A obtained this way will not result in an excess because the body will not produce more than it needs. However, Vitamin A is also present in animal products (retinol) such as meat, liver, dairy products, eggs and fish. The livers of carnivores can detoxify excess Vitamin A. The liver of human beings cannot. (16)
Last but certainly not least, atherosclerosis, the problematic plaques of fats and cholesterols that coat the inside our arteries and cause heart attacks and strokes, affect only herbivores. Carnivores and omnivores cannot develop atherosclerosis no matter how much animal protein, saturated fat and cholesterol they eat. The liver and biliary system of a carnivore has unlimited capacity to process and expel cholesterol with the stool for elimination. Human beings have very limited capacity for cholesterol removal. Human beings do not need to obtain cholesterol from their diet because human bodies can synthesize all that they need. It seems clear that herbivores and human beings are not designed to eat meat (8,9,10,18). Humans that eat meat pay the price with a much higher incidence of chronic health problems such as heart disease, cancer and diabetes (5,6).
The editor of the American Journal of Cardiology, Dr. William C. Roberts sums it up in this way. “Although we think we are, and we act as if we are, human beings are not natural carnivores. When we kill animals to eat them, they end up killing us, because their flesh, which contains cholesterol and saturated fat, was never intended for human beings, who are natural herbivores.” (10)
Observations such as these suggest that, anatomically and physiologically, human beings are herbivores. However the actual definition of an herbivore is an organism that derives its energy and nutrient requirements from a diet consisting mainly or exclusively from plants (11). We are aware that human beings can indeed metabolize animal-sourced foods and gain energy and nutrients from them. Humans have been behaving like omnivores for a long time and throughout their existence have displayed a great ability to survive on vastly varied diets – diets both high in animal-based food and high in plant-based food. As far as species survival is concerned, an animal that is able to eat from a wider palate gains a large survival advantage over those who can’t.
If we look back at the digestive systems of our ancestors, including those of our nearest relatives, the great apes including chimpanzees and gorillas, we see that they are very similar to ours. What do living primates with guts like ours eat? Fruits, nuts, leaves, insects the occasional animal-sourced food such as a bird or lizard. The percentage of animal foods that chimpanzees eat is very low, about 2 to 3%, and mostly in the form of termites and other insects. Gorillas are almost pure vegetarians. The majority of food eaten by primates today and over the last thirty million years is vegetable. In other words, our guts evolved eating plants (2,4). Even Neanderthals, long thought to be mostly meat eaters, are revealing in recent research that examined fossil material trapped in their teeth that they actually cooked and ate a wide range of plant foods (1).
Anthropologist Nathanial Dominy studies the diets of hominins, our early human ancestors who lived two million years ago. Present research is looking for the nutritional jackpot that spurred our forebears to stand upright on two feet and enlarge their brains. Some suggest meat was the stimulus but Dominy believes it was bulbs and tubers. These nutrient-rich underground plant storage structures are the foundation of the diet of cultures around the world that thrive to old age without the chronic conditions that plague our society today. Dominy and his colleagues point out that “form follows function”. The low flat molars with thick enamel seen in our ancestors are efficient for chewing hard foods such as bulbs and tubers. Dental similarities and molecular biology helped us pinpoint that chimpanzees and gorillas are our nearest relatives, not orangutans as was once believed. Now, compelling new isotopic evidence suggest that our earliest ancestors subsisted on primarily underground storage organs (tubers and bulbs). Dominy and his colleagues also used the recently clarified human genome to show that humans stand apart from apes in their ability to digest starch. This distinction has emerged during the last two million years with humans now possessing many copies of the gene that regulates starch digestion making this an adaptive shift towards eating more starchy foods. (7)
It is clear that humans are not carnivores but are we herbivores? Or omnivores? It seems to depend on how you approach the question. What is certain is that in body form and function human beings have evolved to thrive on plant foods. In the past we may have been forced by circumstances to eat whatever was available in order to survive. But in today’s modern world in which our food selections are almost unlimited, what should we be eating? What is the optimal diet for human health? The evidence is overwhelming that humans are at their healthiest, living the longest and avoiding the chronic diseases that are becoming more and more common in today’s population by eating solely plant-sourced foods (19,20,21,22,23).
1 Hardy, K., Buckley, S., Collins, M.J., Estalrrich, A., Brothwell, D., Copeland, L. et al. Neanderthal medics? Evidence for food, cooking, and medicinal plants entrapped in dental calculus. Naturwissenschaften August 2012; 99(8): 617 to 626.
2 Dunn, Rob. Human Ancestors Were Nearly All Vegetarians. Scientific American. July 23, 2012.
3 Mills, Milton. The Comparative Anatomy of Eating. November, 2009.
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8 Roberts, William C. Twenty questions on atherosclerosis. Proceedings: Baylor University Medical Center. 2000 Apr; 13(2): 139–143.
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10 Roberts, W.C. We think we are one, we act as if we are one, but we are not one. Am J Cardiol. 1990;66:896.
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15 NOLA.com White-tailed deer shown to raid nests, eat eggs and baby birds, USGS reports. March, 2016.
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20 Barnard, N.D., Cohen, J., Jenkins, D.J.A. et al. A Low-Fat Vegan Diet and a Conventional Diabetes Diet in the Treatment of Type 2 Diabetes: A Randomized, Controlled, 74-week ClinicalTrial. American Journal of Clinical Nutrition May 2009; 89: 1588S – 1596S.
21 Tuso, P.J., Ismail, M.H., Ha, B.P., Bartolotto, C. Nutritional Update for Physicians: Plant-Based Diets. The Permanente Journal Spring 2013; 17: 61-66.
22 Rock ,C.L., Doyle, C., Demark-Wahnefried, W. et al. Nutrition and Physical Activity Guidelines for Cancer Survivors. CA: A Cancer Journal for Clinicians July-August, 2012; 62: 243-274.
23 Marsh, K., Zeuschner, C., Saunders, A. Health Implications of a Vegetarian Diet: A Review. American Journal of Lifestyle MeddicineMay-June 2012; 6: 250 – 267.