The comparative anatomy of eating
A presentation by Milton Mills, M.D.
The major causes of death in Western countries are cardiovascular diseases and cancers. Abundant medical research linking these diseases to dietary and lifestyle factors, guidelines advanced by the American Heart Association, the American Cancer Society, and the Surgeon General, among others, counsel Americans to sharply reduce animal foods consumed and replace them with fruits, vegetables, and whole grains. In effect, they are recommending a more plant-based diet, which begs the question:
Are humans designed to eat meat?
Milton Mills, M.D. has an extensive background in nutrition research, focusing on the role nutrition plays in the development of chronic diseases. He is a graduate of the Stanford University School of Medicine and is a practicing physician in the Washington, D.C. area. He also serves as the Associate Director of Preventive Medicine for the health policy group Physicians Committee for Responsible Medicine. He has lectured extensively throughout North America and is a Nutrition Health Education Video Spokesperson for the Discovery Health Channel.
The following is the introduction, human characteristics, comparative outline, and conclusion of Dr. Milton R. Mills’ analysis of human anatomy and physiology: “The Comparative Anatomy of Eating”:
Humans are most often described as “omnivores.”
This classification is based on the “observation” that humans generally eat a wide variety of plant and animal foods. However, culture, custom and training are confounding variables when looking at human dietary practices.
Thus, “observation” is not the best technique to use when trying to identify the most “natural” diet for humans. While most humans are clearly “behavioral” omnivores, the question still remains as to whether humans are anatomically suited for a diet that includes animal as well as plant foods.
A better and more objective technique is to look at human anatomy and physiology.
Mammals are anatomically and physiologically adapted to procure and consume particular kinds of diets. It is common practice when examining fossils of extinct mammals to examine anatomical features to deduce the animal’s probable diet.
Therefore, we can look at mammalian carnivores, herbivores (plant-eaters) and omnivores to see which anatomical and physiological features are associated with each kind of diet. Then we can look at human anatomy and physiology to see in which group we belong.
The human gastrointestinal tract features the anatomical modifications consistent with an herbivorous diet. Humans have muscular lips and a small opening into the oral cavity. Many of the so-called “muscles of expression” are actually the muscles used in chewing. The muscular and agile tongue essential for eating, has adapted to use in speech and other things. The mandibular joint is flattened by a cartilaginous plate and is located well above the plane of the teeth. The temporalis muscle is reduced. The characteristic “square jaw” of adult males reflects the expanded angular process of the mandible and the enlarged masseter/pterygoid muscle group. The human mandible can move forward to engage the incisors, and side-to-side to crush and grind.
Human teeth are also similar to those found in other herbivores with the exception of the canines (the canines of some of the apes are elongated and are thought to be used for display and/or defense). Our teeth are rather large and usually abut against one another. The incisors are flat and spade-like, useful for peeling, snipping and biting relatively soft materials. The canines are neither serrated nor conical, but are flattened, blunt and small and function like incisors. The premolars and molars are squarish, flattened and nodular, and used for crushing, grinding and pulping noncoarse foods.
Human saliva contains the carbohydrate-digesting enzyme, salivary amylase. This enzyme is responsible for the majority of starch digestion. The esophagus is narrow and suited to small, soft balls of thoroughly chewed food. Eating quickly, attempting to swallow a large amount of food or swallowing fibrous and/or poorly chewed food (meat is the most frequent culprit) often results in choking in humans.
Man’s stomach is single-chambered, but only moderately acidic. (Clinically, a person presenting with a gastric pH less than 4-5 when there is food in the stomach is cause for concern.) The stomach volume represents about 21-27% of the total volume of the human GI tract. The stomach serves as a mixing and storage chamber, mixing and liquefying ingested foodstuffs and regulating their entry into the small intestine. The human small intestine is long, averaging from 10 to 11 times the body length. (Our small intestine averages 22 to 30 feet in length. Human body size is measured from the top of the head to end of the spine and averages between two to three feet in length in normal-sized individuals.).
The human colon demonstrates the pouched structure peculiar to herbivores. The distensible large intestine is larger in cross-section than the small intestine, and is relatively long. Man’s colon is responsible for water and electrolyte absorption and vitamin production and absorption. There is also extensive bacterial fermentation of fibrous plant materials, with the production and absorption of significant amounts of food energy (volatile short-chain fatty acids) depending upon the fiber content of the diet. The extent to which the fermentation and absorption of metabolites takes place in the human colon has only recently begun to be investigated.
In conclusion, we see that human beings have the gastrointestinal tract structure of a “committed” herbivore. Humankind does not show the mixed structural features one expects and finds in anatomical omnivores such as bears and raccoons. Thus, from comparing the gastrointestinal tract of humans to that of carnivores, herbivores and omnivores we must conclude that humankind’s GI tract is designed for a purely plant-food diet.
The next document contains the complete analysis by Dr. Milton R. Mills