A gastroenterologist explains the similarities between carnivorous animals and humans, finding little difference.
January 1, 1975
The Stone Age Diet
This book is a study of the ecology of Man, as his environment has changed with (relatively) lightning-like rapidity from prehistorical to modern times, and to delineate the effect these changes have had on nutrition. An attempt will be made to answer the question: "Is modern Man actually better or worse off nutritionally than was his Stone Age forbear?"
Writing this book has indeed been fun. While collecting material for the early chapters I was able to add greatly to my knowledge of comparative anatomy and physiology, how all various sorts of animals are constructed, and how their digestive tracts function. Later chapters led me into a fascinating world of the past, of anthropology and archaeology, which I embraced enthusiastically though amateurishly. Finally I ventured into the shadowy sphere of philosophy, explored some aspects of future food production, and have set down the dire predictions of population ecologists for the arrival of the 21st century. It has been thrilling to see how each bit of scientific data from such widely separated disciplines fitted together into a mosaic of such undeniable clarity that the aphorism: "That contrary to Nature cannot be fact"-was again verified, this time in the field of human dietetics and nutrition.
WHAT IT’S ALL ABOUT
Of course you are’ a human being! Everybody is. But did you know that you are also an animal–a carnivorous animal? All humans are. Did anybody ever tell you that your ancestors were exclusively carnivores for at least two and possibly twenty million years? Were you aware that ancestral man first departed slightly from a strictly carnivorous diet a mere ten thousand years ago? Well, he was and he did, and discussion of these salient points relevant to man’s diet will be the first task of this book. Over the next 9,950 years Man continued to make minor changes in his victuals as new plant substances were discovered and cultivated. These changes contributed nothing nutritionally but a more abundant supply of calories. They did promote cultural progress from feeding to eating. However, few if any of the novelties introduced to his palate during thc following centuries of dietary experimentation were sufficiently indigestible, or eaten in so great a quantity, as to incite rebellion by his digestive tract. As a consequence, man continued to nibble at them and experiment cautiously with still newer foods. He actually even grew to like some of them, especially when he was hungry and there was no meat in the larder. The low carbohydrate diet is an old diet. It was the choice of man for two million Stone Age years. He first departed from his meat-fat diet only a few thousand years ago. This was partly because of an increasing population. More important were climatic changes which decimated the large game animals, his chief source of food. Strictly from hunger, man began to eat some plant substances which were, of course, carbohydrate.
Since it is easier to begin any task with its simplest component, a comparison of man’s digestive tract with that of the meat eater and that of the grazing animal, respectively, we will begin with the former, a carnivorous animal: the dog. The digestive tracts of all carnivores are remarkably similar in structure and function. If one could be plucked from its nest in the abdomen and stretched out full length, one could see that it was rather short, being only about six times the animal’s body length. >It is composed of an uninterrupted tube, enlarged in certain areas, to which are appended certain solid organs, the glands of digestion. The dimensions of this tube will of course vary with the size of the animal. Since we plan to make comparison eventually with man, a carnivorous animal of relative size will be described. Such an animal would be a large dog-a Great Dane or a Saint Bernard-which frequently attains a weight of 150 pounds, and has other measurements comparable to a human.
Let us start with the mouth. First we would encounter the jaws, set with incisor, canine, and molar teeth. The dog possesses incisor teeth in both upper and lower jaws. Thc jaw movements are up and down which, together with the ridged character of the molar teeth, indicate a tearing or crushing function rather than that of grinding or mastication. The salivary glands do not have an important digestive function in the dog, serving merely to lubricate the chunks of meat this animal normally “wolfs,” or swallows whole. >Food, when swallowed, enters the esophagus, a tube-like structure which extends from the mouth through the chest to the stomach. When distended with food, peristaltic waves are set up in the esophagus which gradually “milk” the food into the stomach. The Great Dane’s stomach is rather small, having a capacity of only two quarts or less. It is the only bulbous enlargement encountered along the alimentary way. When distended it has somewhat the shope of a small football, and its size indicates the amount of food the animal can ingest at one time without discomfort. The chief functions of the carnivorous stomach are:
1) to serve as a reservoir; and
2) to dissolve the aliment (ingested food), which grently facilitates its complete and rapid digestion as it passes down the intestine. The reservoir function allows the animal to eat a fairly large volume of food at one time. Since the food of these animals is normally meat and fat-highly concentrated foods of small bulk-sufficient amounts may be ingested at intervals of once daily to serve their nutritional needs. Dissolving the meat and fat in the stomach is possible because this organ has the ability to manufacture and secrete into its lumen a strong mineral acid called hydrochloric acid. This same material, under the name of muriatic acid, is used widely in industry 8S a solvent of many substances, including metals, minerals, and organic material. Foodstuffs are held back in the stomach until solution has occurred. The solubility of different materials varies widely and as a consequence, some foods leave the stomach quickly, while others are retained for a longer period.Some insoluble substances, such as cellulose, large pieces of bone and cartilage, or raw vegetable material, if it should be ingested, are eventually emptied into the small intestine and pass through the remainder of the tract unchanged. If they are too large to pass through the exit from the stomach they are vomited. The carnivorous stomach, which has been filled with its normnl ration of meat and fat, wilt be able to dissolve the entire meal and evacuate it into lhe small intestine within three hours. The stomach then enters a period of rest until the next meal is eaten.The dissolved food, called “chyme” is fed into the upper end of the small intestine in a series of small suprts under the control of the pylorus, a muscular valve separating the stomach from the intestine. Very little actual digestion of food occurs in the carnivorous stomach. There is some slight brenking down of highly emulsified fats, such as cream, and insignificant digestion of some protein by the pepsin of the gastric juice. In the carnivore the stomach is not a vital organ; the animal get” along quite well even though much, or even all, of its functions has been lost through disease or surgical removal. Food leaving the stomach enters the small intestine and begins a most eventful journey through the remainder of the digestive tract.
The diameter of the small instestine is similar to that of the esophagus–about the size of a garden hose. Its length is difficult to determine with accuracy, since this dimension depends largely upon the state of its contraction or relaxation. If the small bowel (or intestine) is completely relaxed, as after death, it may he stretched out for thirty feet. Contrariwise, if a rubber tube is fed through the small intestine of a living animal, the leading tip of the tube will have traversed its entire length by the time scarcely ten feet of the tube have entered the mouth. The true functional length of our dog’s small bowel is thought by most anatomists to be about twenty feet. However, this is a most important twenty feet, for it is during transit through this portion of the tract that virtually all digestion and absorption of food must occur. The small intestine is a vital organ, for no carnivore can live without it.
Several inches from the stomach a duct or tube enters the small intestine from the side. About an inch from its point of entry into the bowel, this tube branches; the shorter branch lends to the pancreas, and the longer leads to the liver. There are the digestive glands which furnish the digestive juices to the alimentary tract. They are of vital importance to the carnivore. The gallbladder is a sac-like structure attached to and communicating with the tube leading from the liver to the intestine (bile duct). The gallbladder is well-developed and functions strongly in all carnivores. The presence of the first few spurts of chyme in the duodenum, or first division of the small intestine, alerts chemical sensors or hormones. These stimulate the production of digestive enzymes by the pancreas, which are in turn emptied into the intestine through the pancreatic duct. There they mingle with the chyme and immediately begin to break it down into its component parts. These chemical processes are continued, the chyme moves down the digestive tract, so that by the time it has reached the distal end of the small bowel, virtually all the material that can be digested has been absorbed from the digestive tube, leaving only a small indigestible residue to be emptied as a liquid suspension into the colon or large intestine for disposal as waste. The process of digesting the food by pancreatic juice is most important because meat, fat, or carbohydrate cannot be absorbed by the small intestine as such. First they must be broken down into their basic components, the only forms in which they can be assimilated.
Proteins are broken down into a variety of amino acids, fats to fatty acids and glycerol, and digestible carbohydrates to glucose. The action of the pancreatic juice and a few weak and inconstant intestinal enzymes, constitute the entire digestive potential of the carnivorous animal. These enzymes are all produced by the animal itself. This point is of great importance, as will be seen when tho carnivorous and herbivorous digestive mechanisms are compared. Meanwhile, back in the duodenum, other activities have taken place. Certain other hormones are produced which increase the muscular activities of the intestine, and produce mixing and churning movements which facilitate digestion by the enzymes. Since digestion by enzymes takes place on the surfnce of the material, the process is greatly facilitated if the size of the food particles are reduced by chewing, grinding, powdering, or-most ideally of all-by actually dissolving it. Thus, the stomach’s function of dissolving food before it enters the intestine, makes chewing or other processing of the normal carnivorous diet of ment and fat virtually unnecessary. The digestion of fat requires some special handling by the alimentary system. It has heen shown that digestion occurs efficiently only when the aliment has been dissolved before being exposed to the assault of the enzymes. Fat, however, will not dissolve in water. Bile, which is manufactured by the liver, contains certain substances called bile salts, which act very much like modern industry detergent to render fats soluble in the watery chyme, and thus render them susceptible to the action of fat-digesting enzymes. Fat in the carnivorous diet is present in large amounts on occasions and, since the need for bile is limited to times when there is considerable fat in the diet, the bile is not allowed to drain off, to be wasted during the interdigestive periods (between meals) of the carnivorous animals. Instead it is diverted to the gallbladder, where it can be stored until the presence of fat in the intestine again signals the need for its presence, whereupon a hormone, produced by the presence of fat in the intestine, causes the gallbladder to contract strongly and deliver great amounts of concentrated bile to the intestine. Digestion and absorption of the normal carnivorous diet-protein and fat with but little carbohydrate-is remarkably efficient. If “balance studies” are accomplished, which will merely measure the amount of a certain nutriment administered in the diet, and then determine how much of that same material appears in the animal’s excreta, it is found that the healthy animal never loses more than 4% of the ingested fat and only a trace of dietary protein. The distal end of the dog’s small intestine terminates by emptying into the large intestine, or colon. This connection between the two is of an “end to side” sort; that is, they join each other at a right angle. In the dog there is here a blind pouch, or cul-de-sac, which is two or three inches in length and is called the cecum. While the cecum is functionless in carnivores, it would be well to keep one’s eye on this area of the colon, for differences in it, and also the stomach, constitute two of the major points of variation between carnivorous and herbivorous design. The length of the colon, like that of the small intestine, is subject to considerable variation, according to different authorities. In a large dog it is generally considered to be about four feet. The colon terminates in the globular rectum, which is the area where fecal material is stored until enough has accumulated to make expelling it worthwhile. The opening of the rectum to the outside is called the anus. The volume of the rectum is about that of a baseball. Since digestion of food is complete by the time the small intestinal contents are emptied into the colon, the latter organ has no digestive function. The indigestible residue of the carnivorous diet is small; therefore the colon of these animals is short, of small capacity, and with a physiological activity confined to the transport of indigestible waste to the outside and reabsorption of water and a few minerals from it.
The contents of the small intestine are emptied into the colon as a water suspension. As this material is slowly moved along the colon the water is absorbed; its consistency becomes more firm and its volume smaller. By the time it reaches the rectum this waste material has become a small firm mass, possibly about the size of an egg or tennis ball. When expelled, this material constitutes the bowel movement. When on a nonnal diet of meat and fat, the dog’s stool is firm and practically odorless. Evacuation of the rectum occurs once in each twenty-four to forty-eight hours. In the carnivore the colon is by no means a vital organ; carnivorous animals get along well (albeit with less conveence) after the entire large bowel has been removed. The healthy carnivorous digestive tract furnishes residence for relatively few bacteria and no microprotozoa. These are microscopic-sized organisms; the former are plant material, while the latter are animalcules. The strong ocid of the stomach guarantees that most microorganisms swallowed with the food or otherwise will be killed, or at least be attenuated and not allowed to multiply in that area. Those escaping the stomach are rarely able to withstand the digestive activity of the small intestine. However, toward the lower end of the small intestine, where digestive activity has almost disappeared, a few surviving bacteria are to be found. But in the large intestine, myriad organisms thrive and serve some function in forming certain vitamins: pyridoxine, biotin, folic aeid, Vitamin K, and possibly others. The bacteria of the carnivorous colon are of the putrefactive type; they nourish in on alkaline medium. The digestive tract we have been considering is practically sterile (devoid of organisms) except in the large intesline. The coefficient of digestion is the percentage of ingested food thut is digested, absorbed, and utilized by the animal. It is a measure of nutritive efficiency and, in the carnivore eating its normal diet of meat and fat, the coefficient approaches 100%.
This, then, is the carnivorous digestive tract. It is simple, short, and of small capacity. A small variety of concentrated food is ingested at infrequent intervals. Food is digested only by enzymes which are manufactured by the animal itself. The meat-eating animals have no dependence upon microorganisms to assist in digesting the food. The food is almost completely digested and absorbed, leaving but little excretory bulk. Digestion is rapid, complete, and intermittent. The entire alimentnry canal functions for a few hours, then enters upon an interdigestive period of rest. Significant digestive activity is confined to the small intestine. The carnivore is able to maintain life even after losing both stomach and colon, but cannot survive a loss of the small intestine.
And A Man
To describe the structure and function of a human digestive tract would be mercly repetitious, for it is practically identical with that of the dog already delineated. The sole difference between the two is the presence in man of a rudimentary structure springing from a functionless cecum; this is called the appendix. This organ hos long been considered lo be a degenerated structure and is often cited as evidence that man was originally herbivorous and then became carnivorous. Supposedly, as this dietary change occurred, he gradually lost the use of his cecum, since he was no longer digesting vegetable material, and it gradually shriveled into the vestige we know today. Others, however, suggest that man might not be losing his appendix at all, but is attempting to gain a functioning cecum. This would suggest that he was an original carnivore and that centuries of increasing plant food consumption caused this adaptive change in his digestive tract, in an effort to afford greater digestive capability for his civilized diet. Since this is a purely philosophical matter which will not be solved in less thnn another ten thousand years, it is pointless to pursue it further.