The Physiology Of Nutrition
( Originally Published 1921 )
Broadly considered, all physiological or life processes relate to nutrition, and are affected, directly or remotely, by food. Digestion is most immediately and wholly related to food, since digestion is the process of converting food into those substances which then become the living tissue, or which supply the living tissue with materials for its activities. A second group of physiological functions or activities most directly connected with food or nutrition, includes the transformation of food elements in the liver, the distribution of these elements of food in the muscles, and the elimination of waste products of the body through the lungs and through the kidneys.
The process of digestion is primarily a chemical one, but there are also mental or nervous factors and physical factors to be considered. The chemical process of digestion begins in the mouth and continues throughout the length of the alimentary tract.
The transformation which the food undergoes in the different digestive organs varies with the nature of the food. Thus the digestion of starch begins in the mouth and is checked in the stomach, but is completed in the small intestine. On the other hand, the digestion of protein occurs chiefly in the stomach. Fat is digested almost wholly in the intestines.
The chemical process of digestion is carried on by means of enzymes or ferments, secreted by the digestive glands. With these ferments; which are highly complex chemical substances, the digestive glands secrete simpler substances, the purpose of which is to give an alkaline or an acid reaction to the material being digested.
The saliva or digestive juice of the mouth is weakly alkaline and contains a ferment known as ptyalin, which has the power of converting starch into sugar. This may be demonstrated by the fact that dry bread, when thoroughly masticated, develops a sweet taste.
When the food passes into the stomach it meets the gastric secretions, the strong hydrochloric acid of which counteracts the alkaline effect of the saliva and gives the contents of the stomach an acid reaction. This acid, it seems, is necessary to enable the ferment pepsin to get in its work—dissolving the protein elements of our food. Digestion, in the stomach, is not completed, however, even for protein. The main function of the stomach seems to be to act as a warehouse to take care of our irregularly eaten food and to dole it out in a slow and carefully regulated stream to the more important digestive organ, the small intestine. During this period of temporary storage, a certain churning about and thorough intermixing of the food occurs. The chemical transformations, however, are of a preliminary nature. We are inclined to give the stomach more credit and attention because of its prominence and because when overloaded it makes its presence known.
Shortly after the food passes into the small intestines, it encounters the very powerful digestive ferments secreted by the pancreas and also the bile from the liver. Other ferments are secreted from the walls of the intestines, the total effect of these secretions in the intestine being to give a strong alkaline reaction and to re-continue the digestion of both starch arid protein as well as to commence and complete the digestion of fat.
This process of digestion continues through-out the length of the small intestine which is also the chief organ of absorption of the digested food elements into the blood stream.
Digestion is nearly completed by the time the stream of material reaches the colon or large intestine. The function of this latter organ is chiefly that of retaining the unabsorbed material or food waste.
The changes which occur in the digestion of the various groups of food material are essentially as follows: Water, whether taken separately, or the water contained in moist food is absorbed without chemical change. This absorption of water may take place in any portion of the alimentary tract. Water drunk between meals is absorbed directly from the stomach. The rate of absorption of water will depend upon the degree of moisture of the food, or the amount of water drunk with it. If the food is eaten dry, water will be secreted from the blood to bring to the food a suitable liquid condition. . It was formerly given as a generally hygienic advice not to drink with meals. More recent investigation has shown that moderate drinking with meals aids digestion, provided the drinking of water or other liquid is not for the purpose of washing down foods and thus preventing sufficient mastication and salivation.
Next to water, the sugars are absorbed with the least digestive change. True fruit sugars undergo no chemical change for they exist in fruits in the same form as the sugar in the blood.
Cane sugar (that derived from beets is chemically the same) is a more complex substance, which must be broken up into the simpler sugars such as exist in fruits or in the blood.
Starch is a still more complex substance, corn-posed of the same primary chemical elements as the sugars. Starch is not soluble. In digestion it undergoes a complex process of being "hydrolized," which merely means that more hydrogen and oxygen, in the form of water enters into chemical combination with the starch and so changes it into sugar. There are several steps of this change, the intermediate products being gum-like substances called dextrins. This process of the simplification of the starch molecule can be partly brought about by heat. This occurs in the toasting of bread, or more completely in the manufacture of zwieback. Certain manufactured cereals are similarly treated and are known as pre-digested foods. There is no evidence, however, that this partial performance of the natural digestive function outside the body is any advantage to a healthy man.
We were formerly taught that the human being could not digest raw starch; a view which seems rather absurd as it assumes that man is by nature a cooking animal. The moist cooking of starch does not change it chemically but only results in the dry starch grains swelling up to a pasty-like mass. The result is that digestion may occur somewhat more rapidly, but undesirable fermentation may also occur more readily. Man has power to digest starch either raw or cooked, and difficulties which occur in its digestion are probably due to the use of too great a proportion of starch in the diet.
Another artificial process of "digesting" starch is by means of treating it with strong acids, as in the manufacture of glucose from corn-starch. Chemically pure glucose is a wholesome product, for it is indeed the same sugar that occurs in fruits and in the blood. The commercial product may contain residues of chemicals used in its manufacture, though these should be properly neutralized. Moreover, the commercial glucose contains the gummy dextrins. The prejudice against glucose as food is founded upon ignorance of its nature; it is just as good a food as ordinary starch or sugar, and these are whole-some food substances. The practical trouble is that the present-day civilized diet already contains too much food of this sort which crowds out other essential food. elements. Hence, though starch, sugar, glucose, etc., are all good foods, their use should be discouraged as the tendency is to over-use them.
Though we do not list soap as an article of food—and only feed it to small boys who have been telling lies—yet a substance very akin to soap is found in food as an intermediate stage in the digestion of fat.. Fat is insoluble, and hence cannot pass through the walls of the intestine, but fat treated with alkali becomes soap or is saponified, and in this soluble form passes through the intestinal wall; then the alkali is removed again and the fat restored, existing in the blood in the form of tiny fat globules.
The digestion of protein is a very complex process. Like fat, protein is not soluble, and hence it is broken down into its simpler ingredients known as amino-acids. There are a large number of these and they are not alike; the different combinations and proportions of these amino-acids account for the different. kinds of proteins. This explains why all proteins are not of equal value for the nourishment of the body. These various amino-acids are recombined, after absorption, into the various proteins needed by the body. These may be like the proteins of the food, but are more apt to be entirely different proteins which have been made out of the food proteins but with the discarding of considerable portions of their substance. The amount of protein actually needed to nourish the adult is small, and that amount depends upon the nature of the protein in the food. Some proteins taken alone will not support life at all. Gelatine is one of these, and for this reason it was formerly thought to be without food value. It is now known that gelatine has food value when combined with other proteins which supplement the particular amino-acids which the gelatine lacks.
Chemically considered, the processes of digestion seem to be exactly like similar processes which may be conducted by the scientist in his test-tubes. But into the chemistry of life processes a factor enters which does not exist in the laboratory processes. This factor is a nervous or mental one. 'We have long known that the sight, smell or taste of food causes the "mouth to water," but it is only more recently that scientists have discovered that the secretions of digestive juices are influenced in quality as well as quantity by such nervous or mental stimulation.
Still more remarkable, as it at first seems, not only is the secretion of saliva affected in this manner, but the secretion of the gastric juice is also affected by the offering of food to the senses, and before such food enters the stomach. Thus, if meat be held up before a hungry dog, the dog's stomach immediately begins the secretion of gastric juice—and a more acid juice is secreted than if the dog be offered bread. From such experiments we can reason that the entire process of digestion is very skillfully adapted to the nature and quantity of the food. Obviously, the appeal of food to the senses has only a preliminary effect, and such adaptation by means of nerve stimuli to the secreting glands must go on throughout the entire process of digestion, as it does indeed throughout all physiological processes.
The practical application of such knowledge would seem to argue in favor of the simplification of the diet and of the use of foods in their more elementary or natural form. How these physiological instincts can adapt 'themselves to the highly artificial and complicated civilized diet is a mystery! Indeed, they probably do not adapt themselves completely, which is doubtless one of the reasons why the highly complicated and over-seasoned bill of fare is not as digestible and wholesome as a simpler diet derived from natural foods.
We are frequently told that appetizing foods and the enjoyment of our meals is conducive to good digestion and proper assimilation. This is unquestionably true, in so far as unpalatable food cloys the appetite and fails to bring forth the proper secretion of digestive ferments. It is also true that anger, intense sorrow, or other distressing emotions will check or even entirely stop the process of digestion. But this argument in favor of appetizing foods may lead to trouble, if it is used to encourage us in the use of too highly flavored or over-seasoned foods. Such foods over-stimulate the jaded appetite, and result in over-eating. Obviously, such artificial flavors, which disguise the true nature of the food, can serve no good purpose in the adaptation of the digestive secretions to the nature of the food. But the worst feature of the use of over-seasoned food is that the man fed upon a highly stimulating diet loses the power to enjoy, and hence the power to digest simpler food. There is a very easy cure, however, for this condition, and that is genuine hunger. The over-fed gormand, who has lost all power to enjoy his meals, and who would sniff contemptuously at bread and butter, can very readily develop an appetite for old boots when a wise Providence casts him adrift at sea in an unprovisioned boat.
The third factor in the process of digestion is a mechanical one. Our teeth are given us for the purpose of chewing food, but entirely too much of our civilized dishes have already been chewed by the grinding burrs of mills, or by the chemical processing of food factories. The result is that. these ground-up, mixed-up and pre-digested foods not only discourage the use of our teeth and the accompanying process of insalivation, but a mass of food enters the stomach which is too finely ground and too readily soluble. In the natural process of digestion, the digestive solvents gradually chip off or dissolve the external portions of the food particles. But when food, instead of consisting of granules or solid particles of the natural food substance, is in a mushy, semi-soluble condition, the entire mass is. attacked too rapidly by the digestive ferment, but the chemical process is not completed quickly enough. The result is that unwholesome fermentations, due to the presence of bacteria,. occur. Such bacterial fermentation or decomposition may produce various toxins or poisons.
Similar undesirable fermentations with resulting developments of poisoning or auto-intoxication may occur, merely because the mass of digesting food moves too slowly through the intestines, or because the residue is retained too long in the colon.
The remedy for both evils is to be found in the use of coarser and more natural foods. The outer coatings of grain, most notably wheat bran, and the fibrous portions of vegetables, partial larly of leafy vegetables, contain a woody fibre known as cellulose. This cellulose fibre is not digestible and does not ferment, but passes through the alimentary tract unchanged. The presence of such fibre increases the bulk of the food waste, especially in the latter stages of the digestive process. Man was fitted by nature for a diet containing a considerable portion of such fibre, and when deprived of it, and particularly when all food has been finely ground or pre-dissolved, the result is that digestion occurs too -quickly in the upper portion of the digestive -canal, and the small undigested residue remaining fails to move along with sufficient rapidity. This is the explanation of the common civilized complaint of constipation, and the associated evil results of bacterial fermentation and auto-intoxication.
The functions of digestion thus far considered in this chapter are generally understood because the subject is presented in the ordinary school physiology. But the processes of nutrition that occur after the food elements have been absorbed into the blood are not so commonly understood.
The function of the liver, we are told, is to convert the sugar, which results from the digestion of all carbohydrate foods, into a substance called glycogen. This material may be stored by the liver in moderate quantities, and in this capacity the liver acts as a sort of temporary warehouse to store the fuel food as digested until it is required for the production of heat or energy of the muscles. Fat, which serves the same ultimate use, is not stored in the liver, but, if taken in excess of the body's immediate power to consume it, must be stored as fat throughout the various fatty deposits of the body. Either sugar or fat may be oxidized to produce heat and energy. Moreover, when carbohydrate foods are eaten in excess of the body's needs, or the liver's capacity for temporary storage, the resulting blood sugar may also be converted into fat and stored in the fatty tissues, in the same way as fat derived directly from the food.
Sugar in the blood, and hence ultimately fat, can also be derived from protein foods when these are eaten greatly in excess of our needs. This is true because the protein molecule contains carbon, hydrogen and oxygen, the elements of both sugar and fat. But the protein also contains nitrogen, and sometimes other elements which cannot be oxidized or burned. When protein is eaten in excess of the body's needs, a portion of it is thus wastefully used, the same as carbohydrates or fats, but the nitrogen is useless and must be excreted as a waste product. This excretion occurs through the kidneys in the form of urea. This disposition of excess protein must not be confused with the more normal use of protein, which is to build up the protoplasm of active cells and tissues. In the case of growth, such protein actually becomes a part of the living tissue. The individual cells are constantly breaking down and being replaced by others, so that there is a certain process of cell growth always going on, even in the adult. The protein that has gone through this cycle and become part of the living tissue, only later to be discarded, is ultimately disposed of in the same manner as the excess protein taken with food.
It is because of this fact that scientists are very slow in finding out the true protein requirements of the body. They formerly assumed that the amount of urea excreted by the kidneys indicated the amount of protein that the life processes really demanded. Hence their surprise and scepticism when it was discovered a few years ago that a man could live, and seemingly be the better for it, on from one-fourth to one-half of the amount of protein formerly thought necessary. The scientists had been particularly positive that the body required these larger amounts of protein, because, when the amount taken in the food was decreased to less than the accutomed figure, the result was that more nitrogen seemed to be excreted than was being taken in the food. They reasoned that this nitrogen must come from the living tissue, and that the man was therefore consuming himself, and would ultimately waste away and die of starvation. The error was in the fact that the observations did not continue long enough. The extra nitrogen being excreted was derived from a sort of floating surplus, and when this was disposed of the amount of nitrogen excreted was reduced, or, as the scientists say, the nitrogen balance was re-established, and maintained on a lower level.
Excess protein is not only wasted in the sense that it is not utilized, but it wastes other food substances because it stimulates the rate of metabolism, or physiological change, and causes the rate of oxidation of the body to increase during its period of protein digestion and absorption. This effect of protein was only recently discovered, but it has now been shown that a man, after eating heavily of meat, will generate from ten to thirty per cent more heat for a period of six to eight hours after such a meal of meat than he would on an empty stomach. Excess protein therefore wastes other foods, as this extra oxidation does not serve any useful purpose.
Some scientists still argue that there is an advantage to be derived in this wasteful and excess use of protein. These arguments, how-ever, seem to be based upon the natural prejudice in favor of the maintaining of established habits. By the same line of reasoning, many people argue that the eating of all food in excess of our actual needs is a good thing, as it makes us "fat and prosperous" with a sort of surplus bank account of nutriment always on hand.
The fallacy of this view will be fully considered in our chapters on "How Much to Eat" and also in the chapter on "Eating to Reduce Weight."