The nutrition of man

by R. H. Chittenden

CHAPTER II

ABSORPTION, ASSIMILATION, AND THE PROCESSES OF METABOLISM

Topics: Physiological peculiarities in absorption. Chemical changes in epithelial walls of intestine. Two pathways for absorbed material. Function of the liver as a regulator of carbohydrate. Absorption of proteid products. Assimilation of food products. Anabolism. Katabolism. Metabolism. Processes of metabolism. Older views regarding oxidation. Discoveries of Lavoisier. The views of Liebig. Theory of luxus consumption. Oxidation in the body not simple combustion. Oxygen not the cause of the decompositions. Oxidation not confined to any one place. Intracellular enzymes. Living cells the guiding power in katabolism. Some intermediary products of tissue metabolism. Chemical structure of different proteids. Decomposition products of nucleoproteids. Relation to uric acid. Action of specific intracellular enzymes. Creatin and creatinin. Relation to urea. Proteid katabolism a series of progressive chemical decompositions. Intracellular enzymes as the active agents.

Digestion being completed, and the available portion of the foodstuffs thereby converted into forms suitable for absorption, the question naturally arises, In what manner are these products transported from the alimentary tract to the tissues and organs of the body? In attempting to answer this question, we shall find many illustrations of the precise and undeviating methods which prevail in the processes of nutrition. For example, it would seem plausible to assume that the different forms of sugar entering into man’s ordinary diet, all of them being soluble, would be directly absorbed and at once utilized, but such is far from being the case. Milk-sugar and cane-sugar, both appearing in greater or less degree in our daily dietaries, if introduced directly into the blood, are at once excreted through the kidneys unchanged. The body cannot use them, and they are gotten rid of as speedily as possible, much as if they were poisons. When taken by way of the mouth, however, they are utilized, simply because in the intestine two enzymes are present there, known as lactase and invertase, which break each of the sugars apart into two smaller molecules. In other words, milk-sugar and cane-sugar are disaccharides, and if they are to be absorbed in forms capable of being made use of by the body they must be split apart into simpler sugars, viz., monosaccharides, such as dextrose, levulose, etc. The great bulk of the carbohydrate food consumed by man is in the form of starch, and this, as we have seen, is converted into maltose by the action of saliva and pancreatic juice. Maltose, however, like cane-sugar, is a disaccharide, and the body has no power to burn it or utilize it directly; but in the intestine and elsewhere is an enzyme termed maltase, which breaks up maltose into two molecules of the monosaccharide dextrose, and this the body can use. Man frequently consumes starch to the extent of a pound a day, and if utilized it must all undergo transformation into maltose, and then into dextrose. There is no apparent reason why maltose should not be absorbed and assimilated as readily as dextrose, but so urgent is the necessity for this conversion into dextrose that in the blood itself there is present maltase, to effect the transformation of any maltose that may gain entrance there. We are here face to face with a simple fact in nutrition. The body cannot utilize disaccharides directly. Why it is so we cannot say, but the fact is a good illustration of the principle that nothing can be taken for granted in our study of nutrition.

For years, physiologists assumed that the ordinary physical laws of osmosis, imbibition, and diffusion were quite adequate to explain the passage of digested food materials into the blood and lymph. If a substance was soluble and diffusible, that was sufficient; it would quite naturally be absorbed in harmony with its diffusion velocity. This, however, is not wholly true, since experiment shows that the rapidity of absorption of diffusible substances through the wall of the intestine is by no means always proportional to the diffusion velocity of the substance. The lining membrane of the small intestine, where absorption mainly takes place, is not to be compared to a dead parchment membrane. On the contrary, it is made up of living protoplasmic cells; absorption is not a physical, but a physiological, process, in which the living epithelium cells stand as guardians of the portals, ready to challenge and, if need be, modify the rate of passage. Osmosis and diffusion undoubtedly play some part in absorption, but they alone are not sufficient to account for what actually takes place in the absorption of digestion products, and other substances from the living intestine.

The primary products formed in the digestion of proteid foods—the proteoses and peptones—afford another illustration of physiological peculiarity in absorption. These bodies are readily soluble and quite diffusible, yet they are never found to any extent in the circulating blood and lymph during health. It is of course possible, as has been previously suggested, that as soon as formed they undergo transformation into simpler decomposition products in the small intestine; but this is by no means certain. If proteoses and peptones are injected directly into the blood, they cause a marked disturbance, influencing at once blood-pressure, affecting the coagulability of the blood, and in many other ways exhibiting a pronounced deleterious action which at once indicates they are out of their normal environment. They are not at home in the circulating blood, and the latter medium gets rid of them as speedily as possible; they behave like veritable poisons, and yet they are the primary products formed in the digestion of all proteid foodstuffs. On the basis of all physical laws governing diffusion they should be absorbed, and help to renew the proteids of the blood and later the proteids of the tissues. Yet, as we have said, they are not normally present in the blood or lymph. Apparently, in the very act of absorption, as they pass through the epithelial cells of the intestinal wall, before they gain entrance to the blood stream, they undergo transformation into serum-albumin and globulin, the characteristic blood proteids. The other alternative is that, as previously mentioned, they are completely broken down in the intestine into amino-acids, etc., and these simpler products synthesized, as they pass through the intestinal wall toward the blood, into serum-albumin and globulin. Certainly as yet, there is no evidence that the amino-acids, as such, go through the epithelial cells of the intestine; they are not found in the blood or lymph to any appreciable extent, yet the proteids of the blood are reinforced in some manner by the products of proteid digestion. Whichever view is correct, one thing is perfectly obvious, viz., that in the act of absorption the products resulting from the gastric and pancreatic digestion of proteid foods are exposed to some influence, presumably in the epithelial cells of the intestinal wall, by which there is a reconstruction of proteid. Further, the proteid substances so formed are of the type peculiar to the blood of that particular species of animal. The proteids of beef, mutton, chicken, oatmeal, or bread go to make the proteids of human blood.

From these statements, it is obvious that what we term absorption is something more than a simple diffusion of soluble substances from the alimentary tract into the blood current. The process is much more complex than appears on the surface, and our lack of definite knowledge, in spite of numerous efforts to unravel the mystery, merely strengthens the view that we are dealing here with an obscure physiological problem, and not a simple physical one. Digestion induces a splitting up of the food proteid into fragments, large or small, while incidental to absorption there is apparently a reconstruction, or synthesis, of proteid from the fragments so formed. The process seems somewhat costly, physiologically speaking, yet when one considers the variety of proteids consumed as food, it is easy to comprehend how essential it is that in some manner, as in absorption, there be opportunity for construction of the specific proteids of the blood and lymph.

We find an analogous process in the absorption of fats. As we have seen, the fats of the food are broken apart in the small intestine into glycerin and free fatty acid, a portion of the latter, and perhaps all, combining with the alkali of the intestinal juices to form soluble soaps, or sodium salts of the respective fatty acids. The neutral fats present in animal and vegetable foods are all alike in containing the glyceryl radicle, but they differ in the character of the fatty acids present. Further, one form of animal fat, like that from beef, may contain quite a different proportion of stearin, palmitin, and olein than is present in the fat of another animal, like mutton. By digestion, however, they are all broken apart into fatty acid and glycerin. These acids and their salts can be readily detected in the intestine, but they are not found in the blood or lymph, yet shortly after fatty food is taken the lymph is seen to be milky from fat. Obviously, the fatty acids liberated in the intestine are absorbed, either as soluble soaps or as free fatty acids dissolved in bile, but as they pass through the epithelial cells of the intestine into the lacteal radicles, there is a synthesis or reconstruction of fat; and as a result, neutral fats and not soaps are found in the lymph. Here, then, we have a process quite analogous to what apparently occurs in the absorption of proteid, though less complex; and it is possible that this is one of the factors which aids in the formation of a specific fat mixture corresponding, in a measure, to the type of fat present in the particular species. It is well understood that the fat of an animal’s tissues may be modified somewhat by the character of the fat fed, yet in spite of this there is a certain degree of constancy in composition which calls for explanation. Sheep and oxen feeding in the same pasture have fat widely different in the proportion of stearin, palmitin, etc. The fat of man’s tissues is fairly definite in composition, yet he eats a great variety of fatty foods. One man may consume large amounts of hard mutton fat with its relatively large content of stearin, while another individual may take his fat mainly in the form of the soft butter fats, with their relatively large content of olein and palmitin. In both cases, the fat of the man’s tissues will be essentially the same. To be sure, the changes that take place in the tissue cells, reinforced by the construction of fat from other sources, may be partly responsible for this constancy of composition, but the transformations incidental to absorption are quite possibly, in some measure, helpful thereto.

The great bulk of the digested food material is absorbed from the small intestine, and there are two pathways open through which the absorbed material can gain access to the blood. The one path leads directly to the liver, and substances taking this course are exposed to the action of this organ, before they enter into the general circulation. The other path is through the lacteal or lymphatic system, and constitutes a roundabout way for substances to enter the blood stream, since they must first pass through the thoracic duct before entering the main circulation. As a general truth, it may be stated that fats are absorbed through the latter channel, while carbohydrates and proteids follow the first path. The innumerable blood capillaries in the villi of the intestine take up the products resulting from the digestion of proteids and carbohydrates, through which they are passed into the portal vein, and thereby distributed throughout the liver. This means that both carbohydrates and proteids—or their decomposition products—are exposed to a variety of possible changes in this large glandular organ, before they can enter into the tissues of the body. As we have seen, practically all carbohydrate food is converted into a monosaccharide, principally dextrose, in the alimentary tract; and it is in this form of a simple sugar that the carbohydrate passes into the blood. This might easily mean a pound of sugar absorbed during the twenty-four hours, and would obviously give to the blood a high degree of concentration, unless the excess was quickly disposed of. Sugar is very diffusible, and if it accumulates to any extent in the blood it is quickly gotten rid of by excretion through the kidneys. This, however, is wasteful, physiologically and otherwise, and does not ordinarily occur except in diseased conditions. Further, physiologists have learned that a certain small, but definite, amount of sugar in the blood is a necessary requirement in nutrition, and it is the function of the liver to maintain the proper carbohydrate level.

We must again emphasize the great importance of carbohydrate food; there is a far larger amount of starchy food consumed than of any other foodstuff, and it is more readily available as a source of energy. Its presence in the blood, in the form of sugar, is constantly demanded, but it must be kept within the proper limits for the uses of the different tissues and organs of the body. The liver serves as an effective regulator, maintaining, in spite of all fluctuations in the supply and demand, a definite percentage of sugar such as is best adapted to keep the tissues of the body in a normal and healthy condition. This regulation by the liver is rendered possible through the ability of the hepatic cells to transform the sugar brought to the gland into glycogen, so-called animal starch, which is stored up in the liver until such time as it is needed by the body. The process is one of dehydration, the reverse of what takes place in the intestine when ordinary starch is converted into maltose and dextrose. The efficiency of this regulating mechanism depends also upon the ability of the liver to transform glycogen into sugar, presumably through the agency of an enzyme in the hepatic cells. Hence, glycogen may be looked upon as a temporary reserve supply of carbohydrate, manufactured and stored in the liver during digestion, when naturally large amounts of sugar are passing into the portal blood, and to be drawn upon whenever from any cause the content of sugar in the blood threatens to fall below normal. Obviously, there must be some delicate machinery for the adjustment of these opposite changes in the liver, and we may well believe that it is associated with the composition of the blood itself, which in some fashion stimulates and inhibits, as may be required, the functional activity of the liver, or its component cells. In any event, we have in this so-called glycogenic function of the liver a most effective means for accomplishing the complete and judicious utilization of all the sugar formed from the carbohydrates of the food, after it has once passed beyond the confines of the alimentary tract into the blood; preventing all loss, and at the same time guarding against all danger, from undue accumulation of sugar in the circulation. We see, too, how wise the provision that all sugar should pass from the alimentary canal into the portal circulation and not by way of the lymphatics, since by the latter channel the regulating action of the liver would be mainly lost. Further, recalling how soluble and diffusible sugar is, we may well marvel that it practically all passes from the intestine by way of the blood, and escapes entry into the lymphatics. Surely, this marked shunning of the other equally accessible pathway affords a striking illustration of selective action such as might be expected in a physiological process, but not in harmony with the ordinary physical laws of osmosis or diffusion. In conformity with this statement, it may be mentioned that appropriate experiments have clearly demonstrated that the different sugars available as food are not absorbed from the intestine in harmony with their diffusion velocity, but show deviations therefrom which can be explained only on the ground that the intestinal wall exercises some selective action, due to the living cells composing it. Likewise interesting in their bearing on nutrition are the observations of Hofmeister,[13] who finds by experiments on dogs that the assimilation limit of the different sugars shows marked variation. Thus, dextrose, levulose, and cane-sugar have the highest assimilation, while milk-sugar is far less easily and completely assimilated. If this is equally true of man, it indicates that starchy foods, with their ultimate conversion into dextrose, are to be ranked as having a high assimilation limit, thus affording additional evidence of their high nutritive value.