Absorption of Food


   Before food can be of any value to the body it must be carried to the cells. In order to do this, it is necessary that it be removed from the intestinal canal and be taken up by the blood and lymph. The process by which this is accomplished is termed absorption.

   Some absorption takes place in the stomach and a small amount, chiefly of water, takes place in the colon, in the cecum, to be exact. The small intestine, due to its peculiar structure is specialized for the work of absorption. The greater part of the food is absorbed from this.

   Some part of the process of absorption may be explained by the laws of diffusion and osmosis, but only a small part of it may be so explained. We are forced to make due allowance for the fact that the cells lining the small intestine are living. The walls of the intestine do not behave like a dead membrane. Every epithelial cell lining the digestive tract is in itself a complete organism, a living being, with the most complete function. These exercise a selective capacity by which, in a normal condition, they permit the absorption of good food and prevent the absorption of a whole series of poisons which are readily soluble in the digestive juices. Absorption is a physiological, not a physical process; one of active selection and absorption and not mere osmosis.

   This is well illustrated by the following well known facts. Certain substances which are rated by ordinary standards as highly diffusible are not permitted to pass through the intestinal lining. Magnesium sulphate (epsom salts) and grape sugar will serve as an excellent example of this. When a test is made with parchment or any ordinary membrane the sugar is found to be less diffusible than salt. In the intestine this is reversed. The sugar is readily absorbed while the salt is excluded almost entirely.

   This selection of the good and useful and rejection of the injurious and useless is done by the cells lining the walls of the intestine as these take up the food from the intestine and secrete it into the blood stream.

   Some of the salts seem to be absorbed from the stomach. However, most of these together with the monosaccharides, amino-acids, fatty acids and glycerol are absorbed from the small intestine. By this is meant that these compounds disappear from the intestine. It by no means follows that they enter the blood as such. It seems probable that they undergo some changes during their passage through the cells. This is known to be true of the fatty acids and soap for these are changed in their passage through the cells and enter the blood as neutral fat.

   Proteins belonging to one class of animals will not nourish another class of animals if injected directly into the circulation without undergoing digestive changes. The protein (albumen) of an egg, if injected directly into the blood, acts as a poison and is immediately expelled. The proteins of nuts, wheat, cheese or milk, mutton, beef, eggs, chicken, etc., are all different and distinct but each and all of these may be used to nourish the human body. Before they can be used, however, they must first be converted into a particular class of proteins. If they are not so converted they not only do not nourish the body, but if they are forced into the blood stream they act as real poisons.

   In spite of the many changes the proteins of the various foods undergo in the stomach and intestine they still remain the protein peculiar to those foods--eggs, beef, mutton, beans, etc. During their passage through the intestinal walls these proteins undergo some change (of a nature wholly unknown) which fits them for entrance into the body. For no sooner than these have passed through the intestinal wall into the circulation, than their nature is changed. They are now "human proteins."

   This is of sufficient importance to justify a little further explanation. The protein molecule in the chyle, as it exists in the lumen of the intestinal canal, is known as peptone. It is a highly toxic substance in this form, yet it is the only form in which protein can be absorbed by the intestinal mucous membrane and passed on to the blood. As it passes through the cells of the intestinal wall the peptone undergoes further changes. It loses its toxicity and appears in the blood in one or another of at least three forms: namely, serum albumen, serum globulin and fibrinogen.

   These proteins undergo further transformation in being built into cell substance. No one knows how many forms of protein can be produced by these transformations, nor can they always be fully identified with chemical accuracy for they often exist in very minute quantities.

   Once the food is in the blood stream it is carried to all parts of the body to nourish the cells and to be used for the various purposes which food serves. Food material may be conveyed directly to the liver, or it may be carried through the lymphatic system. It appears, as a general rule, that proteins and fats are conveyed directly to the liver, while the carbohydrates are sent by the other route. When the food reaches the cell it is subjected to still further changes which are apparently due to enzymatic action, before it is finally incorporated by the cell.