Food and You
Dallas E. Boggs, PhD


Absorption of Foodstuffs from the Small Intestine

Absorption of Protein

THE DIGESTION OF PROTEINS is a necessary step in liberating the amino acids that are locked up in the food. The body then selects the desired ones and joins them together again (in new combinations) to make the special proteins found in blood, muscle, skin and other tissues of man—it builds the proteins according to its own special patterns. For this purpose it is desirable to have the amino acids, or building blocks, supplied from the digestive system as separate units. That is also a first rate defense against possibly dangerous reactions—proteins are rarely absorbed without previous digestion; but, when this does happen, the body may become sensitized and respond to further intrusions of the same  molecules with rather severe reactions. A strange protein in the body can act as a violent poison, and a food allergy may be evidence of previous sensitization to it.

      The splitting of food proteins takes place in the small intestine, where the amino acids are dissolved in the watery contents of the bowel and transported through the gut wall and into the blood capillaries. All of the blood that comes from the stomach and intestines is collected in the portal vein and passed through the liver before returning to the heart for another circuit of the body. The liver is instrumental in the utilization of the foodstuffs; and, in many ways, it is a most important organ. Its location squarely across the portal route gives it first choice of the new materials from the small intestine.

     The liver selects from the portal blood those amino acids that are needed immediately for the synthesis of new proteins. (One of these is concerned with the clotting of blood.) The amino acids not removed at once are carried to the heart and distributed from there to all other tissues of the body. Each tis­sue (or organ) has its own special requirements, which are satisfied by removal from the blood of the materials needed for mainte­nance and repair. Thus the amino acids absorbed from the in­testine during the digestion of a meal are gradually removed from the blood. Most of the time the assortment of amino acids supplied from the intestine is not in the exact proportions needed by the various tissues, and this leaves an excess of some of them that cannot be used in the immediate processes of making new proteins or repairing old ones. Spare amino acids finally get their nitrogen clipped off in the liver and are burned as fuel. (It will be re­called that the extent of amino acid wastage determines the bio­logical value of a protein.)

      During the normal digestion of a meal, amino acids are absorbed as fast as they are liberated from the proteins by the digestive enzymes. The capacity of the intestine to absorb them is very great, and they are absorbed at approximately equal rates—so that the blood, as long as digestion continues, is provided with a steady stream of the amino acids representative of the protein being digested. This is important be­cause, if the body is to use them to the great­est advantage, all of the essential amino acids must be present in the blood at the same time. If five of the essential ones are fed at one time and the remaining five a short time later (one hour), much of the benefit of the feeding is lost. This means that two pro­teins cannot supplement each other unless they are consumed at the same meal. For example, zein (the incomplete protein in corn) will not be improved in biological value by milk or meat eaten at the next meal.

      Patients are often too ill to eat enough food to insure their proper convalescence. In such cases it is customary to feed them by introducing into a vein a solution containing glucose, amino acids, lipids (fat) and salts (minerals). It is important not to introduce these substances more rapidly than they would normally enter from the intestine—if an injection of this type is too rapid, the nutrient materials accumulate in the blood faster than the tissues can use them; and this condition leads to their excretion through the kidneys. For maximum benefit to be de­rived from intravenous feeding, solutions of amino acids must contain the ten essen­tials in the right proportions.

Absorption of Carbohydrate

      By the action of digestive enzymes, the carbohydrates are all broken down to simple sugars that can be absorbed from the small intestine. They follow the same route as the amino acids—after emerging from the intestinal capillaries into the portal sys­tem, they pass first to the liver and later to the rest of the body. Practically all of the carbohydrate in the average diet is di­gested to yield glucose, fructose and galactose. These three sim­ple sugars have identical molecular weights and numbers of carbon, hydrogen and oxygen atoms. They differ slightly in the way they are put together. For some unaccountable reason, galactose is absorbed faster than any other sugar; glucose, in turn, is absorbed more rapidly than fructose. There is a special absorptive mechanism for these three sugars that speeds them through the intestinal wall much faster than any of the other sugars. (Some rare sugars are smaller in molecular size but are absorbed at much slower rates than the three mentioned above.) Phos­phorus, one of the mineral elements, takes an active role in the rapid absorption of galactose, glucose and fructose—by combining with them to form phosphorylated sugars.

      The absorption of sugars, as well as other nutrients, is measured by feeding a known amount—and then (by chemical analysis) measuring what remains in the intestine at the end of a specified time interval. (When sugar molecules are labeled with carbon-14, it is easy to trace their movement in and out of the cells.) The difference between the amounts fed and recovered is obviously the amount absorbed in the given time. Regardless of the kind of carbohydrate fed, the princi­pal sugar found in blood is glucose. The transformation of other sugars to glucose takes place in the liver. The sugars, like the amino acids, enter the blood through the portal vein and must pass through the liver before being distributed to other tissues of the body. The liver has considerable storage capacity for carbohydrate—and it always puts it away as glycogen (animal starch), which, in time of need, is broken down to meet the demand for sugar. The breakdown of glycogen always yields glucose.

      Ordinary table sugar (sucrose) cannot be absorbed as such; it must first be split into its two component simple sugars (glu­cose and fructose). If injected through a needle directly into the blood stream, sucrose cannot be utilized by the body—it will be excreted unchanged in the urine. The same is generally true of other complex carbohydrates. Both proteins and carbohydrates in the diet must be taken apart, down to their simplest units before absorption and utilization by the tissues.

      The intestine can absorb huge quantities of sugar; but too much at one time can flood the system. If an excessively large amount of table sugar or candy is eaten in a short time, the absorption is so rapid that the tissues are unable to remove the excess. It spills over into the urine and is wasted.

The Absorption of Fat

      Fats (or lipids) present some unique features that are associated with the fact that they are not soluble in water. The digestive juices plus the water that is taken with meals makes the intestinal contents a very watery medium. All of the foodstuffs, except fat, are soluble in water and pass through the intestinal wall in watery solution. The nearest ap­proach to a water solution of fat is a fine emulsion made by subdividing the fat droplets to microscopic size. (A familiar example of this is homogenized milk in which the fat is so finely divided that it does not separate and collect on the top as cream.) The intestine forms a stable emulsion of fat by the vigorous mixing of its contents with bile and some of the products of fat digestion.

      The enzyme li­pase splits food fat into glycerin and fatty acids. Glycerin and some of the fatty acids of small molecular size are soluble in water, and these substances are absorbed into the portal vein and travel the same route as the amino acids and sugars. The small particles of fat that are not attacked by lipase make their way into the lymph vessels of the intestinal wall. The lymphatic system in the abdomen empties into a large vessel called the thoracic duct, which follows the spinal column through the chest cavity and empties into one of the large veins at the base of the neck—this is a by‑pass around the liver. The lymph system is a network of vessels found in all tissues of the body. It serves to collect the excess water and nutrients that have found their way out of the blood stream. The blood capillaries are "leaky" enough to per­mit passage of nearly all components of blood except red cells. This fluid (lymph) contains the nutrients required by the cells, and they remove from it what they need. The remainder must be disposed of—and one function of the lymphatic system is to collect left‑overs and return them to the blood for use elsewhere.

      As noted above, patients who are too ill to eat are often nourished by introducing nutrient solutions directly into their veins. This procedure is called “Total Parenteral Nutrition”, or “TPN”. It is often limited to water‑soluble nutrients such as glucose, amino acids and some of the mineral salts. The amount that can be given at one time is small; and, additionally, the solution must be rather dilute in order to avoid complications in the blood stream. For these reasons it is very difficult to administer enough calories by intravenous feeding to maintain body weight. Since fats furnish more than twice as many calories per unit weight as either amino acids or glucose, it often necessary to include them. This is possible only because a method has been found for making a very fine and stable emulsion of fat in water. (This is exactly what the body accomplishes normally when it absorbs an emulsion of fat and pours it into the blood stream by way of the thoracic duct.) In a liter (approximately 1 quart) of solution it is possible to emulsify three hundred grams of fat, which supplies 2700 calories.

Absorption of Vitamins

      Detailed information about the absorption of vitamins is not available. In the early days of vitamin research, there were no chemical methods for the detection of vitamins—their presence or absence in a diet could only be determined by feed­ing young animals (usually rats) and observing the effect on their growth. An interesting modification of the feeding technique (bioassay) is the substitution of bacteria for rats. The bacteria reproduce much faster than animals, and they respond equally well to vitamin deficiencies in their diet. Instead of weighing the bacteria, as is done with rats, a chemical analysis is used to traces the growth of the culture. Many bacteria normally produce a weak acid as a by­-product of their growth—to determine how much growth has occurred, it is only necessary to make a simple titration of the acid. Biological assays are still used for some pur­poses, but many chemical  procedures have been developed for the quantitative determination of vitamins. It is now possible to accumulate much more information about the absorption of these micronutrients from the intestine.

      The digestive enzymes destroy the cell structures in foods and thus release the vitamins as well as the other foodstuffs. Some of the vitamins (such as Vitamin B-12) may be combined with other substances such as proteins; and these combinations are broken before the vitamins are absorbed. Some of the water‑soluble vitamins are assisted across the in­testinal lining by making a new combination with phosphorus. The water‑soluble vitamins enter the blood by the same route as the other water soluble foodstuffs, e.g., the amino acids, sugars and minerals. The fat‑soluble vitamins enter with the fats—during digestion, they can be found in the thoracic duct.

      The mere fact that a food contains a large amount of a vita­min is no guarantee that it is available to the body. An out­standing example of this is fresh baker's yeast. This material, like all yeasts, is very rich in the B vitamins; but the digestive enzymes of man are unable to break down many of the live yeast cells to liberate the vitamins. The result is that fresh yeast passes through the whole digestive tract without giving up its vitamin content. In fact, the situation may even be worse than this; the yeast continues to grow within the intestine and may even steal some of the vitamins from other food sources to satisfy its own needs. These facts should not be taken as a condemnation of yeast as a food. It is only necessary to heat or dry the product sufficiently to kill the cells in order to make their vitamins completely avail­able. Another virtue of yeast as a food is that the protein that it contains is quite good—it is probable that extensive yeast culture in the future may supply a considerable portion of the world's need for food protein.

Absorption of Minerals

      The mineral salts needed by the body are soluble in water and easily absorbed into the portal vein; but the speed of ab­sorption varies considerably with the type of salt. One of the simple salts of iodine can be detected in the saliva in less than five minutes. This means that the salt in this brief time had to pass the intestinal wall, and then be transported in the blood to the salivary gland and make its way across the membrane lining the gland before appearing again in the mouth. The salts of calcium on the other hand require much more time for absorption.

      The absorption of iron presents a special and very interesting story. The main function of iron is to form an important part of hemoglobin, the red coloring material in blood that carries oxygen from the lungs to the other organs. If the body has no need for the manufacture of new hemoglobin, iron will not be absorbed from the intestine. This curious fact makes it appear as if the intestine can sense the body's need and decide whether or not to admit iron to the blood stream. No other example is known of this kind of behavior. With other minerals and food­stuffs the intestine is completely indifferent and absorbs every­thing that is presented whether it is useful or harmful. After much research on animal subjects, it was discovered that the in­testinal lining contains a peculiar protein, called ferritin, which has a special affinity for iron. Ferritin is able to pass along the atoms of iron from intestine to blood stream in a hand‑to‑hand manner much like the old‑fashioned bucket brigade at a fire. If all hands are holding a bucket, no more can be passed; but as soon as one is used at the fire, there can be a movement all down the line. In the body, the need for more iron seems to be met by taking it from the inside end of the bucket brigade. This leaves a vacancy on the outside end that can then be filled by the iron in the intestine. The adult healthy woman needs very little dietary iron because the body uses what it has many times over. There is a small unavoidable loss in the feces; and, in a case of accidental bleeding, or of menstrua­tion in a woman, the loss of iron in the blood is made good at once by absorption of replacement iron from the intestine. Thus, the need for iron sets in motion the bucket brigade, which remains at rest except in cases of need.