Food and You
Dallas E. Boggs
Utilization of Vitamins
Utilization of Vitamins
VITAMINS must be furnished ready‑made in the diet—the body is unable to make them from simpler materials. The total amount eaten daily is so minute that it could easily be contained in a small teaspoon—its fuel value is insignificant. Therefore, the importance of the vitamins is not measured in terms of calories but in their remarkable property of aiding or catalyzing the many chemical reactions necessary for the maintenance of life. A mechanical analogy may be helpful in thinking about their usefulness. The engine in a car will run for a limited time if it is supplied only with gasoline. But unless lubricating oil is present, the engine wears out or breaks down long before it should. The oil does not furnish any of the fuel nor does it become part of the machine, but its presence permits the parts to run smoothly for a long time. Like the lubricating oil, vitamins furnish no energy for the work of the body and they do not seem to be part of its structure. But if they are not present, many of the body's chemical reactions fail to occur and others are incomplete.
The Fat‑Soluble Vitamins
Vitamins A, D, E and K are usually associated with the fats in plants and animals and are easily soluble in ether or other fat solvents. Hence this group of vitamins is labeled fat‑soluble. This is about the only property they have in common. Some of the consequences of eating diets deficient in this class of nutrients are well known. Lack of vitamin A results in a dry, cracked skin and an inability to see in dim light. This "night blindness" is due to a lack of vitamin A in the visual purple of the eye. The rapidity of alternate bleaching and coloring of this compound determines the keenness of vision in dim light.
Lack of vitamin D—also known as the "sunshine vitamin" because exposure of the skin to ultra-violet light allows the body to manufacture it—can cause rickets, and this deficiency leaves its mark (especially in growing children) in the form of bow legs and pigeon breast. It is perfectly plain that bone development is faulty in such cases. This means that something is wrong with the utilization of calcium and phosphorus, the two most important mineral elements found in bone. It is known that vitamin D helps in the absorption of calcium from the small intestine, but just how the vitamin takes part in the building of new bone is complicated by the fact that the hormone of the parathyroid gland also takes part in the transport and use of calcium and phosphorus in the body.
Vitamin E deficiency was first observed in rats. The most striking sign of this deficiency in that species is sterility in both sexes. Fertility can be restored in the female by feeding some source of vitamin E. In the male rat, however, sterility due to lack of vitamin E is permanent; and no amount of medication will repair the damage. These results have not been confirmed in the human being, but it is doubtful that anyone has ever seen a clear‑cut case of vitamin E deficiency in man. The evidence, though meager, does indicate that this vitamin is needed by the human, but practically nothing is known about its function in the body. Some experimental evidence suggests that vitamin E acts as a regulator of tissue oxidations. Here again is another complication because one of the hormones, namely thyroxin, is especially active in this respect. There is some reason to think, however, that these two substances are on opposite sides of the balance. Vitamin E tends to slow down oxidations, especially in muscle, while thyroxin tends to make them go faster.
Vitamin K is essential because without it blood fails to clot and there is danger of bleeding to death from an otherwise insignificant injury. Serious bleeding may occur even without a break in the skin. The blood capillaries spring leaks very easily in vitamin K deficiency, and blood may escape through these leaks and find its way out of the blood vessels into other tissues, such as the muscles and skin. This condition was first seen in young chicks—and it was soon discovered that "hemorrhagic disease" of the newborn human infant could be caused by deficiency of vitamin K. Some of this nutrient is obtained directly from the diet, but a considerable portion is manufactured by the bacteria that normally live in the digestive tract.
If the mother eats a vitamin K deficient diet or is unable to absorb enough of the vitamin for herself and unborn baby, the infant will be born with a deficiency. Since it requires some time for the new‑born to acquire a normal bacterial population in his intestine, vitamin K must be furnished to the infant at birth to prevent hemorrhage. The safest procedure is to make sure that the pregnant woman has enough vitamin K in her diet to get her baby off to a good start. The mechanism by which this vitamin assists blood clotting is not known in detail; it is believed that it enables the liver to form one of the proteins needed in clot formation—part of a very complex process that includes several proteins in the blood as well as the mineral element calcium.
It is evident from this brief discussion that the fat‑soluble vitamins are necessary for health. The signs of a deficiency are easily recognized in some instances but not in others. The chemical structure of all four of these vitamins is known in every detail, and they can be synthesized by an organic chemist. The unsolved riddles lie in the field of the mechanisms by which these interesting substances carry out their work in the body. It seems fairly certain that one of the great obstacles in solving these problems is the insolubility of the fat‑soluble vitamins in water. The body is essentially a watery medium, and all of the biochemical reactions occur in this medium. The biochemists have not yet learned how to reconstruct the chemical environment of the cell, and until they do it will be very difficult to discover just how the fat‑soluble vitamins work.
The Water‑Soluble Vitamins
The water‑soluble vitamins are more numerous than the fat-soluble ones. They include the members of the "B‑complex" and vitamin C. This sort of classification is the result of the historical development of the subject. Vitamin A was the first to be given a name, and shortly thereafter a water‑soluble substance was found necessary for health, and hence it was called vitamin B. After this came vitamin C, etc. It soon developed, however, that A and B were not single compounds. Vitamin D was found associated with vitamin A in cod liver oil. What was once thought to be a simple vitamin B turned out to be a whole family of vitamins that nearly always occurred together in the plant kingdom. This group then became known as the "B‑complex." Vitamin C has remained as a single substance; and after its isolation and synthesis, it was named ascorbic acid.
Three vitamins of the B‑complex (thiamine, riboflavin and nicotinic acid) have received much attention. The absence of each of these in a diet leads to certain signs of deficiency which are well recognized. Thiamine deficiency is manifested in a disease called beri‑beri. Lack of riboflavin is indicated by cracks in the skin at the corners of the mouth. A deficiency of nicotinic acid causes pellagra, a disease characterized by inflammation of the mouth, diarrhea and a roughness of the exposed parts of the skin. These three vitamins play an important part in the utilization of carbohydrate. They form essential links in the enzyme systems that make it possible to burn carbohydrate at body temperature.
All enzymes are proteins; but they require certain other substances in order to be active. By coupling with a vitamin the enzyme protein is capable of removing hydrogen from a molecule of glucose and finally assisting in its combination with the oxygen that is obtained from the lungs. Thus the oxidation of glucose, as well as other foodstuffs, is ultimately accomplished by removing the hydrogen in gentle stages. It is possible to isolate the proteins and vitamins concerned in these reactions, and they have been studied in the laboratory outside of the body. On this account a great deal is known about the mechanisms of their action in the body. This is a good example of a remarkable advance in the science of nutrition. When a chemical reaction of the body can be reproduced in the laboratory, it is not long before the details are worked out. It is the scientists' firm conviction that every process in the body can be explored in this manner.
The other members of the B‑complex (pyridoxine, pantothenic acid, folic acid, biotin, inositol, and para‑aminobenzoic acid) have all been identified chemically and can be synthesized by the organic chemist. Their physiological functions are not well known. The most recent and most exciting member of the group is vitamin B-12. This vitamin contains the mineral element cobalt, and it is necessary for the normal manufacture of hemoglobin and red blood cells. It has been used successfully in the treatment of pernicious anemia, a disease in which the bone marrow is unable to produce enough hemoglobin and red blood cells to keep up with their destruction.
Scurvy develops in people who live on a diet deficient in vitamin C. The only other animals known to develop this disease are the monkey and the guinea pig. (Rats, cats, dogs and other laboratory animals are able to synthesize enough of this vitamin for their own needs.) Until a dietary deficiency disease can be produced in laboratory animals, very little can be learned about its causes and cure. For this reason it is most fortunate that the guinea pig will develop scurvy in the absence of vitamin C. With the use of this animal it was possible to test the potency of various sources of vitamin C and finally to isolate the pure substance from lemon juice. After chemical identification, it was synthesized in the laboratory, and the synthetic product was shown to have the same potency as the one found in lemon juice. Scurvy is characterized by loosening of the teeth, bleeding gums and very painful joints.