Food Requirements Vitamins

There is no clear-cut definition of vitamins, no sure way of deciding whether a substance important in nutrition is to be regarded as a vitamin or not. Vitamins are organic substances capable in small concentration of affecting the health or influencing the rate of growth of organisms. There are numerous substances that act in this way. Some of them are regarded as vitamins, others not, or there may be differences of opinion. Thus it was noted in the last chapter that certain unsaturated fats were necessary for the growth and well-being of rats. These fats can be regarded as a fat requirement or they can be called vitamin F. Similarly, various insects require cholesterol. This may be regarded simply as a sterol requirement, or the cholesterol may be considered a vitamin in the same sense that calciferol (vitamin D) is a vitamin. The fact that vitamins are capable of exerting a powerful influence when present in extremely low concentrations is an indication that they act in some way as catalysts. Certainly this is true of some vitamins.

It is by no means an easy task to present an up-to-date summary of our information concerning vitamins. The earlier studies of vitamins were essentially practical. Men suffered from diseases due to vitamin deficiency. It was important to discover which foods contained the necessary vitamins, and in what amount. Lacking a chemical knowledge of the vitamins it was essential to develop biological methods of assay. Thus, it was found that a certain amount of vitamin was necessary to cure pigeons of polyneuritis or to permit normal growth in rats. On the basis of such studies on rats, pigeons, and guinea-pigs, various types of vitamin units were established, and the vitamin content of all sorts of foods was determined. To raise colonies of rats and guinea pigs is rather an expensive and time-consuming business. Fortunately, the presence of vitamins and the amount of a certain vitamin contained in a given sample of food can be tested with organisms much easier and less expensive to raise than mammals and birds. Insects require many of the known vitamins and they might be used as test animals. But much more suitable are various bacteria and fungi. They can be raised in test tubes in a minimum of space. Obviously one must first determine the exact vitamin requirements of a given type of bacterium or fungus. The assay of vitamins by the use of bacteria is now well established practice and there is a large literature in the field.

The chemical composition of most vitamins is now known. This makes possible direct chemical analysis. Often such analysis is aided by spectroscopic study. Thus the A vitamins give a blue color with antimony trichloride and this color is the basis for spectroscopic tests. Some vitamins give characteristic absorption bands in the ultraviolet. Likewise, some vitamins are fluorescent, and the fluorescent color can be tested spectroscopically. Thus in addition to ordinary chemical assay methods, spectroscopic tests are of great value.

In many cases a vitamin may be replaced by other chemical substances related to it chemically. Thus various substances related to ascorbic acid (vitamin C) behave like ascorbic acid in preventing scurvy. There are many substances with vitamin D or vitamin K activity.
The fact that a substance has a chemical structure similar to that of a vitamin does not necessarily indicate that the substance will act like the vitamin. Often, indeed, a substance which resembles a particular vitamin in its chemical composition is an antagonist to it and tends to prevent its action. So, for example, Dicumarol is chemically close to vitamin K; and yet whereas feeding of vitamin K favors blood clotting in higher animals, Dicumarol prevents clotting. Similarly, various analogues of pyridoxine (a B 6 vitamin) act to prevent its action. Other cases of anti-vitamins could be cited. Studies of antivitamin action may give clues as to the mechanism of the action of the vitamin they antagonize. The subject is also of considerable interest to the theoretical pharmacologist. More and more cases are being discovered in which the harmful action of a particular drug is due to its chemical affinity to some substance important for the vital process.

In some cases, the need for a vitamin is lessened by feeding a substance which can be converted into it. In some organisms, tryptophan can be converted into nicotinic acid. Thus, chicks do not require nicotinic acid if fed enough tryptophan. Moreover, isotope studies show that tryptophan can be converted into nicotinic acid. Another instance in which a necessary vitamin can be replaced by an amino acid has been described for the lactic acid bacteria, Streptococcus faecalis and Lactobacillus casei. Certain strains of these species require vitamin B 6, but they can grow without this vitamin in the presence of dalanine (the dextro-rotatory, naturally occurring form of the amino acid). In this instance, however, the amino acid is not the precursor of the vitamin, but apparently the vitamin is necessary in order to produce the essential amino acid. These cases, interesting in themselves, indicate also that the need of a particular organism for a given vitamin may to some extent depend on the diet.

As a result of our knowledge concerning the chemical nature of the vitamins, many of them can be synthesized and are actually being manufactured on a large scale. We also have much knowledge concerning the specific vitamin requirements of many higher animals, of the isolated roots of green plants, of bacteria, fungi and protozoa. More and more information is becoming available concerning the vitamin needs of various insects. Vitamin studies are relatively simple for terrestrial forms-mammals, birds, insects. Aquatic forms are difficult material. Perhaps if fish could be grown in sterilized water and kept sterile, correct data on their vitamin requirements could be obtained. With ordinary methods of attack, one can never be certain of the contribution that bacteria and other microorganisms are making to the vitamin content of the food taken in by the fish. Even in terrestrial animals, microorganisms may play a part in providing vitamins. The bacteria in the intestinal tract can and do manufacture vitamins. In experiments with rats, it is sometimes necessary to prevent the animals from eating their feces, for such fecal material may be a source of vitamins. In many instances a mammal may not need a particular vitamin as food because it is manufactured by the bacteria of its intestine. By giving the animal antibiotics such as sulfa drugs and thus eliminating or reducing its intestinal bacterial flora, it is possible to investigate the need for a vitamin such as folic acid. The nutritional requirements of various types of animals may depend largely on the parasites or symbionts they harbor. Many species of insects, especially among the Hemiptera, have a constant association with symbiotic fungi. Even protozoa may contain symbionts or parasites within their single-celled bodies.