VITAMINS. - They are regulatory principles, largely known chemically, contained in animal and plant tissues used as foods. The name "vitamins," due to Funk, is the most widespread, although improper because not all of these principles contain nitrogen; they have also been called "accessory dietary factors" (Mc Collum), "cutonne" (Pugliese), "nutramine" (Abderhalden), "completine" (R. Berg).
I. HISTORY
The history of vitaminology is recent, even though as far back as 1720 the Austrian Kramer had made interesting observations on the relationship between scurvy and diet. Generally one starts from the findings of Lunin (1881) and of the Italians Coppola and Pasqualis (1895-96), which suggested that natural foods, such as milk, contain principles necessary for metabolic balance, distinct from the known classes of food materials. From 1897 come the fundamental observations of Eijkman, who discovered the relationship between a diet exclusively of polished rice (as is used in many regions of the Far East) and beriberi; in 1906 Hopkins published exemplary research on the insufficiency of synthetic diets, even when combined with full respect for energy and amino-acid requirements. Stefani, in 1910, put forward the hypothesis that pellagra depended on the absence in a maize-based diet of something "imponderable" yet necessary to the organism. In 1912 Holst and Froelich described the picture of experimental scurvy in the guinea pig, recognizing in it the deficiency of a factor of this kind, found especially in certain plant tissues and fresh animal organs, but absent from cereal grains; they thus placed the old empirical observations of sailors and explorers on a secure experimental basis. In 1914 Funk published a fundamental monograph on vitamins and on the related deficiencies or "avitaminoses." To complete these remarks on the development of vitaminology, after recalling the work of Mc Collum and Davis, who distinguished vitamins into water-soluble and fat-soluble (1915), it must be added that Lorenzini and co-workers, as early as 1917, had begun the study of methods for the extraction and stabilization of vitamins and had thought of various therapeutic applications, recognizing the importance of those dysvitaminoses linked to the organism's inability to make proper use of vitamins, even when these are normally supplied by the diet. The study of such dysvitaminoses in particular, together with avitaminoses from imbalance, is approached with particular interest by the most modern lines of research. By now chemistry has taken over vitaminological studies and, thanks above all to eminent chemists such as Windhaus, Karrer, Szent-György, Williams, R. Kuhn, etc., the synthesis of almost all the vitamins known today has been achieved.
There exist at least 14 vitamins, but in the syllabus presented by the "Council on foods and nutrition" of the "American Medical Association," published in June 1946, only the manifestations of seven vitamin-deficiency states are noted, that is, only those avitaminoses that have been reliably observed in man and that are of dietary origin. Some of the omitted vitamins have been shown to be essential only under special experimental conditions. Many vitamins, since they are not synthesized by the tissues of the human body, are ordinarily formed in sufficient quantity by the bacteria of the intestine. Sulfonamides, for example, which inhibit the growth of intestinal bacteria, can give rise to various forms of avitaminosis.
Today, in assessing the vitamin quantities needed in the diet, the tendency is to ensure a wide margin of safety, in order to avoid the possible onset of deficiency symptoms under unfavorable conditions. Diseases from vitamin deficiency are often the result of multiple factors, and the lack of vitamins in the diet is only one of their causes, as happens, for example, in rickets; until a better understanding of the accessory causal factors has been reached, a generous administration of vitamins is to be recommended as the only safe route of prophylaxis, especially in periods of increased requirement and of illness.
Alongside pictures of hypovitaminosis from deficient vitamin intake, deficiency pictures from complete utilization may also be observed; thus, for example, a disorder of intestinal fat absorption may entail poor absorption of fat-soluble vitamins, even though the dietary supply of these is entirely normal. The metabolic imbalance induced by vitamin deficiency can cause irreparable and non-reversible lesions: Mouriquand has called "paravitaminosis" the morbid picture represented by lesions resistant to the administration of the specific vitamin. It is now established that one deficiency aggravates another and that a good supply of vitamins allows the organism to maintain its balance with a lower protein quota.
II. DISTINCTIONS
Vitamins are commonly distinguished into fat-soluble, that is, bound to fats, soluble in them and extractable with fat solvents, and water-soluble, that is, soluble in aqueous media and extractable by aqueous or hydroalcoholic extraction.
The most important fat-soluble vitamins are: 1) vitamin A or antixerophthalmic vitamin, contained above all in cod-liver oil, raw spinach, butter, egg yolk, fresh tomato; it is very important for visual function, anti-infective, epithelium-protective; 2) the group of D vitamins or calcium-fixing or antirachitic vitamins, little widespread in nature; cod-liver oil is especially rich in them; for the onset of rickets, besides the deficiency of the vitamin in the diet and of solar irradiation on the organism, the calcium-phosphorus balance in the diet also counts; 3) vitamin E or antisterility vitamin, which promotes the fertility of animals, is present in the embryos of cereals, in green leaves, in nuts and in peas; it also seems to be a neurotropic factor of great importance; 4) the K vitamins or antihemorrhagic vitamins, whose deficit produces hemorrhages and a lowering of the coagulability of the blood: K_{1}, of vegetable origin, is contained above all in green leaves, in oats, in wheat; K_{2}, of bacterial origin, is found in the liver, coming from intestinal saprogenic germs; 5) the so-called vitamin F, an unsaturated fatty acid, whose deficiency produces above all disturbances of cutaneous trophism.
The most important water-soluble vitamins are: numerous factors extractable from yeasts, which constitute the B_{1} complex; the many factors that compose it, distinguishable according to resistance to heating, are not yet all sufficiently known; the most studied are: 1) vitamin B_{1} or aneurin or thiamine, antipolyneuritic, a vitamin of fundamental importance, since around it is centered a complex of indispensable enzymatic functions (decarboxylation, oxidation, phosphorylation), it promotes alcoholic fermentation and the multiplication of bacteria; as Williams says, it is one of the earliest inventions for evolving life; 2) vitamin B_{2} or riboflavin or lactoflavin, contained in lactic and butyric bacteria, in egg white, in milk serum; it is antistomatic, antieczematous and perhaps also antipellagrous; 3) vitamin B_{6} or adermin or pyridoxine, contained in rice bran, in egg yolk, in beef liver, is antidermatitic; 4) vitamin PP, antipellagrous, nicotinamide, of fundamental importance in the protection of the skin, mucous membranes and nervous system, is a constituent of the active group of enzymes engaged in oxide-reductive processes (dehydrogenases); 5) vitamin B_{12}, active at the dose of a few gamma (γ=thousandth of a milligram) against pernicious anemia; and, finally, 6) folic acid, against alimentary and gravidic perniciosiform anemias; 7) pantothenic acid; 8) vitamin H or antiseborrheic or biotin. Also belonging to the water-soluble ones are: 9) vitamin C or ascorbic acid, antiscorbutic; it is very widespread in the vegetable kingdom (especially peppers, lemons, asparagus); it cannot be synthesized by either man or the monkey, which therefore must take in ascorbic acid preformed from outside; the fundamental function of vitamin C is that of transporting hydrogen in the oxide-reductive processes of the cells; its requirement is increased in fever and in all infectious diseases that require an increase in the adrenal protective function; 10) vitamin P or citrin or of capillary permeability is a complex of flavonoid bodies, with a generically antihemorrhagic action; its action overlaps only in part with that of ascorbic acid, in that it regulates above all vascular permeability.
All these exogenous regulators, introduced as such or as precursors (provitamins) that the organism knows how to elaborate into vitamins, are as a rule engaged in vital processes fundamental for all cells. Some vitamins certainly influence the central and peripheral nervous system and perhaps in a special way that of vegetative life; also very extensive are the relations of some chapters of dermatology and ophthalmology with vitaminology. Much studied is the relation of vitamins with natural immunity.
In the case of dietary restrictions imposed for any reason on a community, vitamin deficiency can cause more serious damage than the caloric reduction of the ration. Vitamins, which are true plant hormones, having often the significance of growth factors for the plants themselves, therefore perform a function of fundamental importance in the rational nutrition of modern man (Virtanen), and constitute a vast and important chapter, still developing, of the science of nutrition.