Vitamins. Recognized Vitamins and Growth-factors

Vitamins are organic compounds essential in minute quantities for the maintenance of health and normal development in most animals and some plants. Many vitamins act as coenzymes or precursors of coenzymes to aid in regulating metabolic processes to produce body tissues or to store or release energy. others are related to the sterols and hormones.

Many vitamins are synthesized by plants and are present in the food intake of animals, though a few are synthesized in the animal body. The existence of such ‘‘special factors’’ in food has long been suspected; for example it has been known since the seventeenth century that fresh fruits and vegetables in the diet prevent scurvy.

In 1890 the Dutch physician Christiaan Eijkman showed that beriberi resulted from eating ‘‘polished’’ rice, or rice from which the husks had been removed. When the husks were added to the diet, the disease disappeared, and it was recognized in 1901 that beriberi was due to a dietary deficiency.

Modern vitamin theory dates from 1912 when Sir Frederick Gowland Hopkins showed that animals did not thrive when fed on carefully purified fats, proteins, carbohydrates, mineral salts, and water; but that a very small addition of milk was enough to render the purified diet adequate. In the same year, Casimir Funk at the Lister Institute in London found that the antiberiberi factor in rice-husks was an amine and propounded his ‘‘vitamine’’ (vital amine) theory, linking vitamin deficiency to other diseases including pellagra and rickets (rachitis).

The term was applied to ’’accessory factors’’ generally, but as various chemical structures and functions were identified among these compounds and many were found not to be amines at all, they were called vitamins.

In 1915, Americans E. V. McCollum and M. Davis showed that there were at least two kinds of vitamins, one soluble in fatty and the other in nonfatty foods. They were later called vitamin A and vitamin B, respectively, and each has since been found to be a group of compounds with related functions.

Table 1. Recognized vitamins and growth-factors

The water-soluble group includes vitamins B and C. The vitamin B complex contains a large collection of compounds, all of which are essential constituents in various enzyme systems. Similarly the fat-soluble vitamin group is now known to include vitamins A, D, E, and K. The physiological functions of most of the vitamins have been examined in detail and their molecular structures have been determined, but their dietary importance has ensured the everyday use of the original alphabetical classification.

Water-soluble vitamins are absorbed in the intestine and carried in the blood to the tissues. They are distinguished from each other by the degree of solubility, a factor that influences their route in the body. In their free state, the B vitamins are inactive, and they must go through several chemical processes before they can perform their functions in the body. When combined with other substances, they are changed into their functional, or coenzyme, form and can then combine with proteins to form active enzymes that catalyze various metabolic and regulatory processes.

The fact that vitamin C prevents and cures scurvy is well known, but the vitamin is also essential for the growth of bones and teeth, for the maintenance of subcutaneous tissues and the walls of blood vessels, and for the healing of wounds. A controversial theory suggests that the intake of large quantities of vitamin C can prevent or cure the common cold, but there is no clear evidence to support this claim. When the intake of water-soluble vitamins exceeds the bodily requirements, they are stored to a limited extent in body tissues, but most of the excess is excreted in the urine.

The fat-soluble vitamins are absorbed with the help of bile salts and are carried through the body in the lymphatic system. The body stores larger quantities of fat-soluble vitamins than of water-soluble ones. The liver provides the chief storage tissue for vitamins A and D; vitamin E is stored in body fat and in the reproductive organs. Vitamins of this group perform various functions. For example, vitamin A combines with proteins in the retina of the eye to aid night vision, though it may have other functions as yet uncertain.

The antirachitic factor was labeled vitamin D; it is essential to growth, especially in calcium metabolism for bone formation and the avoidance of ricketts. Produced in the skin on exposure to sunlight, vitamin D is one of the few vitamins synthesized by animals. In 1922 a new factor called vitamin E, which facilitates animal growth and maintains fertility in rats and some other species, was identified. Vitamin K is essential for the enzymatic processes of blood clotting.

The number and variety of recognized nutritional factors is continually growing, though not all are essential to animal or human nutrition. Moreover, the definition of a vitamin is imprecise because the metabolic functions of enzymes, coenzymes, sterols, hormones and vitamins are closely related, and much detailed research is required to differentiate between these and other groups of biochemical molecules.

Vitamins are not distributed equally throughout nature, nor do they perform the same functions in all species. Sometimes a compound that can act as a vitamin is present in combination with another compound that prevents its absorption and so destroys its activity. Both plants and animals are important natural sources of vitamins for human health, and the more restricted the diet the more likely it is that one or more vitamins will be lacking.

The vitamin requirements of most organisms are fairly well known, but there is not uniform agreement about the requirements for a healthy human diet. Differences arise due to the various ways requirements are determined and to the scanty data available for some of the vitamins. Studies in this field give rise to the subject of human nutrition in which the quantities of the various vitamins figure importantly, along with minerals and other essential trace elements in the diet. It is generally thought that a balanced diet supplies all the vitamins needed for a healthy lifestyle, but this is not necessarily true.

Long storage of fruits and vegetables after harvesting may result in the loss of vitamin C due to oxidation. Washed vegetables lose water-soluble vitamins; heating and overcooking destroys them. Manufactured foods therefore often require added vitamins to replace such losses. Sometimes extra vitamins are added to raise the proportion above its natural level, and many people also take vitamin supplements.

Milk fortified with vitamin D was first introduced by the Borden Company in the U.S. in 1933. As vitamins were isolated and then synthesized, they could be manufactured by pharmaceutical companies. Vitamin C was first to be synthesized in a laboratory in 1933, followed by vitamin B2 in 1935. By 1936, vitamin and iron supplement sales were widely available in the U.S. In Britain, though not in the U.S., vitamin supplements are classified as drugs.

The chemical structures of all the known vitamins have been determined and most can now be manufactured synthetically by chemical or biochemical processes. The quantity of each product is governed by economic considerations and wide variations in annual production occur. Thus, only about 10 tons of vitamin B₁₂ are manufactured each year, but 50,000 tons of vitamin C (ascorbic acid) are made. Manufacturers have developed variations in the synthetic processes, most of which are the subject of trade secrets.

Synthetic vitamins produced in bulk are used in the food industry, and large quantities are added to animal feed, but there are also other uses. For example, some vitamins act as antioxidants, and this property has been used in the manufacture of plastics. A small but growing market is also developing for vitamins that benefit the skin, and vitamins are beginning to appear in cosmetics, skin creams, lotions, and shampoos. Research to identify new vitamins and growth factors continues along with efforts to discover more economical methods of manufacture and new uses for the known vitamins.