CLASSIFICATION AND STORAGE OF VITAMINS IN THE BODY

CLASSIFICATION

In 1913, McCollum and Davis described a lipid-soluble essential food factor in butter fat and egg yolk. In 1915, a water-soluble factor in wheat germ necessary for the growth of young rats was also described. Since then, two categories of vitamins are usually recognized : fat-soluble and water-soluble. These two groups discharge rather different functions.

A. Fat-soluble vitamins: These are oily substances, not readily soluble in water and their biochemical functions are not well understood. They contain only carbon, hydrogen and oxygen. Their examples are vitamins A, D, E and K. They, however, play more specialized roles in certain group of animals and in particular type of activities. For instance, they function in the formation of a visual pigment (vitamin A), in the absorption of calcium and phosphorus from the vertebrate intestine (vitamin D), in protecting mitochondrial system from inactivation (vitamin E) or in the formation of a blood clotting factor in vertebrates (vitamin K). The individual fat-soluble vitamins bear a closer resemblance to each other chemically. In fact, the 4 fat-soluble vitamins can be regarded as lipids. Vitamins A, E and K are terpenoids, and vitamin D is a steroid. All four areisoprenoid compounds, since they are synthesized biologically from units of isoprene, a building block of many naturally-occurring oily, greasy or rubbery substances of plant origin. Unlike the water- soluble vitamins (B and C), fat-soluble vitamins can be strored in the body, e.g., an adult's liver can store enough vitamin A to last several months or longer. However, because fat-soluble vitamins are storable, their excessive intakes can result in toxic conditions (hypervitaminoses).

B. Water-soluble vitamins. Most of these are universally vitamins since they perform the
same general functions wherever they occur. Besides C, H and O, they also contain nitrogen. They are catalytic factors and as such form vital links in the chains of biochemical reactions characteristic of all living objects. For instance, thiamine is required whenever sugars are oxidized aerobically to release energy. The individual water-soluble vitamins bear no closer resemblance to each other chemically. The biochemical or coenzyme function of nearly all of these is known. The common water-soluble vitamins are vitamins of B complex such as B1 through B12 (vitamins B4, B8, B10and B11, however, do not exist) and the vitamin C. Choline, inositol, p-aminobenzoic acid, bioflavonoids and α-lipoic acid are frequently included in this category. Many nutritionists, however, do not consider them as true vitamins, although their dietary deficiencies in animals lead to the development of characteristic symptoms. Moreover, none of them except α-lipoic acid is a part of the coenzyme system. The B-series of vitamins, being water-soluble and excretable, are required daily in meagre amounts (in milligrams or even less) for the normal growth and good health of humans and many other organisms. It is virtually impossible to ‘overdose’ on them.

STORAGE OF VITAMINS IN THE BODY

The vitamins can be stored in the body to a slight extent. The liver cells are, however, rich in certain fat-soluble vitamins. For instance, the amount of vitamin A contained in the liver is sufficient enough to meet its requirement without any additional intake for about 6 months. Similarly, the quantity of vitamin D stored ordinarily in the liver is sufficient to maintain a person without any additional intake of vitamin D for about 2 months. The storage of vitamin K is, however, relatively slight.

The water-soluble vitamins are stored even in lesser amounts in the cells. Evidently, in cases of deficiency of vitamin B compounds, clinical symptoms appear rather early, that is within a few days. Similarly, absence of vitamin C can induce deficiency symptoms within a few weeks. Vitamin C is stored in the adrenal cortex.