Absence or insufficiency of minute traces of certain substances other than protein, carbohydrate, fat and mineral salts present in natural foodstuff causes certain well-defined symptoms of diseases and ultimately produces death. These substances were called vitamins. The name was first given to an impure chemical compound isolated from rice polishing by Funk. That compound contained nitrogen, and as it was essential for life it was termed vitamine-life-mine. Later investigations on these compounds have shown that most of them (so far isolated) do not contain any nitrogen. So the name vitamine is a misnomer. But as the term was a current coin for a long time, only the terminal 'e' was dropped at the suggestion of Drummond and these compounds are now designated by the general term 'vitamin'.Existence of vitamins was first recognized by the well-known principle of cause and effect. Even in 1928 McCarrison wrote:
"Though these substances are essential to life and are builders of our bodies, we cannot as yet make them nor see them nor weigh them, as we can make, see and weigh the other constituents of our food: the proteins, the fats, the starches, the sugars, the mineral salts and water."
But the painstaking series of researches of scientists during the past few years have altered the position considerably. Of the eight or nine vitamins known today four have been isolated in a pure state. These are vitamins A: the fat soluble factor of growth, B: the antineuritic vitamin, C: the antiscorbutic vitamin, and D: the anti-rachitic vitamin. Not merely these four have been isolated, the scientists have gone further. Through their efforts vitamin C can now be obtained in any desired amount in a pure crystalline condition. It has also been synthesized. Vitamin D has been obtained in a pure crystalline condition by irradiating ergosterol present in animal and vegetable fats.
"It is of interest to learn that vitamin D purchased as pure crystals now costs only one-eighth of its price in cod-liver oil and the daily- dose can be brought for less than one-tenth of a penny."(Carr)
Although vitamin A has not yet been synthesized, its reduction product Perhydro-vitamin A has been synthesized. Of the four vitamins structural formulas can only be advanced for vitamins A and C, the credit for which is mainly due to Karrer and his co-workers of the Chemical Institute of Zurich.With these preliminary remarks I shall proceed to discuss the four vitamins in detail.
It was recognized that night-blindness and Xerophthalmia (a disease of the eye) occur when fresh green vegetables, pure good milk and certain other things are eliminated from the diet, and they are cured when cod-liver oil is administered for some time. Previous administration of cod-liver oil also prevents the occurrence of these diseases. This indicates the presence of certain substance in cod-liver oil as well as those natural foods which is a preventive as well as curative for night-blindness and Xerophthalmia. It was also recognized that the absence of this factor inhibits the growth of the animal. It was designated as vitamin A. Although cod-liver oil is a very good source of vitamin A, it is present in such a minute quantity there that vitamin A could not be isolated from it. It may be mentioned here that cod-liver oil contains 99 per cent fat and of the 1 per cent non-fatty substances vitamin A is only a minute trace. This shows what minute quantities of this vitamin we require for a healthy life.It was only after the discovery by Von Euler and Karrer that certain fish-liver oils were found to contain 200-2000 times the amount of vitamin A present in common cod-liver oil that the isolation of vitamin A was possible. One such fish is Halibut. But fish-liver oils are much richer in vitamin A in summer than in winter. For instance, Halibut oil contains 20 times more in summer. This depends on the nature of food taken.Vitamin A is a viscous light oil, containing only carbon, hydrogen and oxygen. It is a complex alcohol soluble in fat and is very little soluble in water. As it is a compound containing four conjugated double bonds it is readily oxidised.Only the animal world contains vitamin A, but it is derived from the vegetable source which supplies carotene and similar other plant pigments. Therefore carotene is called Pro-Vitamin A. One molecule of carotene is converted into two molecules of vitamin A in the animal and human systems. Up-till now carotenes and kryptoxanthin (pigment of the yellow maize) have been recognized as Pro-vitamin A. Carotene is present in all green leafy vegetables, peas and beans, carrots, orange or yellow fruits and vegetables. Excellent sources of vitamin A are fish-liver oils, butter, cream, eggs and cow's milk. But the vitamin A content of milk depends on the nature of food taken by the cow. An abundant supply of green fodder is to be ensured in order to get milk rich in vitamin A. Of course it goes without saying that butter and cream contain more vitamin A than milk, but their vitamin content also depends on the nature of milk from which they are prepared. Generally town cow's milk contains less vitamin A as they get more of cereals than of green fodder.But no one need go to these animals and costly sources for vitamin A. It is immaterial for men in general whether the vegetable world contains carotene or vitamin A, so long as the ultimate effect is the same. Let me quote from Vitamins: A Survey of the Present Knowledge (1932 edition):
"Animal sources of vitamin A such as milk-fat, eggs, mammalian-liver and cod-liver oil are expensive, but when it is remembered that this vitamin is supplied by all green vegetables and others which contain the yellow colouring matter carotene, the problem can be cheaply solved wherever vegetables can be grown. For example, green spinach and green cabbage (but not white) are weight for weight as good as the best New Zealand butter. "
We have in the Annual Review of Biochemistry for 1935, p.396 also:
"Green leafy vegetables (spinach, kale, collards, turnip greens, beet tops, mustard greens), peas, asparagus and snap beans are economical sources of iron and vitamins A and G. Carrots, apricots and other coloured (orange or yellow) fruits or vegetable furnish vitamin A."
Moreover, long-continued feeding of foods rich in cholesterol, vitamin D and fats (eggs, cod-liver oil and other animal fats) may produce arteriosclerosis in man.Ripe mangoes and papayas contain a good deal of carotene. Sweet pumpkins which grow abundantly in many parts of India also contain carotene.It is not essential to take everyday the required amount of vitamin A as it is necessary to take carbohydrate for supplying required calories. When the intake of carotene is in excess of the requirement it is converted into the vitamin and stored up in the liver for future use, specially when there is deficient intake.