Vitamins

In absence of air, materials containing the vitamin may be heated to a comparatively high temperature without serious loss, espe cially in acid solution. On storage after this treatment, however, the potency disappears much more rapidly than from the untreated material.

Fat-soluble Vitamins, A, D, and E.—After the discrimina tion of the water-soluble B and fat-soluble A vitamins (1915, McCollum and Davis) much attention was paid to the study of the latter. A very rich source of it was found to be cod-liver oil (1921, Zilva and Miura), whilst it occurred in much smaller pro portion in the body fat of many mammals and fishes, in egg yolk, milk and butter and cereal seed embryos.

In the absence of such food materials from their diet, young experimental animals ceased to grow, lost weight and finally died.

Frequently a characteristic condition of the eye (xerophthalmia) developed before death. When this diet was supplemented by small quantities of material rich in the vitamin, growth was resumed, the eye symptoms cleared up and the animal became normal. It was also found (1915-1919 Mellanby) that some of the foodstuffs containing the fat-soluble vitamin A were effective in preventing the onset of rickets (q.v.) in puppies kept on certain special diets, and in curing the condition both in dogs and man. Nevertheless, certain discrepancies were soon observed, which led to the suggestion that two principles were concerned, and it was finally proved (1922 McCollum) that two fat-soluble vitamins exist, vitamin A which cures and prevents xerophthalmia, and vitamin D which cures and prevents rickets by promoting the proper calcification of bone. This conclusion has since received striking confirmation in many other ways. The simultaneous pres ence of both vitamins is essential for the normal growth and well being of the animal.

It has also been shown (1922, Evans and Scott) that a third fat soluble vitamin exists, the presence of which in the diet is essential for the fertility of rats.

Vitamin A (the Anti-xerophthalmic Vitamin).—The richest natural sources of this vitamin are the liver fats of many mammals and fishes. In some of these (liver oils of salmon and halibut, cod-liver oil) it is accompanied by vitamin D but in others not (liver fat of sheep, calf and ox). The liver oils of salmon and halibut contain zoo times as much vitamin A as cod liver oil and the liver fats of sheep, etc., ten times as much, whereas butter only contains about of the amount present in cod-liver oil. Almost free from vitamin D it is found in green

plants, its formation being greatly accelerated by the influence of light. This is the ultimate source from which mammals'and fish alike derive their store of vitamin A. It is also present in smaller amount in the body fat of mammals and some fish (but is exceptionally plentiful in eel oil, where it is accompanied by vitamin D) and in butter in which its content depends on the diet of the animal ; it is absent from, or only present in low concentration in, vegetable fats and many fruits. The vitamin is slowly destroyed by oxidation even at air temperatures, more rapidly at high temperatures, although in the absence of air fats containing it can be heated for a considerable time to i2o° C without serious loss. It is not itself a fat, and when the oils in which it occurs are saponified it is found in the unsaponifiable matter, which usually amounts to 3% of the oil. Green and Mellanby (Brit. Med. bourn. 1928, i. 691) brought forward im portant evidence that vitamin A acts as an anti-infective agent.

Vitamin D (the Antirachitic Vitamin).

Whilst investi gators were gradually approaching the conclusion that a special vitamin was concerned with the prevention and cure of rickets, knowledge concerning this disease had progressed in a different direction. It had been found (1919, Huldschinsky), that rickets could be cured by exposure of the patient to the ultra-violet rays of the mercury vapour lamp and the use of sunlight as the source of radiation was afterwards (1921, Hess and Unger) found to be equally successful, not only with human subjects but also with rats. The connection between radiation and vitamin D has now been made clear. The remarkable observation was made that irradiation (with ultra-violet light) of the food of rats conferred upon it the power of preventing the occurrence of rickets, or, in other words, produced in it vitamin D (1924, Steenbock and Black). Inactive vegetable oils also became activated when treated in a similar manner (1924, Hess and Weinstock, Steen bock and Black). Once this was known, it was soon discovered, independently by a number- of observers, that cholesterol, which accompanies most animal fats, and the analogous constituents of vegetable oils, became active antirachitically when they were exposed to ultra-violet radiation (1924).