Minimum Vitamin A Requirements with Particular Reference to Cattle

Abstract

The total storage of vitamin A and carotene in the liver and body fat of cows 2 to 18 years old, which had access to green feed in abundance throughout life, was estimated to be about 0.6 to 0.7 gm. in the younger animals and up to 3.6 gm. in aged cows. From 67 to 93 per cent of the storage was in the liver. The amount in other organs was negligible. In the liver most of the storage was in the form of vitamin A, whereas in the fat, carotene predominated. A comparison between the estimated total storage, including that in the blood stream, and the time required to deplete similar animals on a carotenoid-deficient ration, indicated a daily withdrawal of 9 to 11 micrograms per kilogram live weight. Cattle may ingest more carotene in a few days' grazing on green pasture than is stored in the body of a very fat old cow that has had continual access to green feed or alfalfa hay. A cow whose reserves were depleted stored about 400 mg. of carotene in a 13-day period, during which she ingested 240 kg. of freshly cut alfalfa containing about 15 gm. of carotene. Although the percentage recovery was low (2.7 per cent), a relatively rapid storage was shown. The concentration of the reserves of young animals as compared with that of aged adults, however, suggests that reserves above a certain level are accumulated slowly. Subcutaneous injections of carotene, dissolved in olive oil, into a vitamin A deficient calf were followed by disappearance of a corneal lesion and by slight improvement in physical condition; but gain in weight did not occur. Apparently the injected carotene was slowly absorbed or poorly utilized. Night blindness was found to be the first detectable clinical symptom of vitamin A deficiency and to constitute a delicate index upon which minimum requirements could be based. The progressive development of other symptoms of deficiency is presented in the discussion. The daily minimum carotene requirement of the bovine is 26 to 33 micrograms per kilogram live weight. Carotene intake at the level of 29 micrograms per kilogram daily prevented or cured clinical symptoms, including night blindness, and promoted normal weight increases; yet it resulted in no storage. When the intake fell below this level, night blindness reappeared, and gains decreased. Normal reproduction occurred in one instance in a cow that began gestation with no reserve and whose carotene intake was maintained at about the minimum level until the last month of pregnancy, when it was increased about threefold. Vitamin A deficiency that had progressed to the point of night blindness and convulsions did not inhibit the occurrence of oestrus. From a study of the minimum requirement to prevent or cure symptoms of deficiency and to permit growth at approximately the normal rate in rats, swine, sheep, and cattle, the hypothesis is advanced that vitamin A requirement is related to body weight rather than to energy requirement and that the minimum requirement of mammals is in the order of 20 to 30 micrograms of vitamin A or carotene daily per kilogram of body weight. This implies that, to compensate for lower food consumption per unit of weight, large animals require a higher percentage of vitamin A in the diet than do small animals. The carotene requirement per unit of body weight of chickens and turkeys appears to be equal, but the requirement of both species is considerably higher than that of the mammals studied, thus indicating a difference between birds and mammals in this regard.