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Prompted by the critical food needs and the accompanying problems of rationing during the war, the British Medical Research Council in 1942 undertook to determine the vitamin A and carotene requirements of human adults.
Twenty-three healthy young adults (20 men and 3 women) between the ages of 19 and 34 years received a diet deficient in vitamin A (containing less than 70 international units of beta carotene daily) for six and one-half to twenty-five months. Sixteen subjects received the diet unsupplemented until they showed unmistakable signs of deficiency or left the experiment for reasons unrelated to the study. They were then given graded doses of vitamin A or carotene. Seven subjects served as positive controls, 2 of them receiving 2,500 international units of vitamin A daily as a natural concentrate and 5 receiving about 5,000 international units daily of carotene as beta carotene in oil or margarine or as carrots,
Vitamin A Requirement of Human Adults: An Experimental Study of Vitamin A Deprivation in Man. A Report of the Vitamin A Sub-Committee of the Accessory Food Factors Committee. JAMA. 1949;140(16):1310. doi:10.1001/jama.1949.02900510060037