This second paper (1) that E. V. McCollum and his colleagues published in the American Journal of Hygiene (the first was in Volume I of the Journal the year before (2)) is typical of McCollum's meticulous use of carefully controlled diets in small animal models to identify essential nutrients-the approach he pioneered and with which he revolutionized nutritional science. Having discovered "fat-soluble A" (vitamin A) in 1913, he began searching for the cause of rickets soon after joining the faculty of the Johns Hopkins School of Hygiene and Public Health in 1917 as its first professor of "Chemical Hygiene." The paper makes clear that by 1921 he was aware that cod liver oil may, in fact, contain "a second dietary essential associated with certain fats" (1, p. 98). In this paper, he reproduces rickets in animals deprived of cod liver oil and calcium. Within 2 years, he had completed the study of over 300 different experimental diets and was convinced of the existence of the second and distinct "dietary essential" and of its primary role in preventing rickets. This second of the fat-soluble vitamins, the "antirachitic vitamin," was to be named "vitamin D."
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