Originally we were accustomed to think of deficiency diseases as caused by the lack of specific vitamins. Thus, pellagra was caused by the deficiency of nicotinic acid; scurvy by the deficiency of ascorbic acid; and beriberi by the deficiency of thiamine. This conception was at least partly owing to experimental work on animals fed synthetic diets supposed to be deficient in single vitamins. But so far as humans are concerned, deficiency diseases arise from the use of more or less definite diets which are monotonous and usually can be easily shown to be lacking in several vitamins.
The complete clinical picture of pellagra, as we have learned largely from the work of Spies and his collaborators, although chiefly due to the deficiency of nicotinic acid, is modified by the additional deficiencies of thiamin, riboflavin, and almost certainly other vitamins of the B2 complex.