The discovery of insulin brought about a revolution in the dietary management of diabetes mellitus. Whereas in the preinsulin era the low carbohydrate-high fat diet was universally employed, now the majority of authorities advocate the high carbohydrate-low fat diet. Indeed, the revolution has been even more marked. In the preinsulin era carbohydrate was considered the bete noire of diabetic patients; now this role has been assigned by some investigators to fat. The advent of insulin made possible the discovery of certain fundamental principles of the metabolic disturbance in diabetes mellitus. The subject, however, is still to a large degree in a bedeviled and controversial state. In this paper the literature of the subject will be reviewed and discussed, and some experimental work will be reported. In 1923 Allen 1 reported the results of an exhaustive study of this subject with a large number of patients and stated the following fundamental