Protein Metabolism in Diabetes Mellitus: Implications for Clinical Management

Diabetes mellitus is characterized by derangements in the metabolism not only of glucose and fat but also of protein (1). However, protein has always received less attention than fat and glucose, both in terms of alterations in its metabolism and in its nutritional implications. Although hyperglycemia and its consequences have always been the hallmark of the disease and plasma glucose the main index of diabetic control, altered protein metabolism was recognized even in the preinsulin era because of the severe muscle and other protein depletion that occurred, even with an apparently adequate protein intake. In 1906, the German investigator Bernhard Naunyn observed an increase in glycosuria with increased dietary protein and recommended that protein intake as well as carbohydrate intake should be restricted (2). In the preinsulin era (1922), Marsh et al. addressed the issue as to whether "the laws that govern protein metabolism of non-diabetic subjects applied equally to those with diabetes" (3). These investigators concluded, in contrast to Benedict and Joslin [cited in (3)] who preceded them, that protein requirements for achieving nitrogen equilibrium were similar to those of nondiabetic persons. However, this was in a setting in which glycosuria had been eliminated by the high-fat, low-carbohydrate diet that was then the only way of achieving metabolic control. There intervened a long period in which a disproportionately small amount of attention was paid to the question of protein metabolism. It is informative to trace the trends in nutritional recommendations for persons with diabetes from the preinsulin era to the present. With the advent of insulin and oral agents and with data emerging from clinical and experimental research, nutrition recommendations have evolved from low-carbohydrate, high-fat to high-carbohydrate, high-fibre, low-fat, low-cholesterol diets (4,5). From 1935 until the current decade, guidelines for protein intake had remained at 85-90 g/day or approximately 18% of energy intake. One report of intakes during World War II gave values of 68 g protein and 2150 kcal/d, which were associated with general improvement of "adult diabetes" (diabete gras) (6). The benefit was most likely to have been related to the energy restriction. The