Effects of specific macronutrients

John Miles (Rochester, MN) discussed effects of meal ingestion on protein metabolism, acute effects of dietary protein and amino acids on glucose metabolism, and effects of protein in persons with diabetes, noting that insulin decreases endogenous proteolysis during a mixed meal. In a study of protein ingestion in persons with diabetes, plasma amino acids and, subsequently, urea levels increased. Usually, increasing protein ingestion is associated with protein oxidation, which is important as there is no physiologic mechanism for accumulation of protein stores, while conversely we adapt to decreasing dietary protein by decreasing amino acid oxidation. The effect of protein on glucose metabolism can be studied by measurement of arterial-venous balance, with evidence that amino acids displace glucose as metabolic fuels. Assessing independent effects of protein and of insulin on systemic and regional glucose metabolism, Miles showed evidence that insulin decreases circulating amino acid levels, and that with addition of amino acid infusion, there is decreased insulin-mediated glucose uptake and decreased insulin-mediated suppression of hepatic glucose output. Thus, amino acids cause a form of insulin resistance. Following acute protein ingestion in normal persons, however, there is little change in glucose homeostasis, presumably reflecting increases in insulin and in glucagon levels. As far as chronic effects of isocaloric high- or low-protein diets administered for 5 weeks in persons with diabetes, there is evidence of a decrease in glucose and triglyceride levels with a high-protein diet, as well as a trend to weight loss. The energy cost of fat ingestion and storage is low, and that for carbohydrate is modest, but there is a 25–30% thermic effect …