Metabolic Aspects of Low Carbohydrate Diets and Exercise Sandra J. Peters, Ph. D. (Faculty of Applied Health Sciences, Brock University, St Catharines, ON) Following a low carbohydrate diet, there is a shift towards fat oxidation to provide energy, both at rest and during exercise. However, the underlying mechanisms underlying this metabolic shift in skeletal muscle remain obscure. Such a change in fuel selection could either be due to an up-regulation of enzymes and transporters which regulate triglyceride breakdown, free fatty acid uptake and fat oxidation, or due to a down-regulation of key enzymes which regulate carbohydrate oxidation. To date, there is a paucity of information regarding regulation of fat oxidation in human skeletal muscle following a low carbohydrate diet, but such changes appear to be slower, requiring many weeks for documented increases in maximal activities of key fat burning enzymes. However, adaptive regulation of carbohydrate oxidation, which will indirectly increase the utilization of fat fuel, occurs more rapidly following a low carbohydrate diet (in the order of hours and days). Pyruvate dehydrogenase (PDH) catalyzes the oxidative decarboxylation of pyruvate, thereby producing acetyl-CoA to feed the tricarboxylic acid cycle. It acts as a regulatory gateway, indirectly determining the proportion of acetyl-CoA derived from fat and carbohydrate. It is inactivated by reversible phosphorylation through the activity of an intrinsic PDH kinase. PDH kinase undergoes an adaptive up-regulation of maximal activity and gene expression following a low carbohydrate diet, rendering the PDH complex less sensitive to activation during exercise, and decreasing the capacity for skeletal muscle carbohydrate oxidation. There is still much to learn about the mechanisms, time course, and consequences of this diet-induced skeletal muscle adaptation.