Dietary carbohydrate and its effects on metabolism and substrate stores in sedentary and active individuals

This review of carbohydrate (CHO) ingestion and exercise addresses three major issues: (a) how CHO ingestion influences CHO and fat stores, (b) how exercise, by changing CHO stores, alters the responses to CHO or fat ingestion, and (c) the roles of CHO in exercise performance and metabolism. Dietary manipulation is not a simple issue; increasing the dietary content of any specific nutrient alters the entire diet composition. High CHO diets are often low fat diets, hence changing the metabolism and storage of both fat and CHO. Acute CHO ingestion increases CHO oxidation and the "spared" fats are deposited as fat. Chronic high CHO ingestion (without an active lifestyle) leads to muscle becoming insulin-insensitive, adipose tissue processing CHO to fatty acids, and the liver increasing production of VLDL triglycerides. CHO ingestion prior to and during prolonged exercise can increase endurance. It has been suggested that moderate or low glycemic index forms be used prior to and during the exercise, but there is no consensus as to whether it should be a recommendation. The physiological nature of the regulation of CHO stores is poorly understood, but the recent identification of a key enzyme, glycogenin, and two forms of glycogen (pro- and macroglycogen) show promise of a deeper understanding.