Metabolic substrate utilization during exercise

Substrate utilization during prolonged exercise in normal subjects. Muscle glycogen breakdown is important initially but stores are rapidly depleted. Glucose (derived from circulating glucose and hepatic glycogenolysis and gluconeogenesis) and FFA then become increasingly important. However, the fall in plasma glucose concentration reduces the secretion of insulin and raises that of counterregulatory hormones (such as epinephrine and norepinephrine). As a result, prolonged exercise is associated with a gradual fall in utilization of glucose and increase in that of FFA.