Marathon fatigue: the role of plasma fatty acids, muscle glycogen and blood glucose.

The role of carbohydrate depletion in marathon fatigue was examined in 6 marathon runs. Four of the runs were potentially 'fast-time' marathons and culminated in fatigue. The utilization of carbohydrate, lipid and protein, and plasma concentrations of free fatty acids (FFA), glucose and lactate were measured at intervals throughout the runs. The contribution from protein to energy output was low (1-2%). The utilization of lipid was dependent upon plasma concentrations of FFA, which rose throughout the run. The utilization of carbohydrate mirrored that of FFA and thus fell throughout the run. Fatigue was characterized by a drop in running speed, a drop in carbohydrate utilization, an unchanging FFA utilization and a fall in blood glucose. The fall in blood glucose was not seen in the non-fatigued runners. These results are consistent with carbohydrate depletion being the cause of fatigue. The implications of these data are that lipid is the preferred fuel, but is rate-limiting, and that carbohydrate depletion, even though it causes fatigue, ensures an optimal-time marathon.