Failure of caffeine to affect substrate utilization during prolonged running.

Nine sub-3-h male marathoners performed three 45-min monitored treadmill runs at approximately 75% of VO2max during a 2-wk period. The men were assigned in a random, double-blind fashion following the control run to receive either 350 ml of decaffeinated coffee or 350 ml of decaffeinated coffee with 400 mg of caffeine added 1 h before the second run with crossover to the other beverage for the third run. Venous blood was analyzed for free fatty acids, triglycerides, glucose, and lactic acid before beverage consumption and before and after each run. Oxygen consumption (VO2), carbon dioxide production (VCO2), respiratory exchange ratio (R), ventilation (VE), and perceived exertion were measured at 15, 30, and 45 min of each run. Of the blood parameters, free fatty acid and lactic acid concentration increased following caffeine ingestion. There was no difference in VO2, VCO2, or R between the three runs. Perceived exertion showed a significant decrease (P less than 0.05) at each time point in caffeine added and decaffeinated compared to control. Triglycerides, glucose, and lactic acid increased similarly in all three runs. In these well-trained marathoners, although plasma free fatty acids were elevated significantly prior to exercise after caffeine ingestion, there was no indirect evidence of altered substrate utilization during subsequent treadmill running.