On the first day of a trial, participants will arrive at the lab in the morning after a day of dietary control and a standardized breakfast including the treatment beverage. Prior to beginning exercise, participants will complete the physical and mental performance tests. A small catheter (flexible plastic tube) will be inserted into a forearm vein by Dr. Phillips or a medically trained and qualified member of the laboratory group; Dr. Tarnopolsky is responsible for the individuals inserting the venous catheters. The needle will be removed promptly after the insertion of each catheter; only the catheters will remain in their arms until the end of the trial. The catheter will be used for repeated blood sampling over the next six hours (see Figure 1). Additionally, breath samples will be taken at the same time as the blood samples. Following the performance tests, and prior to the start of exercise, a baseline blood sample will be drawn. Participants will then perform a bout of endurance exercise that consists of treadmill walking for 180 minutes at 40% of their peak VO2. Following the start of exercise, participants will consume a treatment beverage (or placebo) every 20 minutes after the start of exercise and continuing until 20 minutes after exercise (see Figure 1). Following the exercise bout, participants will repeat the physical and mental performance tests.