Two-a-Day Workouts Increase Fat Use

Exercising below 65 percent of maximum effort uses mainly fat as fuel. Above this intensity, carbohydrates become increasingly important, and they're used almost exclusively for fuel at extreme exercise intensities.

            Swiss researchers found that fat use during exercise is influenced by prior physical activity. Moderately-trained subjects took two maximal treadmill tests: one to determine maximal oxygen uptake and estimate fat use and a second to determine the effects of prior exercise on fat use. During the second test, subjects exercised on a treadmill for one hour at 57 percent of maximum effort. After a short break, they took the second treadmill test to exhaustion. Fat use was higher when the subjects took the test after the initial one-hour run. Two-a-day workouts will trigger more fat-burning during the second workout. (Metabolism Clinical and Experimental, 58: 1778-1786, 2009)