Carbonhydrate is the principle fuel for exercise above 65% of maximum effort. However, metabolic rate increases 12 to 25 times above rest during exercise, so fat use increases tremendously. South African scientists showed that use was greater during running than cycling. They measured fat use indirectly during prolonged cycling and running at 60, 65, 70, 75, and 80% of maximum effort. Cycling places a more concentrated load on the leg and thigh muscles, which would increase the use of carbohydrates and decrease the use of fats. (International Journal of Sports Nutrition and Exercise Metabolism, 20: 44-55, 2010)
