The body uses mainly fat as fuel at exercise intensities below 50 percent of maximum effort, and uses mainly carbohydrates at higher intensities. Conventional wisdom is that prolonged, low-intensity exercise promotes fat loss best. However, well-controlled studies from Canada in middle-aged women showed that weight loss was greatest at higher exercise intensities.
An Australian study might help explain the confusion. Researchers found that metabolism was elevated in overweight adults for three days following a high-intensity walk lasting 60 minutes, but was not elevated at lower intensities. In other studies, similar results were found in people performing high-intensity weight training. Intense exercise, whether aerobic or resistance, is more effective for weight loss than low-intensity exercise. (Medicine Science Sports Exercise, 43: 624-631)