For years exercise physiologists advised that the best way to burn fat during exercise was to use lower-intensity work. That advice mainly applied to aerobic exercise, since fat can only be burned in the presence of the increased oxygen intake typical of aerobics. Early studies showed that as exercise intensity increased, the body relied less on fat stores and more on carbohydrate sources of energy, such as stored glycogen in liver and circulating blood glucose.
More recent studies show that, contrary to popular belief, higher-intensity exercise may still be superior for purposes of fat loss. Though you burn less fat during higher-intensity exercise, you also burn more calories. That makes considerable inroads into existing energy stores, which must be replaced following exercise. Such processes include glycogen replenishment and other recovery functions that are energy intensive. The increased energy requirement forces the body to continue to use stored fat at a higher rate. Add it all up, and you wind up losing more fat with higher-intensity than with lower-intensity exercise.
A recent study compared two levels of exercise intensity during varying amounts of time.1 The time factor is important because you tap into more fat during longer exercise sessions. The six subjects in the study engaged in two workouts:
1) exercising for 90 minutes at 40 percent of maximum oxygen uptake.
2) exercising at 60 percent of maximum oxygen uptake for 60 minutes. (The exercise intensity was limited to 60 percent of maximum oxygen uptake because the body begins to use greater carbohydrate stores at 65 percent.)
The results showed that both levels of exercise intensity and time led to similar fat use during exercise. Immediately following exercise, fat breakdown ceased but rose again an hour after exercise and remained elevated for another three hours, at which time the experiment ended. The study confirmed that fat use following exercise was greater after the higher intensity’60 percent’bout.
The study confirms that higher-intensity exercise is superior for maximum fat burning. The most effective way to accomplish that is with interval training, in which you do high-intensity exercise alternated with lower-intensity exercise for a set time period. Thus, if you’re using a treadmill, you may run or walk fast for three minutes, then walk slower for another three minutes, then run again, and so on for 30 minutes or more. That type of aerobics is most effective for fat-burning purposes. IM
1 AlMulla, N., et al. (2000). Postexercise adipose tissue skeletal muscle lipid metabolism in humans: the effects of exercise intensity. J Physiol. 524:919-928.