How exercise effects fat burn
Many are under the impression that their workout turns them into a fat burning machine 24 hours following their workout. It’s a widely held belief that regular workouts result in accelerated fat-burning around the clock.
Not so fast, at least not for moderate-intensity workouts. According to Edward Melanson, PHD, and associate professor of medicine at the University of Colorado
“ Moderate duration exercise of and hour or less has little impact on 24-hour fat oxidation.”
Most studies regarding fat burning—from exercise—have been short-term studies, which spanned only several hours and looked at people who were in an unfed state. Melanson’s team looked at a more true to life scenario where they followed the subjects over a 24-hour period; they exercised and ate normally or they did no exercise and ate.
It’s not that we don’t burn fat through exercise; it’s that we replace the calories with the food we eat. Exercise increases your body’s ability to burn fat, but if you replace the calories, you’re back to square one.
This information shouldn’t dissuade you from exercise; however, it should let you know that you need to be more realistic about calories and calories out.
Melanson's team evaluated fat burning in 10 lean, endurance-trained participants, 10 lean but untrained people, and eight untrained and obese people during exercise conditions and sedentary conditions.
Participants were fed a diet that was 20% fat, 65% carbs, and 15% protein for three days before each session and on the day they exercised or did not exercise. On the exercise day, participants rode a stationary bike at a moderate intensity for one hour, burning about 400 calories.
When Melanson's team measured calorie expenditures, they were higher in each group when they exercised compared to when they did not, not surprisingly, but they found that burning of carbohydrate, not fat, seemed to increase in the 24-hour period after exercising.In the journal report, Melanson reports additional fat-burning studies, including one that compared seven men ages 60-75 with seven other men ages 20-30, with no differences in fat burning between groups for the 24 hours after exercise or no exercise.
Why don't we become long-term fat burners after a good workout? The most likely reason is that we eat, and what we eat affects fat burning. For instance, eating as little as 240 calories of carbohydrate during the hour before exercise can reduce fat burning during exercise, and the boost in fat burning during exercise can be "blunted" for up to six hours after eating a meal, says Melanson, citing other research.
To maintain their low body fat, endurance-trained exercisers may simply eat less fat than they burn habitually, he says.The study findings are ''dispelling the myth that you can create a 24-hour fat-burning situation after exercise," says Pete McCall, an exercise physiologist and spokesman for the American Council on Exercise. But, he tells WebMD, the findings were limited to exercisers who did moderate-intensity exercise, and for an hour or less. "These results might not apply to different forms of exercise or higher-intensity exercise," McCall says.
Still, he says, the research results might be a crucial wake-up call. "The point of this study, I think, is [that] he is trying to get people out of that mind-set: 'I just worked out and I can eat whatever I want.'" At least for people trying to lose weight, McCall says, that's certainly not true.Melanson says that the take-home message from his research depends on whether you are trying to lose weight or just maintain. "If you are using exercise to lose body weight or body fat, you have to consider how many calories you are expending and how many you are taking in," he says. The goal is a negative fat balance.
"If your body mass index is below 25, you shouldn't be concerned about losing more body fat," he says.