Exercise not likely to rev up your metabolism

Jacqueline Stenson MSNBC contributor

Start exercising and you’ll become a round-the-clock, fat-burning machine, right? That’s long been a commonly held belief among exercisers and fitness experts alike. But a new report finds that, sadly, it’s not very likely.

The notion that exercise somehow boosts the body’s ability to burn fat for as long as 24 hours after a workout has led to a misperception among the general public that diet doesn’t matter so much as long as one exercises, says Edward Melanson, an exercise physiologist and associate professor of medicine at the University of Colorado in Denver.

“People think they have a license to eat whatever they want, and our research shows that is definitely not the case,” he says. “You can easily undo what you set out to do.”

In the new report, published in the journal Exercise and Sport Sciences Reviews, Melanson and colleagues discuss research to date on the issue of burning fat during and after exercise. The authors conclude that while people do burn more fat when they are exercising than when they are not, they have no greater ability to burn fat over the next 24 hours than on days when they are couch potatoes.

“If you exercise and replace the calories you burn, you’re no better — with regard to how much fat you burn off — than if you didn’t exercise,” says Melanson.

Experts ‘flabbergasted’
In their own research, Melanson and his team studied moderately active people who, on separate days, performed low-intensity or high-intensity cycling, or no structured exercise at all. They repeated their experiment with endurance athletes (competitive runners and triathletes), while comparing sedentary obese people with sedentary lean people, and then again while comparing older men with younger men.

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None of these studies, which involved a total of 65 exercisers whose dietary intake was closely controlled, showed that people burned substantially more fat in the 24 hours after they exercised than on days when they didn’t exercise. Melanson says other experts in his field have been “flabbergasted” by the results.

“Bottom line is that we once thought that exercise would burn calories, especially fat calories, for a long period after a bout of exercise,” says exercise physiologist Gerald Endress, fitness director for the Duke University Diet and Fitness Center who was not involved in the research. “This does not seem to be the case.”

But both Melanson and Endress say it can’t be ruled out that longer, harder and possibly different types of exercise performed regularly on consecutive days could lead to a more lasting post-workout fat burn. In Melanson’s research, for instance, participants all cycled for under an hour, burning up to 400 calories.

The new paper offers additional evidence that exercise does not boost metabolism as much as widely believed, Endress says. In addition to the misperception that exercise greatly hikes fat burning after exercise, there is also the false belief that weight training dramatically increases metabolism by adding muscle, he notes.

While it’s true that a pound of muscle burns more calories than a pound of fat — about seven to 10 calories a day versus two calories — most people don’t put on enough muscle to make much of a difference, Endress says.

“Building muscle is very difficult for most individuals because it requires heavy weight workouts and a higher intake of calories,” he says. “Average fitness enthusiasts [who are working out to gain muscle] will only add four to five pounds of lean mass,” he says, and burn an additional 28 to 50 calories a day. (Men tend to gain more muscle, on average, than women.)

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