Low-impact exercise helps obese boys burn more fat

NEW YORK (Reuters Health) - For obese boys, lower-intensity exercise like walking may be better at burning fat than more-vigorous workouts, a small study suggests.

In exercise tests of 30 thin or obese 12-year-old boys, French researchers found that obese boys burned the most fat when they worked out at a modest intensity -- akin to riding a bike on level ground.

Once the activity became more challenging, their bodies began to use substantially less fat compared with normal-weight boys.

The findings suggest that low-intensity exercise may be the best way for obese boys to shed body fat, the researchers report in the online edition of the British Journal of Sports Medicine.

For the study, researchers led by Dr. Gauthier Zunquin, of the University of the Opal Coast in Dunkerque, had 30 boys pedal on a stationary bike at a gradually building intensity.

During the tests, the researchers measured the amount of oxygen the boys inhaled and the amount of carbon dioxide they exhaled, which allowed them to estimate how much fat the boys were burning at a given exercise intensity.

The researchers found that at lower-intensity levels, thin and obese boys burned fat at a similar rate. But at higher intensity levels, obese boys' fat-burning rate lagged behind the thinner boys' rate. Moreover, obese boys hit their maximum fat-burning potential at a fairly low intensity of exercise.

The reason for the disparity may have to do with muscle, according to Zunquin's team.

Obese people's muscles tend to have a higher percentage of so-called "fast-twitch" fibers, which predominately use carbohydrates for fuel.

They also generally have a lower percentage of slow-twitch muscle fibers, which preferentially burn fat. This muscle-fiber balance may limit the amount of fat obese people can burn during higher-intensity workouts, Zunquin and his colleagues speculate.

Whatever the reason for the findings, the researchers suggest that obese children might be better served by regular, moderate exercise than by more grueling workouts.

SOURCE: British Journal of Sports Medicine, online April 1, 2008.