Obesity Reduces Ability to Oxidize Fat During Exercise

April 4, 2008

Among pubertal boys, those who are obese have a reduced ability to oxidize fat during moderate exercise compared to those who are lean, possibly because of differences in muscle fiber distribution, according to a report published online April 2 in the British Journal of Sports Medicine.

FRIDAY, April 4 (HealthDay News) -- Among pubertal boys, those who are obese have a reduced ability to oxidize fat during moderate exercise compared to those who are lean, possibly because of differences in muscle fiber distribution, according to a report published online April 2 in the British Journal of Sports Medicine.

Gautier Zunquin, Ph.D., of the Laboratoire REcherche Littorale en Activites Corporelles et Sportives in Dunkerque, France, and colleagues assessed fat oxidation in 17 obese boys (mean age 12.1, mean body mass index 26.68) and 13 lean boys (mean age 12, mean body mass index 18.56) during a graded leg cycle ergometry test.

The researchers found the fat oxidation rate was identical in both groups during low-intensity exercise (0-30 percent of peak oxygen consumption). But they found that the rate was significantly higher among lean boys during moderate exercise (40-60 percent of peak oxygen consumption), suggesting that obese boys should exercise at a lower intensity to enhance lipolysis and reduce fat mass.

"Muscular modifications induced by obesity and being sedentary may partially explain the differences in fuel oxidation mechanisms," the authors write. "Muscle fiber type distribution is modified with obesity. Compared with lean subjects, obese subjects have a higher percentage of type II fibers (fast twitch fibers where carbohydrate oxidation predominates) and a lower percentage of type I fibers (oxidative slow twitch fibers where fat oxidation predominates). Changes observed in muscle fiber type distribution in relation to obesity could explain the decrease in fat oxidation rates in obese children."

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