Getting more exercise may be key to staying healthy, but its effect on body fat may depend on whether the exerciser is male or female, according to a study of 17-year-old boys and girls.

“Taking up regular physical activity of 30 to 60 minutes of moderate intensity (equal to brisk walking) per day is, besides quitting smoking, the most important change an individual can make to improve his or her general health and wellbeing,” study author Dr. Ulf Ekelund, of the Medical Research Council, Epidemiology Unit, in the United Kingdom told AMN Health.

“In children, regular physical activity improves bone health and is associated with improvements in all ‘adult risk factors’ for coronary heart disease and type 2 diabetes,” he added.

Yet, “physical activity may not be as strongly related to obesity as we might think,” Ekelund said.

He and colleagues had a total of 445 adolescents and their mothers, complete questionnaires about the frequency, length, and intensity of physical activity they had engaged in during leisure time, at school or as a means of transportation during a 7-day period. All of the subjects are involved in the ongoing Stockholm Weight Development Study.

Overall, about 10 percent of the girls and 15 percent of the boys were overweight or obese as were roughly 34 percent of the mothers.

Teen boys were generally heavier, taller and more active than their female counterparts and their fat mass decreased as their physical activity level increased, Ekelund and colleagues report in the American Journal of Clinical Nutrition.

The same was not true for adolescent girls, who had a much higher percentage of body fat than did the boys. Their level of physical activity did not affect their body fat.

So simply increasing one’s physical activity level may not be the key to reducing his or her level of obesity or preventing it.

“Successful weight loss is clearly a combination between reduced energy intake and increased physical activity,” Ekelund said. “Limiting calorie intake is a more effective way of preventing weight gain than trying to burn them off afterwards.”

Further, “physical activity is not likely to counteract a poor diet,” according to Dr. Dennis M. Styne of the University of California, Davis, who authored an editorial accompanying Ekelund’s study.

He notes in the article that “it would take (more than) one to two hours of extremely vigorous activity to counteract a single large-sized… children’s meal at a fast food restaurant, and there are few children (or adults) who can maintain such a pace.”

Yet, Dr. Eric Small, chair of the American Academy of Pediatrics’ sports medicine committee, who was not involved in Ekelund’s research, warned, “we should not conclude that physical activity is no good.”

“As a medical community we recommend daily physical exercise for all children and adolescents,” Small told Reuters Health, explaining that regular exercise is known to lower a person’s risk of diabetes, high blood pressure and cholesterol and, in most cases, it also helps to lessen fat mass.

SOURCE: American Journal of Clinical Nutrition, February 2005.