Waning exercise levels, even more than overeating, may be a major reason that many U.S. girls become overweight as teenagers, new study findings suggest.

In a study of nearly 2,300 girls followed for a decade, researchers found that the proportion who were overweight or obese doubled over time - and declining activity levels looked to be a top reason.

Girls who remained moderately active throughout their teen years gained anywhere from 9 to 20 pounds less than their peers who got little or no exercise. Diet, on the other hand, did not emerge as a key factor, as the girls’ reported calorie intakes changed little as they got older.

“Our results provide evidence that physical activity plays a substantial and independent part in the rate of gain in BMI (body mass index) during adolescence,” the study authors write in a report published online by The Lancet medical journal.

Teenagers’ increasingly sedentary lifestyles, the researchers add, may be a “major factor” in the sharp increase in obesity the U.S. has seen in the past 20 years.

The implication, they say, is that getting kids to stay moderately active - such as walking briskly for 30 minutes on most days of the week - could help combat the “global obesity epidemic.”

Dr. Sue Y. S. Kimm of the University of New Mexico Health Sciences Center in Albuquerque led the study, which followed 2,287 white and black girls from three U.S. cities for up to 10 years, starting at the age of 9 or 10.

Each year, the researchers recorded the girls’ BMI - a measure of weight in relation to height - and used skin fold measurements to gauge their body fat. The girls also periodically reported on their exercise and eating habits.

Girls were considered active if their exercise habits were at least equivalent to 30 minutes of brisk walking per day, 5 days a week - and were maintained throughout adolescence. Just one-third of white girls and 11 percent of black girls met that definition, the researchers found.

Active girls generally had a lower BMI than their inactive peers at the age of 9, and that difference in body mass tripled over time, according to Kimm’s team.

The effect of exercise habits was particularly pronounced among black girls, who tended to gain more weight than white girls did as their activity levels declined.

Diet did not appear to play a large role in the girls’ weight gain over time, since their calorie intakes changed relatively little over time, according to the researchers.

However, they point out, calorie intakes were based on food records that the girls kept periodically. People in studies, particularly white women, are known to commonly under-report their food intake, and this could underlie the lack of calorie change seen in this study, Kimm and her colleagues note.

Regardless, the findings on exercise have “important implications for obesity prevention,” according to Dr. John J. Reilly of the University of Glasgow in the UK. In an accompanying editorial, he writes that doctors “should have no hesitation” in encouraging exercise during adolescence - which, besides possibly combating excess weight gain, makes for a healthier heart and bones.

SOURCE: The Lancet, online July 14, 2005.