In a dispiriting finding for African American girls and women, a new study finds that while engaging in high levels of physical activity is a good bet for preventing obesity in white adolescent girls, it does not give their black peers the same benefit

The study found that among black adolescent girls who moved the most at age 12, obesity at age 14 was nearly as likely as it was for those whose activity rates were far lower. For white girls, by contrast, regular exercise at 12 appeared a nearly sure way to head off obesity at 14. That finding held, even when the calorie intakes of an African American youngster and her white counterpart were the same. The authors, a pair of British researchers using data from a government health study that followed American adolescents for several years, said that their findings pointed to a significant metabolic disadvantage for African American girls hoping to maintain a healthy weight. They concluded that obesity-prevention interventions may need to be adapted to account for the finding that black girls are less sensitive to the effects of physical activity than their white counterparts. In the national effort to stem a crisis of obesity in the United States, the state of African American women stands out as a particular challenge. At 39.4%, their rate of obesity is the highest of any single ethnic or gender group measured. Four in five black women are overweight or obese when measured by the most widely accepted gauge of fatness, the body mass index, or BMI. The run-up in weight probably begins in adolescence, experts say, which is why many efforts to address the epidemic of obesity and its related diseases in African American women are heavily focused on girls. Michelle Obama's "Let's Move" campaign, for example, has focused intensively on getting African Americans girls and adolescents to become more active. The study, which draws from a database of 1,148 adolescents, is the first to explore differences between white and black girls' physical activity rates and their effect on weight. (Just under half — 538 — identified themselves as African American.) But it falls in line with research that finds black women oxidize fat more slowly in response to exercise, and that their resting metabolic rates are lower than those of white women. Such racial differences may predispose black girls to retaining fat accumulated during puberty. The results suggest that prompting adolescent girls to be active may be important to preventing obesity but that using different approaches (e.g. emphasizing reductions in energy intake) may be necessary to prevent obesity in black girls. The study compared white and black girls' physical activity and food intake as measured in three-day stretches where they wore a pedometer and kept track of what they ate. They also reported participation in physical activities through the year. Based on activity, each group was divided into upper and lower halves. By BMI and two other obesity measures (a measure of body fat adopted by the International Obesity Task Force and a gauge of skin-fold thickness), the 12-year-old black girls in the top half of the physical activity continuum were only 15% less likely to be obese by age 14 than ones in the lower half. For white girls, those in the upper half were 85% less likely to become obese over the next two years than were those in the bottom half.