A new study finds that while exercise is an almost certain way to prevent… (John Moore / Getty Images )
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, published in the Archives of Pediatrics and Adolescent Medicine, 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 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 sisters.
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. First LadyMichelle Obama's"Let's Move" campaign, for example, has focused intensively on getting African Americans girls and adolescents to become more active.
The new finding "makes our life more challenging," said Ginny Ehrlich, chief executive of the Alliance for a Healthier Generation, which works in more than 14,000 U.S. schools to encourage exercise and healthier eating. It underscores the need to stress the fundamental skill of balancing energy intake and output.
"It's not just physical activity," Ehrlich said. "That's particularly important for African American girls."
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," wrote the authors, James White of Cardiff University and Russell Jago of the University of Bristol. "Our 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.
For 28-year-old Toni Carey of Norfolk, Va., co-founder of the national running group Black Girls Run!, that harsh reality compounds an already challenging situation for African American girls. When Carey took up running to improve her health, she said her mother's "first reaction was, 'That's something that black people don't do.' She said, 'Your uterus is going to fall out' and all sorts of things."
In African American households headed by a single-parent struggling to feed kids on a limited income, she said, "if you have to eat off the dollar menu, that's what you do." In others, she added, family cooking traditions that emphasize less healthful foods and food preparation can be difficult to change.
Beyond that, "if you aren't seeing your peers out there running and exercising, or you hear them say, 'I don't want to mess up my hair,' it's more than likely you're not going to engage in that physical activity."
Ehrlich, of the Alliance for a Healthier Generation, said that she would be wary of sharing the study's results with young black women, for whom body size is "already a really sensitive subject."
"It creates yet another barrier to what might already feel like a struggle," she said. "When we talk with young people, we talk about healthy living — eating better and moving more. We're trying to stay away from messaging around obesity."
Linda Bacon, an associate nutritionist with UC Davis and a critic of the nation's focus on reducing obesity, said activists were right to focus on healthy lifestyles and not on BMI.
"We should just be encouraging activity for the sake of activity and good health," said Bacon, author of "Health at Every Size." "If we encourage it as a weight management technique, when it doesn't work for that, people won't see the value in it."