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NEW YORK (Reuters Health) - In a new study of U.S. preteen and teen girls, daily exercise was strongly linked to weight and obesity in white girls but not black girls.
Although it's still important to promote physical activity in all young people, according to the researchers that may not be enough to prevent black girls -- who have a higher rate of obesity to begin with -- from gaining weight.
"I think everyone would agree we need people to be active. It's not sufficient on its own to prevent weight gain, but it's really an important part of the equation," said Alison Field, who studies weight in adolescents and women at Harvard Medical School and Children's Hospital Boston.
Still, the new findings "would suggest that… what we've been recommending may not be the perfect fit for African Americans," Field, who wasn't involved in the new research, told Reuters Health.
One possibility is that along with other lifestyle changes, black girls need to get a lot more exercise than white girls to start making a difference in their obesity risk, she added.
But it's unclear why that would be the case, and just how much physical activity they would need.
Field said the results are "sobering" given that black girls typically are less active to begin with and getting teen girls involved and engaged in new types of exercise is particularly challenging.
The findings come from a second analysis of data originally collected by the National Institutes of Health, which followed girls in Cincinnati, Berkeley, California and Rockville, Maryland starting when they were nine or ten years old in 1985.
For the new analysis, James White of Cardiff University and Russell Jago of the University of Bristol, both in the UK, used data on physical activity, eating habits, weight and height from when the girls were 12 and 14 years old.
Physical activity was measured over three days with a device called an accelerometer, which is kept in a pouch above the hip and calculates how much time the wearer spends walking, running or otherwise being active.
At age 14, close to 16 percent of the black girls qualified as obese, compared to just five percent of white girls.
The researchers found the most active white 12-year-olds were 85 percent less likely to be obese at the second reading than the least active. That held up when they took into account girls' diets and how much time they spent sitting.
But for black girls, there was no clear link between physical activity at age 12 and obesity at 14.
The study, published Monday in the Archives of Pediatrics & Adolescent Medicine, included 1,148 girls, split roughly evenly between black and white adolescents.
Another report published in the same journal found certain genes may affect a person's chance of being obese as a teen and young adult -- but that those influences may also start as early as a few years of age.
Based on 32 small genetic sequences that have been tied to obesity in adults, people in New Zealand who had a higher genetic risk were heavier starting at age three, all the way through age 38 -- the end of the study period.
Researchers led by Daniel Belsky at Duke University in Durham, North Carolina found that in their preteens and teens, people were more than twice as likely to be obese if they had more of those risky gene patterns than average.
That study involved 856 people of European descent -- and the new findings in teen girls question whether those genes would be equally important in non-whites, Jose Fernandez from the University of Alabama at Birmingham wrote in a commentary published with the two studies.
Belsky said one way researchers can use the new information is to look at how interventions such as exercise or diet changes affect people with different genes differently.
Genetics certainly isn't everything, researchers agreed.
"The magnitude of genetic risks that we're (seeing) in this profile is small," Belsky told Reuters Health.
"Some of the kids at higher levels of genetic risk didn't become obese, and some of the kids at lower levels of genetic risk did become obese."
Field said most genetic differences related to obesity, including those between whites and blacks, can be overcome.
"There are certain people who are more susceptible for obesity and weight gain, but data suggest that at least for most people, there are things they can do in their lifestyle to lessen their genetic disadvantage."