A new study finds that the amount of regular exercise girls get in their spare time drops by more than 80 percent between elementary school and late adolescence. Black teens reported a 100 percent falloff in regular physical activity each week. Their white peers fared a little better, but even their exercise scores dropped by nearly two-thirds over the 10-year period.
The findings help explain why obesity, Type II diabetes and inactivity are higher among black women than among white women, which raises their risk of heart disease, stroke, colon cancer, osteoporosis and other health problems tied to exercise. Some 15 percent of American youth are overweight, triple the number since 1970s.
Dr. Sue Kimm of the University of Pittsburgh led the study, which appears in tomorrow's New England Journal of Medicine.
Kimm and her colleagues followed 1,213 black girls and 1,166 white girls from the age of 9 and 10 to 18 and 19. The girls were queried regularly about their exercise habits during out-of-school hours, as well as about other health habits and their family's social and economic status.
At the start of the study period, the two groups reported roughly the same amount of weekly exercise, as estimated in units called metabolic equivalents, or METs, in the form of swimming, biking, dance, gymnastics, and other sports. Black girls had 27.3 and white girls had 30.8 METs, respectively.
But 10 years later, black girls reported getting 0 METs in their free time each week, on average, a decline of 100 percent from the earlier figure. Whites got 11 MET-times a week, 64 percent fewer than before.
The number of girls in each group who reported getting no regular leisure exercise by age 16 or 17 was 56 percent for blacks and 31 percent for whites. For the entire study, the average activity score plunged 83 percent.
Black girls had higher average body mass -- a marker of overweight and obesity -- than the white girls in all age brackets. High body mass index (BMI) was associated with steeper declines in physical activity for both groups.
The rate of smoking was more than four times greater among white girls, 27 percent vs. 5.7 percent, and smoking predicted who exercised less.
Black girls were three times as likely to get pregnant by the eighth year of the study -- 22.3 percent vs. 7.5 percent. Pregnancy was cited as a reason for lack of exercise for blacks, but not for whites.
White girls whose parents were poorly educated had greater drops in exercise between childhood and adolescence. But that effect held only for black girls in their teens. Other work has found a similar link between years of education and amount of physical activity among women in this country.
Richard Troiano, an epidemiologist at the National Cancer Institute and author of an editorial accompanying the journal article, said the study underscores how exercise-unfriendly American culture has become.
People take elevators rather than climb stairs, they drive to work and school instead of walk. Neighborhood playgrounds are no longer seen as safe havens for local kids, meaning those who want to participate in sports have to be taken to gyms, fields, or parks -- trips that require sacrifices of time and often money from parents.
"We need to re-emphasize the need for physical activity," Troiano said. "We have to put it back in because we've essentially taken it out."
Bernard Gutin, an exercise expert at the Medical College of Georgia in Augusta, called the latest findings "pretty scary. The implication is that after school [teen girls] just don't do a blessed thing."
Gutin and his colleagues have studied fitness interventions in both boys and girls, with encouraging results. A half-hour or more daily of moderate-to-vigorous exercise can help them shed pounds and take off fat even without changing their diet, he said. His group is now looking at the impact of regular 80-minute workouts on body and abdominal fat.
Sumru Ekrut, a Wellesley College researcher who studies the exercise habits of girls, said the trends in the latest analysis aren't surprising, though the magnitude of the reduction in physical activity is larger than she has found.
"With increasing age, fewer kids are physically active; this is true for both boys and girls," Ekrut said.
The phenomenon of the athletic girl is relatively recent. So researchers know little about the negative effects of stopping exercise in this group, Ekrut said.
"No one has looked at what happens to girls who stop being physically active -- whether their developmental trajectory changes. It might, but one could also argue that the protective effects linger."
While participation in sports is generally perceived as a good thing for girls, Ekrut said it's not always an unmitigated plus. Team sports can expose younger girls to the more adult, and potentially dangerous, behaviors of older students, like drinking alcohol. These in turn can make them vulnerable to risky sexual practices and other hazardous activities.
What To Do
For more on the importance to the skeleton of exercise in childhood, visit the National Institute of Child Health and Human Development.