The amount of regular exercise girls get falls off dramatically as they move through their teenage years, dropping to practically zero in many cases, especially among blacks, a study found.
By the time they were 16 or 17, more than half of the black girls in the study and nearly a third of the white girls reported they got no regular exercise at all outside school.
With obesity at epidemic levels, "it's a cause for alarm," said Dr. Sue Y.S. Kimm of the University of Pittsburgh School of Medicine. "We cannot sit complacent anymore."
Kimm and her colleagues reported their findings in Thursday's New England Journal of Medicine.
She said that there has been no similar study of boys but that they are generally more active because of their greater participation in sports.
The girls' decline in physical activity was affected by lower levels of parental education, heavier weight, smoking and pregnancy. Girls with better-educated parents may be better informed and more encouraged to exercise, the researchers suggested.
The study followed 1,213 black girls and 1,166 white girls from the Washington, D.C., Cincinnati and San Francisco areas for 10 years, beginning at age 9 or 10.
Through a series of questionnaires and interviews, the researchers recorded after-school exercise such as sports, bicycling, dancing and gymnastics.
By the end of the study, the activity score for the whole group dropped 83 percent.
Kimm said yet-unpublished data from the group shows that obesity doubled even though no significant increase in calorie consumption was reported.
"We have to surmise that physical activity appears to be a major suspect," she said.
Dr. Paul D. Thompson of Hartford Hospital said the study confirms suspicions that teenage girls in particular are getting little exercise.
In another study in the journal, researchers found that women who sit for less than four hours a day have a lower risk of heart disease than those who sit for prolonged periods.
The large study of 73,734 postmenopausal women also confirmed earlier research that showed brisk walking is just as effective as more vigorous exercise such as aerobics, jogging or tennis in reducing the risk of heart disease.
Women in the study who spent 2.5 hours a week walking or exercising vigorously cut their risk by about 30 percent.
"The good news is that 30 minutes, five days a week will do it, will confer tremendous cardiovascular benefits. This is an amount of exercise that is feasible and accessible for nearly everyone," said one of the researchers, Dr. JoAnn Manson, chief of preventive medicine at Harvard's Brigham and Women's Hospital.
Thompson said: "The take-home message for physicians or any health professional dealing with patients is to encourage them to do something. And walking was a good physical activity."
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New England Journal: http://www.nejm.org
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