The September 4, 2002 issue of the New England Journal of Medicine has both good and bad news for the health of American women and girls. One article (the good news) presents the results of a study on post-menopausal women showing that regular physical activity diminishes the risk of heart disease. The bad news is that a second article in the same issue reports that as girls advance from childhood to mid-adolescence, their levels of physical activity drop precipitously. This pattern does not bode well for their health.
Over 70,000 women between fifty and seventy-nine years old were the subjects of the first report from Women's Health Initiative. They included both Caucasian and minority women who were followed for up to 5.9 years. The subjects provided estimates of their walking frequency and intensity, as well as of other recreational activity levels.
Study authors found that whether they looked at the women's total physical activity or their amount and pace of walking, the results were similar. The 20% who exercised most had significant reductions in their risk of cardiovascular disease (CVD) compared to the women who exercised the least. And the degree of risk reduction was impressive: white women who expended the most energy in total physical activity reduce their risk of CVD by 45%; for black women the reduction was slightly greater 52%. Even women whose Body Mass Index* suggested overweight or obesity benefited from exercise. Women whose BMI was 30 or greater significantly reduced their risk of CVD by 35%! Exercise benefited women in all the age groups, too. Even women between seventy and seventy-nine years old had lowered their risk of CVD.
Unfortunately, if the results of the second study are any indication, the beneficial health effects of regular exercise do not seem to be influencing the lifestyles of girls and adolescents. This study followed 1,213 black girls and 1,166 white girls from the ages of eight or nine until they were eighteen or nineteen years old. Based on questionnaires the girls completed, the researchers calculated an activity score MET or metabolic equivalents for each girl. Initially, the activity scores for black girls were 27.3 and for white girls 30.8.
By the time the girls were sixteen or seventeen years old, over half of the black girls and nearly one third of the white girls reported that they participated in no habitual leisure-time activity. At the end of the study (when participants were eighteen or nineteen years old), the scores reported by black girls had declined to zero, and those of white girls to 11. As might be expected, the greater the decrease in activity, the higher the BMI.
Not only was this decrease in activity bad for body weight, it was also indicative of the acquisition of non-healthy behaviors. For white girls, the activity decrease was significantly associated with cigarette smoking, while for black girls it was associated with pregnancy.
While the news media are rife with messages about the American epidemic of obesity, and the food police insist on blaming so-called "junk foods" for heart disease and other ills, little attention has been paid to the exercise part of the energy balance equation. These two reports should serve notice that the number of calories consumed is not the only, nor perhaps even the most important, determinant of fatness and health. Older women may be getting the message, but girls and adolescents are not. Unless this decline in activity with maturity can be effectively addressed, it is unlikely we'll end the rise in childhood obesity and its associated ailments.
Ruth Kava is Director of Nutrition for the American Council on Science and Health.
*A BMI of 25-29.9 is considered overweight, and a BMI of 30 or greater is considered obese. One caveat is in order, however. Because BMI doesnÃt directly measure fatness, a very muscular person (e.g. a bodybuilder) could have a high BMI without having excess amounts of body fat. While the women in this study were unlikely to be bodybuilders, the most frequent and intensive exercisers among them might not be as overly fat as the BMI numbers suggest.