Exercise Helps Girls Avoid Excess Weight Gain in Adolescence

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A new study suggests that increasing physical activity may be the key to fighting the obesity epidemic -- despite all the recent emphasis on food consumption and schemes to tax or ban "junk food."

Adolescent girls who are habitually physically active are less likely to gain excess fat as they mature, according to a report in the July 14 issue of The Lancet. This study examined the activity levels, food intake, and changes in body weight and fatness of 1,152 black and 1,135 white girls over a ten-year period, between the ages of nine-or-ten and eighteen-or-nineteen. The results indicate that emphasizing food intake alone will not ameliorate the increases in obesity that have occurred over the past few decades among the young.

Girls in the present study were separated into three groups -- active, moderately active, or inactive -- according to how much exercise they habitually engaged in at the beginning of the study. Activity levels were reassessed at years three, five, and seven-to-ten of the study, and food intake was estimated from three-day food diaries in years one-to-five, seven, eight, and ten. Participants' Body Mass Indexes (BMI) were calculated yearly, as a rough index of body fat. For a more direct measure of body fat, the researchers also measured the girls' skinfold thickness -- which directly indicates the amount of fat under the skin in several areas of the body.

The researchers found that activity levels of both black and white girls declined as they matured, a result that they had previously reported. They found that the changes in activity level in both races were inversely related to the changes in BMI -- as the girls became less active, their BMIs increased. Similarly, the more direct measure of fatness, the skinfold thicknesses, also increased as activity levels declined. Black girls decreased their activity to a greater extent than did white girls, and both indices of fatness also increased more in black than in white girls. Black girls reported eating significantly more calories than white girls, and black girls' intake increased over time by about 120 calories per day. White girls' reported energy intake remained unchanged over time.

Although the authors of the report noted that the lack of increase in the white girls' energy intake might be suspect (it is known that food intake is often underestimated and underreported), the strength of the association between activity and fatness indicates that changes in food intake were not the major determinant of increased fatness.

What do these results mean in terms of dealing with the increasing prevalence of obesity in American youth? One important lesson is that the emphasis that many place on consumption of particular foods may well be misguided, as we at ACSH have suggested on numerous occasions (see, for example, http://www.acsh.org/factsfears/newsID.586/news_detail.asp ). Our society must recognize that sedentary lifestyles are having a negative impact on the health and wellbeing of our youth. Unless the tendency to decrease activity levels with age can be altered, the effect on public health will be vast.

Ruth Kava, Ph.D., R.D., is Director of Nutrition at the American Council on Science and Health.