Activity, dietary intake, and weight changes in a longitudinal study of preadolescent and adolescent boys and girls

Objective. To examine the role of physical activity, inactivity, and dietar y patterns on annual weight changes among preadolescents and adolescents, t aking growth and development into account. Study Design. We studied a cohort of 6149 girls and 4620 boys from all over the United States who were 9 to 14 years old in 1996. All returned questio nnaires in the fall of 1996 and a year later in 1997. Each child provided h is or her current height and weight and a detailed assessment of typical pa st-year dietary intakes, physical activities, and recreational inactivities (TV, videos/VCR, and video/computer games). Methods. Our hypotheses were that physical activity and dietary fiber intak e are negatively correlated with annual changes in adiposity and that recre ational inactivity (TV/videos/games), caloric intake, and dietary fat intak e are positively correlated with annual changes in adiposity. Separately fo r boys and girls, we performed regression analysis of 1-year change in body mass index (BMI; kg/m(2)). All hypothesized factors were in the model simu ltaneously with several adjustment factors. Results. Larger increases in BMI from 1996 to 1997 were among girls who rep orted higher caloric intakes (.0061 +/- .0026 kg/m(2) per 100 kcal/day; bet a +/- standard error), less physical activity (-.0284 +/-.0142 kg/m(2)/hour /day) and more time with TV/videos/games (.0372 +/-.0106 kg/ m(2)/hour/day) during the year between the 2 BMI assessments. Larger BMI increases were a mong boys who reported more time with TV/videos/games (.0384 +/-.0101) duri ng the year. For both boys and girls, a larger rise in caloric intake from 1996 to 1997 predicted larger BMI increases (girls:.0059 +/-.0027 kg/m(2) p er increase of 100 kcal/day; boys:.0082 +/-.0030). No significant associati ons were noted for energy-adjusted dietary fat or fiber. Conclusions. For both boys and girls, a 1-year increase in BMI was larger i n those who reported more time with TV/videos/games during the year between the 2 BMI measurements, and in those who reported that their caloric intak es increased more from 1 year to the next. Larger year-to-year increases in BMI were also seen among girls who reported higher caloric intakes and les s physical activity during the year between the 2 BMI measurements. Althoug h the magnitudes of these estimated effects were small, their cumulative ef fects, year after year during adolescence, would produce substantial gains in body weight. Strategies to prevent excessive caloric intakes, to decreas e time with TV/videos/games, and to increase physical activity would be pro mising as a means to prevent obesity.