摘要：Objectives. We examined the effect of physical education instruction time on body mass index (BMI) change in elementary school. Methods. We examined data from a national sample of 9751 kindergartners in the United States who were reported on for 2 years. We used a difference-in-differences approach to examine the effect of an increase in physical education instruction time between kindergarten and first grade on the difference in BMI change in the 2 grades, using the same child as the control. Results. One additional hour of physical education in first grade compared with the time allowed for physical education in kindergarten reduces BMI among girls who were overweight or at risk for overweight in kindergarten (coefficient = −0.31, P < .001) but has no significant effect among overweight or at-risk-for-overweight boys (coefficient = −0.07, P = .25) or among boys (coefficient = 0.04, P = .31) or girls (coefficient = 0.01, P = .80) with a normal BMI. Conclusions. Expanding physical education programs in schools, in the form in which they currently exist, may be an effective intervention for combating obesity in the early years, especially among girls. Children spend many hours in school, making physical education (PE) programs in schools a potentially important channel through which physical activity and fitness may be promoted among young children. 1, 2 How effective are school PE programs in preventing obesity and promoting physical activity? Research studies tend to paint a pessimistic picture. Although guidelines recommend that students have daily classes, receive a substantial percentage of their weekly amount of in-school physical activity in PE classes, and be physically active for at least half of the PE class time, only a small minority of children have daily classes, and active class time is far below 50%. 3– 7 In response, a number of programs have been developed to improve physical education, often in combination with other health education or environmental interventions. 8– 12 Some intervention trials have indeed demonstrated that carefully designed programs can improve youth fitness and may reduce obesity. 10, 13 However, no national study has evaluated the role of PE classes—in the form in which they are currently implemented in American schools—in preventing obesity. Even if actual classes fall short of standards or exemplary programs, their overall role may be more important for public health than the incremental addition to the baseline that possible interventions could provide. We evaluated the effect of currently existing PE programs in US elementary schools by following a nationally representative cohort of kindergartners in the United States. The longitudinal data allowed us to study whether increases or decreases in physical education over time affect changes in body mass index (BMI).
