摘要：Objectives. To determine whether school or nonschool environments contribute more to childhood overweight, we compared children’s gains in body mass index (BMI) when school is in session (during the kindergarten and first-grade school years) with their gains in BMI when school is out (during summer vacation). Methods. The BMIs of 5380 children in 310 schools were measured as part of the Early Childhood Longitudinal Study, Kindergarten Cohort. We used these measurements to estimate BMI gain rates during kindergarten, summer, and first grade. Results. Growth in BMI was typically faster and more variable during summer vacation than during the kindergarten and first-grade school years. The difference between school and summer gain rates was especially large for 3 at-risk subgroups: Black children, Hispanic children, and children who were already overweight at the beginning of kindergarten. Conclusions. Although a school’s diet and exercise policies may be less than ideal, it appears that early school environments contribute less to overweight than do nonschool environments. Over the past 2 decades, the prevalence of overweight among young US schoolchildren has tripled, from 5% to 15% among the 6- to 11-year-old population. 1 – 3 Overweight is especially common among young Black and Hispanic schoolchildren, approximately 20% of whom are now overweight. 2 (Following conventional usage, we apply the label “overweight” to children whose body mass index [BMI] exceeds the 95th percentile on the Centers for Disease Control and Prevention’s [CDC’s] BMI-for-age charts 4 ; these charts give the age-specific BMI distribution that prevailed before recent increases in BMI. Some researchers use the label “obese” for certain overweight children, but the word “obese” is not as clearly defined for children as it is for adults. 5 ) In seeking explanations for childhood overweight, some observers have pointed to schools, which 1 critic has called “obesity zones.” 6 , 7 Schools have been faulted for serving fattening lunches, 8 for scheduling inadequate time for exercise, 9 and for allowing packaged-food and soft drink companies to install vending machines. 7 , 10 , 11 Other observers, by contrast, have pointed to influences outside the school walls, suggesting that childhood overweight results from children overconsuming fast food and energy-dense convenience foods, 12 , 13 from a lack of sidewalks or recreational areas in many housing developments, 14 from excessive television viewing, 15 and from reductions in parental supervision as more mothers enter the workforce. 16 Although each of these specific factors may have some effect, it is unclear in general whether childhood overweight arises primarily from school or nonschool influences. This issue is fundamental because it can help to focus future efforts. For example, if the major sources of overweight reside inside school walls, then interventions should focus on improving the school environment. By contrast, if the major sources of overweight are found outside of schools, then interventions that improve or compensate for the nonschool environment may be more promising. Disentangling the effects of school and non-school environments poses a methodological challenge. It is difficult to measure—or even to identify—all of the school and nonschool influences on body mass index (BMI). And it is both impractical and unethical to run a clinical trial in which the school “treatment” is offered to some children but withheld from others. Fortunately, the structure of the school calendar allows us to observe children under both school and nonschool conditions. During the school year, children are exposed to a mix of school and nonschool influences, but during summer vacation they are exposed to non-school influences alone. 17 , 18 If overweight arises primarily from school influences, we would expect accelerated BMI gains during the school year. By contrast, if overweight arises primarily from nonschool influences, we would expect accelerated BMI gains during summer vacation. Our main objective, then, was to compare school and nonschool influences on children’s BMI by estimating children’s rates of gain when they are in school (during the academic year) and when they are out of school (during summer vacation). Our study design was roughly analogous to a crossover trial, in which every participant is exposed to a period of school treatment and a period of nonschool treatment. The natural experiment afforded by the school calendar, though, differs from an ideal crossover trial in 2 important ways. First, in a crossover trial, different groups would be rotated through the school treatment at different times; however, in US schools, nearly all children are exposed to the school treatment at about the same time, so the school treatment is confounded with the season of the year. Second, some children attend school during summer and thus cannot be observed outside the school environment. We excluded such children from our primary analyses, although later we discuss secondary analyses in which they were compared with other children.
