摘要：Although preventing youth overweight and obesity is a public health priority, quality physical education (PE) is marginalized in practice. In May 2014, we estimated energy expenditure (EE; derived from PE frequency, duration, and intensity; mean student mass; and class size) from national recommendations and data from the 19 US states with PE duration guidelines, under 3 scenarios: potential (quality PE, defined as 50% moderate-to-vigorous physical activity [MVPA]), reality (MVPA = 35%), and classroom instruction only. Students in schools following nationally recommended PE standards from grades 1 through 10 could expend from 35 000 to 90 000 more kilocalories than students who received classroom instruction instead. PE's potential for increasing student EE will only be realized with stronger school policies and increased accountability. The prevalence of overweight and obesity among school-aged youths in the United States remains high. 1 Of particular concern is that obesity in childhood tracks into adulthood and is associated with a host of chronic morbidities. 2 Although most of these diseases typically are not manifest until adulthood (e.g., stroke, cancer), the prevalence of type 2 diabetes and metabolic syndrome during childhood is increasing. 3 Prevention of overweight and obesity among youths requires that energy expenditure (EE) exceed energy intake. Engaging in physical activity (PA) is a widely accepted approach to preventing and treating childhood obesity, and 2008 United States Physical Activity Guidelines , published by the US Department of Health and Human Services (USDHHS), recommends that youths engage in at least 60 minutes of mostly moderate-to-vigorous PA (MVPA) daily. 2 Schools have long been identified as important venues for the provision of PA, especially during physical education (PE) classes. 4,5 PE, a mandated curriculum area in 43 states for elementary grades, 41 for middle schools, and 44 for high schools, 6 includes substantial amounts of PA during class time and is considered a public health resource for combating and preventing overweight and obesity. 4–10 Although numerous health organizations and publications, including Healthy People 2020 , 11 recommend daily PE, only 19 of 50 states reported having policies identifying the frequency of delivery and specific number of minutes for PE. 6 Of these states, only 5 reported having these policies at all 3 levels of primary and secondary school. A 2012 Institute of Medicine report, Accelerating Progress in Obesity Prevention: Solving the Weight of the Nation , identified requiring school PE as a key strategy for obesity prevention in the United States. 7 Analyses of data from the Early Childhood Longitudinal Study showed that PE reduced body mass index among first-grade girls who were overweight or at risk for overweight in kindergarten 8 and reduced the probability of obesity among fifth-grade boys. 9 Recently, the Institute of Medicine recommended an average daily dose of 30 minutes (elementary school) and 45 minutes (middle and high school) of activity per PE lesson that is composed of at least 50% MVPA. 10 Although some PE lessons surpass the 50% MVPA criterion, researchers have demonstrated that most fall short of this mark. Fairclough and Stratton reviewed 44 studies of elementary school PE and found that children averaged 37.4% (±15.7%) of lesson time in MVPA (mean = 12.6 min MVPA/lesson; 21.0% of USDHHS total daily guidelines). 12 Fairclough and Stratton also reviewed 40 studies of middle and high schools and found that students averaged 27.7% to 46.8% of PE time in MVPA, depending in part on the method of assessment. 13 These authors did not report mean lesson length, but at the recommended 45 minutes per lesson at the secondary level, 10 students on average could have accumulated between 12.5 and 21.1 MVPA minutes (20.8% and 35.1% of USDHHS total daily guidelines, respectively). Although PE has many objectives (e.g., physical fitness; motor skills; cognitive, social, and emotional development), not all of which require engaging in PA during lesson time, many medical and professional communities consider sufficient MVPA to be a useful proxy for PE quality. 14,15 This approach has yielded positive changes in professional development policy, as reported in the School Health Policies and Practices Study 2012. 16 Between 2000 and 2012, the percentage of states that funded or provided professional development for teachers to learn methods to improve MVPA during PE increased from 28.0% to 66.7%. Despite this positive trend, only Idaho specifies the percentage of a lesson to be spent in MVPA (i.e., 33% in grades K–5; 50% in grades 6–12), and only Texas specifies required minutes of a lesson in MVPA (30 min/d for grades K–8). 17 Carlson et al. noted that no policy addressed funding or enforcement; that policy was not uniform across states, within school levels, and within states across school levels; and that only Idaho specified a means for objectively evaluating whether MVPA goals were met. 17 Lesson frequency and length, percentage of lesson time in MVPA, and steps per minute have all been used to define PE policy for quantity and quality. These metrics, however, do not readily reveal the potential public health impact that PE has on controlling childhood overweight and obesity. 9 In public health forums, EE is frequently expressed in METs (metabolic equivalents of task) and MET hours, and in the PE context, 4.5 METs have been equated with MVPA. 18 Bassett et al. computed a weighted mean EE of 1.70 MET hours gained per day for studies where PE was a mandatory policy. 19 This mean value equates to a gain of 23 minutes of MVPA per day (38.3% of USDHHS guidelines) versus no PE. Although this latter metric may be more comprehensible to practitioners than a MET-based one, it still provides only derivative information about EE. We used the Bassett et al. data, 19 along with the mean weights of children in elementary, middle, and high school, and calculated that children in middle school (98 kcal/day) and high school (121 kcal/day) PE could surpass the additional caloric expenditure of 64 kilocalories per day deemed necessary to reach overall youth obesity prevalence targets by 2020. 20 These predicted EE estimates provide a useful starting point for discussion, but they are based on a limited number of large-scale interventions that do not reflect the potential or real impact of existing PE policy on EE. We aimed to quantify the potential caloric impact of quality PE (i.e., at 50% MVPA) on the estimated EE of individual students and classes under conditions (frequency, duration, class size) recommended by professionals and by those states that have education policies mandating number of minutes for PE. We also compared estimated EE under these conditions to calculations derived from (1) data from studies where the intensity of PA was observed during PE lessons (i.e., the reality) and (2) estimations of student EE if they spent equivalent periods in classroom activities instead of PE.
