Department of Exercise Science, University of South Carolina, Columbia, SC 29208, USA.
Department of Health Promotion, Education, and Behavior, University of South Carolina, Columbia, SC 29208, USA.
J Sport Health Sci. 2020 Jan;9(1):18-27. doi: 10.1016/j.jshs.2019.05.006. Epub 2019 May 23.
To evaluate the potential of a year-round school calendar (180-day school year distributed across 12 months) as an intervention compared to a traditional school calendar (180-day school year distributed across 9 months) for mitigating children's weight gain and fitness loss via a natural experiment.
Height, weight, and cardiorespiratory fitness (CRF) (i.e., Fitnessgram Progressive Aerobic Cardiovascular Endurance Run) were measured in children (5-12 years old) in 3 schools (2 traditional, 1 year-round, = 990 students, age = 8.6 ± 2.4 years, 53.1% male, 68.9% African American) from 1 school district. Structure (represented by the presence of a school day) was the independent variable. Changes in body mass index (BMI), age- and sex-specific BMI z-scores (zBMI), BMI percentile, percent of overweight or obese children, and CRF (Progressive Aerobic Cardiovascular Endurance Run laps completed) were assessed for summer 2017 (May-August 2017), school year 2017/2018 (August 2017-May 2018), and summer 2018 (May-August 2018). Primary analyses examined the overall change in weight and CRF from summer 2017 until summer 2018 via multilevel mixed effects regression, with group (traditional year-round calendar), time, and a group-by-time interaction as the independent variables. Secondary regression analyses estimated differences in change within and between groups during each time period, separately.
Year-round students gained less BMI (difference in ∆ = -0.44, 95% confidence interval (CI): -0.67 to -0.03) and less CRF (difference in ∆ = -1.92, 95%CI: -3.56 to -0.28) than students attending a traditional school overall. Compared with traditional students, during both summers, year-round students gained less BMI (summer 2017 difference in ∆ = -0.15, 95%CI: -0.21 to -0.08; summer 2018 difference in ∆ = -0.16, 95%CI: -0.24 to -0.07) and zBMI (summer 2017 difference in ∆ = -0.032, 95%CI: -0.050 to -0.010; summer 2018 difference in ∆ = -0.033, 95%CI: -0.056 to -0.009), and increased CRF (summer 2017 difference in ∆ = 0.40, 95%CI: 0.02-0.85; summer 2018 difference in ∆ = 0.23, 95%CI: -0.25 to 0.74). However, the opposite was observed for the school year, with traditional students gaining less BMI and zBMI and increasing CRF compared with year-round students (difference in BMI ∆ = 0.05, 95%CI: 0.03-0.07; difference in zBMI ∆ = 0.012, 95%CI: 0.005-0.019; difference in Progressive Aerobic Cardiovascular Endurance Run laps ∆ = -0.43, 95%CI: -0.58 to -0.28).
The year-round school calendar had a small beneficial impact on children's weight status but not CRF. It is unclear if this benefit to children's weight would be maintained because gains made in the summer were largely erased during the school year. Trajectories of weight and CRF gain/loss were consistent with the structured days hypothesis.
通过自然实验，评估全年制校历（180 天的学年分布在 12 个月中）作为一种干预措施的潜力，与传统校历（180 天的学年分布在 9 个月中）相比，以减轻儿童的体重增加和体能下降。
在一个学区的 3 所学校（2 所传统学校，1 所全年制学校）中测量了 5-12 岁儿童的身高、体重和心肺功能（即健身有氧心血管耐力跑）。结构（由学校日的存在表示）是自变量。评估了 2017 年夏季（2017 年 5 月至 8 月）、2017/2018 学年（2017 年 8 月至 2018 年 5 月）和 2018 年夏季（2018 年 5 月至 8 月）期间的体重指数（BMI）、年龄和性别特异性 BMI z 分数（zBMI）、BMI 百分位数、超重或肥胖儿童的百分比和心肺功能（完成的渐进式有氧心血管耐力跑圈数）的变化。主要分析通过多层混合效应回归评估了 2017 年夏季至 2018 年夏季期间体重和心肺功能的总体变化，其中独立变量为组（传统-全年制校历）、时间和组-时间交互作用。二次回归分析分别估计了每个时间段内和组间的变化差异。
与上传统学校的学生相比，全年制学生的 BMI 增加较少（差异为 ∆= -0.44，95%置信区间（CI）：-0.67 至 -0.03），心肺功能下降较少（差异为 ∆= -1.92，95%CI：-3.56 至 -0.28）。与传统学生相比，在两个夏季期间，全年制学生的 BMI 增加较少（2017 年夏季差异为 ∆= -0.15，95%CI：-0.21 至 -0.08；2018 年夏季差异为 ∆= -0.16，95%CI：-0.24 至 -0.07）和 zBMI（2017 年夏季差异为 ∆= -0.032，95%CI：-0.050 至 -0.010；2018 年夏季差异为 ∆= -0.033，95%CI：-0.056 至 -0.009），心肺功能增加（2017 年夏季差异为 ∆= 0.40，95%CI：0.02-0.85；2018 年夏季差异为 ∆= 0.23，95%CI：-0.25 至 0.74）。然而，在学年期间观察到的情况正好相反，与全年制学生相比，传统学生的 BMI 和 zBMI 增加较少，心肺功能增加（BMI 差异为 ∆= 0.05，95%CI：0.03-0.07；zBMI 差异为 ∆= 0.012，95%CI：0.005-0.019；渐进式有氧心血管耐力跑圈数差异为 ∆= -0.43，95%CI：-0.58 至 -0.28）。
全年制校历对儿童的体重状况有较小的有益影响，但对心肺功能没有影响。目前尚不清楚这种对儿童体重的益处是否会持续下去，因为夏季的增长在学年期间大部分都被抹去了。体重和心肺功能增加/减少的轨迹与结构天数假说一致。
