1. Study Design And Study Sample
The Childhood Obesity Study in China Mega-cities (COCM) was a US NIH-funded longitudinal study aimed at examining the etiology of childhood obesity and chronic diseases in China, especially in major cities, which have been experiencing rapid socioeconomic changes over the past three decades. These transitions have resulted in many dramatic changes in social environments and people’s lifestyles, which have led to an increase in obesity and other health problems. Initially, four major cities were included in the 2015 (baseline) survey for the study, including Beijing (China’s capital, in North China), Shanghai (the largest and most economically developed city in China, in Southeast China), Nanjing (China’s old capital before 1949, the capital of Jiangsu province, in Southeast China), and Xi’an (the capital of Shaanxi province, in Northwest China, which has served as the capital of China for 13 dynasties and over 1,300 years). Chengdu (in Southwest China, the capital of Sichuan province, one of the national central cities) was added in 2016. In each city, two primary schools and two middle schools were included. In each school, a class from each grade (grades 3–6 in primary schools, and grades 7–9 in middle schools) was included. Written informed consent was obtained from parents or children.
Children with missing data on age, gender, weight, height, and PA- and SB-related measurements were excluded from analysis (n = 171). For cross-sectional data analysis, sample characteristics of 5,535 participants were described across the three waves; for PA- and SB-related factors, the children’s first available observation during 2015–2017 (3,268 different children) were presented by gender. For longitudinal analysis of the associations, we included 2,762 students with the needed data from all three waves from 2015 to 2017.
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Assessment and measures
1) Anthropometric measures
Height was measured by a Seca 213 Portable Stadiometer Height-Rod with a resolution of 0.1 centimeter; body weight was measured by a Seca 877 electronic flat scale with a resolution of 0.1 kilogram. Waist circumference (WC) was measured by a tape with a resolution of 0.1 centimeter.
Children’s BMI (body mass index, kg/m2) was calculated as body weight divided by height squared (kg/m2). Overweight and obesity were defined based on age-gender-specific BMI curves developed by the Working Group on Obesity in China (WGOC) that correspond to BMI levels of 24 and 28 at age 18 [26], respectively. Abdominal obesity was defined as WHtR ≥ 0.46 [27].
2) Physical activity (PA) and sedentary behaviors (SB)
PA factors: PA time in school education classes (minutes/week), school physical examination (yes/no), extracurricular PA requirement (yes/no), access to sporting equipment at school (“unavailable”, “not enough”, “sometimes enough” and “fair enough”), and the walking time on the way to school (“<5 min”, “5 ~ 15 min” and “> 15 min”).
Considering the influence of parental support on the associations between PA, SB, and children’s weight status, the following measurements were included: mothers’ paired PA (yes/no), fathers’ paired PA (yes/no).
The SB outcome measures included: (1) screen viewing (hours/week) and (2) homework (minutes/day). Activities in the screen viewing category included watching TV, video games, videotapes, VCDs, DVDs, and movies/videos online, but did not include computer time (e.g., surfing the Internet) for homework purposes per week. Children reported on homework time in a separate question.
3) Parental and household characteristics
To consider the influence of family and household characteristics on the associations among PA, SB, and children’s weight status, self-reported height and weight of mothers and fathers were used to calculate parental BMI, and overweight and obesity were defined based on the standards set by the Working Group on Obesity in China (WGOC) (BMI ≥ 24 kg/m2) [22]. Highest maternal education (“middle school or lower”, “high and vocational schools”, and “college or above”), and family home ownership (rent or share residency with relatives, “own an apartment”, and “own a house”) were collected using questionnaires.
4) Covariates
Covariates included age (in years), parental BMI (in kg/m2), maternal highest education (“middle school or lower”, “high and vocational schools”, and “college or above”), family home ownership (“rent or share residency with relatives”, “own an apartment”, and “own a house”), and family living environment (“very safe”, “safe”, and “unsafe”).
3. Statistical Analysis
For participants’ characteristics over the survey years, continuous variables were reported as means (standard deviation, SDs) and compared with the Mann-Whitney-Wilcoxon test. Categorical variables were reported as percentages, and the differences were compared with the chi-square test.
Then mixed-effects models were fitted to investigate the associations between ov/ob (also central obesity) and PA/SB-related measures while adjusting for other covariates. The models adjusted for age, parental BMIs, maternal highest education level, living environment, the city, and the year. Analysis stratified by gender was conducted to test gender-based differences in the associations.
All statistical analyses were conducted using Stata 15.0 (Stata Corp, College Station, Texas, USA). The effected size was reported as odds ratios (ORs) and 95% confidence interval (95% CI).