Measurement of multiple obesity-related factors
Five obesity-related risk factors were investigated at age 11-12, where suitable genetic instruments were available for inclusion in the analysis: obesity, physical activity, cardiopulmonary physical fitness, SDB, and sleep quality.
Physical activity was assessed using a questionnaire and evaluation at school. We used the Chinese version of the International Physical Activity Questionnaire13 to determine a child’s physical activity levels. Participants were asked a series of questions pertaining to intensity, duration, and frequency of physical activity in the preceding week. Physical activities were categorized into vigorous, moderate, and mild, according to their metabolic equivalent (MET) values, namely >6 MET, 3–6 MET, and <3 MET. Physical activities were then quantified according to their average MET expenditure per day14. Cardiorespiratory physical fitness was assessed through an 800-m sprint, a surrogate marker of predicted maximal oxygen consumption15, during school visits by using a standardized protocol16. It was first converted into z scores by using sex- and age-specific means and standard deviations. Hence, a higher cardiorespiratory physical fitness index signifies greater physical fitness.
SDB was determined by using a Chinese version of the Children’s Sleep Habit Questionnaire, which was completed by the parent. Researchers compared this questionnaire with objective polysomnography-defined obstructive sleep apnea and found it to be a valid tool for detecting the presence of SDB17. Sleep quality was evaluated through the Chinese version of the Pittsburgh Sleep Quality Index (PSQI) questionnaire. The questionnaire consists of 19 self-rated questions that are grouped into seven subscale components: sleep quality, sleep latency, sleep duration, habitual sleep efficiency, daytime dysfunction, sleep disturbances, and use of sleep medications18. Each component score has equal weight and is measured on a Likert scale of 0 (no difficulty ) to 3 (severe difficulty ). The seven component scores are summed to yield a global PSQI score, with a range of 0–21; higher scores indicate poorer sleep quality.
We used body mass index (BMI) as a marker of obesity and calculated it by dividing body weight in kilograms by height in meters squared (kg/m2). Subsequently, BMI was converted into age- and sex-specific z scores (z-BMI) according to the World Health Organization growth standards for school-aged children and adolescents19.