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.