Measurement of Mediators
Spirometry was performed as per a previously standardized protocol.16 In our previous study, the ratio of forced expiratory volume in 1 second (FEV1) to forced vital capacity (FVC) showed the strongest association with body mass index (BMI).17 Hence, FEV1/FVC was used as a surrogate for pulmonary function performance.
FeNO, a marker of airway inflammation, was measured using a portable NO analyzer NIOX MINO® Airway Inflammation Monitor (Aerocrine AB, Solna, Sweden) according to standardized protocols.18
Physical fitness was assessed based on a combination of all physical fitness tests (800 m sprint, jump and reach, sit-ups, and sit-and-reach) administered during school visits using a standardized protocol.19 Physical fitness parameters included cardiorespiratory endurance, muscular strength, endurance, and flexibility. We added them up to formulate a physical fitness index and transformed them into z-scores using age- sex- specific reference. A higher physical fitness index indicates better physical fitness.
Sleep behaviors were assessed using a parent-administrated questionnaire—the Children’s Sleep Habit Questionnaire (CSHQ). The CSHQ was designed and developed to assess the sleep behaviors of school-aged children.20 It was proven to be a valid tool for detecting the presence of SDB by comparing it with objective polysomnography-defined obstructive sleep apnea.21
A validated Chinese version of the Tanner-derived composite stage (TDCS)22 was used to evaluate the Tanner stage. Children with early puberty were defined as those reaching a certain pubertal stage earlier than the median age for that stage23 based on large-scale population-based Chinese studies.24,25