Study design and participants
This longitudinal study was conducted in a private primary and secondary school located in adjacent facilities in Tokyo, Japan. Data were collected annually (in February or March) from 2015 to 2020. The two study groups were: 1) the group initially surveyed in 2015 and followed longitudinally each year between February and March for 5 years (n = 32) and 2) the group initially surveyed in 2016 and followed longitudinally each year between February and March for 5 years (n = 31). This study started with baseline data collection of fourth grade children (9-10 years old) in 2015 (group 1, not= 32) and 2016 (group 2, not= 31). The children attended the same class, chosen at random by the school administration. We administered annual surveys to these children for five consecutive years until they reached eighth grade (secondary school; ages 13-14). A total of 63 healthy children (56% girls) provided baseline data in 2015 and 2016 and were followed over the five-year study period.
Prior to the start of the study and at each follow-up survey and data collection point, parents received comprehensive information about the study’s purpose and methods through their children’s teachers. Written parental consent was returned to teachers after participants’ parents had had an opportunity to consider their child’s participation. All procedures involving human participants were performed in accordance with institutional and/or national research committee ethical standards and the 1964 Declaration of Helsinki and subsequent amendments or comparable ethical standards. The study protocol was approved by the Tohoku Gakuin University Graduate School of Human Informatics Study Ethics Committee (reference number 2017R001). Only children with written informed parental consent for participation were included.
Measurements
Demography and anthropometry
The age, grade, sex, weight and height of each participating child were obtained using a self-administered questionnaire. Weight status, defined as percentage overweight (POW), was assessed using Japanese cut-off values based on national benchmark data for Japanese children33. Briefly, POW is commonly used to assess childhood obesity in medical institutions and schools in Japan and is calculated as the ratio of weight to standard weight based on gender, age, and height. The POW criteria for obesity and underweight are ≥20% and ≤-20%, respectively, with the normal range being -20% to 20%. These criteria for obesity and underweight have been shown to correspond to age-adjusted BMI percentiles of >86-89% and <2-6%, respectively33.
AP measurements
Participants’ usual PA was assessed using a triaxial accelerometer (HJA-750C Active style Pro, Omron Healthcare, Kyoto, Japan) measuring 40 × 52 × 12 mm and weighing 23 g including batteries. This triaxial accelerometer collects information on time spent in ambulatory and non-ambulatory activities of varying intensities. The accuracy of this device has been reported in previous studies34.35. Based on default predictive equations established for adults and results of a previous study in children36, the following conversion equations were used. Ambulatory activities were calculated as 0.6237 × MET value + 0.2411, and non-ambulatory activities were calculated as 0.6145 × MET value + 0.5573. Time spent on activities requiring ≥3 METs, 1.6–2.9 METs, and ≤1.5 METs was considered MVPA, LPA, and SB, respectively. We used the macro program (ver. 190829) developed and distributed by the Japan Physical Activity Research Platform (http://paplatform.umin.jp) to process the accelerometer data.
Accelerometry methods, including epoch duration, non-wearing time, and valid wearing minutes and days, were defined based on previous studies32.37. Participants were required to wear the waist accelerometer for ≥7 consecutive days during all waking hours except while showering, bathing, or swimming. The accelerometers were set to record data for 10 s sampling intervals (epochs) throughout the wearing period. Wear-free time in a day was defined as any period with > 10 min of consecutive zeros. Valid accelerometry data including >600 min/day for at least four days, including one non-school day, was analyzed at each data collection point.
statistical analyzes
We used LMM methods with REML to examine whether daily MVPA, LPA, and SB times changed during the five-year transition from primary to secondary school. Based on previous evidence, the model included school grade (time-coded: 0, 1, 2, 3, and 4) and grade2 (timecode: 0, 1, 4, 9, and 16) in each wave of annual time, gender, POW, and daily time spent wearing the accelerometer as independent covariates in the fixed part of the model19.
We considered two LMM models, LMM I and II, which were run without and with adjustments for gender fixed effects and the gender-grade interaction, respectively. These models allowed for random effect within the group where the initial assessment time differed. We also fit the models with the random effect of time-coded school grade in each participant. A quadratic trend was used to estimate the periods during which MVPA, LPA and SB changed. All valid data at each collection point were included in the analyzes because LMMs are robust to missing data and can estimate longitudinal trends with incomplete datasets38. LMMs were used because they are robust to missing data and can estimate longitudinal trends with incomplete datasets. This method allowed us to include all valid data collected at any time in the analyses. A total of 52, 54, 52, 53, and 53 participants were analyzed in evaluations 1, 2, 3, 4, and 5, respectively, of this five-year longitudinal study (Table 2).
All statistical analyzes were performed using Stata for Windows version 15.1 (Stata, College Station, TX, USA). All significance tests were two-sided and results were considered statistically significant at P<0.05.
Ethics approval
All procedures performed in studies involving human participants complied with institutional and/or national research committee ethical standards and the 1964 Declaration of Helsinki and subsequent amendments or comparable ethical standards. The study design was approved by the Tohoku Gakuin University Graduate School of Human Informatics Study Ethics Committee (reference number 2017R001).
Informed consent
Informed consent was obtained from the parents of all individual participants included in the study.
