Exercise time positively correlates with physical fitness, regardless of overweight, obesity and underweight among primary and secondary students: a Japanese nationwide cross-sectional study

More research is needed to understand how exercise time and physical fitness relate to body size. In particular, there are no studies that have examined the relationship between physical activity and physical fitness, taking into account both overweight/obesity and thinness, using national samples. This study aimed to analyze exercise time by body size and physical fitness using a Japanese national sample of Japanese children and adolescents. The survey included 19,471 primary and 10,596 secondary schools in Japan. After excluding participants with missing data, the analysis included 878,348 Year 5 primary school students and 757,358 Year 8 secondary school students. Body mass index (BMI) was calculated based on participants’ height and weight, and they were subsequently categorized into four BMI groups: underweight, normal weight, overweight, and obese. Physical fitness was classified as high or low based on the total score from eight physical fitness tests. Exercise time was assessed via a questionnaire and compared between the high- and low-fitness groups within each BMI category. Among the boys and girls in primary and secondary schools, exercise time was greater in the high-fitness group compared to the low-fitness group across all four BMI categories (underweight, normal weight, overweight and obesity). In conclusion, this study suggested that exercise time is positively associated with higher physical fitness regardless of whether an individual is obese, overweight, normal weight or underweight. These results may indicate the importance of promoting exercise time among all children, and further longitudinal studies are needed.