Risk of obesity enhanced by poor physical activity in high school students.

Background: The aim of this study was to elucidate the interactions between a family history of obesity and poor physical activity.

Methods: A case-control study was performed based on medical check data for all first year high school students admitted to Aichi Prefectural public high schools in the 2004 academic year. Prior to the post-admission medical check up, all students received an interview sheet containing questions on family history of obesity and extent of physical activity. Experienced nurses measured body mass and height. Valid answers and measurements were obtained from 20,155 boys and 19,682 girls.

Results: A family history of obesity, reluctance to exercise, current and recent past sedentary lifestyles each significantly increased risk of obesity (body mass index [BMI] >or= 25) and severe obesity (BMI >or= 30) in both boys and girls. Two-way ANOVA revealed that a family history of obesity, associated with poor exercise, positively affected BMI in both boys and girls. Moreover, significant positive interaction was found between a family history of obesity and each of the unfavorable physical activity conditions. Synergistic effects (synergy indexes >1) increasing risk of obesity were noted in both boys and girls when a family history of obesity overlapped with reluctance to exercise, current physical inactivity, or recent physical inactivity.

Conclusion: Overlapping of family history of obesity and exercise risk factors synergistically increase the BMI and risk of obesity in both boys and girls. Intervention to promote exercise in adolescents who have obese family member(s) should be encouraged.