Effects of changes in physique and lifestyle on bone mineral density in the early teens

The purpose of this longitudinal study was to reveal the effects of physique, body composition, and lifestyles on bone mineral density (BMD) in the early teens. Subjects of the current study were 73 boys and 73 girls. Osteo-sono assessment index (OSI), body mass index (BMI), body fat percentage and fat-free mass index (FFMI) were measures. All subjects were measured at two points; 5th grade and 8th grade. The association between changes in these values and the OSI at 8th grade were evaluated with multiple regression analysis. Positive correlations were observed between the OSI and exercise time/FFMI in both sexes. Furthermore, this trend was also observed between the OSI and BMI in boys. The results suggested that an increase in exercise time and FFMI are factors that may enhance BMD in both sexes. However, the increase in BMI was found to increase BMD only in boys. Such gender difference was suggested to be the result of what BMI represents for each group. For boys, increased BMI reflects the increasing amount of muscle mass, whereas for girls, it mainly reflects an increase in body fat. Therefore, muscle mass needs to be increased by long periods of exercise in order to increase BMD, especially in girls.