Body mass index, percent body fat, and bone mass in a cohort of Chinese twins aged 6 to 18 years

Introduction: This report examines the relationship of body mass index (BMI), percent body fat (%BF), and bone mass in a cohort of male and female twins recruited from Anhui province, China, ages 6-18 years-577 male pairs (mean age=11.4) and 478 female pairs (mean age=11.6). Methods: Whole body bone mineral content (WBMC) in (g), whole body bone area (WBA) in (cm²), and %BF were measured using DEXA (Lunar Prodigy, USA). Regression analysis of within-pair differences was used to assess the strength of the association, and the analysis was stratified by gender and age group, where age cut-offs were based on ages at spermarche or menarche estimated from large population based studies in China. Males were stratified at ages before 14 and age 14-18, and females at ages prior to 12 and age 12-18. Results: Univariately, BMI and %BF were associated with WBMC and WBA in the younger males and females, and in older males; %BF was significant only in older females. Multivariate models included both BMI and %BF. Among the younger males, age <14, BMI and %BF were significantly associated with WBMC and WBA. In the younger females, age<12, %BF was only significant to WBA. In the older age group, only BMI was significant to WBMC and WBA in females, but in males, BMI was positively associated, and %BF was negatively associated with both bone measures. Discussion: These findings show that association between BMI and %BF and bone mass differ across gender and developmental stages, and %BF appears to be beneficial at younger ages, but detrimental or non-beneficial at older ages of development.