Association between body composition and bone mineral density in young and middle-aged adults

In this cross-sectional study, we aimed to explore the correlation between body composition and bone mineral density (BMD) in young and middle-aged people with different gender and different BMI groups. We selected 1,295 young and middle-aged people (20 to 50 years old) who underwent physical examination at the Health Management Center of the International Medical Service (Xidan Branch) of Peking Union Medical College Hospital from January 2020 to June 2024 as the study subjects, and demographic characteristics, laboratory examinations, body composition(assessed by bioelectrical impedance analysis, including measurements of skeletal muscle mass, skeletal muscle index, body fat mass, percent body fat, fat-free mass, and visceral Fat Area), and dual-energy X-ray bone absorptiometry (DXA) results were collected. Based on gender and body mass index (BMI), the study participants were categorized into three groups: normal weight (18.5 kg/m² ≤ BMI < 24 kg/m²), overweight (24 kg/m² ≤ BMI < 28 kg/m²), and obese (BMI ≥ 28 kg/m²). One-way ANOVA was employed to assess differences among the three groups. The relationship between BMI and BMD was analyzed using multiple regression models along with natural cubic spline models. Additionally, multiple regression models were used to examine the association between body composition and BMD. There were 774 males (216 in the normal body composition group, 380 in the overweight group, and 178 in the obese group) and 521 females (386 in the normal body composition group, 99 in the overweight group, and 36 in the obese group), and there was a significant difference in the laboratory results, body composition, and BMD among the three groups of males and females (all P < 0.05). Multiple regression analysis, after adjusting for age, smoking and alcohol consumption history, hypertension, and diabetes mellitus, revealed a significant positive correlation between BMI and BMD at all skeletal sites (all P < 0.05). The obese group exhibited a more pronounced increase in BMD compared to the overweight group (lumbar spine BMD: β = 0.04, 95% CI 0.02-0.07; femoral neck BMD: β = 0.09, 95% CI 0.07-0.11; total hip BMD: β = 0.11, 95% CI 0.09-0.13; all P < 0.05). Further analysis of the nonlinear relationship between BMI and BMD demonstrated that in women, BMD at all sites increased with higher BMI. In contrast, men exhibited an inverted U-shaped nonlinear association, with a decline in BMD observed in the high BMI range (BMI ≥ 30 kg/m²). The correlation between body composition and BMD, skeletal muscle mass (SMM), skeletal muscle index (SMI), and fat free mass (FFM) were significantly positively correlated with BMD at all sites in both males and females (P < 0.05), and percent body fat (PBF) was negatively correlated with BMD at all sites in the obese group of males, the overweight groups of males and females (P < 0.05). Among young and middle-aged individuals, men show a decline in BMD when BMI reaches 30 or above, whereas women continue to experience a gradual increase in BMD with rising BMI. Additionally, elevated PBF is negatively correlated with BMD. This suggests that maintaining an appropriate BMI and PBF may be beneficial for bone health in this age group.