Background: Obesity has a complex relationship with bone mineral density (BMD) and fracture rate.1 The respective effects of lean mass (LM) and fat mass (FM) on BMD is debated. LM has a positive correlation with BMD,2 whereas FM and FM percentage (FM%) have shown negative associations with BMD,3 although studies show positive relationships in women.4 Limited studies explore how LM and FM interact with BMD in complex obesity.
Methods: Cross-sectional study of patients with complex, severe obesity referred to the Blacktown Metabolic and Weight Loss Program who underwent dual-energy x-ray absorptiometry scans. Population subgroups were based on gender and menopausal status. Spearman correlations compared BMD at the lumbar spine (L1-L4), femoral neck (FN) and total hip (TH) with anthropomorphic variables and body composition.
Results: The study consisted of 121 patients (40 men, 81 women), with mean age 49.8±12.4 and BMI 43.4±6.9 kg/m2. The osteoporosis rate in men ³50 years and post-menopausal women (n=57) was 3.5%. Increasing age expectedly correlated with worse BMD at L1-L4 (p=0.039), FN (p<0.001) and TH (p<0.001). Weight significantly positively correlated with FN (p=0.004) and TH (p=0.016) BMD, but the relationship was non-significant when corrected for age. BMI did not correlate with BMD. In men, LM positively correlated with FN (p=0.003) and TH (p=0.017) BMD, whereas mixed relationships were demonstrated in pre- and post-menopausal women. In the whole cohort, FM was not correlated with BMD, whereas FM% negatively correlated with FN BMD (p=0.042). In post-menopausal women, FM% had a significant positive relationship with L1-L4 BMD (p=0.009).
Conclusion: In patients with complex, severe obesity, LM is potentially a better predictor of osteoporosis risk compared to BMI or FM, particularly in men. However, evidence remains mixed in women. Further longitudinal research with larger cohorts is required to explore the complex effects of severe obesity and body composition on BMD.