Fat and lean mass association with osteoporosis and bone mineral density: the ELSA-Brasil cohort

The relationship between body composition, especially fat mass (FM), and bone mineral density (BMD) is still a matter of debate.

The aim of this study was to test whether FM and lean mass (LM) are predictors of osteoporosis and if the association between body composition and BMD varies by sex, age, and body mass index (BMI) status.

We analyzed 1316 participants from the ELSA-Brasil Cohort who underwent dual-energy X-ray absorptiometry. Osteoporosis prevalence was stratified by BMI category and sex. Multivariable regression analyses were performed with lean and FM indexes as predictors of osteoporosis and BMD at different sites according to sex, age, and BMI category.

Osteoporosis was prevalent in 18.6% of postmenopausal females and 8% of males aged ≥50 years. In the fully adjusted model, LM was inversely associated with osteoporosis in the overall male population and in females with BMI ≥30 kg/m² and positively associated with BMD in males and in postmenopausal females with BMI ≥25 kg/m². FM was positively associated with BMD in postmenopausal females with BMI < 25 kg/m².

In postmenopausal women with a BMI < 25 kg/m², BMD is positively associated with FM, whereas in postmenopausal females with a BMI ≥25 kg/m², BMD is positively associated with LM. These findings support the theory that FM is an important predictor of BMD-possibly due to its effect on estradiol levels through aromatization-up until a certain "fat threshold", after which other factors, including LM and its effect on mechanical loading, likely become more dominant factors affecting BMD.