High Levels of Body Fat Mass Related to Lower Bone Density in Men

Bone density is negatively associated with fat mass and positively associated with lean mass, according to study findings published in the Journal of Clinical Endocrinology & Metabolism.

Previous research has found an inconsistent relationship between the effect of fat mass on bone density, ranging from a positive to a negative association. Small numbers, referral bias, such as recruiting patients with osteoporosis, lack of racial/ethnic diversity, and/or the use of estimated rather than measured body composition may have limited past studies, the researchers explained. Furthermore, prior work may have been limited due to the fact body composition is not a routine clinical measure.

To evaluate the association of body composition with total body bone mineral density (TB BMD), the researchers used data from NHANES 2011-2018, which included participants with various body mass index (BMI) levels and who had TB dual-energy x-ray absorptiometry (DXA).

Lean mass index (LMI) and fat mass index (FMI) were used to assess the effects of body composition on TB BMD. Linear regression models were developed with BMD as the outcome and controlling for age, sex, race/ethnicity, height, and smoking status.

A total of 10,814 participants aged 20 to 59 years were included and categorized into 3 groups based on BMI—BMI <30 kg/m² (n = 6795; mean age, 38.7 ± 9.8 years; 51.4% male); BMI 30-39.9 kg/m² (n = 3248; mean age, 40.8 ± 9.3 years; 51.5% male); and BMI ≥40 kg/m² (n = 771; mean age, 40.4 ± 9.7 years; 33.6% male).

Univariate analyses found that TB BMD was significantly associated with BMI and LMI (0.025 higher T-score and 0.14 higher T-score per 1 kg/m² increase, respectively, P <.001 for both). TB BMD had a slight negative association with FMI (0.025 lower T-score per 1 kg/m² increase, P <.001), although this association was weak and became slightly positive after adjustment for age and sex.

The associations for LMI and FMI were substantially affected after adjustment for one another. Each 1 kg/m² of LMI was associated with a 0.19 higher T-score, and each 1 kg/m² of FMI was associated with a 0.09 lower T-score (P <.001 for both) when LMI and FMI were included in the same model.

The associations of LMI and FMI with TB BMD remained in multivariable modeling that included age, sex, race/ethnicity, height, smoking status, LMI, and FMI. Every additional 1 kg/m² LMI was associated with a 0.19 higher T-score, and every additional 1 kg/m² FMI was associated with a 0.10 lower T-score (P < .001 for both). The magnitude of association for LMI was substantially more positive and the association for FMI substantially more negative after adjustment for one another.

The negative effect of FMI was stronger in men compared with women. Multivariable modeling showed that every additional 1 kg/m² of FMI was associated with an average of 0.08 lower BMD T-score in women and 0.13 lower BMD T-score in men (P for interaction <.001).

Based on BMI strata, the effect of LMI on TB BMD was attenuated with increasing strata (0.24 T-score vs 0.17 T-score vs 0.11 T-score per 1 kg/m² for BMI <30, BMI 30 to 39.9, and BMI ≥40 kg/m², respectively, P for interaction <.001 for higher strata vs lowest strata). The effect of FMI was not significantly different based on BMI strata when accounting for the change in LMI with BMI.

The researchers noted that studies have shown that BMD at high levels of fat may be affected by soft tissue artifacts and could be unreliable. Also, TB DXA regions, rather than regional scans, were used owing to availability, and the participants younger than 60 years of age.

“While lean mass had an overall stronger effect than fat, the negative effects of fat on BMD were particularly pronounced in men and in those with the highest levels of fat,” the researchers stated. “Our results emphasize the importance of bone health as obesity trends worsen in the United States and may explain higher rates of fracture at certain sites in at least some studies of obese subjects.”

Disclosure: Some of the study authors declared affiliations with biotech, pharmaceutical, and/or device companies. Please see the original reference for a full list of authors’ disclosures

Reference

Jain RK, Vokes T. Fat mass has negative effects on bone, especially in men: a cross-sectional analysis of NHANES 2011-2018. J Clin Endocrinol Metab. Published online February 10, 2022. doi: 10.1210/clinem/dgac040