Linear and non-linear relationships between body fat mass distribution and bone mineral density in adults: The NHANES, 2011–2018

The relationship between body fat mass and bone mineral density (BMD) remains controversial. This research aimed to explore the linear or non-linear relationship between body fat mass and BMD among adults in the United States. This cross-sectional study identified adults aged 18 years or older in the National Health and Nutrition Examination Survey from 2011 to 2018. After adjusting for covariates, linear relationships between body fat mass and BMD in different genders were tested by generalized linear models, and potential non-linear relationships were explored by generalized additive models and piecewise linear regression models. The research included 4691 (57.9% of the total sample) males and 3417 (42.1% the of total sample) females. In both males and females, we found a negative association between android or total body fat mass and lumbar spine BMD and a positive association between appendicular, android, gynoid, or total body fat mass and whole body BMD (all P < 0.05). The relationships between body fat mass in all regions and lumbar spine BMD were U-shaped in males and inverted U-shaped in females (all P non-linear < 0.05). Inverted U-shaped relationships existed between body fat mass in all regions and whole body BMD in females (all P non-linear < 0.05). Body fat mass was negatively and linearly associated with lumbar spine BMD, but positively associated with whole body BMD. Body fat mass had a U-shaped relationship with lumbar spine BMD in males and an inverted U-shaped association with lumbar spine and whole body BMD in females.