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Muscle fat infiltration but not muscle cross-sectional area is independently associated with bone mineral density at the lumbar spine.

OBJECTIVES
Although sarcopenia and osteoporosis are inter related conditions that are common with advancing age, few studies have explored relationships between muscle quality and bone mineral density (BMD). We investigated age and sex-specific paraspinal muscle fat infiltration (MFI), muscle cross-sectional area (CSA), and spine volumetric BMD (vBMD) in healthy Chinese adults.


METHODS
605 healthy adults aged 20-59y (340 women, mean age 39.2y; 265 men, mean age 38.8y) had axial T2WI MRI imaging of the lumbar spine and CSA (cm2) and MFI (%) were measured in the psoas and multifidus and erector spinae (MF-ES) muscles (L3-L4). MFI measurements were calibrated against a region of interest in an adjacent area of subcutaneous pure fat. L2-L4 vBMD was measured by quantitative computed tomography. Age and sex-specific subgroups were compared using the Mann-Whitney test. Multiple regression was used to test independent associations of MFI and CSA with vBMD.


RESULTS
Females had lower CSA and higher MFI than males in both the psoas and MF-ES muscles (p < 0.001). In females and males, MF-ES MFI increased with age (p < 0.001) and in females age-related increases were observed for the psoas muscles (p < 0.05). Greater fat infiltration of the MS-ES muscle unit was associated with lower vBMD in both sexes (p < 0.001) but not with CSA. Following adjustment for demographic variables and CSA, MS-ES MFI remained predictive of vBMD (β = -0.408 to -0.157, p < 0.001).


CONCLUSIONS
We have demonstrated that, independent of CSA and demographic variables, MFI of the MF-ES muscles is predictive of lower lumbar spine vBMD in both sexes.


ADVANCES IN KNOWLEDGE
This is the first study to demonstrate that, independent of muscle size and demographic variables, MFI of the paraspinal MF-ES muscles is predictive of lower lumbar spine vBMD in both sexes.