Background

Sarcopenia is defined by low function and low muscle quantity. Functional measures are not muscle-specific or sensitive to aetiology. Further, how to adjust muscle quantity for body size is debated and sarcopenic obesity is not well-described. Muscle-specific biomarkers strongly linked to functional outcomes are needed to properly address sarcopenia.

Methods

Current thresholds were used to investigate sarcopenia prevalence in different BMI-classes in 9615 subjects. Fat-free muscle volume (FFMV) and muscle fat infiltration (MFI) was quantified using MRI. For each subject, a sex-BMI matched virtual control group (VCG) was created and the FFMVi (FFMV/height2) z-score extracted from each VCG-distribution (FFMVi[VCG]). The value of combining FFMViVCG and MFI was investigated through hospital nights, hand grip strength, stair climbing, walking pace, and falls.

Results

Current definition showed decreased sarcopenia prevalence with increased BMI (normal weight 4.3%/overweight 1.1%/obese 0.1%) and previously suggested adjustments (e.g. division by height2) did not normalize this BMI-association. In contrast, the prevalence of low functional performance increased with increased BMI. The association between FFMVi and BMI (R2=0.66) was however normalized by FFMVi[VCG] (R2=0.01). Hospital nights, low hand grip strength, slow walking pace, no stair climbing were positively associated with MFI (p<0.05) and negatively associated with FFMVi[VCG] (p<0.01). Falls were positively associated with MFI (p<0.01), non-significant for FFMVi[VCG]. FFMVi[VCG] and MFI combined had highest diagnostic performance for detecting low function.

Conclusions

Today’s sarcopenia definitions are BMI-dependent, limiting detection in overweight/obesity where functional performance was lower. VCG-adjusted muscle volume (FFMVi[VCG]) enabled BMI-invariant detection, enabling proper assessment across BMI-classes. FFMV and MFI provides a more complete, muscle-specific description linked to function, enabling objective sarcopenia detection.