Background/Purpose:

Patients with SLE are at risk for both skeletal muscle loss and increased adiposity, which may predispose to worse health-related quality of life. Decreased skeletal muscle mass, particularly after adjustment for fat mass (FM), has been associated with disability in RA, but this relationship is not well understood in SLE. We aimed to determine the range of body composition in women with SLE relative to a national reference population without SLE and the associations of fat-adjusted and unadjusted appendicular lean mass (ALM) with self-reported disability.

Methods: Cross sectional dual energy x-ray absorptiometry (DXA) data from 2 prospective cohorts of women with validated SLE were analyzed. Age, sex, and race/ethnicity-specific ALM index (ALMI, kg/m²) and FM index (FMI, kg/m²) Z-scores, as well as fat-adjusted ALMI Z-scores, reflecting the number of standard deviations from average for a given level of adiposity, were generated using National Health and Nutrition Examination Survey reference ranges. Associations between fat-adjusted and unadjusted ALMI Z-Scores, as well as FMI Z-scores, and natural log of Valued Life Activities disability were determined using univariate and multivariate linear regression models and adjusted for age, race/ethnicity, natural log of high-sensitivity CRP, and study center. A kappa statistic was calculated to assess concordance between low and normal fat-adjusted and unadjusted ALMI scores, with low lean mass for age defined as fat-adjusted or unadjusted ALMI Z-Score ≤-1 (< 16th percentile for reference data).

Results: In the combined cohort of 237 women with SLE, mean unadjusted ALMI, FMI, and fat-adjusted ALMI Z-scores were -0.34, -0.34, and -0.18, respectively, suggesting modest deficits compared to national reference data (Table 1). Although there were significant associations between ALMI Z-score and disability (p=0.05) and separately FMI Z-score and disability (p< 0.001), fat-adjusted ALMI Z-score was not significantly associated with disability (p=0.07) (Table 2). The association of ALMI Z-score with disability was attenuated after adjustment for multiple covariates (p=0.37) while the association of FMI Z-score with disability remained significant (p=0.02). The correspondence between fat-adjusted and unadjusted ALMI definitions of low lean mass for age was fair (κ 0.28, p< 0.001) (Table 3). 

Conclusion: Women with SLE had muscle deficits relative to a national reference population without SLE before and after adjustment for adiposity. However, these muscle deficits, unlike fat mass, were not independently associated with self-reported disability. Further study is needed to determine the association of body composition with additional measures of health-related quality of life.

Table 1

Table 2

Table 3

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/application-of-a-novel-method-for-determination-of-muscle-mass-relative-to-fat-mass-in-women-with-systemic-lupus-erythematosus-and-association-with-disability/