Assessment of muscle mass relative to fat mass and associations with physical functioning in rheumatoid arthritis

Abstract

Objectives: To determine whether a novel measure of appendicular lean mass relative to fat mass is associated with physical functioning in RA.

Methods: In a cross-sectional design, three independent RA cohorts were retrospectively analysed. Whole-body DXA measures of appendicular lean mass index (ALMI, kg/m 2 ) and fat mass index (FMI, kg/m 2 ) were converted to age, sex and race-specific Z-scores using published National Health and Nutrition Examination Survey reference ranges. Adiposity-adjusted ALMI Z-scores (ALMI FMI ) were determined using a published method to adjust for normal associations between ALMI and FMI Z-scores. Associations between ALMI Z-scores, ALMI FMI Z-scores and physical functioning were assessed after adjusting for age, sex and study. Functional outcomes assessed included the HAQ, Valued Life Activities assessment and Short Physical Performance Battery. Low lean for age was defined as a Z-score of -1 or less.

Results: Our sample consisted of 442 patients with RA. The combined cohort had a mean ALMI Z-score of - 0.51 (1.08) and a mean ALMI FMI Z-score of - 0.58 (1.53), suggesting muscle mass deficits compared with a nationally representative sample. Greater ALMI FMI Z-scores demonstrated stronger associations with better functional outcomes compared with ALMI Z-scores. Associations were not attenuated with adjustment for systemic inflammation or pain. The FMI Z-score was independently associated with physical functioning, with a stronger association seen among patients with greater FMI Z-score. Adiposity-adjusted definitions of low lean mass more clearly identified those with functional impairment.

Conclusion: Estimates of appendicular lean mass that are adjusted for adiposity demonstrate stronger positive associations with functional outcomes compared with unadjusted estimates.

Keywords: disability; fat mass; lean mass; physical function; rheumatoid arthritis.

Fig. 1

HAQ disability scores among individuals classified as low lean mass for age and normal Those who were low by both definitions had high HAQ scores, while those who were normal by both definitions had low HAQ scores. Those who had a low appendicular lean mass index Z-score but normal adiposity-adjusted lean mass index Z-score had similar HAQ scores compared with individuals who were normal by both definitions. Those who had a normal appendicular lean mass index Z-score but a low adiposity-adjusted lean mass Z-score had HAQ scores that were similar to individuals who were low by both definitions.

Fig. 2

Illustration of associations between body composition estimates and disability scores The LOWESS curve illustrates associations between fat mass index Z-scores and adiposity-adjusted lean mass index Z-scores, with physical functioning as measured by HAQ (predicted from regression model in Table 4). The association between fat mass index Z-score and HAQ was greater among individuals with greater fat mass index Z-scores.