Adjustment of Skeletal Muscle Mass Estimates for the Extent of Adiposity Strengthens Relationships with Functional Outcomes in Rheumatoid Arthritis

Joshua Baker¹, Jon Giles², Mary Leonard³, David Weber⁴, Jin Long⁵, Erik Jorgenson⁶ and Patricia P. Katz⁷, ¹Medicine/Rheumatology, University of Pennsylvania, Philadelphia, PA, ²Rheumatology, Columbia University Medical Center, NY, NY, ³Stanford University, Palo Alto, CA, ⁴Rochester University, Rochester, NY, ⁵Children's Hospital of Philadelphia, Philadelphia, PA, ⁶University of Pennsylvania, Philadelphia, PA, ⁷Rheumatology, UCSF, SF, CA

Meeting: 2015 ACR/ARHP Annual Meeting

Date of first publication: September 29, 2015

Keywords: adipose tissue, Disability, muscle strength and rheumatoid arthritis (RA)

Session Information

Date: Monday, November 9, 2015

Title: Rheumatoid Arthritis - Clinical Aspects II: Infection, Malignancy and Other Comorbidites in RA

Session Type: ACR Concurrent Abstract Session

Session Time: 2:30PM-4:00PM

Background/Purpose: Skeletal muscle loss in rheumatoid arthritis (RA) has
been described in association with poor physical functioning. Greater adiposity
is simultaneously associated with both greater muscle mass and worse physical
function and therefore is a potential confounder in the assessment of
relationships between muscle and physical function. We aimed to determine
if adjustment of muscle mass estimates for adiposity improves correlations with
physical function in patients with RA.

Methods: Three large independent RA cohorts from academic
institutions in the US were retrospectively analyzed in a cross-sectional
design. Whole-body Dual-Energy Absorptiometry (DXA) measures of appendicular
lean mass index (ALMI, kg/m²) and fat mass index (FMI, kg/m²)
were converted to age, sex, and race-specific Z-Scores using NHANES reference
ranges. Fat-adjusted ALMI Z-Scores (ie. standard
deviations below average for that level of adiposity) were determined using a
novel, comprehensive residual method to adjust for the normal age, sex, and
race-specific associations between ALMI and FMI Z-Scores within NHANES.
Associations between ALMI Z-Scores (standard and fat-adjusted) and physical
functioning were assessed over the range of adiposity adjusting for age, sex,
study, and disease activity measures, and assessing for interaction. Functional
outcomes assessed included the Health Assessment Questionnaire (HAQ), Valued
Life Activities assessment (VLA), and Short Physical Performance Battery
(SPPB). Low lean for age was defined as an ALMI or fat-adjusted ALMI Z-Score of
≤ -1.

Results: A total of 415 patients were studied across the
cohorts. The combined cohort had a mean ALMI Z-Score of -0.54, mean FMI Z-Score
of -0.21, and mean fat-adjusted ALMI Z-Score of -0.53 (all p<0.001)
suggesting significant muscle deficits compared with national reference ranges
before and after adjustment for adiposity. Fat-adjusted ALMI Z-Scores
demonstrated stronger associations with all three functional outcomes across
all cohorts after adjustment for age and sex (Table). Associations were not
attenuated with adjustment for CRP or pain scores. There was an independent
association between FMI Z-Score and physical functioning outcomes with a
stronger association seen among patients with greater FMI Z-Score (p for
interaction <0.05). Fat-adjusted definitions of low lean mass for age more
clearly identified those with functional impairment as measured by the HAQ, VLA
or SPPB.

Conclusion: Fat-adjusted estimates of skeletal muscle mass
deficits demonstrate stronger correlations with physical functioning in RA and
thus represent a valid and potentially useful and important outcome measure.
Fat mass also demonstrates independent associations with physical function.
These observations have far-reaching implications for how we interpret, treat,
and study body composition and its impact on physical functioning in RA.

Table 1: Age and sex-adjusted associations between unadjusted/fat-adjusted measures of appendicular lean mass with physical functioning (HAQ, VLA, SPPB) in combined cohort. Observations were similar across all 3 cohorts.
	HAQ (N=415)		Ln(VLA) (N=291)		Ln(SPPB) (N=356)
	β (95% CI)	p	β (95% CI)	p	β (95% CI)	p
Standard ALMI Z	-0.081 (-0.14, -0.019)	0.01	-0.047 (-0.11, 0.013)	0.12	-0.0021 (-0.043, 0.047)	0.93
Fat-adj ALMI Z	-0.17 (-0.22, -0.12)	<0.001	-0.12 (-0.17, -0.076)	<0.001	0.071 (0.11, 0.036)	<0.001
Abbreviations: HAQ= Health Assessment Questionnaire; VLA= Valued Life Activities; SPPB= Short Physical Performance Battery; ALMI= Appendicular Lean Mass Index; OR= Odds Ratio; CI= Confidence Interval

Disclosure: J. Baker, None; J. Giles, None; M. Leonard, None; D. Weber, None; J. Long, None; E. Jorgenson, None; P. P. Katz, None.

To cite this abstract in AMA style:

Baker J, Giles J, Leonard M, Weber D, Long J, Jorgenson E, Katz PP. Adjustment of Skeletal Muscle Mass Estimates for the Extent of Adiposity Strengthens Relationships with Functional Outcomes in Rheumatoid Arthritis [abstract]. Arthritis Rheumatol. 2015; 67 (suppl 10). https://acrabstracts.org/abstract/adjustment-of-skeletal-muscle-mass-estimates-for-the-extent-of-adiposity-strengthens-relationships-with-functional-outcomes-in-rheumatoid-arthritis/. Accessed .

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