Fat depots within and around the skeletal muscle in rheumatoid arthritis and their role in physical function

Altered body composition towards greater body fat mass and lower lean mass is a well-known manifestation of Rheumatoid Arthritis (RA). However, there is limited information on fat accumulation in and around the skeletal muscles, and whether they contribute to functional limitations and low physical activity levels that persist in this population despite well-controlled disease. The main objectives of this dissertation were to 1) characterize accumulation of skeletal muscle fat (SMF), intermuscular adipose tissue (IMAT) and subcutaneous adipose tissue (SAT) in individuals with RA and determine the associations of SMF, IMAT and SAT with physical function and physical activity measures; and 2) compare SMF, IMAT and SAT between individuals with RA and healthy individuals. This cross-sectional ancillary study used data from previously conducted studies in adults with RA and healthy adults. SMF, IMAT and SAT were measured from computed tomography images of the mid-thigh region. Physical function in the RA cohort was measured using self-report and performance-based tests, and physical activity was assessed using an accelerometer-based activity monitor. Associations between each fat depot, and physical function and activity were assessed by multiple linear regression models. To compare SMF, IMAT and SAT in RA and non-RA subjects, those with RA were matched in sex and BMI to healthy adults of same age and to older healthy adults who were 10-20 years older. The differences between groups were assessed using related samples tests. In subjects with RA, higher SMF significantly contributed to lower physical function and activity (R2Δ range=.08-.25, p<0.05), whereas IMAT and SAT accumulation were not associated with physical function or activity. Individuals with RA had significantly higher SMF (10% difference, p < 0.05) whereas IMAT and SAT accumulation were similar to healthy individuals of same age. There were no differences in SMF, IMAT and SAT between the RA and matched older healthy groups. Study findings demonstrate that SMF accumulation is an important contributor to low physical function and activity, and accumulates more in RA. These results provide preliminary evidence for future longitudinal studies to investigate the impact of SMF on disability and promote health in individuals with RA.