Low muscle density due to the accumulation of intramuscular fat has been observed in patients with rheumatoid arthritis (RA). It has also been associated with higher disease activity and higher interleukin 6 (IL-6) levels than present in healthy controls. Prior research in RA patients has indicated that low muscle density correlates with poor physical function. But these studies have not ascertained if the relationships between muscle density, physical function and strength are independent of other relevant factors in body composition, such as total and visceral adiposity.
New research from Joshua F. Baker, MD, MSCE, of the Philadelphia Veteran’s Affairs Medical Center and University of Pennsylvania, Philadelphia, and colleagues sought to characterize the relationship between muscle density, physical functioning and strength independent of other comprehensive body composition assessments in RA patients. The results were published in Arthritis Care & Research in December.
During the study, 103 RA patients and 428 healthy controls underwent whole-body dual X-ray absorptiometry and peripheral quantitative computed tomography. These assessments quantified patients’ appendicular lean mass index (ALMI), fat mass index (FMI), visceral fat area and muscle density. Measurements were also taken for hand grip strength, muscle strength at the knee and lower leg, disability and physical function.
“Patients with RA had substantially lower muscle density compared with the reference group as evidenced by low (negative) muscle density Z scores, consistent with a greater intramuscular fat area,” write the authors. “Patients with RA also had excessive visceral adiposity.”
Among RA patients, low muscle density was associated with higher disease activity, C-reactive protein and interleukin 6 levels. It was also associated with greater total and visceral fat, lower ALMI Z scores, physical inactivity and long-term use of glucocorticoids. Greater muscle density was associated with less disability as measured by the Health Assessment Questionnaire (HAQ).
Additionally, RA patients with low ALMI Z scores had lower muscle density scores than healthy controls who had similarly low ALMI scores. After adjusting for ALMI and FMI Z scores, low muscle density was independently associated with lower muscle strength, higher HAQ scores and lower Short Physical Performance Battery scores.
“These results provide further support for the hypothesis that fat infiltration of muscle contributes to muscle dysfunction in patients with RA, over and above the effects of low muscle mass and excess total and visceral adiposity,” write the authors in their discussion. “Clinical interventions or therapies that result in improvements in muscle density might, therefore, reasonably be expected to result in improvements in physical function.”