Does Loss of Skeletal Muscle Mass Increase the Risk of Knee OA?

Obesity, a state of excess adiposity, is a major risk factor for knee osteoarthritis (OA). Prior studies of obesity and knee OA have mostly defined obesity using anthropometric measures, such as body weight or body mass index (BMI). However, these measurements reflect the composite of fat, muscle and bone mass and are not exclusive measures of adiposity. Thus, it is not clear whether the effects of BMI, typically interpreted as effects of obesity, are truly due to excess adiposity rather than to overall loading due to the combined weight of body mass.

Objective
Misra et al. undertook this study to examine the longitudinal association of body composition categories based on fat and muscle mass with the risk of incident knee OA.

Methods
The researchers included participants from the Multicenter Osteoarthritis Study, a longitudinal cohort of individuals with or at risk of knee OA. Based on body composition (i.e., fat and muscle mass) from whole-body dual X-ray absorptiometry, subjects were categorized as obese nonsarcopenic (obese), sarcopenic obese, sarcopenic nonobese (sarcopenic) or nonsarcopenic nonobese (the referent category). They examined the relationship of baseline body composition categories with the risk of incident radiographic OA at 60 months using binomial regression with robust variance estimation, adjusting for potential confounders.

Results
Among 1,653 subjects without radiographic knee OA at baseline, significantly increased risk of incident radiographic knee OA was found among obese women (relative risk [RR] 2.29 [95% confidence interval {95% CI} 1.64–3.20]), obese men (RR 1.73 [95% CI .08–2.78]) and sarcopenic obese women (RR 2.09 [95% CI 1.17–3.73]), but not among sarcopenic obese men (RR 1.74 [95% CI 0.68–4.46]). Sarcopenia was not associated with risk of knee OA (for women, RR 0.96 [95% CI 0.62–1.49]; for men, RR 0.66 [95% CI 0.34–1.30]).

Limitations
The researchers acknowledge several limitations, including the fact that the sample size of the sarcopenic obesity category was small, limiting the ability to precisely estimate the relationship of sarcopenic obesity with the risk of knee OA in men; the subjects were primarily Caucasians, meaning these findings may not be generalizable to other racial groups; and physical activity levels can affect body composition.

Conclusion
In this large longitudinal cohort, the researchers found body composition-based obesity and sarcopenic obesity, but not sarcopenia, were associated with the risk of knee OA. They suggest that weight loss strategies for knee OA should focus on obesity and sarcopenic obesity.

Excerpted and adapted from:
Misra D, Fielding RA, Felson DT, et al. Risk of knee osteoarthritis with obesity, sarcopenic obesity, and sarcopenia. Arthritis Rheumatol. 2019 Feb;71(2):232–237.