You’ve likely heard that carrying around excess pounds can make rheumatoid arthritis (RA) worse, but how do you know if you’re too heavy? The usual shortcut to determining whether someone is overweight is to rely on body mass index (BMI), a simple calculation that factors in your weight in relationship to your height. 

BMI is still used by most health care providers, yet many people have questioned its accuracy. 

One major criticism: BMI doesn’t distinguish between muscle and fat (and muscle actually weighs more). That means it’s possible to have a BMI in the overweight range despite having very little fat on your body. It’s also possible to have a normal or overweight BMI despite having too much fat (thanks to a lack of muscle). 

That latter issue may be a serious issue for people with RA, according to a new Chinese study published in the Journal of Cachexia, Sarcopenia, and Muscle. The study found that nearly half of Chinese RA patients had myopenia (decreased muscle mass), and that RA patients with this problem “had worse disease characteristics, including significantly higher indicators of disease activity, higher rate of functional limitation, and [worse] radiographic scores.”

To conduct this research, investigators assessed body composition with a technology called bioelectric impedance analysis. This non-invasive test involves running a mild electrical current through the body and measuring how quickly it flows through different types of tissue. The specific device they used, InBody 230, measures overall body fat percentage as well as the distribution of fat in the body.

After analyzing the body composition of 457 RA patients and 1,860 control patients with the InBody 230, researchers reported that 45 percent of RA patients met the criteria for myopenia. What’s more, they also found that those with myopenia had higher disease activity and more functional limitations. 

“Chronic inflammatory response in RA is a highly energy‐consuming process… which results in loss of muscle mass despite adequate dietary intake,” the authors wrote. “Previous studies showed that decreased muscle mass is associated with worse outcomes and even higher mortality in some circumstances.” 

The scientists also used height and weight to calculate the BMI of each participant, but found that this commonly used score was not associated with disease activity; it only seemed related to morning stiffness and JSN (joint space narrowing) scores. 

“In conclusion, myopenia is very common in Chinese RA patients, which is associated with functional limitation and joint damage,” the authors wrote. “Further research on the underlying mechanism and the effect of skeletal muscle mass improvement in RA management are worth exploring in a larger study.”

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