In this cross-sectional study, we explored the association between LBM and osteoarthritis. In regression models adjusted for all covariates, we observed a non-linear relationship between LBM and osteoarthritis, with an inflection point of 52.26 kg. Before the inflection point, LBM was negatively associated with osteoarthritis, whereas after the inflection point, the association was not statistically significant. In addition, stratified analysis by gender and age revealed that this non-linear relationship was also present among women and those over 60 years of age.
Although the relationship between LBM and osteoarthritis has been explored, their results are not consistent and their small sample sizes have lacked further analysis of stratification. A meta-analysis of osteoarthritis and body composition suggested that LBM was positively associated with osteoarthritis[20]. Contradictory to this, a Swedish randomized clinical trial showed that patients with osteoarthritis of the knee phenotypically exhibited a low proportion of whole-body LBM[21]. Whereas, a study of middle-aged and older Korean adults showed that lower extremity LBM was negatively correlated with radiographic osteoarthritis of the knee[22]. In contrast, our study found a non-linear relationship and a negative correlation before the inflection point.
Although the exact mechanisms between LBM and osteoarthritis are unknown, there are some studies that appear to provide a possible explanation for the correlation between LBM and osteoarthritis. One study suggests that this relationship between LBM and osteoarthritis may be due to the effect of muscle on oxidative stress in articular cartilage[23]. LBM gain enhances the body's antioxidant defense mechanisms, which in turn reduces damage to articular cartilage from oxidative stress[24]. Studies have shown that oxidative stress is one of the main causes of cartilage matrix degradation and chondrocyte apoptosis, whereas the secretion of antioxidant enzymes such as superoxide dismutase (SOD) and glutathione peroxidase (GPx) in muscle tissues is increased during exercise and these enzymes are effective in scavenging reactive oxygen species (ROS) in the body, protecting articular cartilage from oxidative stress damage[25, 26]. Another study showed that LBM increase may affect chondrocyte metabolism and promote cartilage matrix synthesis[27]. Increased levels of insulin-like growth factor-1 (IGF-1) secreted by muscle during LBM gain promote chondrocyte proliferation and matrix synthesis and inhibit cartilage matrix degradation[28, 29]. At the same time, LBM gain may imply an increase in muscle mass, and evidence suggests that enhanced muscle strength improves the distribution of mechanical loads on the joints, improves joint stability, and reduces wear and degradation of the articular cartilage[30]. In the joints of the lower extremity such as in the knee, muscles can help absorb impact forces and reduce cartilage damage. In summary, the relationship between lean body mass and osteoarthritis still has many unknown underlying mechanisms that need more research to further elucidate.
There were several advantages of the present study over previous studies. First, this study included 31,172 individuals, which was the largest sample size to date. Second, NHANES included a representative population of non-institutionalized American civilians, so our findings would be broadly applicable to the entire American population. Third, NHANES has comprehensive health data collection, enabling researchers to conduct more sophisticated analyses of outcomes and associations among various factors. There are also some limitations to this study. First, some self-reported data may be subject to recall bias. Second, NHANES is a cross-sectional survey, and the data were collected at a specific point in time, limiting the ability to make causal inferences. Moreover, the dynamics of population health and nutritional status over time may not be adequately captured by cross-sectional data.
In conclusion, we found a non-linear association between LBM and osteoarthritis in this cross-sectional study of the general U.S. adults. Differences in age and gender were further explored. Our study might provide information for public health policy makers and clinicians to intervene in osteoarthritis by controlling body measurements. Further randomized controlled trials are needed to verify this finding.