In this nationwide prospective cohort study, we explored the associations between sarcopenia, obesity, and depression in the middle-aged and elderly Chinese population based on CHARLS. Our results reconfirmed that sarcopenia was associated with increased risk of depression, and that low BMI and WC was associated with increased risk of depression. And in the male population, the association was partially mediated through obesity biomarkers of BMI and WC.
Previous studies confirmed an association between sarcopenia and mental health, but results remained controversial. Hsu et al. reported an independent positive relationship between sarcopenia and depression(Hsu et al., 2014). However, the study by Bertoni et al. pointed no significant association between muscle weakness at baseline and incidence of depression at follow-up(Bertoni et al., 2018). Sarcopenia combined indicators of grip strength, muscle mass and physical performance. There was growing evidence that grip strength and muscle mass were associated with mental health conditions(Kim et al., 2011; Marques et al., 2020). A UK study found that grip strength was negatively correlated with depression and anxiety, and a significant interaction between grip strength and walking speed was found(Cabanas-Sánchez et al., 2022). Skeletal muscle mass was also inversely associated with the risk of depression in the elderly male population(Kim et al., 2011). Although most studies pointed to the possible adverse effects of sarcopenia on mental function through metabolic and endocrine mechanisms, a reverse pathogenic pathway was possible(Li et al., 2022c). Depression might contribute to sarcopenia by affecting social mobility. Therefore, we demonstrated the causal association between sarcopenia and the onset of depression in a prospective cohort study.
Our finding confirmed that sarcopenia was a risk factor for the onset of depression in a middle-aged and elderly Chinese population. Not only expanding the existing evidence, but our results further suggest that this association exists only in the male population. And a previous cross-sectional study in South Korea had similar results that depression was associated with sarcopenia and low body mass, especially in the male population(Kim et al., 2011). One possible reason for the gender difference is the effect of sex steroids (eg, estrogen, testosterone), which played an important role in the growth and maintenance of muscle mass and strength, and the metabolic function of skeletal muscle(McClung et al., 2006). Research pointed to the sex difference in muscle homeostasis. Low testosterone concentrations lead to a reduction in muscle mass and strength in men(Bhasin et al., 1996). And low testosterone levels were associated with incident depression(Ford et al., 2016). Furthermore, men were more likely than women to notice the loss of muscle mass and associated weakness, which might contribute to the development of depression(Ida et al., 2018).
Previous studies have analyzed sarcopenia and obesity in conjunction with depression. For example, a cross-sectional study in Japan found that participants with sarcopenic obesity had a significantly increased risk of depression and that sarcopenia and obesity had a synergistic effect(Ishii et al., 2016). The results of a study in China showed that the association between sarcopenia and depression varied among different BMI subgroups(Gao et al., 2021). However, these studies restricted the population to individuals with different obesity and did not investigate the contribution of obesity to the association between sarcopenia and depression. But some studies have pointed out the relationship between sarcopenia and depression, sarcopenia and obesity, obesity and depression(Ida et al., 2018; Kim et al., 2011; Lee et al., 2007). From this, it can be suggested that abdominal obesity and general obesity play an important role in the pathway between the two. Sarcopenia might adversely affect mental function through metabolic and endocrine mechanisms. Our study was the first to evaluate the mediating role of obesity between sarcopenia and depression. We found that BMI and WC significantly mediated the association between sarcopenia and the onset of depression, especially for BMI. Underweight might increase the risk of depression in population with sarcopenia.
At the population level, BMI and WC were reliable measures that captured the biological roles of adipose tissue, including inflammatory processes and the effects of leptin(Li et al., 2022a), which explained the mediating role of BMI and WC in our study. Previous studies showed that the severity of sarcopenia decreased linearly with BMI(Lee et al., 2007), which was consistent with our research results. This was also consistent with typical hallmarks of malnutrition in older adults, with low BMI, low skeletal muscle mass, or muscle strength being one of the criteria for malnutrition phenotypes recommended by the Global Leadership Initiative on the Malnutrition working group(Cederholm et al., 2019). Our study also pointed to an inverse association between obesity and depression. Similar results have been obtained in previous studies(Kim et al., 2011). A possible reason for this phenomenon was that people with a low BMI might still have as much fat as people with a high BMI, which could be explained by the obesity paradox. The obesity paradox is the unexpected phenomenon of reduced mortality in overweight or mildly obese individuals(Flegal et al., 2013). Furthermore, the severity of obesity may be underestimated due to the inability of BMI to distinguish adipose tissue from lean body mass(Romero-Corral et al., 2008). Therefore, this study included WC, which could reflect the accumulation degree of abdominal obesity, into the study. The results showed that WC still could regulate the association between sarcopenia and depression, which further indicated that malnutrition in the middle and elderly population will increase the risk of depression in the sarcopenia population.
There are several strengths of our study. First, this is the first mediation study to assess the effect of obesity on the association between sarcopenia and depression, which could help develop targeted intervention strategies to prevent and treat depression. Second, the present study was based on a long-term follow-up of a large, prospective national cohort of middle-aged and elderly Chinese populations, which allowed us to comprehensively assess the underlying mechanisms of the observed associations and to illustrate the causal associations of the study variables with outcomes. However, some limitations should be considered in the interpretation of our findings. First, we excluded all missing values instead of imputation, which might reduce the representativeness of the sample. Second, the CES-D10 was used to examine the number of self-reported depression in the past week, which could introduce recall bias. But previous studies also proved that CES-D10 had high reliability and validity in Chinese population(Boey, 1999). Given that the etiology of depression is complex and multifactorial, it cannot necessarily be assumed that overweight or obese middle-aged and elderly adults are protected from sarcopenia and depression.