Results of this large, prospective cohort study of middle-aged and older adults over 45 years of age indicate that sarcopenia is associated with a faster increase in CES-D scores and increased risks of incident depression. Besides, we found a stronger association between sarcopenia and trajectory of CES-D scores in nonsmokers compared to smokers.
Our results showed that baseline sarcopenia status was cross-sectionally associated with severer depressive symptoms at baseline, longitudinally associated with an accelerated trajectory of depressive development, and prospectively associated with increased risks of incident depression during follow-up. Our findings were consistent with the results of several previous cross-sectional studies 14,15,27 and a prospective study with a mean follow-up of 3.7 years 11. Compared to the previous prospective study 11, the present study had a longer follow-up period of 5.42 years. More importantly, associations between sarcopenia and rate of change in depressive symptoms have not been explored previously.
Potential mechanisms underlying the association between sarcopenia and depression include multiple factors, such as neurotrophic factors, chronic inflammation, oxidative stress, and physical activity 28. Skeletal muscle tissue can produce neurotrophic factors, especially brain-derived neurotrophic factor (BDNF), which can increase synaptic plasticity and promote neurogenesis, especially in the hippocampus 29,30. Neuroplasticity plays an important role in the development of depression, and the hippocampus is a key region of the brain associated with psychiatric disorders 29,30. Inflammatory cytokines have been shown to affect sarcopenia and depression by interfering with metabolism or the activity of other cytokines, a common influence on both 28. The primary cause of sarcopenia is lack of exercise 31, while randomized controlled studies have confirmed the role of exercise in improving depressive symptoms 32. A growing number of studies have shown that exercise enhances the expression of BDNF, improves antioxidant capacity, and has anti-inflammatory effects 33,34.
The three-way interactions between smoking, sarcopenia, and CES-D scores were statistically significant, suggesting potential effects of smoking on the sarcopenia-depression association. Results of stratification analyses indicated that associations between sarcopenia and rate of change in CES-D scores were stronger in nonsmokers than in smokers. The corresponding mechanisms have not been elucidated until now. This discrepancy by smoking status could be explained by the following. First, participants with past- and current-smoking status had a smaller sample size compared to nonsmokers, which might induce a poorer statistically power. Second, evidence from previous epidemiological studies has indicated that smoking is a risk factor for depression 35,36. As a result, effects of sarcopenia on depression might be offset by the detrimental effects of smoking among smokers. Besides, it has been proposed that hormonal dysregulation in hypothalamic-pituitary-adrenal (HPA) axis is a risk factor shared by sarcopenia and depression 29. Results of animal studies indicate that chronic nicotine exposure affects the secretion of cortisol and activity of HPA-related monoamine neurotransmitter system 37. Studies focusing on the biological mechanisms were needed in the future. It can be hypothesized that smoking, which affects the hormonal dysregulation in HPA axis, may mask the effect of sarcopenia on depressive symptoms. Comparatively, quantitative data from the CES-D score are more sensitive as an indication of outcome and can swiftly show changes in outcome during follow-up than depressed symptoms, which are evaluated based on the scale.
This study has several advantages. First, the CHARLS study has a big sample size, covers a large area, has strong population representation, and has a high degree of confidence in inferring the features of the complete population from the sample information. Second, the study made full use of database information, not only for the long follow-up period of the longitudinal study, but also for the findings of both dichotomous indicators of depression status and quantitative indicators of CES-D scores. We found consistent results with different methods of data analyses suggesting that our findings were robust. In addition, this is the first study in Asia to examine the association between sarcopenia status and the rate of change in CES-D scores, and finds the important result that the effect of sarcopenia status on the rate of change in CES-D scores is predominantly in the nonsmoking population, providing new ideas for depression prevention and early intervention.
However, there are some limitations in this study. First, rather than 6 m as stated in AWGS2019 criteria, the distance measured by the CHARLS database to measure step speed is 2.5 m. The length of the walk during the gait speed test, however, had no influence on the recorded gait speed, according to a comprehensive review of 48 studies that evaluated gait speed in older adults 38. Consequently, a 2.5 m walk would be suitable for measuring the walking speed of senior Chinese citizens. Second, the present study, while informative, omitted an examination of the relationship between alterations in sarcopenia status and the rate of change in CES-D scores and depressive symptoms throughout the follow-up period.
In conclusion, sarcopenia was associated with an accelerated rate of increase in CES-D scores among Chinese adults aged 45 years and older, especially for nonsmokers. Sarcopenia was also associated with increased risks of incident depression. Sarcopenia may be used as an early sign for the development of depression. Interventional studies are warranted to confirm our findings in the future.