With a rapid increase in the number of older adults in Korea, new concepts and approaches are emerging regarding the treatment of conditions in older patients; one such condition is sarcopenia. Sarcopenia is a syndrome of progressive skeletal muscle loss [1]. In 2010, the European Working Group on Sarcopenia in Older People first proposed a diagnostic algorithm for sarcopenia to foster advances in identifying and caring for people with sarcopenia [2]. However, to diagnose sarcopenia in Asian people, it is necessary to identify differences from their Western contemporaries in terms of anthropometric, cultural, and lifestylerelated factors, such as relatively smaller body sizes, higher adiposity, and less mechanized and more physically active lifestyles [3,4]. In 2014, the Asian Working Group for Sarcopenia (AWGS) consensus defined sarcopenia as “age-related loss of muscle mass, plus low muscle strength, and/or low physical performance” and specified cut-offs for each diagnostic component [3]. With advancements in research in Asia, AWGS updated their guidelines for diagnosing sarcopenia in 2019; they retained the previous definition of sarcopenia but revised the diagnostic algorithm, protocols, and some criteria [4]. AWGS 2019 also introduced the term “possible sarcopenia,” which is defined as low muscle strength with or without low physical performance, specifically for use in primary health care or community-based health promotion to enable early lifestyle interventions [4]. Muscle mass peaks in the age of 20s and 30s, and then it gradually decreases. During the age of 40–70 years, muscle mass decreases by 8% every 10 years; after 70 years of age, it decreases by nearly 10% in 10 years. Additionally, muscle strength decreases faster than muscle mass. After 70 years of age, it decreases by 25%–35% in 10 years, and both muscle mass and strength decrease faster in men than in women [5]. Based on the AWGS 2019 criteria, one study found that 21.3% and 13.8% of Korean men and women aged 70–84 years had sarcopenia, respectively [6]. Additionally, the coronavirus disease 2019 was an extraordinary emergency, which became a pandemic. While quarantine and social distancing appeared essential at the time, the harmful physical and psychological health effects of such measures cannot be ignored [7]. Sedentary lifestyle and a lack of a healthy diet brought upon by these measures can significantly increase the prevalence of sarcopenia [8]. In fact, sarcopenia is strongly associated with adverse outcomes, such as falls, fractures, frailty, morbidity, mortality, and high healthcare utilization [1,9]. Despite its clinical importance, sarcopenia remains underrecognized and poorly managed in routine clinical practice. Furthermore, while many studies have evaluated the association between sarcopenia and physical health, there is a lack of studies that have evaluated the association between sarcopenia with mental health conditions, including depression.

Depression, a major mental health disorder, is a mood disorder with symptoms that include persistently depressive mood, loss of motivation or interest, and hopelessness about the future, which could lead to suicide [10]. Since 2001, epidemiological surveys on mental disorders in Korea have been conducted every 5 years with a gradually increasing reported lifetime prevalence of depression (2001, 4.0%; 2006, 5.6%; and 2011, 6.7%) [11]. Depression are closely associated with various health problems in older adults, such as chronic disease, disability, morbidity, and mortality [10,12]. Considering the various adverse outcomes associated with sarcopenia and depressive symptoms in older people, it is important to understand the association between these disorders. To our knowledge, some studies in Asia (e.g., China and Japan) have evaluated the association between sarcopenia and depressive symptoms; however, such studies are lacking in Korea [13,14]. Particularly, no studies have evaluated the relationship between possible sarcopenia and depressive symptoms in Korea. Additionally, the contribution of possible sarcopenia toward specific depressive symptoms remains unexplored. Therefore, we investigated the association between possible sarcopenia and specific depressive symptoms in Korean older adults. Using the nationally representative data from the 2018 Korea National Health and Nutrition Examination Survey (2018 KNHANES), we conducted cross-sectional analyses to estimate the association between possible sarcopenia and depressive symptoms in Korean community-dwelling older adults. If an association is identified, patients should be screened for possible sarcopenia using tests that can be performed quickly and easily in the clinic when they visit the primary health care centers. If possible sarcopenia is identified in them, they should also be screened for depressive symptoms. Consequently, primary care physicians can advise early lifestyle interventions for such patients. The purpose of this study was to examine the relationship between possible sarcopenia and depressive symptoms so that Korean older adults can receive early interventions for these disorders at primary health care centers.

In the present study, we used the data from 2018 KNHANES. To our knowledge, our study was the first attempt to identify associations between possible sarcopenia and depressive symptoms in Korean older adults using a large nationally representative survey of the Korean population. We found that possible sarcopenia, assessed using the AWGS 2019 algorithm, was positively associated with higher odds of depressive symptoms. Individuals with low hand-grip strength were more likely to have a higher PHQ-9 score than those without sarcopenia. These findings are consistent with those of many other cross-sectional studies, suggesting that sarcopenia is closely correlated with higher odds of depressive symptoms. A meta-analysis of 15 observational studies indicated that sarcopenia was independently and positively associated with depression both in the narrative review and quantitative analysis with and without adjustments for covariates [20]. Furthermore, China Health And Retirement Longitudinal Study 2015 reported that possible sarcopenia was associated with higher odds of depressive symptoms in 2,627 Chinese older people [14]. The Toyota Prevention Intervention for Cognitive Decline and Sarcopenia study reported that both low physical performance and muscle strength were significantly associated with depressive mood in 432 Japanese older adults; however, no association was noted with decreased muscle mass [13]. Other than studies from Asia, several other studies have evaluated the association between sarcopenia and depressive symptoms. For example, the Brazilian Longitudinal Study of Adult Health suggested that depression was associated with sarcopenia as defined by the National Institutes of Health criteria in 5,927 middle-aged and older Brazilian adults; this association was mainly due to low muscle strength [21]. Few studies have been conducted in Korea on this subject [22,23]. However, none of the them have evaluated the association between possible sarcopenia and depressive symptoms in Korea using the AWGS 2019 algorithm. Byeon et al. [22] in 2016 evaluated the association between sarcopenia and depression; they defined sarcopenia as a decrease in muscle mass only. They reported no association between sarcopenia and depression in 7,364 Korean adults aged ≥20 years [22]. However, decreased muscle strength is the most important diagnostic parameter in guidelines for diagnosing sarcopenia since it mainly predicts adverse outcomes [9]. Therefore, we assumed that muscle strength is more important than muscle mass in the present study. This assumption led us to find that possible sarcopenia was positively associated with higher odds of depressive symptoms. Furthermore, we found the association between possible sarcopenia and depressive symptoms based on age cut-off, as well as possible sarcopenia and specific depressive symptoms. Compared with participants with no sarcopenia, possible sarcopenia in participants in their 60s and those over 70 years of age was positively associated with depressive symptoms. Possible sarcopenia was significantly associated with four of the nine depressive symptoms on PHQ-9.

Several possible mechanisms might help explain the association between sarcopenia and depressive symptoms. First, chronic inflammation is common to both sarcopenia and depressive symptoms. Agerelated chronic low-grade inflammation characterized by proinflammatory cytokines, such as tumor necrosis factor-α and interleukin-6, has been reported as an important biological event in the pathogenesis of sarcopenia and depressive symptoms [24,25]. Second, excessive oxidative stress related to chronic diseases might also cause sarcopenia [24]. Excessive oxidative stress might also possible mediators of known environmental risk factors in depressive symptoms [25]. Third, people with sarcopenia or depressive symptoms are more likely to have similar lifestyle factors, such as malnutrition and physical inactivity [1,10]. Among essential amino acids, branched-chain amino acids (BCAAs) are the most involved in the direct stimulation of muscle protein synthesis, and the mammalian target of rapamycin (mTOR) regulates cell growth, cell proliferation, cell survival, cell motility, protein synthesis, autophagy, and transcription [26]. These facts suggest that BCAA deficiency and dysregulated mTOR pathway might be involved in the pathogenesis of sarcopenia. Reduced activation of mTOR due to BCAA deficiency could play a crucial and unrecognized role in the etiology of depressive symptoms and lower energy metabolism in patients with major depression [26]. Therefore, good nutrition (especially proteins and amino acids) is important for the resolution of depressive symptoms and sarcopenia. Particularly, intake of leucine alone can activate several intracellular signaling proteins involved in the initiation of mRNA translation, including mTOR, which is associated with a decrease in depressive symptomatology while stimulating muscle protein synthesis [26,27]. Physical activity can reduce the permeability and function of the blood-brain barrier as it reinforces antioxidative capacity, reduces oxidative stress, and produces antiinflammatory effects [25,28,29]. Exercise improves sarcopenia and depressive symptoms [1,25,29]. Upregulation of mRNA translation is an important regulator of acute increase in exercise-induced muscle protein synthesis [27]. The activation of the mTOR signaling pathway through exercise is associated with an increased rate of muscle protein synthesis and a decrease in depressive symptomatology [26,27].

However, there were limitations to this study. First, there was no information about muscle mass in 2018 KNHANES. Muscle mass is needed to diagnose sarcopenia according to the AWGS 2019 algorithm; therefore, we could not investigate any association between sarcopenia and depressive symptoms. Second, according to the AWGS 2019 guidelines, possible sarcopenia could be diagnosed based on low muscle strength or physical performance. However, in the data from 2018 KNHANES, there was no information about physical performance as proposed by the AWGS 2019 guidelines. Although several studies have proven that low muscle strength, a component of possible sarcopenia, is independently associated with depressive symptoms, the diagnosis of possible sarcopenia based only on low muscle strength might have resulted in errors in identifying participants with possible sarcopenia [13,14,21]. Third, the study used a cross-sectional design; therefore, we could not determine causality. Fourth, this study used observational data, which might include biases due to confounding factors. To reduce such biases, we considered as many factors as possible in the analysis. However, other potential confounding factors, such as nutritional status, food habits, and sleep behavior, could not be ruled out. Finally, Koreans tend to respond subconsciously and negatively to mental health-related questions due to sociocultural factors [30]. Therefore, the prevalence of depressive symptoms might have been underestimated.

Despite these limitations, there were several strengths of this study. First, we used a large and nationally representative sample, thereby enabling the generalizability of our findings to the overall Korean older population. Second, the study was the first one to examine associations between possible sarcopenia and items of depressive symptoms in PHQ-9 in Korean older adults. Third, the scarcity of studies on the topic makes the findings of this study important in determining criteria for associations between possible sarcopenia and other diseases. More importantly, the AWGS 2019 guidelines recommend a new entity “possible sarcopenia” to facilitate early lifestyle interventions. Our findings support the validity of the current major algorithm of possible sarcopenia proposed by AWGS 2019. Therefore, maintaining sufficient muscle strength might be important in combating depressive symptoms in older adults. Additionally, screening for possible sarcopenia and depressive symptoms in routine clinical practice could promote healthy aging.

In conclusion, possible sarcopenia was significantly associated with depressive symptoms in Korean community-dwelling older adults. The results of this study demonstrated that early interventions for possible sarcopenia and depressive symptoms in routine clinical practice could enable Korean older adults to have healthy aging. Additionally, future longitudinal studies, including large-scale cohort studies, are needed to explore any causal relationship between possible sarcopenia and depressive symptoms in Korean community-dwelling older adults.