In this study, our main finding was that the prevalence of sarcopenia in rural elderly in China was 25.4%, in which 20.7% for men and 28.5% for women. The subjects with sarcopenia had a lower BMI than the non-sarcopenic older subjects. There were no significant differences in age, sex, or walking speed between non-sarcopenic and sarcopenic subjects. Compared with non-sarcopenic subjects, sarcopenic subjects had higher levels of GDF11 and lower levels of ALB, FGF19, and TNF-α. In addition, inflammatory cytokine IL-6 and β-superfamily cytokine GDF15 are not independent factors of sarcopenia.
In recent years, epidemiological survey results of sarcopenia in Chinese population show that the prevalence rate of sarcopenia in elderly people aged 60 and above is 3.1%-62.9%[12]. A study in Taiwan showed that the prevalence of sarcopenia was 18.6% in 302 aged 65 and older men and 23.0% in women, which is similar to our results[13]. A New Mexico senior health survey found that the prevalence of sarcopenia in older men and women was 28.5% and 33.9%, respectively[14]. However, other studies have reported low prevalence of sarcopenia, and differences in prevalence results may be due to differences in the definition of sarcopenia, diagnostic cut-off values, or ethnic background of the population.
BMI, waist circumference, and/or waist-to-hip ratio are commonly used to define obesity, and BMI in older adults needs to be interpreted with caution because of reduced physiological height and lack of correlation between BMI and body fat percentage, distribution, or body composition in older adults.That is why we discussed here is just sarcopenia and non-sarcopenia groups of relatively high low BMI, and overweight and obese relations with less muscle disease needs further research.Our finding that a lower BMI is a risk factor for sarcopenia is consistent with the findings of multiple previous studies suggesting that the higher the BMI in older adults, the lower the risk of sarcopenia[15, 16].While obesity is considered a risk factor for many adverse outcomes, in older adults, being slightly overweight may be beneficial. Studies have also shown that older men are resistant to the dangers of overweight and obesity; Mild overweight, obesity, and even central obesity were beneficial for survival[17].Similarly, a study of hospitalized older adults found that fat mass was associated with a lower risk of death or complications[18].In addition, studies have shown that BMI is positively correlated with muscle mass and fat mass[19–21].It has been pointed out that fat is an energy store for the elderly and helps individuals survive in disease or poor physical condition. The amount of fat can affect lean body mass for several ages, and people with high fat amount may have a higher protein intake, which is a protective mechanism against muscle loss[22, 23].Given this, we believe that high BMI may play a protective role against loss of muscle mass and strength in older adults. The results of this study further support the negative association between BMI and sarcopenia, so maintaining a healthy BMI in older adults may help maintain muscle mass and strength.
Studies have shown that the serum albumin of the elderly with sarcopenia is lower than that of the non-sarcopenia elderly participants[24, 25]. A meta-study of sarcopenia involving 4,904 community-living and institutional older adults (68-87.6 years) from 9 countries and another meta-analysis of 4,071 participants from 5 countries showed that regardless of age and setting, Albumin is negatively associated with sarcopenia, and given that there is no conclusive evidence that serum albumin can be definitively used as a biomarker for sarcopenia, which is easier and less costly to implement than other approaches and ensures timely assessment and early intervention in elderly patients with sarcopenia, Therefore, serum albumin decline can be considered as a reference indicator for biomarkers of sarcopenia. However, albumin concentrations are also affected by processes such as inflammation and infection, so there is no consensus on optimal limits and reference ranges for serum values to assess nutritional status in older adults[26–28]. Albumin is the most abundant plasma protein in the body, and its basic role is to regulate the passage of water and solutes through capillaries by maintaining colloidal pressure in the vascular system. Albumin has long been considered integral to the assessment of nutritional status, it is considered to be a very important plasma protein in the assessment of nutritional status, reducing it can alter wound healing, cause immune problems, and reduce lean body mass[29]. Improving the nutritional status of the elderly can increase the serum albumin content, which may play a certain role in the treatment of sarcopenia in the elderly.
In elderly patients with sarcopenia, the level of GDF11 was higher in the sarcopenia group than in the non-sarcopenia group, while GDF15 was not statistically different between the two groups. GDF11 and GDF15 are members of the transforming growth factor β superfamily of cytokines, and GDF11 is closely related to myostatin (MSTN). GDF15 is also known as MIC-1[30]. However, some scholars prefer to believe that GDF15 belongs to the glial cell derived neurotrophic factor (GDNF) -like growth factor or cytokine family, because GDF15 receptor GFRAL is a co-receptor of Ret tyrosine kinase[31, 32]. Multiple studies have suggested that GDF11 inhibits skeletal muscle regeneration[33–35]. In a recent study, Juli E. Jones found that elevated GDF11 could induce the expression of oxia in mice with decreased muscle mass, food intake and body weight, and that GDF11 could also up-regulate plasma GDF15. Blocking the GDF15 receptor alleviates anorexia but does not reduce muscle loss, whereas blocking the GDF11 receptor ActR II prevents muscle loss and reduces anorexia[33]. Therefore, elevated GDF11 can be considered as an independent predictor of sarcopenia.
We also found that FGF19 was lower in the sarcopenia group than in the non-sarcopenia group. FGF19 is a member of the FGF family, a family of proteins involved in differentiation, development and metabolism[36]. FGF19 is similar to endocrine hormone and is secreted by ileum intestinal epithelial cells. The pathogenesis of sarcopenia is related to skeletal muscle metabolism, and FGF19 has been found to play a role in muscle metabolism[37]. Therefore, FGF19 may be an emerging factor for the treatment of sarcopenia, and can contribute to the diagnosis and prevention of sarcopenia. The negative association between FGF19 and sarcopenia was also confirmed by a recent cross-sectional study in Turkey of 88 elderly outpatient participants aged 65 years or older[38].
A number of literatures have shown that sarcopenia is related to inflammatory cytokines, but the existing studies are controversial on whether inflammatory factors play a positive or negative role in sarcopenia. Studies have shown that chronic inflammation leads to a loss of muscle mass by affecting protein synthesis and catabolism. However, most of the people in these studies were obese or had other chronic conditions, so the role of inflammatory factors in sarcopenia remains controversial[39]. In a recent study, 299 Japanese residents (127 males and 172 females) participated in urban health check-ups. It was found that IL-6 is not an independent factor in sarcopenia[40]. This conclusion is similar to ours. A recent community sarcopenia study in Taiwan also found no significant correlation between serum IL-6 levels and ASMI, grip strength, or gait speed[41]. IL-6 is an effective regulator of human fat metabolism, which can increase lipolysis and fat oxidation. IL-6 acts in an anti-inflammatory way during muscle contraction. Because IL-6 has pro-inflammatory and anti-inflammatory effects, and the IL-6 secreted into the blood cannot be determined whether it is derived from muscle tissue[42].
The role of TNF-α played in sarcopenia remains controversial. Results in this study demonstrated that a low TNF-α level in sarcopenic old adults. In vitro studies have shown that TNF-α has a direct inhibitory effect on insulin signaling and can cause insulin resistance in skeletal muscle, thereby increasing free fatty acids and causing chronic inflammation. A clinical study showed the level of TNF-α in the elderly sarcopenia group was significantly lower than that in the non-sarcopenia group. In multiple logistic regression analysis, low level of TNF-α was identified as an independent risk factor for sarcopenia[40, 43]. Further research is needed to better understand the role of TNF-α in sarcopenia.
There are some limitations to our study. First, because the participants came from rural communities, most of whom may have different occupations and living standards than older people in urban environments, the study has population limitations. Secondly, there are some unmeasured or unknown confounding factors that may affect the results of the study. Sarcopenia is an emerging and complex syndrome, and its influencing factors and pathogenesis are complex and diverse. Therefore, it is impossible to make a definitive diagnosis of sarcopenia through a single serum marker. Another, the lack of investigation of the clinical characteristics of the participants with noncommunicable diseases is also a limitation of this study, and the noncommunicable diseases of the participants may also affect the conclusions of the study. In addition, this is a cross-sectional, single-center study that cannot definitively establish a causal relationship between serum biomarkers and sarcopenia. Further longitudinal and multi-center collaborative studies are needed to confirm our results.