This study investigated the relationship between sarcopenia and insulin among individuals aged over 60 within Chinese communities, utilizing nationally representative data. Our cross-sectional analysis revealed a positive correlation between ASMI and insulin. Moreover, our longitudinal analysis demonstrated that older adults with lower insulin level were at an elevated risk of developing new-onset sarcopenia.
In this study, the prevalence of sarcopenia falls within the intermediate range compared to previous research [20, 21]. There are several reasons that could explain this discrepancy [22]. First, estimates of sarcopenia prevalence are influenced by the diagnostic criteria employed. Second, prevalence estimates may differ based on the assessment techniques utilized. Third, prevalence estimates can vary across different populations and regions. On the other hand, the incidence of new-onset sarcopenia observed in this study is similar to that reported in a previous study [23].
As an anabolic hormone, insulin promotes protein synthesis by facilitating the uptake of amino acids into muscle tissues [24]. Our study results indicate that insulin levels are positively correlated with ASMI and serve as a protective factor against sarcopenia. A previous study has reached similar conclusions, but our larger sample size strengthens the evidence in this area [25]. The relationship between insulin levels and the decline in muscle mass and function in older adults, particularly those with DM, is complex. Traditionally, insulin resistance has been considered central to the onset of DM, leading to opposing hypotheses [26]: one suggests that insulin resistance contributes to the development of sarcopenia, while the other posits that sarcopenia is a risk factor for insulin resistance and DM. However, mounting evidence indicates that disordered insulin secretion, rather than insulin resistance, plays a crucial role in the progression of DM [27, 28]. In aging and diabetes, diminished insulin signaling impairs muscle protein synthesis and enhances muscle protein degradation, resulting in muscle mass loss and eventual sarcopenia. Therefore, insulin therapy slows the progression of sarcopenia in individuals with DM [29]. However, in a cohort study from Mexico involving community-dwelling older adults without other chronic health conditions, hyperinsulinemia, an early indicator of insulin resistance, was linked to a reduction in ASM [30]. Given the considerable heterogeneity of sarcopenia across diverse populations, further investigation is warranted to determine whether this could elucidate the conflicting results observed in different population.
In addition to insulin, ASMI exhibited positive correlations with glucose and CR, while demonstrating negative correlations with HDL-C and LPR among the blood indicators. Similar findings have also been reported in a previous study [31]. Therefore, ASMI is also associated with liver function, and renal function other than β cell function. Sarcopenia correlates with fibrotic burden in individuals diagnosed with chronic hepatitis B. Moreover, ASMI experiences a notable decrease during antiviral therapy for chronic hepatitis B [32]. Progressive renal dysfunction is linked to diminished muscle strength and physical performance. Among older men residing in the community, even mild-to-moderate renal impairment at the outset is correlated with deteriorations in grip strength, gait speed, and overall muscle function over time [33].
Our longitudinal analysis, utilizing nationally representative data, indicated that the protective factors of sarcopenia include hypertension besides insulin, while the risk factors include age and HDL-C. Within this study, hypertension was found to reduce the risk of sarcopenia, a finding consistent with prior research [34]. Generally, nutritional and exercise therapies are advocated for hypertension management [35], both of which have been shown to mitigate sarcopenia [36]. Nevertheless, further investigation is warranted to elucidate the relationship between hypertension and sarcopenia prevention. In middle-aged and older Chinese adults, each incremental unit rise in HDL-C levels corresponds to a 42% increase in the likelihood of developing sarcopenia at 4 years follow up, emphasizing the importance of effectively managing high HDL-C levels in sarcopenia prevention [37]. A study from China indicates that the prevalence of sarcopenia among males aged 60–69 years, 70–79 years, and over 80 years is 1.5%, 9.6%, and 33.1%, respectively. Therefore, prior to reaching 80 years of age, preserving muscle mass warrants primary consideration, whereas after surpassing this age threshold, emphasis should shift towards enhancing muscle strength and function to mitigate disability risk [38].
It is important to acknowledge the limitations of this study. Firstly, while we adjusted for a comprehensive set of potential confounders based on existing knowledge, certain additional confounding factors, such as physical activity and dietary intake, were not accounted for in our analysis. Secondly, the observational nature of our study made it susceptible to recall bias inherent in questionnaire surveys. Thirdly, while our longitudinal study revealed a stronger correlation between sarcopenia and insulin compared to the cross-sectional analysis, the underlying biological mechanisms remain unclear. Therefore, further experimental studies are warranted to elucidate and confirm this association.