In this study, we estimated the prevalence of early-onset sarcopenia and sarcopenic obesity in the US by the well-designed nationally representative database NHANES 2015–2018. We found that OSA was a significant independent risk factor for early-onset sarcopenia and early-onset sarcopenic obesity. Chronic inflammation, body mass, IR, and dietary quality had a significant mediating effect on this association.
Sarcopenia has become an urgent issue for healthcare systems all over the world. Loss of appendicular skeletal muscle not only obstacles old people’s activities of daily living (ADL) but is also associated with other complications such as osteoporosis, diabetes, and cardiovascular disease[25][26][27]. Early-onset sarcopenia, as an early term of sarcopenia, might bring more adverse outcomes but not get sufficient attention. In our study, we estimated that the prevalence of early-onset sarcopenia in the US has reached 5.6% and 4.6%, according to the multistage weighted survey design of NHANES. We found that patients with early-onset sarcopenia suffered from a higher prevalence of diabetes, hypertension, thyroid disease, heart disease than their healthy counterparts. And the adverse synergetic effect of sarcopenia and obesity also existed in those with early-onset sarcopenic obesity[28]. Besides, participants with early-onset sarcopenia or sarcopenic obesity were under poorer sleep quality. Compared to the healthy group, they had longer sleep duration(P < 0.05), more snore times (P < 0.01), and more breath-stopping times (P < 0.01). Meanwhile, the prevalence of OSA was estimated to be 31.1% in the sarcopenia group and 49.7% in the sarcopenic obesity group.
Existing studies have indicated some sleep-related problems are tightly associated with muscle mass and function. For example, Takuma Shibuki et al. found that long sleep was positively associated with sarcopenia. Moreover, they found poor sleep quality (quantified by times of insomnia per night) was associated with sarcopenia in normal sleepers but not in long sleepers[29]. Zhang et al. further indicated the U-shape nonlinear association between sleep duration and grip strength and pointed out that this association was meditated by BMI[30]. However, the association between OSA and sarcopenia has not reached a consensus yet. Some studies indicated that sleep deficiency and intermittent hypoxia status favored increased body mass, while others observed that OSA was only associated with obesity but not with sarcopenia[31]. Meanwhile, there was a relative scarcity of research on early-onset sarcopenia conducted in middle-aged adults compared to elderly adults previously. Our study supported and extended these previous studies. We found that participants with OSA had higher BMI but lower ALM/BMI than those without OSA. Our weighted multivariable logistic regression analyses further supported that OSA was an independent risk factor of early-onset sarcopenia and early-onset sarcopenic obesity after adjusting for many potential confounding variables. Our studies attached the importance to find the susceptible of early-onset sarcopenia or sarcopenic obesity among those with OSA because early-term interventions are of more use in the prevention and treatment of sarcopenia or sarcopenic obesity[32].
Several possible mechanisms may account for the relationship between OSA and early-onset sarcopenia. Interruption of breathing by OSA results in repeated intermittent hypoxia, hypercapnia and arousals from sleep. Such short and high-frequency bouts of blood O2 desaturations activates hypoxia-inducible factor-1 (HIF-1) and HIF-2, which belong to the HIF family of transcriptional activators. And HIF-1α-dependent β cell dysfunction may induce hypersecretion and insulin resistance by increased generation of reactive oxygen species(ROS)[33]. On the other hand, insulin resistance obstacles peripheral glucose utilization and dysregulates protein turnover, especially in skeletal muscle and dietary nutrition quality[35]. We performed a mediation analysis to estimate the mediating effect of insulin resistance on the association between OSA and early-onset sarcopenia. HOMA-IR, as an indicator for insulin resistance, it mediated 10.32% of the potential effects of OSA on sarcopenia in our mediation models.
Intermittent hypoxia also induces the activation of the hypothalamic-pituitary-adrenal (HPA) axis and hypothalamic-pituitary-thyroid (HPT) axis, which may affect muscle catabolism through low-grade inflammation and oxidative stress[35]. And it was confirmed that clustered systemic and muscle oxidative stress and inflammation can accelerate the onset of sarcopenia in individuals with obesity[36]. CRP was an important chronic inflammation marker and mediated 30.28% of this association. Therefore, the inflammation hypothesis may be proposed as an important potential mechanism linking OSA and early-onset sarcopenia.
Given that early-onset sarcopenia was associated with obesity and poor dietary quality, we also estimated their mediating effect on this association. BMI was an indicator of obesity and mediated 53.64% of the potential effects after adjusting for confounders. This result indicated that a higher BMI score was related to a higher risk of early-onset sarcopenia. Because early-onset sarcopenia was often secondary to obesity and other chronic diseases, this may partly explain why the association between OSA and sarcopenia was not significant among the elderly in our study for there were a relatively small number of patients with obesity among those aged 40–60. Therefore, reducing excess adiposity remains the fundamental pathogenetic treatment for those with early-onset sarcopenic obesity. With respect to diet nutritional factors, HEI-2015 was an indicator of dietary quality and mediated 8.64% of the potential effects of OSA on sarcopenia. Although nutritional intervention was considered as therapy in priority to preserve muscle mass for sarcopenia and sarcopenic obesity, optimal dietary options and medical nutritional support strategies need to be confirmed for these young patients[37].
Our study has several strengths. First, data from NAHANES was collected cautiously and widely accepted because of its scientific design. Such a nationally representative dataset enabled us to minimize the sampling bias and take many confounders into consideration to get the precise estimate. Second, this study was the first to identify the prevalence of early-onset sarcopenia in the US and its odds ratio caused by OSA. And we performed a mediation analysis, which enables an insightful understanding of the inter-relationship of these factors with sarcopenia from a comprehensive perspective. We find out the mediating role of insulin resistance, poor dietary quality, chronic inflammation, body mass and obesity on the association between OSA and early-onset sarcopenia, which may provide new strategies for the prevention and treatment of early-onset sarcopenia.
There are also some limitations existing in our study as well. First, due to the cross-sectional nature of the study, we were unable to determine the causal relationship between the OSA and sarcopenia. And further longitudinal studies are necessary. Second, sleep-related problems and some chronic comorbidities were assessed by self-reported methods, which might cause recall bias although there is a good correlation between subjective perception of sleep quality and objective measures was reported[38]. Third, our study was also limited by the unmeasured variables. We selected a relatively simple method from previous studies to define early-onset sarcopenia which only focused on muscle mass and ignored muscle function. However, according to a previous study, these younger group with early-onset sarcopenia may still preserve muscle function. Including altered muscle functional parameters as a necessary component of the diagnostic process may even lead to potential borderline conditions[39]. Fourth, considering the complex and multifactorial etiology of early-onset sarcopenia, the association reported in this study could be biased by other unknown confounders although we had adjusted sociodemographic features, socioeconomic status, dietary quality, relevant comorbidity in our logistic models. And other biological mechanisms, such as epigenetic changes and neuroendocrine responses, may also play a role in the pathophysiology of sarcopenia. Therefore, future research is needed to incorporate other intermediate mechanisms and guide treatment decisions in the clinical practice.