Bidirectional Communication Between Circadian Rhythm Disruption and Frailty: A Hypothesis From Gut Microbiota Metabolites

Introduction

Circadian rhythmicity constitutes a critical physiological phenomenon essential for sustaining metabolic homeostasis and normal biological processes within organisms.¹ In mammals, the circadian rhythm operates as a self-sustaining, interlocking transcriptional-translational feedback loop, comprising a network of genes such as BMAL1, CLOCK, the period genes (PER1/2/3), and the cryptochrome genes (CRY1/2).² While light signals serve as the primary cues for synchronizing the master clock with the external environment, other factors, including diet, feeding behaviors, hormones, and short-chain fatty acids (SCFAs) produced by the gut microbiota, also play significant roles.^3,4 Notably, the gut microbiota exhibits circadian rhythms analogous to those of the host, and the rhythmicity of microbial metabolites is disrupted in mice deficient in the BMAL1 gene.^5,6 This interplay and coexistence between the rhythmicity of the gut microbiota and host circadian rhythms are pivotal in maintaining regular rhythms and biological processes within the organism.

Once the circadian system of an organism becomes disrupted, these mechanisms of communication and coexistence are compromised, leading to alterations in the metabolites of the intestinal microbiota. This disruption results in the dysregulation of the organism’s intestinal ecology and the accumulation of chronic low-grade inflammation throughout the body. Consequently, this can initiate or exacerbate frailty, a geriatric syndrome characterized by a decline in physiological reserves and increased susceptibility to stressors.⁷ The clinical phenotype and pathophysiology of frailty are complex, involving multiple physiological systems and their interactions throughout the body. A commonly employed tool for assessing frailty in clinical practice is the Frailty Phenotype Score, as proposed by Fried et al.⁸ By examining clinical data from more than 5000 cases of frail individuals aged≥ 65 years, Fried’s team proposed a definition of frailty characterized by five primary manifestations. They established five diagnostic criteria: weight loss, slow gait, poor grip strength, low physical activity, and a sense of fatigue. Individuals meeting three or more of these criteria were diagnosed with frailty, those meeting one or two criteria were classified as pre-frail, and those meeting none were considered non-frail. Although several epidemiological studies have validated this model’s ability to predict adverse clinical outcomes, its primary limitation lies in its exclusive focus on somatic functioning, neglecting important aspects of daily life such as emotional and psychological well-being, sleep, and socialization. The frailty index model developed by Rockwood’s team addresses these limitations and has become another widely used tool for assessing frailty.⁹ The frailty index model was developed based on the theory of health deficits, also known as the cumulative assessment of deficits. This theory refers to the proportion of potentially unhealthy measures of an individual relative to all measures at a given point in time. The model is capable of simultaneously considering a wide range of health variables that influence changes in frailty, including somatic functioning, mental state, psychological, and social factors. Interestingly, frailty and circadian rhythm disruption are intersecting “shadows” of these considered health variables and their downstream pathophysiological mechanisms, suggesting a bidirectional relationship exists.¹⁰ Logically, it can be hypothesized that the gut microbiota, which has been confirmed to be correlated with both circadian disruption and frailty, can provide a rational explanation for this correlation as well as the bidirectional relationship.

In recent years, the “microbiota-gut-brain axis” and “microbiota-gut-muscle axis” doctrines have established two bi-directional homeostatic systems connecting the gastrointestinal tract, the central nervous system, and the muscular system,^11,12 and various pathways involved, such as the hypothalamic-pituitary-adrenal axis,¹³ the endocrine system,¹⁴ and muscle mass control,¹² have been implicated in the pathology of frailty and circadian disorders, suggesting that it is highly likely that the gut microbiota may play a mediating role between frailty and circadian disruption through dietary modalities. For instance, a single-arm, two-sample clinical trial involving 19 participants with metabolic syndrome and a baseline mean daily eating window of ≥14 hours demonstrated that a 10-hour time-restricted diet enhanced cardiometabolic health in patients with metabolic syndrome who were receiving standard medical care, including high utilization of statins and antihypertensives. This finding suggests that time-restricted diets represent a potent lifestyle intervention that may be pertinent to the restoration of normal circadian rhythms in individuals.¹⁵ Additionally, an animal study employing multiple lung cancer cell lines, two xenograft mouse models, and a chemically-induced mouse lung cancer model revealed that an intermittent 6-hour time-restricted diet inhibited lung cancer progression and reprogrammed circadian gene expression in the host, in contrast to normal ad libitum feeding.¹⁶ Correspondingly, while there is no specific treatment option for frailty, the necessity for nutritional support is well recognized. A cohort study involving the oldest participants demonstrated a dose-dependent negative correlation between improvements in diet quality and the incidence of frailty.¹⁷ Another prospective cohort study analyzing a total of 180 older adults demonstrated that significant increases in energy, protein, vitamin D, vitamin C, and folate intake were associated with significant improvements in individual frailty and subsequent reductions in malnutrition rates.¹⁸ However, although the single link between gut microbiota metabolites and frailty or circadian rhythm disruption has been well established in previous studies, however, the role of gut microbiota metabolites in the bidirectional communication between circadian rhythm and frailty has not been addressed. Based on this, we propose hypotheses to emphasize the important role of gut microbiota metabolites in the bidirectional relationship between circadian rhythm disruption and frailty in the organism and to speculate on the possible mechanisms.

Hypothesis

Here we hypothesize that circadian disruption can induce individual frailty by disrupting gut microbiota composition and metabolites. Correspondingly, alterations in gut microbiota composition and metabolites in the frail status will also induce or exacerbate a state of circadian imbalance. When frailty occurs or when circadian rhythms produce abnormal oscillations, the gut microbiota and its metabolites respond, leading to abnormalities in a variety of important physiological pathways in the body, such as neuroendocrine, metabolic, and skeletal muscle mass. Rather than merely asserting the involvement of gut microbiota in frailty or circadian rhythm disorders, our hypothesis seeks to highlight the critical role of gut microbiota metabolites in mediating the bidirectional relationship between these conditions.

Discussion

The bidirectional relationship between circadian rhythm disruption and frailty encompasses numerous intriguing features and mechanisms. Sleep disorders, including insomnia, poor sleep quality, and reduced sleep duration, represent significant phenotypes of circadian disruption that both predict and directly influence frailty.^19,20 Furthermore, circadian rhythm disruption can induce oxidative damage to the brain, accelerate brain aging, and cause neuronal damage, all of which may impact frailty and contribute to the onset of cognitive frailty.^21,22 Consequently, frail older adults frequently exhibit psychological disorders such as anxiety and depression, which in turn exacerbate sleep-wake disturbances, disrupt normal eating patterns, and ultimately lead to the development of circadian disorders.²³

Although the precise mechanisms underlying this bidirectional relationship between circadian rhythm disruption and frailty have yet to be systematically elucidated, it can be speculated that metabolic function is one of the significant answers (Figure 1). Circadian rhythms are believed to regulate metabolic function, with many metabolic processes demonstrating circadian rhythmicity, suggesting they are subject to direct or intermittent circadian control. Moreover, individuals possessing variants and polymorphisms in the CRY2 and PER2 genes demonstrate metabolic dysfunction, evidenced by elevated blood glucose levels.²⁴ A study assessing the body’s circadian rhythm through melatonin level monitoring has revealed that disruptions in the central circadian rhythm, indicated by reduced melatonin levels, are linked to insulin resistance, diabetes and obesity.²⁵ Another study corroborated that metabolic syndrome, characterized by insulin resistance, is significantly associated with frailty,²⁶ suggesting that impaired metabolic function and the development of metabolic syndrome, potentially due to reduced melatonin levels during disruptions of the circadian rhythm, may constitute a mechanism through which such disruptions promote frailty. Cohort studies involving community-dwelling older adults have further substantiated the validity of this hypothesis.^27,28 Moreover, research on rodent models has shown that genetic disruption of circadian genes leads to abnormalities in both central and peripheral clocks, resulting in a range of symptoms indicative of metabolic syndrome, such as adipocyte hypertrophy, fatty liver, dyslipidemia, hypoglycemia, hyperglycemia, and obesity.^29,30 These alterations may accelerate the decline in muscle function and overall fitness, thereby contributing to the onset of frailty.³¹

Figure 1 Brief introduction of mechanisms for the role of gut microbiota metabolites (eg, butyrate) in circadian rhythm disruption mediating frailty onset.

Gut microbial metabolites represented by SCFAs imply another significant answer (Figure 1). SCFAs, as endocrine mediators of the gut microbiota, are not only capable of influencing skeletal muscle metabolism and function, but also have a circadian rhythm.^32,33 During the circadian rhythm disruption, such as insomnia or fragmented sleep, there is a consequent decrease in butyrate levels, a compound recognized for its anti-inflammatory properties and its role in maintaining mitochondrial function and which itself follows a circadian pattern.^34,35 This decrease is accompanied by an increased production of acetate and propionate.³⁶ This distinct process directly contributes to elevated levels of reactive oxygen species and dysregulated mitochondrial homeostasis. The ensuing upregulation of oxidative stress and mitochondrial dysfunction disrupts the microbiota-gut-muscle axis, ultimately altering the composition and structure of the gut microbiota. Such alterations facilitate an inappropriate increase in bacterial-derived material, leading to the accumulation of skeletal muscle hypoplasia. In conjunction with systemic chronic low-grade inflammation, these changes contribute to the onset of frailty.^37–39 Moreover, the decline in butyrate levels due to disruptions in circadian rhythms also specifically leads to impaired intestinal barrier function. This impairment facilitates the translocation of bacteria and their byproducts, such as lipopolysaccharides, into the circulatory system through a “leaky gut”, allowing their migration to the mesenteric lymph nodes, liver, and brain.^40,41 This process triggers a systemic inflammatory response that ultimately contribute to the development of frailty. Furthermore, both in vitro and in vivo studies have shown that butyrate can enhance the levels of regulatory T cells in the intestinal tract and promote the T cells’ polarization in the immune organs.⁴² This action helps prevent the accumulation of chronic inflammation in the intestinal tract, thereby reducing the risk of frailty. Additionally, butyrate has been shown to activate the PI3K/Akt/mTOR signaling pathway, which inhibits oxidative stress and autophagy in skeletal muscle cells, and promotes macrophage polarization towards the M2 phenotype, thereby preventing the onset of myasthenia gravis, which can reduce the incidence of sarcopenia and frailty.^32,43 In a separate animal study, butyrate was demonstrated to function as a histone deacetylase inhibitor, markedly enhancing histone acetylation at the corticotropin-releasing hormone receptor 2 (CRHR2) promoter in vitro.⁴⁴ This action normalized corticosterone levels and CRHR2 expression in vivo, resulting in improvements in brain function and behavior, potentially preventing cognitive frailty and exerting beneficial effects on the circadian system. Furthermore, SCFAs, exemplified by butyrate, can also modulate satiety through G-protein-coupled receptors (GPCR) 41 and 43, thereby interacting with the circadian system. Specifically, SCFAs decrease appetite by promoting the secretion of satiety peptides, such as peptide YY and glucagon-like peptide 1, from enteroendocrine cells via GPCRs.^45,46 This targeted regulatory influence of the circadian system on dietary rhythms, mediated by gut microbiota metabolites, effectively mitigates the decline in somatic function due to undernutrition or the development of metabolic syndrome due to overnutrition, both of which impact the state of frailty.

In summary, while existing research has identified a unidirectional relationship between gut microbiota and disruptions in circadian rhythm or frailty, the critical role of gut microbiota metabolites in the bidirectional interaction between circadian rhythm disruption and frailty remains unexplored. We hypothesize, based on a body of evidence and established pathophysiological mechanisms, that disruptions in circadian rhythm or the presence of frailty can alter normal physiological processes and functional pathways by impacting the composition and structure of gut microbiota metabolites. Notably, butyrate may play a pivotal role in this process.

Expectations

We suggest that subsequent research examining this hypothesis should comprehensively account for the potential impacts of significant covariates, including various circadian rhythm disorders such as insomnia, sleep-wake cycle disorders and sleep-wake delay, as well as other critical factors like age, physical activity, smoking habits, medication use, and comorbidities on the gut microbiota metabolites. This consideration is essential to minimize errors that may arise. In managing confounding variables, future studies should initially employ a stratified randomized grouping method to ensure baseline equilibrium. Furthermore, given that confounding variables may encompass both categorical and continuous types, it is crucial to utilize multifactor regression analysis techniques and propensity score analyses, the latter of which can effectively reduce the number of independent variables and address the limitations associated with the number of independent variables in both stratified and multifactor analyses by consolidating multiple confounders into a single composite score.

Research models, such as the recently proposed zebrafish model, are of significant interest due to their skeletal muscles exhibiting remarkable histological and molecular similarities to human muscles, which makes them an invaluable model for studying the gut-muscle-brain axis. Furthermore, zebrafish possess the unique ability to absorb substances from water through their skin and gills, which facilitates the use of drug-induced disease models and the subsequent testing of newly developed drugs.⁴⁷ In addition, assessing the concentrations of gut microbiota metabolites, such as butyrate, is also crucial for validating this hypothesis. It is recommended that future research employ more robust methodologies for sequencing gut microbiota metabolites. Techniques such as microbial metabolomics analysis based on gas chromatography-time-of-flight mass spectrometry (GC-TOF-MS) technology,⁴⁸ which facilitates high-throughput absolute quantification of over 150 significant gut flora metabolites, should be considered to enhance the identification and detection of specific gut microbial metabolites associated with circadian rhythm disorders and frailty, thereby fully substantiating the significance of butyrate. Additionally, future studies should also focus on polyamines, a class of metabolites produced by both the mammalian host and gut microbiota, which exhibit circadian characteristics. As versatile signaling molecules, polyamines participate in numerous physiological and pathological processes, and their levels exhibit significant fluctuations in frailty mouse models,⁴⁹ indicating that polyamines may play an equally critical role in frailty. Unfortunately, the currently available evidence does not allow for conclusions or speculation regarding the relationship between polyamines and the bidirectional connection between frailty and circadian disturbances. Further research is needed to elucidate these relationships.

In considering interventions, the most direct approach involves supplementation with exogenous butyrate or gavage therapy with Clostridium butyrate to evaluate its effects on the host’s circadian system or frailty status. Oral supplementation with exogenous probiotics, such as Lactobacillus casei, is believed to maintain intestinal ecological balance and stabilize the intestinal barrier, and should also be considered as a potential therapeutic intervention.³⁷ Furthermore, when designing the study, researchers should consider using clinical assessment scales, wearable devices, somatic function tests, and melatonin levels to assess changes in circadian rhythm disturbance and frailty following reductions in gut microbiota products such as butyrate. Subsequently, it should also be evaluated whether gut microbiota metabolite levels similarly improve after nutritional support and exercise therapy to improve a frail state or exogenous melatonin supplementation to improve circadian disorders. Equally important is the consideration of the study population. When designing studies, it is crucial to assess whether individuals in the “advanced stages of frailty” (high or very high frailty index scores) and those who develop frailty at a younger or middle age exhibit improved responses to interventions. Additionally, centenarians represent another valuable study population, often regarded as a model of healthy aging. It is important to acknowledge that the status of being a centenarian does not automatically equate to optimal health; rather, centenarians often exhibit higher morbidity rates and endure prolonged periods of illness compared to younger adults. Future research is advised to thoroughly examine the significance of this distinct population to enhance understanding of the critical role that gut microbiota metabolites play in the bidirectional relationship between circadian rhythm disruption and frailty. In this context, meticulous consideration of healthy aging populations is crucial, as it can significantly aid in eliminating potential confounding factors arising from the natural effects of aging on the circadian system, frailty, and gut microbiota metabolites. Additionally, given the reversible nature of frailty and the dynamic characteristics of circadian rhythms, it is imperative for future studies to incorporate extended follow-up periods and longitudinal data. This approach will facilitate the observation of changes in the composition and structure of gut microbiota and its metabolites throughout disease progression, thereby enabling a more robust establishment of causal relationships between gut microbiota metabolites, frailty, and circadian rhythm disruption. Such studies hold the potential to provide compelling evidence to test these hypotheses and may contribute to enabling frail individuals to reverse their frail status, thereby improving their quality of life.