Key Points
- Disruption of circadian rhythms and circadian clocks is an emerging risk factor for cardiometabolic disease. Skeletal muscle has an intrinsic circadian clock that regulates essential functions, including substrate metabolism.
- Skeletal muscle circadian clock disruption promotes a preferential shift toward lipid oxidation while reducing carbohydrate oxidation. The effects are apparent at the whole-body level evidenced by peripheral insulin resistance and glucose intolerance, increased energy homeostasis, and fasting hyperglycemia.
- The circadian system exhibits plasticity, suggesting that its disruption may be modifiable through lifestyle interventions. Exercise targets skeletal muscle tissue, potentially serving as a countermeasure to these metabolic derangements that increase cardiometabolic disease risk.
- Exercise modifies the skeletal muscle clock and may revert substrate utilization toward carbohydrate oxidation, as well as promote internal metabolic synchrony among tissues. Ultimately, these improvements in muscle function may reduce cardiometabolic disease risk.
INTRODUCTION
Temporal regulation of physiology and metabolism is governed by the circadian system. The core of this system is the molecular clock mechanism, which exists in every mammalian cell (1). Molecular clocks in both central and peripheral tissues work in a synchronized manner to temporally orchestrate the expression of thousands of genes in anticipation of upcoming physiological events. The collective output of these molecular clocks consists of approximate 24-h biological cycles, known as circadian rhythms (2). In humans, aspects of metabolism follow circadian rhythmicity. For example, endogenous plasma glucose concentrations peak early in the morning before waking (3). Whole-body insulin sensitivity is greatest in the morning and decreases in the late afternoon (4). Substrate utilization also is rhythmic; carbohydrate oxidation is greater in the morning and decreases at night, whereas lipid oxidation displays a reciprocal pattern (5). In addition, resting energy expenditure is greatest during the day and is lowest during the biological night (5). Notably, these rhythms reflect a typical and mostly healthy circadian pattern, but circadian rhythms are susceptible to disruption, which negatively impacts health. For example, disrupted molecular clock mechanisms cause temporal disorganization in physiological and metabolic processes, resulting in the development of chronic conditions including cardiometabolic disease (6).
Contributing to approximately 40% of body mass, skeletal muscle is one of the largest organ systems in the body and is responsible for force production, locomotion, and posture (7). In addition, skeletal muscle contributes to systemic physiology and metabolic processes. For example, it is the primary regulator of macronutrient metabolism (8) and also participates in abundant crosstalk mechanisms with other tissues such as the liver, adipose, pancreas, and skeletal system (9,10). The skeletal muscle circadian clock is upstream of many cellular processes and temporally orchestrates gene expression that is necessary for fundamental functions, such as myogenesis, transcription, and metabolism (11,12). Disruption of the skeletal muscle circadian clock alters substrate metabolism, which subsequently leads to peripheral insulin resistance and glucose intolerance, increased energy expenditure, and fasting hyperglycemia (13–15) observed at the whole-body level. Thus, proper function of the skeletal muscle circadian clock may be necessary for long-term cardiometabolic health.
Early evidence suggests that disruption of circadian rhythms may be actionable — an important point for the development of interventions designed to improve human health. Interventions aimed to restore the function of the molecular clock network, or optimize circadian timing, may lead to the restoration of circadian rhythms, thus, ameliorating cardiometabolic disease. For example, independent of diet composition, alterations to meal timing improves indices of cardiometabolic health in adults including reduced postprandial glucose (16), reduced appetite (17), and improved insulin sensitivity, β cell function, blood pressure, and markers of oxidative stress (18). Exercise may be another behavioral strategy with strong potential to optimize the circadian network and promote health and longevity (19). Although exercise has long been recognized for its disease-mitigating effects, all of the mechanisms by which exercise improves health are yet to be fully elucidated. It is possible that some exercise-induced health benefits are mediated through actions on the circadian clock in skeletal muscle. The skeletal muscle circadian clock is targetable with exercise (20,21) and, thus, is a viable candidate for restoring circadian function and improving health. Additional muscular properties improve with increased skeletal muscle contractility, including strength, function, and metabolic capacity. Thus, we hypothesize that exercise protects against cardiometabolic disease in part through actions on the skeletal muscle circadian clock. The objective of this narrative review is to highlight existing knowledge and gaps related to the connection between skeletal muscle circadian clock disruption to the development of cardiometabolic disease, as well as considerations of exercise as a therapeutic strategy for health restoration.
SKELETAL MUSCLE CLOCK IMPACTS WHOLE-BODY PHYSIOLOGY AND METABOLISM
Circadian clocks provide an evolutionary advantage to an organism by optimizing the timing of cellular metabolism to correspond with day and night cycles (6), and all skeletal muscles have an intrinsic molecular clock mechanism (12). The core mechanism of the molecular clock consists of a transcriptional-translational feedback loop; it is through these molecular feedback loops that physiological processes respond to timing information from organismal behaviors, as well as the outside environment (22). In brief, the positive arm of the of the molecular clock is composed of two core molecules: Circadian Locomotor Output Cycles Kaput (CLOCK) and brain and muscle Arnt-like protein-1 (BMAL1). These molecules heterodimerize and bind to E-box protein-binding DNA sequences, leading to the activation of core clock gene transcription including Period genes (Per1, Per2, and Per3) and Cytochrome genes (Cry1 and Cry2). The negative arm is composed of PER and CRY proteins, which inhibit transcriptional activity of the CLOCK/BMAL1 heterodimer (22).
The energetic demands of skeletal muscle dynamically oscillate across the 24-h cycle as an organism transitions from a resting/fasting phase to an active/feeding phase. The skeletal muscle clock supports this process by fine-tuning transcriptional control of metabolic genes in anticipation of upcoming metabolic challenges. Studies in mouse models reveal a temporal clustering of clock-controlled metabolic gene expression (13,14). During the rest/fasting phase, expression of genes that regulate fatty acid uptake and β-oxidation peaks (13,14). This oscillatory pattern of skeletal muscle gene expression also is reflected in circulating free fatty acids (23,24). In anticipation of waking/feeding, the skeletal muscle clock preferentially shifts substrate utilization toward carbohydrate metabolism by activating the transcription of genes that promote insulin sensitivity and glucose utilization while coordinately inhibiting genes involved in lipid oxidation, protein degradation, and amino acid transport (13,14). Toward the end of the wake/feeding phase, the skeletal muscle clock promotes energy storage by upregulating lipogenic gene expression — potentially indicative of intrinsic preparation for the upcoming rest/fasting phase (13). Although the timing of the skeletal muscle clock runs autonomously, it is influenced by environmental and behavioral cues. One physiological signal that provokes this diurnal substrate switching may be related to nutritional status: BMAL1 gene expression is altered in response to feeding, whereas a circadian clock suppressor, Rev-erbα gene expression, is altered in response to fasting (15).
Translational evidence for skeletal muscle circadian rhythms in humans is available. Human skeletal muscle myotubes exhibit a self-sustained oscillating clock that is detectable through the use of long-term bioluminescence reporter assays on BMAL1 and PER2 genes (25). Follow-up experimental approaches using serial gene expression assessments (every 4 h for a 72-h period) show rhythmic oscillations of clock gene expression in explanted myotubes from groups with diverse metabolic phenotypes, including lean/exercise trained, lean/untrained, obese, and individuals with type 2 diabetes (26). In addition to rhythmic clock gene expression, lipidomic studies reveal that diurnal oscillations of glycerolipids, glycerophospholipids, and spingolipids persist in ex vivo human primary myotubes (27). These diurnal fluctuations in skeletal muscle lipid metabolism may partially underlie the circadian fluctuations that are observed in circulating plasma lipids (28). Elegant studies in human primary myotubes use a perfusion system combined with continuous bioluminescence recordings to reveal that the skeletal muscle clock regulates the circadian secretion of basal myokines, including interleukin-6 (IL-6), IL-8, and macrophage colony-stimulating factor 1 (25). These circadian patterns in myokine secretion are consistent with earlier reports of cytokine diurnal variation in human circulation (29). These studies demonstrate the efficacy and value of ex vivo experimental approaches used to investigate human skeletal muscle clocks and diurnal metabolism with the advantage of being independent of external cues such as exercise or food intake.
Whole-body energy expenditure exhibits a circadian rhythm (5), and diurnal shifts in skeletal muscle mitochondrial metabolism may be reflected in this pattern. Serial skeletal muscle biopsy sampling across the day-night cycle (e.g., 08:00, 13:00, 18:00, 23:00, and 04:00) from untrained, but otherwise healthy, volunteers show robust rhythmicity in skeletal muscle oxidative capacity, assessed using respirometry (30). Specifically, skeletal muscle oxidative capacity peaks at 23:00 and is lowest at 04:00. This trend is consistently observed in whole-body energy expenditure, assessed by indirect calorimetry. Skeletal muscle oxidative capacity fluctuations may be driven by the circadian regulation of mitochondrial dynamics, rather than mitochondrial biogenesis (30). Diurnal patterns in functional and performance outcomes have also been reported, such as higher power and strength in the afternoon compared with in the morning, in untrained individuals (31).
SKELETAL MUSCLE CLOCK DISRUPTION CONTRIBUTES TO CARDIOMETABOLIC DISEASE
Circadian and metabolic networks are inextricably linked, whereas circadian clock disruption is hypothesized to promote pathology (6). Several lines of evidence — spanning from the molecular and genetic underpinnings (32), to highly controlled experimental studies in both animal (15,33,34) and human models (35,36), to epidemiological findings (37,38) — all provide support for the notion that circadian disruption contributes to the development of cardiometabolic disease. One of the earliest links between the circadian system and metabolic disease was the finding that Clock mutant mice develop obesity, hyperleptinemia, hepatic steatosis, hyperglycemia, and hyperinsulinemia (32). Clock mutant mice also are characterized by declines is skeletal muscle mitochondrial physiology, hallmarked by reduced PGC-1α, reduced mitochondrial transcription factor-A protein content, and reduced mitochondrial content leading to poor exercise tolerance (39). This experimental approach (global knockout) complicates interpretation of the role of the skeletal muscle clock, as both central and peripheral clocks are disrupted. Rather, use of the skeletal muscle clock-specific knockout model provides an advantage in this respect, allowing one to decipher the contributing role of the skeletal muscle clock. Indeed, such experiments indicate that skeletal muscle clock disruption contributes to a metabolically dysfunctional phenotype.
The skeletal muscle clock transcriptionally controls the expression of genes that regulate glucose metabolism (13), whereas its disruption may be an underlying cause of insulin resistance and glucose intolerance. Skeletal muscle-specific knockout of BMAL1, a core component of the molecular clock, leads to glucose intolerance and elevated fasting blood insulin levels in animal models (33). This observation may be due to reduced skeletal muscle glucose uptake, as GLUT-4 mRNA and protein levels are significantly reduced (33,34). These findings are mirrored in human skeletal muscle myotubes. Silencing of the Clock gene (siRNA mediated) disrupts the expression of genes involved in GLUT-4 expression, translocation, and recycling (40). Clock-disrupted myotubes have reduced basal, and insulin-stimulated, glucose uptake compared with control myotubes, assessed by radioactive glucose uptake assays (40). Cross-sectional comparisons of myotubes from human donors with varying metabolic phenotypes suggest skeletal muscle circadian rhythms may be dampened in metabolic disease. SIRT and NAMPT, metabolic genes under control of the clock, show robust rhythmic expression when cultured from lean/exercise-trained donors, whereas rhythmic patterns are not observed in myotubes from patients with type 2 diabetes. Moreover, the rhythmic amplitude of Rev-erbα expression positively correlates with insulin sensitivity, measured with the euglycemic-hyperglycemic clamp, across four metabolic phenotypes: lean/exercise trained, lean/untrained, obese, and type 2 diabetes (26). Whether dampened skeletal muscle circadian rhythms are a cause or consequence of metabolic disease cannot be discerned from this study design. Nonetheless, these studies point to an essential role for the skeletal muscle clock in maintaining metabolic health by promoting glucose clearance from the bloodstream via facilitating insulin-stimulated glucose uptake. Given that skeletal muscle mass is responsible for approximately 80% of postprandial (insulin-stimulated) glucose uptake, it is conceivable that skeletal muscle clock disruption contributes to the development of proatherogenenic postprandial glucose excursions, which hallmark insulin resistance and type 2 diabetes (41).
Skeletal muscle clock disruption also impacts whole-body energy homeostasis. A consistent observation among several research groups is that the skeletal muscle clock promotes carbohydrate oxidation, whereas clock disruption leads to a preferential substrate shift toward fatty acid oxidation (14,15,33). An increased reliance on fatty acid oxidation is evidenced in several metrics of whole-body energy metabolism, such as changes in body composition, energy expenditure, and respiratory exchange ratio (RER). For example, skeletal muscle BMAL1 knockout animal models have a leaner body composition compared with wild type, primarily driven by reductions in fat mass. Importantly, this occurs independent of modifications in feeding behavior or physical activity (33). A leaner phenotype is consistent with higher energy expenditure during the active (feeding) phase, assessed by calorimetry (14), as well as lower RER during the rest (fasting) phase. At first glance, fat mass loss may seem to be beneficial for health, in the context of body weight regulation. However, it is important to note that this leaner phenotype comes at the metabolic cost of increased peripheral insulin resistance and reduced skeletal muscle glucose uptake (14,15,33), which are early hallmarks of type 2 diabetes development (42).
Circadian metabolism is a system-wide coordinated effort among clocks in several tissues such as the skeletal muscle, liver, adipose, and pancreas (43). Through these circadian crosstalk mechanisms, clock disruption in one tissue can impact the metabolic function of another. In healthy physiology, skeletal muscle and liver work in a coordinated effort to regulate blood glucose homeostasis. However, studies using skeletal muscle clock knockout animal models indicate that skeletal muscle clock disruption leads to excessive hepatic glucose production (15). For example, skeletal muscle clock disruption leads to a marked reduction in circulating free fatty acids and triglycerides, which mimics a fasting metabolic state. The liver accordingly responds to this perceived fast by increasing hepatic glucose output (15). Through this muscle-liver crosstalk, it may be that skeletal muscle clock disruption influences metabolic function of the liver. Although it is unknown if this phenomenon occurs in humans, it is worth noting that patients with type 2 diabetes experience an exaggerated elevation in morning glucose due to increased hepatic glucose output, known as the “dawn phenomenon” (44).
HUMAN SKELETAL MUSCLE CLOCK DISRUPTION IMPACTS WHOLE-BODY PHYSIOLOGY AND METABOLISM
Studies in preclinical models consistently point to a role for skeletal muscle clock disruption in altering substrate utilization, reducing glucose metabolism, and impacting energy metabolism. Translating these findings to humans requires probing potential mechanisms that may cause skeletal muscle clock disruption in human skeletal muscle. In this regard, the circadian timing of human behavior is emerging as an important factor. Even a short-term shift in sleep and wake patterns is sufficient to impact the skeletal muscle clock. For example, one night of sleep loss, or overnight wakefulness, increases skeletal muscle BMAL1 protein expression, a core component of the molecular clock (45). This is accompanied by downstream changes in metabolic gene expression including an upregulation of genes associated with fatty acid uptake and reduced activation of glycolytic pathways (45). Interestingly, this pattern of substrate shifting is consistent with preclinical findings (14,15,33) and is further reflected in decreases in whole-body glucose metabolism. For example, oral glucose tolerance deteriorates the morning after overnight wakefulness, evidenced by increased fasting and postprandial glucose, whereas insulin concentrations are unaffected, which may be indicative of reduced peripheral glucose uptake (45).
Physiology and metabolism are optimized when the skeletal muscle circadian clock and the external environment/behavior are temporally aligned (6). Thus, a potential explanation for these findings is that overnight wakefulness induces a state of skeletal muscle circadian misalignment, that is, a mismatch between circadian timing intrinsic to skeletal muscle relative to timing in the external environment. These findings (45) are further corroborated by additional controlled experiments in humans that suggest circadian misalignment contributes to poor glucose metabolism. To test this hypothesis, various circadian misalignment protocols have been developed that aim to experimentally misalign internal circadian time and external time (46).
For example, the forced desynchrony protocol refers to monitoring participants in a time-free setting while progressively simulating longer days using controlled light/dark cycles and meal times. Specifically, imposing a 28-h day (rather than a 24-h day) for seven consecutive days achieves a complete inversion of intrinsic circadian time relative to the external environment. A study in 10 adults revealed that experimentally induced circadian misalignment using this forced desynchrony protocol results in a metabolic phenotype consistent with the prediabetic state — that is, increased glucose despite increased insulin, increased mean arterial blood pressure, reduced sleep efficiency, and decreased leptin (35). Similarly, inducing circadian misalignment in healthy adults by rapidly inverting the day-night cycle during inpatient stays reduces insulin-stimulated glucose uptake, assessed using a 40 mU·m−2 euglycemic-hyperinsulinemic clamp (36). The decrease in skeletal muscle insulin sensitivity is attributed to reduced nonoxidative glucose disposal in skeletal muscle, which suggests a reduction in skeletal muscle glycogen synthesis. This finding is accompanied by increased fatty acid metabolic signatures in skeletal muscle biopsies collected during periods of circadian misalignment.
One interpretation is that these alterations in skeletal muscle metabolism may be an elegant example of metabolic dyssynchrony: molecular analysis of skeletal muscle circadian proteins reveals that the skeletal muscle clock does not rapidly adjust to this experimental day/night inversion, as peak BMAL1 and PER2 expression did not differ between circadian aligned versus misaligned conditions. Hence, the skeletal muscle clock orchestrates “sleeping metabolism” even during states of wakefulness. Whole-body energy expenditure also is perturbed during circadian misalignment: sleeping metabolic rate increases, assessed by whole-room calorimetry. Interestingly, these findings are consistent with increased energy expenditure observed in animal models of skeletal muscle circadian disruption, as described above (14,15). It also is interesting to note that short-term circadian misalignment increases sleeping metabolic rate and energy expenditure, which is in direct opposition to the energetic phenotype often pursued in the context of healthy aging in humans. For example, moderate calorie restriction reduces sleeping metabolic rate and energy expenditure in humans (47). Therefore, circadian misalignment contributes to metabolic inefficiency, which is an undesirable health outcome, given that metabolic slowing is hypothesized to promote longevity (48).
Although these studies provide evidence from acute behavioral manipulations, it is crucial to consider the long-term consequences of a lifestyle that chronically predisposes one to circadian misalignment. Perhaps most concerning are the long-term metabolic implications of shift work. Epidemiological evidence supports that shift workers are at an increased risk for obesity, diabetes, and cardiovascular disease (37). These associations are not fully explained by other unhealthy lifestyle factors that often accompany shift work, including poor diet, physical inactivity, smoking, and body mass index; having a history of rotating shift work is an independent risk factor for type 2 diabetes — even after controlling for such variables (38). It is worth noting that these findings are consistent with preclinical investigations, which conclude that disruption of circadian machinery leads to metabolic dysfunction independent of changes in feeding behavior and physical activity (14,15,33). Taken together, these findings further highlight the importance of circadian timing in optimizing cardiometabolic health, above and beyond traditional lifestyle factors. Although the underlying mechanisms between shift work and poor cardiometabolic health are still being determined, these highly controlled experimental studies in humans emphasize the detrimental effects of circadian misalignment, even in the short-term.
EXERCISE TO RESET THE SKELETAL MUSCLE CLOCK
It is well accepted that exercise protects against the development of cardiometabolic disease. Exercise inherently impacts skeletal muscle tissue and also impacts skeletal muscle circadian rhythms. For example, exercise provides a time setting cue forthe skeletal muscle clock. Studies in PER2: LUC mice reveal that 4 wk of exercise training shifts the circadian rhythm phase by approximately 2–3 h in soleus, extensor digitorum longus, and flexor digitorum brevis muscles (20). Interestingly, a circadian rhythm phase shift was also observed in the lungs, indicating that exercise synchronizes circadian clocks across multiple tissues. In humans, an acute bout of exercise, including both endurance (49) and resistance modalities (21), increases skeletal muscle clock gene expression. Exercise intensity may not influence clock gene expression, as no differences occur after exercise bouts completed at 50% versus 70% V̇O2max aerobic exercise (49). On the other side of the activity spectrum, experiments using denervated mouse models, which are characterized by the absence of neural input and lack skeletal muscle contractile function, display altered skeletal muscle clock gene expression (50). The exact mechanisms by which exercise influences the skeletal muscle clock have yet to be fully elucidated, but likely involve motor activation of myofibers, as motor activity influences oscillatory patterns of approximately 15% of skeletal muscle clock gene expression (51). Oxygen sensing may also influence the skeletal muscle molecular clock through the hypoxia-responsible transcription factor (HIF) pathway interaction, suggesting an indirect role of increased skeletal muscle blood flow in altering skeletal muscle circadian rhythms (52). The effects of long-term exercise interventions on skeletal muscle clock genes in humans are not well described. One study indicates that patients with coronary artery disease and type 2 diabetes have increased expression of the clock-associate gene (ALAS1), but no change in clock gene expression, after long-term exercise training (37). There are some key considerations to interpreting findings from skeletal muscle biopsies in humans. The time course of exercise-induced skeletal muscle gene expression may vary by gene, and biopsies only provide a single time point within a dynamic system (53). Exercise-induced changes are muscle group specific (54); hence, generalizing findings from single-site sampling to all muscles is cautioned.
An emerging question is whether the time of day in which exercise is performed is relevant for optimizing cardiometabolic health, and this question certainly extends to skeletal muscle. Exercise modulates skeletal muscle metabolism, and the extent to which this occurs differs by diurnal phase. Experiments in animal models provide evidence that exercise during the early active phase induces a more robust activation of glycolytic pathways compared with exercise during the early rest phase (55). These intriguing findings raise the suspicion of an optimal time window to perform exercise in efforts to augment glucose metabolism. Translating these findings to humans is complicated by the fact that rodent models are nocturnal and may have different feeding patterns than humans. Nonetheless, future investigations are warranted, given the obvious therapeutic implications of augmenting exercise responses by tweaking exercise timing. There are data regarding the importance of exercise timing in humans. A secondary analysis of the Midwest Exercise Trial 2 reports that participants exercising primarily during the first part of the day (>50% of exercise sessions completed between 7:00 and 11:59) had greater weight loss than those exercising later in the afternoon (>50% of exercise sessions completed between 15:00 and 17:00) (56). In contrast, time of day did not augment improvements in glycemic control and postprandial glucose responses in morning exercise training versus evening exercise training in 40 sedentary, overweight adults (57). A key consideration for evaluating the role of circadian timing and exercise in humans will be adequately accounting for differences in nutritional status. The skeletal muscle clock preferentially selects metabolic fuel based on fasting and feeding cues (15) and may subsequently impact the response of skeletal muscle clock to exercise.
FUTURE DIRECTIONS
The data described above advocate for future research that examines the interaction between exercise and skeletal muscle circadian rhythms. Skeletal muscle circadian disruption leads to a preferential shift in substrate utilization toward lipid metabolism while reducing carbohydrate metabolism. This consequentially leads to reduced glucose uptake, metabolic inefficiency, and metabolic dyssynchrony manifesting at the whole-body level such as glucose intolerance, increased energetic homeostasis, and hyperglycemia (Fig. 1). Exercise may be a countermeasure to skeletal muscle circadian disruption. Exercise modifies the skeletal muscle clock mechanism, as well as clock-controlled metabolic pathways that may revert substrate metabolism back toward an optimal balance between carbohydrate and lipid oxidation, thus offsetting these detrimental effects on whole-body cardiometabolic risk (Fig. 2). The circadian system modulates health through an elaborate systemic coordination among circadian clocks. Furthermore, exercise influences circadian clocks in multiple tissues and has the potential to improve metabolic synchrony. The timing in which exercise occurs may be another variable that can be toggled in efforts to further augment exercise-induced improvements in metabolism.
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Figure 1: Conceptual framework illustrating the hypothesis that skeletal muscle clock disruption contributes to the development of cardiometabolic disease. A. Skeletal muscle circadian clock disruption — including molecular knockout animal models and circadian misalignment protocols in humans — results in a preferential shift in substrate utilization toward lipid oxidation while reducing carbohydrate oxidation. B. This has several consequences at the level of the skeletal muscle that subsequently alters whole-body physiology and metabolism including the following: 1. reduced glucose uptake manifesting at the whole-body level as glucose intolerance and peripheral insulin resistance; 2. metabolic inefficiency at the level of skeletal muscle manifesting at the whole-body level as lower respiratory exchange ratio and higher energy metabolism; and 3. metabolic dyssynchrony among tissues manifesting at the whole-body level as fasting hyperglycemia.
Figure 2: Conceptual framework illustrating the hypothesis that exercise resets the skeletal muscle clock leading to improved cardiometabolic health. A. Exercise modifies the skeletal muscle circadian clock mechanism. This stimulus may potentially reshift skeletal muscle substrate utilization back toward an optimal balance of lipid and carbohydrate oxidation. B. This metabolic reshifting is hypothesized to counter changes in skeletal muscle metabolism that are induced by circadian disruption. This leads to metabolic improvements at the level of the skeletal muscle including 1. increased skeletal muscle glucose uptake, 2. improved metabolic efficiency, and 3. enhanced metabolic synchrony among tissues. Ultimately, these improvements will reduce long-term cardiometabolic disease risk.
Circadian-informed exercise prescription is still in its infancy, and little is known about the ways by which exercise parameters — duration, intensity, dose, or type — influence the circadian system. Whether factors such as demographic characteristics, meal timing, training status, chronotype, or disease state influence the association between exercise and circadian physiology has yet to be defined. The optimal timing of exercise implementation in lifestyles characterized by circadian disruption (e.g., shift work) is currently unknown. Given that 16% of the working population engages in irregular shift hours (58), addressing this question has public health implications.
There is a need to translate biological insights learned from preclinical models into the context of integrative human physiology. To support the acceleration of circadian-related translational research, major advancements in methodological approaches for the accurate study of circadian rhythms are needed. For example, there remains a need to quantify the degree of circadian disruption and circadian dyssynchrony in humans. In this regard, the validation of circulating human biomarkers would provide an invaluable tool to assess the efficacy of various interventions aimed to restore circadian health. Tissue-specific insights will continue to be made through the application of in vitro synchronization procedures (59). In addition, luciferase reporter assays are advantageous in that they can provide a temporal domain to cell culture experiments (60). The use of -omics technology will continue to provide important insights along the circadian pathways by combining transcriptomic, proteomic, and metabolomics approaches. Nonetheless, widespread advances are needed, and it is likely that cross-disciplinary approaches among circadian biologists and human physiologists will propel these efforts.
CONCLUSION
Cardiometabolic disease places substantial strain on health care systems worldwide, and there is a growing appreciation for the role of disrupted circadian biology in contributing to this problem. A synthesis of preclinical and human research described within this review reveals a remarkably consistent narrative: disruption of the skeletal muscle clock, through molecular knockout of circadian clock machinery or behavioral misalignment, results in a preferential shift of substrate metabolism toward lipid oxidation while reducing carbohydrate oxidation. The cardiometabolic consequence of skeletal muscle substrate switching is apparent at the whole-body level, evidenced by impaired glucose tolerance, increased energy expenditure, and fasting hyperglycemia. These metabolic derangements occur even in the absence of alterations in physical activity and feeding, which strongly implicates a role for the circadian system in regulating cardiometabolic health. The circadian system exhibits plasticity, providing a potential mechanism whereby behavioral interventions may modify disruption of circadian mechanisms and restore health. Exercise may prove to be an effective strategy in resetting skeletal muscle circadian clocks by reverting skeletal muscle substrate metabolism toward an optimal balance of carbohydrate and lipid metabolism. Although the complexities of the exercise-molecular clock association are still being discovered, there is emerging evidence that exercise impacts molecular clock mechanisms across the body, supporting its potential as a future “chronotherapeutic” for preventing and treating cardiometabolic disease.
Acknowledgment
Illustrations were created with BioRender.
M.L.E. was supported by NIH T32 DK064584. K.A.E. was supported by NIH 1R01AR066082 and U01AG055137. T.W.B.'s effort for this project was partially supported by NIH grants R21AG049974, R01AG056769, and U01AR071133.
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