Differential effects of diet composition and timing of feeding behavior on rat brown adipose tissue and skeletal muscle peripheral clocks

Circadian desynchrony and glucose metabolism

The circadian timing system controls glucose metabolism in a time-of-day dependent manner. In mammals, the circadian timing system consists of the main central clock in the bilateral suprachiasmatic nucleus (SCN) of the anterior hypothalamus and subordinate clocks in peripheral tissues. The oscillations produced by these different clocks with a period of approximately 24-h are generated by the transcriptional-translational feedback loops of a set of core clock genes. Glucose homeostasis is one of the daily rhythms controlled by this circadian timing system. The central pacemaker in the SCN controls glucose homeostasis through its neural projections to hypothalamic hubs that are in control of feeding behavior and energy metabolism. Using hormones such as adrenal glucocorticoids and melatonin and the autonomic nervous system, the SCN modulates critical processes such as glucose production and insulin sensitivity. Peripheral clocks in tissues, such as the liver, muscle, and adipose tissue serve to enhance and sustain these SCN signals. In the optimal situation all these clocks are synchronized and aligned with behavior and the environmental light/dark cycle. A negative impact on glucose metabolism becomes apparent when the internal timing system becomes disturbed, also known as circadian desynchrony or circadian misalignment. Circadian desynchrony may occur at several levels, as the mistiming of light exposure or sleep will especially affect the central clock, whereas mistiming of food intake or physical activity will especially involve the peripheral clocks. In this review, we will summarize the literature investigating the impact of circadian desynchrony on glucose metabolism and how it may result in the development of insulin resistance. In addition, we will discuss potential strategies aimed at reinstating circadian synchrony to improve insulin sensitivity and contribute to the prevention of type 2 diabetes.

Sweet treats before sleep disrupt the clock system and increase metabolic risk markers in healthy rats

AIM

Biological rhythms are endogenously generated natural cycles that act as pacemakers of different physiological mechanisms and homeostasis in the organism, and whose disruption increases metabolic risk. The circadian rhythm is not only reset by light but it is also regulated by behavioral cues such as timing of food intake. This study investigates whether the chronic consumption of a sweet treat before sleeping can disrupt diurnal rhythmicity and metabolism in healthy rats.

METHODS

For this, 32 Fischer rats were administered daily a low dose of sugar (160 mg/kg, equivalent to 2.5 g in humans) as a sweet treat at 8:00 a.m. or 8:00 p.m. (ZT0 and ZT12, respectively) for 4 weeks. To elucidate diurnal rhythmicity of clock gene expression and metabolic parameters, animals were sacrificed at different times, including 1, 7, 13, and 19 h after the last sugar dose (ZT1, ZT7, ZT13, and ZT19).

RESULTS

Increased body weight gain and higher cardiometabolic risk were observed when sweet treat was administered at the beginning of the resting period. Moreover, central clock and food intake signaling genes varied depending on snack time. Specifically, the hypothalamic expression of Nampt, Bmal1, Rev-erbα, and Cart showed prominent changes in their diurnal expression pattern, highlighting that sweet treat before bedtime disrupts hypothalamic control of energy homeostasis.

CONCLUSIONS

These results show that central clock genes and metabolic effects following a low dose of sugar are strongly time-dependent, causing higher circadian metabolic disruption when it is consumed at the beginning of the resting period, that is, with the late-night snack.

Understanding Circadian and Circannual Behavioral Cycles of Captive Giant Pandas (Ailuropoda melanoleuca) Can Help to Promote Good Welfare

Circadian and circannual cycles of behavior regulate many aspects of welfare including metabolism, breeding, and behavioral interactions. In this study, we aim to demonstrate how systematically determining circadian and circannual cycles can provide insight into animals' needs and be part of an evidence-based approach to welfare assessment. We measured and analyzed the observational behavioral data of 13 zoo-housed giant pandas (Ailuropoda melanoleuca), across life stages and between sexes, each month for one year using live camera footage from six zoos across the world. Our results indicate that life stage was associated with changes in overall activity, feeding, locomotion, and pacing, and that sex influenced scent anointing and anogenital rubbing. Overall, the circadian rhythms showed three peaks of activity, including a nocturnal peak, as seen in wild giant pandas. We also found associations between sexual-related, stereotypical/abnormal, and feeding behavior, which are possibly linked to the timing of migration of wild pandas, and elucidated the relationship between a mother and cub, finding that they concentrate maternal behaviors to mainly after closing hours. Understanding these cycle patterns can aid animal care staff in predicting changing needs throughout the day, year, and life cycle and preemptively provide for those needs to best avoid welfare concerns.

Liver and muscle circadian clocks cooperate to support glucose tolerance in mice

Physiology is regulated by interconnected cell and tissue circadian clocks. Disruption of the rhythms generated by the concerted activity of these clocks is associated with metabolic disease. Here we tested the interactions between clocks in two critical components of organismal metabolism, liver and skeletal muscle, by rescuing clock function either in each organ separately or in both organs simultaneously in otherwise clock-less mice. Experiments showed that individual clocks are partially sufficient for tissue glucose metabolism, yet the connections between both tissue clocks coupled to daily feeding rhythms support systemic glucose tolerance. This synergy relies in part on local transcriptional control of the glucose machinery, feeding-responsive signals such as insulin, and metabolic cycles that connect the muscle and liver. We posit that spatiotemporal mechanisms of muscle and liver play an essential role in the maintenance of systemic glucose homeostasis and that disrupting this diurnal coordination can contribute to metabolic disease.

Constitutive Serotonin Tone as a Modulator of Brown Adipose Tissue Thermogenesis: A Rat Study

Brown adipose tissue (BAT), an important regulator of thermogenic and metabolic processes, is considered a promising target to combat metabolic disorders. The neurotransmitter and hormone serotonin (5HT) is a major modulator of energy homeostasis, with its central and peripheral pools acting in opposing ways. To better understand how individual variations in 5HT homeostasis influence the thermogenic functionality of BAT, we used a rat model consisting of two sublines with constitutively increased (high-5HT) or decreased (low-5HT) whole-body 5HT tone, developed by selective breeding for platelet 5HT parameters. We have shown that animals with constitutively low 5HT activity maintained at a standard housing temperature (22 °C) have greater interscapular BAT (iBAT) mass and higher iBAT metabolic activity (as evidenced by measurements of iBAT temperature and glucose uptake), accompanied by increased iBAT mRNA expression of key thermogenic genes, compared to animals with high 5HT tone. In response to further thermogenic challenges-intermittent cold exposure or treatment with a β₃-adrenergic agonist-5HT sublines show several functional and molecular differences linking constitutively low endogenous 5HT tone to higher BAT activity/capacity. Overall, the results support a role of 5-HT in the control of BAT thermogenesis They also suggest that individuals with lower 5HT activity may be more sensitive to β3-adrenergic drugs.

Metabolism and exercise: the skeletal muscle clock takes centre stage

Circadian rhythms that influence mammalian homeostasis and overall health have received increasing interest over the past two decades. The molecular clock, which is present in almost every cell, drives circadian rhythms while being a cornerstone of physiological outcomes. The skeletal muscle clock has emerged as a primary contributor to metabolic health, as the coordinated expression of the core clock factors BMAL1 and CLOCK with the muscle-specific transcription factor MYOD1 facilitates the circadian and metabolic programme that supports skeletal muscle physiology. The phase of the skeletal muscle clock is sensitive to the time of exercise, which provides a rationale for exploring the interactions between the skeletal muscle clock, exercise and metabolic health. Here, we review the underlying mechanisms of the skeletal muscle clock that drive muscle physiology, with a particular focus on metabolic health. Additionally, we highlight the interaction between exercise and the skeletal muscle clock as a means of reinforcing metabolic health and discuss the possible implications of the time of exercise as a chronotherapeutic approach.

Different levels of circadian (de)synchrony -- where does it hurt?

A network of cellular timers ensures the maintenance of homeostasis by temporal modulation of physiological processes across the day. These so-called circadian clocks are synchronized to geophysical time by external time cues (or zeitgebers). In modern societies, natural environmental cycles are disrupted by artificial lighting, around-the-clock availability of food or shift work. Such contradictory zeitgeber input promotes chronodisruption, i.e., the perturbation of internal circadian rhythms, resulting in adverse health outcomes. While this phenomenon is well described, it is still poorly understood at which level of organization perturbed rhythms impact on health and wellbeing. In this review, we discuss different levels of chronodisruption and what is known about their health effects. We summarize the results of disrupted phase coherence between external and internal time vs. misalignment of tissue clocks amongst each other, i.e., internal desynchrony. Last, phase incoherence can also occur at the tissue level itself. Here, alterations in phase coordination can emerge between cellular clocks of the same tissue or between different clock genes within the single cell. A better understanding of the mechanisms of circadian misalignment and its effects on physiology will help to find effective tools to prevent or treat disorders arising from modern-day chronodisruptive environments.

Association of Shift Work with Normal-Weight Obesity in Community-Dwelling Adults

BACKGRUOUND

Shift work is associated with obesity and metabolic syndrome. However, this association in the normal-weight population remains unclear. This study aimed to investigate whether shift work is associated with normal-weight obesity (NWO).

METHODS

From the nationally representative Korea National Health and Nutrition Examination Survey (KNHANES) dataset (2008 to 2011), 3,800 full-time workers aged ≥19 years with a body mass index (BMI) ≤25 kg/m2 were analysed. We defined NWO as BMI ≤25 kg/m2 and body fat percentage ≥25% in men and ≥37% in women. Working patterns were classified into "daytime," "other than daytime," and "shift." Multivariable logistic regression analysis was performed to evaluate the relationship between shift work and NWO.

RESULTS

Shift work was associated with higher odds of NWO than daytime work (adjusted odds ratio [aOR], 1.47; 95% confidence interval [CI], 1.04 to 2.09) and night/evening work (aOR, 1.87; 95% CI, 1.11 to 3.14) after adjustment for type of work, working hours, age, sex, BMI, 25-hydroxyvitamin D levels, homeostatic model assessment for insulin resistance, and other sociodemographic factors. In subgroup analyses, the association between shift work and NWO was more robust in those aged ≥60 years and those working ≥56 hours/week.

CONCLUSION

Shift work was associated with NWO in community-dwelling Korean adults, independent of age, sex, BMI, and other covariates.

Running skeletal muscle clocks on time- the determining factors

Circadian rhythms generate 24 h-long oscillations, which are key regulators of many aspects of behavior and physiology. Recent circadian transcriptome studies have discovered rhythmicity at the transcriptional level of hundreds of skeletal muscle genes, with roles in skeletal muscle growth, maintenance, and metabolic functions. These rhythms allow this tissue to perform molecular functions at the appropriate time of the day in order to anticipate environmental changes. However, while the last decade of research has characterized several aspects of the skeletal muscle molecular clock, many still are unexplored, including its functions, regulatory mechanisms, and interactions with other tissues. The central clock is believed to be located in the suprachiasmatic nucleus (SCN) of the brain hypothalamus, providing entrainment to peripheral organs through humoral and neuronal signals. However, these mechanisms of action are still unknown. Conversely, muscle tissue can be entrained through extrinsic, SCN-independent factors, such as feeding and physical activity. In this review, we provide an overview of the recent research about the extrinsic and intrinsic factors required for skeletal muscle clock regulation. Furthermore, we discuss the need for future studies to elucidate the mechanisms behind this regulation, which will in turn help dissect the role of circadian disruption at the onset of aging and diseases.

Time-restricted feeding during the inactive phase abolishes the daily rhythm in mitochondrial respiration in rat skeletal muscle

Shift-workers show an increased incidence of type 2 diabetes mellitus (T2DM). A possible mechanism is the disruption of the circadian timing of glucose homeostasis. Skeletal muscle mitochondrial function is modulated by the molecular clock. We used time-restricted feeding (TRF) during the inactive phase to investigate how mistimed feeding affects muscle mitochondrial metabolism. Rats on an ad libitum (AL) diet were compared to those that could eat only during the light (inactive) or dark (active) phase. Mitochondrial respiration, metabolic gene expressions, and metabolite concentrations were determined in the soleus muscle. Rats on AL feeding or dark-fed TRF showed a clear daily rhythm in muscle mitochondrial respiration. This rhythm in mitochondrial oxidative phosphorylation capacity was abolished in light-fed TRF animals and overall 24h respiration was lower. The expression of several genes involved in mitochondrial biogenesis and the fission/fusion machinery was altered in light-fed animals. Metabolomics analysis indicated that light-fed animals had lost rhythmic levels of α-ketoglutarate and citric acid. Contrastingly, lipidomics showed that light-fed animals abundantly gained rhythmicity in levels of triglycerides. Furthermore, while the RER shifted entirely with the food intake in the light-fed animals, many measured metabolic parameters (e.g., activity and mitochondrial respiration) did not strictly align with the shifted timing of food intake, resulting in a mismatch between expected metabolic supply/demand (as dictated by the circadian timing system and light/dark-cycle) and the actual metabolic supply/demand (as dictated by the timing of food intake). These data suggest that shift-work impairs mitochondrial metabolism and causes metabolic inflexibility, which can predispose to T2DM.

Chronic Social Defeat Stress Shifts Peripheral Circadian Clocks in Male Mice in a Tissue-Specific and Time-of-Day Dependent Fashion

Uncontrollable stress is linked to the development of many diseases, some of which are associated with disrupted daily rhythms in physiology and behavior. While available data indicate that the master circadian pacemaker in the suprachiasmatic nucleus (SCN) is unaffected by stress, accumulating evidence suggest that circadian oscillators in peripheral tissues and organs can be shifted by a variety of stressors and stress hormones. In the present study, we examined effects of acute and chronic social defeat stress in mice and addressed the question of whether effects of uncontrollable stress on peripheral clocks are tissue specific and depend on time of day of stress exposure. We used mice that carry a luciferase reporter gene fused to the circadian clock gene Period2 (PER2::LUC) to examine daily rhythms of PER2 expression in various peripheral tissues. Mice were exposed to social defeat stress in the early (ZT13-14) or late (ZT21-22) dark phase, either once (acute stress) or repeatedly on 10 consecutive days (chronic stress). One hour after the last stressor, tissue samples from liver, lung, kidney, and white adipose tissue (WAT) were collected. Social defeat stress caused a phase delay of several hours in the rhythm of PER2 expression in lung and kidney, but this delay was stronger after chronic than after acute stress. Moreover, shifts only occurred after stress in the late dark phase, not in the early dark phase. PER2 rhythms in liver and WAT were not significantly shifted by social defeat, suggesting a different response of various peripheral clocks to stress. This study indicates that uncontrollable social defeat stress is capable of shifting peripheral clocks in a time of day dependent and tissue specific manner. These shifts in peripheral clocks were smaller or absent after a single stress exposure and may therefore be the consequence of a cumulative chronic stress effect.

Time-Restricted Feeding in Mice Prevents the Disruption of the Peripheral Circadian Clocks and Its Metabolic Impact during Chronic Jetlag

We used time-restricted feeding (TRF) to investigate whether microbial metabolites and the hunger hormone ghrelin can become the dominant entraining factor during chronic jetlag to prevent disruption of the master and peripheral clocks, in order to promote health. Therefore, hypothalamic clock gene and Agrp/Npy mRNA expression were measured in mice that were either chronically jetlagged and fed ad libitum, jetlagged and fed a TRF diet, or not jetlagged and fed a TRF diet. Fecal short-chain fatty acid (SCFA) concentrations, plasma ghrelin and corticosterone levels, and colonic clock gene mRNA expression were measured. Preventing the disruption of the food intake pattern during chronic jetlag using TRF restored the rhythmicity in hypothalamic clock gene mRNA expression of Reverbα but not of Arntl. TRF countered the changes in plasma ghrelin levels and in hypothalamic Npy mRNA expression induced by chronic jetlag, thereby reestablishing the food intake pattern. Increase in body mass induced by chronic jetlag was prevented. Alterations in diurnal fluctuations in fecal SCFAs during chronic jetlag were prevented thereby re-entraining the rhythmic expression of peripheral clock genes. In conclusion, TRF during chronodisruption re-entrains the rhythms in clock gene expression and signals from the gut that regulate food intake to normalize body homeostasis.

Integration of feeding behavior by the liver circadian clock reveals network dependency of metabolic rhythms

The mammalian circadian clock, expressed throughout the brain and body, controls daily metabolic homeostasis. Clock function in peripheral tissues is required, but not sufficient, for this task. Because of the lack of specialized animal models, it is unclear how tissue clocks interact with extrinsic signals to drive molecular oscillations. Here, we isolated the interaction between feeding and the liver clock by reconstituting Bmal1 exclusively in hepatocytes (Liver-RE), in otherwise clock-less mice, and controlling timing of food intake. We found that the cooperative action of BMAL1 and the transcription factor CEBPB regulates daily liver metabolic transcriptional programs. Functionally, the liver clock and feeding rhythm are sufficient to drive temporal carbohydrate homeostasis. By contrast, liver rhythms tied to redox and lipid metabolism required communication with the skeletal muscle clock, demonstrating peripheral clock cross-talk. Our results highlight how the inner workings of the clock system rely on communicating signals to maintain daily metabolism.

MECHANISMS IN ENDOCRINOLOGY: Human brown adipose tissue as a therapeutic target: warming up or cooling down?

Excessive accumulation of white adipose tissue leads to obesity and its associated metabolic health consequences such as type 2 diabetes and cardiovascular disease. Several approaches to treat or prevent obesity including public health interventions, surgical weight loss, and pharmacological approaches to reduce caloric intake have failed to substantially modify the increasing prevalence of obesity. The (re-)discovery of active brown adipose tissue (BAT) in adult humans approximately 15 years ago led to a resurgence in research into whether BAT activation could be a novel therapy for the treatment of obesity. Upon cold stimulus, BAT activates and generates heat to maintain body temperature, thus increasing energy expenditure. Activation of BAT may provide a unique opportunity to increase energy expenditure without the need for exercise. However, much of the underlying mechanisms surrounding BAT activation are still being elucidated and the effectiveness of BAT as a therapeutic target has not been realised. Research is ongoing to determine how best to expand BAT mass and activate existing BAT; approaches include cold exposure, pharmacological stimulation using sympathomimetics, browning agents that induce formation of thermogenic beige adipocytes in white adipose depots, and the identification of factors secreted by BAT with therapeutic potential. In this review, we discuss the caloric capacity and other metabolic benefits from BAT activation in humans and the role of metabolic tissues such as skeletal muscle in increasing energy expenditure. We discuss the potential of current approaches and the challenges of BAT activation as a novel strategy to treat obesity and metabolic disorders.

Circadian Misalignment and Metabolic Disorders: A Story of Twisted Clocks

Simple Summary

In mammals, many physiological processes follow a 24 h rhythmic pattern. These rhythms are governed by a complex network of circadian clocks, which perceives external time cues (notably light and nutrients) and adjusts the timing of metabolic and physiological functions to allow a proper adaptation of the organism to the daily changes in the environmental conditions. Circadian rhythms originate at the cellular level through a transcriptional–translational regulatory network involving a handful of clock genes. In this review, we show how adverse effects caused by ill-timed feeding or jet lag can lead to a dysregulation of this genetic clockwork, which in turn results in altered metabolic regulation and possibly in diseases. We also show how computational modeling can complement experimental observations to understand the design of the clockwork and the onset of metabolic disorders.

Abstract

Biological clocks are cell-autonomous oscillators that can be entrained by periodic environmental cues. This allows organisms to anticipate predictable daily environmental changes and, thereby, to partition physiological processes into appropriate phases with respect to these changing external conditions. Nowadays our 24/7 society challenges this delicate equilibrium. Indeed, many studies suggest that perturbations such as chronic jet lag, ill-timed eating patterns, or shift work increase the susceptibility to cardiometabolic disorders, diabetes, and cancers. However the underlying mechanisms are still poorly understood. A deeper understanding of this complex, dynamic system requires a global holistic approach for which mathematical modeling can be highly beneficial. In this review, we summarize several experimental works pertaining to the effect of adverse conditions on clock gene expression and on physiology, and we show how computational models can bring interesting insights into the links between circadian misalignment and metabolic diseases.

Communicating clocks shape circadian homeostasis

Circadian clocks temporally coordinate physiology and align it with geophysical time, which enables diverse life-forms to anticipate daily environmental cycles. In complex organisms, clock function originates from the molecular oscillator within each cell and builds upward anatomically into an organism-wide system. Recent advances have transformed our understanding of how clocks are connected to achieve coherence across tissues. Circadian misalignment, often imposed in modern society, disrupts coordination among clocks and has been linked to diseases ranging from metabolic syndrome to cancer. Thus, uncovering the physiological circuits whereby biological clocks achieve coherence will inform on both challenges and opportunities in human health.

The Circadian Clock, Shift Work, and Tissue-Specific Insulin Resistance

Obesity and type 2 diabetes (T2D) have become a global health concern. The prevalence of obesity and T2D is significantly higher in shift workers compared to people working regular hours. An accepted hypothesis is that the increased risk for metabolic health problems arises from aberrantly timed eating behavior, that is, eating out of synchrony with the biological clock. The biological clock is part of the internal circadian timing system, which controls not only the sleep/wake and feeding/fasting cycle, but also many metabolic processes in the body, including the timing of our eating behavior, and processes involved in glucose homeostasis. Rodent studies have shown that eating out of phase with the endogenous clock results in desynchronization between rhythms of the central and peripheral clock systems and between rhythms of different tissue clocks (eg, liver and muscle clock). Glucose homeostasis is a complex process that involves multiple organs. In the healthiest situation, functional rhythms of these organs are synchronized. We hypothesize that desynchronization between different metabolically active organs contributes to alterations in glucose homeostasis. Here we summarize the most recent information on desynchronization between organs due to shift work and shifted food intake patterns and introduce the concept of phenotypic flexibility, a validated test to assess the contribution of each organ to insulin resistance (IR) in humans. We propose this test as a way to provide further insight into the possible desynchronization between tissue clocks. Because different types of IR benefit from different therapeutic approaches, we also describe different chronotherapeutic strategies to promote synchrony within and between metabolically active organs.

Do not neglect the role of circadian rhythm in muscle atrophy

In addition to its role in movement, human skeletal muscle also plays important roles in physiological activities related to metabolism and the endocrine system. Aging and disease onset and progression can induce the reduction of skeletal muscle mass and function, thereby exacerbating skeletal muscle atrophy. Recent studies have confirmed that skeletal muscle atrophy is mainly controlled by the balance between protein synthesis and degradation, the activation of satellite cells, and mitochondrial quality in skeletal muscle. Circadian rhythm is an internal rhythm related to an organism's adaptation to light-dark or day-night cycles of the planet, and consists of a core biological clock and a peripheral biological clock. Skeletal muscle, as the most abundant tissue in the human body, is an essential part of the peripheral biological clock in humans. Increasing evidence has confirmed that maintaining a normal circadian rhythm can be beneficial for increasing protein content, improving mitochondrial quality, and stimulating regeneration and repairing of cells in skeletal muscle to prevent or alleviate skeletal muscle atrophy. In this review, we summarize the roles and underlying mechanisms of circadian rhythm in delaying skeletal muscle atrophy, which will provide a theoretical reference for incorporating aspects of circadian rhythm to the prevention and treatment of skeletal muscle atrophy.

Circadian Rhythm in Adipose Tissue: Novel Antioxidant Target for Metabolic and Cardiovascular Diseases

Obesity is a major risk factor for most metabolic and cardiovascular disorders. Adipose tissue is an important endocrine organ that modulates metabolic and cardiovascular health by secreting signaling molecules. Oxidative stress is a common mechanism associated with metabolic and cardiovascular complications including obesity, type 2 diabetes, and hypertension. Oxidative stress can cause adipose tissue dysfunction. Accumulating data from both humans and experimental animal models suggest that adipose tissue function and oxidative stress have an innate connection with the intrinsic biological clock. Circadian clock orchestrates biological processes in adjusting to daily environmental changes according to internal or external cues. Recent studies have identified the genes and molecular pathways exhibiting circadian expression patterns in adipose tissue. Disruption of the circadian rhythmicity has been suggested to augment oxidative stress and aberrate adipose tissue function and metabolism. Therefore, circadian machinery in the adipose tissue may be a novel therapeutic target for the prevention and treatment of metabolic and cardiovascular diseases. In this review, we summarize recent findings on circadian rhythm and oxidative stress in adipose tissue, dissect the key components that play a role in regulating the clock rhythm, oxidative stress and adipose tissue function, and discuss the potential use of antioxidant treatment on metabolic and cardiovascular diseases by targeting the adipose clock.

After-Effects of Time-Restricted Feeding on Whole-Body Metabolism and Gene Expression in Four Different Peripheral Tissues

OBJECTIVE

Epidemiological studies show that shift workers are at increased risk for type 2 diabetes. As modern societies increasingly require shift work, it seems crucial to determine whether there are long-lasting health effects of rotational shifts.

METHODS

This study examined the after-effects of 4 weeks of time-restricted feeding (TRF) during the light period (= light-fed) in rats, an animal model for shift work. This study also included a TRF-dark (= dark-fed) control group. The aligned and misaligned feeding times of light and dark feeding are associated with poor and good health outcomes, respectively. Several physiological measures were monitored continuously; blood, liver, brown adipose tissue, and soleus and gastrocnemius muscle were collected following 11 days of ad libitum (AL) feeding after ending the TRF.

RESULTS

In the dark-fed animals, the day/night differences in food intake, activity, and respiratory exchange ratio were still enhanced at the end of the experiment. Light-fed animals displayed the smallest day/night differences for these measures, as well as for body temperature. In both the light- and dark-fed animals, rhythms in plasma glucose, nonesterified fatty acids, and gene expression had not fully recovered after 11 days of AL feeding. Importantly, the effects on gene expression were both tissue and gene dependent.

CONCLUSIONS

Our data indicate that rotational shift workers may have an increased risk of long-lasting disturbed rhythms in several physiological measures after a period of shift work. Clearly, such disturbances may harm their health.

Intermittent Fasting Does Not Uniformly Impact Genes Involved in Circadian Regulation in Women with Obesity

OBJECTIVE

This study aimed to examine the effects of intermittent fasting (IF) on mRNA levels of peripheral clock genes in skeletal muscle and subcutaneous adipose tissue (SAT) in women with obesity.

METHODS

Women were randomized to one of two IF protocols and provided with all foods at 100% or 70% of calculated weekly energy requirements for 8 weeks. Breakfast was consumed before a 24-hour fast, which was initiated on three nonconsecutive days per week. Muscle and SAT biopsies were performed at 8 am after an overnight fast at baseline and at week 8 on a refed day and again following a 24-hour fast at week 8 for analysis of the mRNA levels of key genes involved in circadian regulation.

RESULTS

A group-by-time interaction was observed in Per2 in muscle (F = 3.497, P = 0.044) and SAT (F = 6.686, P = 0.008), but significance was lost upon post hoc adjustment. A time effect was observed in Rorα in muscle, which was decreased by refeeding in both groups (F = 7.225, P = 0.003).

CONCLUSIONS

There was no universal effect of IF to alter peripheral clocks, which may be partly because of the alignment of the fasting/feeding cycle with the biological clock. Optimizing intermittent fasting protocols could be important to prevent circadian misalignment in humans.

Mealtime: A circadian disruptor and determinant of energy balance?

Circadian rhythms play a critical role in the physiological processes involved in energy metabolism and energy balance (EB). A large array of metabolic processes, including the expression of many energy-regulating endocrine hormones, display temporal rhythms that are driven by both the circadian clock and food intake. Mealtime has been shown to be a compelling zeitgeber in peripheral tissue rhythms. Inconsistent signalling to the periphery, because of mismatched input from the central clock vs time of eating, results in circadian disruption in which central and/or peripheral rhythms are asynchronously time shifted or their amplitudes reduced. A growing body of evidence supports the negative health effects of circadian disruption, with strong evidence in murine models that mealtime-induced circadian disruption results in various metabolic consequences, including energy imbalance and weight gain. Increased weight gain has been reported to occur even without differences in energy intake, indicating an effect of circadian disruption on energy expenditure. However, the translation of these findings to humans is not well established because the ability to undertake rigorously controlled dietary studies that explore the chronic effects on energy regulation is challenging. Establishing the neuroendocrine changes in response to both acute and chronic variations in mealtime, along with observations in populations with routinely abnormal mealtimes, may provide greater insight into underlying mechanisms that influence long-term weight management under different meal patterns. Human studies should explore mechanisms through relevant biomarkers; for example, cortisol, leptin, ghrelin and other energy-regulating neuroendocrine factors. Mistiming between aggregate hormonal signals, or between hormones with their receptors, may cause reduced signalling intensity and hormonal resistance. Understanding how mealtimes may impact on the coordination of endocrine factors is essential for untangling the complex regulation of EB. Here a review is provided on current evidence of the impacts of mealtime on energy metabolism and the underlying neuroendocrine mechanisms, with a specific focus on human research.

Analysis of Western diet, palmitate and BMAL1 regulation of neuropeptide Y expression in the murine hypothalamus and BMAL1 knockout cell models

Western diets that are high in saturated fat and sugar disrupt circadian rhythms, induce weight gain, and lead to metabolic diseases including obesity. However, the mechanistic link between altered circadian rhythms and energy homeostasis remains poorly understood. In C57BL/6J mice, consuming a Western diet for 16 weeks significantly reduced food intake (at zeitgeber 12-16), in association with decreases in hypothalamic expression of the orexigenic neuropeptides, neuropeptide Y (Npy) and agouti-related peptide (AgRP). To examine the acute effects of the most prevalent saturated fatty acid in a Western diet, palmitate, and the role of the core clock gene, Bmal1, in the regulation of hypothalamic feeding neuropeptides, we used heterogeneous and clonal BMAL1 knockout (KO) immortalized hypothalamic cell lines, expressing specific neuropeptides, derived from male (M) and female (F) mice. Both mHypoA-BMAL1-KO/F and mHypoA-BMAL1-KO/M cells demonstrated a loss of circadian rhythmicity in expression of the clock gene, Per2, as compared to wild-type (control) cultures. Loss of BMAL1 also altered the time-dependent expression of Npy and proopiomelanocortin, and disrupted AgRP rhythmicity. Furthermore, palmitate increased BMAL1 binding to the Npy promotor region, and palmitate treatment (50 μM for 24 h) stimulated Npy expression in a BMAL1-dependent manner in both heterogeneous and clonal NPY-expressing female-derived cell models. The results of this study demonstrate that circadian expression of Bmal1 serves as a mechanistic link between Western diet- and palmitate-induced disruptions of the normal rhythmic patterns in hypothalamic feeding-related neuropeptides.

Feeding Rhythms and the Circadian Regulation of Metabolism

The molecular circadian clock regulates metabolic processes within the cell, and the alignment of these clocks between tissues is essential for the maintenance of metabolic homeostasis. The possibility of misalignment arises from the differential responsiveness of tissues to the environmental cues that synchronize the clock (zeitgebers). Although light is the dominant environmental cue for the master clock of the suprachiasmatic nucleus, many other tissues are sensitive to feeding and fasting. When rhythms of feeding behavior are altered, for example by shift work or the constant availability of highly palatable foods, strong feedback is sent to the peripheral molecular clocks. Varying degrees of phase shift can cause the systemic misalignment of metabolic processes. Moreover, when there is a misalignment between the endogenous rhythms in physiology and environmental inputs, such as feeding during the inactive phase, the body's ability to maintain homeostasis is impaired. The loss of phase coordination between the organism and environment, as well as internal misalignment between tissues, can produce cardiometabolic disease as a consequence. The aim of this review is to synthesize the work on the mechanisms and metabolic effects of circadian misalignment. The timing of food intake is highlighted as a powerful environmental cue with the potential to destroy or restore the synchrony of circadian rhythms in metabolism.

Circadian Clocks Make Metabolism Run

Most organisms adapt to the 24-h cycle of the Earth's rotation by anticipating the time of the day through light-dark cycles. The internal time-keeping system of the circadian clocks has been developed to ensure this anticipation. The circadian system governs the rhythmicity of nearly all physiological and behavioral processes in mammals. In this review, we summarize current knowledge stemming from rodent and human studies on the tight interconnection between the circadian system and metabolism in the body. In particular, we highlight recent advances emphasizing the roles of the peripheral clocks located in the metabolic organs in regulating glucose, lipid, and protein homeostasis at the organismal and cellular levels. Experimental disruption of circadian system in rodents is associated with various metabolic disturbance phenotypes. Similarly, perturbation of the clockwork in humans is linked to the development of metabolic diseases. We discuss recent studies that reveal roles of the circadian system in the temporal coordination of metabolism under physiological conditions and in the development of human pathologies.

Diet-Induced Obesity Disturbs Microglial Immunometabolism in a Time-of-Day Manner

Background: Disturbance of immunometabolic signaling is a key process involved in the progression of obesity. Microglia-the resident immune cells in the brain, initiate local immune responses. It is known that hypercaloric diets lead to microglial activation. Previously, we observed that hypothalamic microglial cells from mice fed high-fat diet (HFD) lose their day/night rhythm and are constantly activated. However, little is known about daily rhythmicity in microglial circadian, immune and metabolic functions, either in lean or obese conditions. Therefore, we hypothesized that HFD disturbs microglial immunometabolism in a day/night-dependent manner. Methods: Obesity was induced in Wistar rats by feeding them HFD ad libitum for the duration of 8 weeks. Microglia were isolated from HFD- and chow-fed control animals at six time points during 24 h [every 4 h starting 2 h after lights on, i.e., Zeitgeber Time 2 (ZT2)]. Gene expression was evaluated using quantitative RT-PCR. JTK_Cycle software was used to estimate daily rhythmicity. Statistical analysis was performed with two-way ANOVA test. Results: Consumption of the obesogenic diet resulted in a 40 g significantly higher body weight gain in week 8, compared to chow diet (p < 0.0001), associated with increased adiposity. We observed significant rhythmicity of circadian clock genes in microglia under chow conditions, which was partially lost in diet-induced obesity (DIO). Microglial immune gene expression also showed time-of-day differences, which were disrupted in HFD-fed animals. Microglia responded to the obesogenic conditions by a shift of substrate utilization with decreased glutamate and glucose metabolism in the active period of the animals, and an overall increase of lipid metabolism, as indicated by gene expression evaluation. Additionally, data on mitochondria bioenergetics and dynamics suggested an increased energy production in microglia during the inactive period on HFD. Finally, evaluation of monocyte functional gene expression showed small or absent effect of HFD on peripheral myeloid cells, suggesting a cell-specific microglial inflammatory response in DIO. Conclusions: An obesogenic diet affects microglial immunometabolism in a time-of-day dependent manner. Given the central role of the brain in energy metabolism, a better knowledge of daily rhythms in microglial immunometabolism could lead to a better understanding of the pathogenesis of obesity.

Time-Restricted Feeding Improves Glucose Tolerance in Rats, but Only When in Line With the Circadian Timing System

Epidemiological studies indicate that shift-workers have an increased risk of type 2 diabetes mellitus (T2DM). Glucose tolerance and insulin sensitivity both are dependent on the circadian timing system (i.e., the time-of-day) and fasting duration, in rodents as well as humans. Therefore, question is whether manipulation of the circadian timing system, for example by changing the timing of feeding and fasting, is a potential preventive treatment for T2DM. Time-restricted feeding (TRF) is well-known to have profound effects on various metabolic measures, including glucose metabolism. However, experiments that directly measure the effects of TRF on glucose tolerance and/or insulin sensitivity at different time points throughout the 24 h cycle are lacking. Here we show, in rats, that TRF in line with the circadian timing system (i.e., feeding during the active phase) improves glucose tolerance during intravenous glucose tolerance tests (ivGTT) in the active phase, as lower insulin levels were observed with similar levels of glucose clearance. However, this was not the case during the inactive phase in which more insulin was released but only a slightly faster glucose clearance was observed. Contrasting, TRF out of sync with the circadian timing system (i.e., feeding during the inactive phase) worsened glucose tolerance, although only marginally, likely because of adaptation to the 4 week TRF regimen. Our results show that TRF can improve glucose metabolism, but strict adherence to the time-restricted feeding period is necessary, as outside the regular eating hours glucose tolerance is worsened.

An Ultradian Feeding Schedule in Rats Affects Metabolic Gene Expression in Liver, Brown Adipose Tissue and Skeletal Muscle with Only Mild Effects on Circadian Clocks

Restricted feeding is well known to affect expression profiles of both clock and metabolic genes. However, it is unknown whether these changes in metabolic gene expression result from changes in the molecular clock or in feeding behavior. Here we eliminated the daily rhythm in feeding behavior by providing 6 meals evenly distributed over the light/dark-cycle. Animals on this 6-meals-a-day feeding schedule retained the normal day/night difference in physiological parameters including body temperature and locomotor activity. The daily rhythm in respiratory exchange ratio (RER), however, was significantly phase-shifted through increased utilization of carbohydrates during the light phase and increased lipid oxidation during the dark phase. This 6-meals-a-day feeding schedule did not have a major impact on the clock gene expression rhythms in the master clock, but did have mild effects on peripheral clocks. In contrast, genes involved in glucose and lipid metabolism showed differential expression. In conclusion, eliminating the daily rhythm in feeding behavior in rats does not affect the master clock and only mildly affects peripheral clocks, but disturbs metabolic rhythms in liver, skeletal muscle and brown adipose tissue in a tissue-dependent manner. Thereby, a clear daily rhythm in feeding behavior strongly regulates timing of peripheral metabolism, separately from circadian clocks.

Circadian misalignment induces fatty acid metabolism gene profiles and compromises insulin sensitivity in human skeletal muscle

Circadian misalignment, such as in shift work, has been associated with obesity and type 2 diabetes. However, direct effects of circadian misalignment on skeletal muscle insulin sensitivity and the muscle molecular circadian clock have never been studied in humans. Here, we investigated insulin sensitivity and muscle metabolism in 14 healthy young lean men [age 22.4 ± 2.8 years; body mass index (BMI) 22.3 ± 2.1 kg/m2 (mean ± SD)] after a 3-d control protocol and a 3.5-d misalignment protocol induced by a 12-h rapid shift of the behavioral cycle. We show that short-term circadian misalignment results in a significant decrease in muscle insulin sensitivity due to a reduced skeletal muscle nonoxidative glucose disposal (rate of disappearance: 23.7 ± 2.4 vs. 18.4 ± 1.4 mg/kg per minute; control vs. misalignment; P = 0.024). Fasting glucose and free fatty acid levels as well as sleeping metabolic rate were higher during circadian misalignment. Molecular analysis of skeletal muscle biopsies revealed that the molecular circadian clock was not aligned to the inverted behavioral cycle, and transcriptome analysis revealed the human PPAR pathway as a key player in the disturbed energy metabolism upon circadian misalignment. Our findings may provide a mechanism underlying the increased risk of type 2 diabetes among shift workers.