Contributions of white and brown adipose tissues and skeletal muscles to acute cold-induced metabolic responses in healthy men

Cold exposure affects glucose metabolism, lipid droplet deposition and mitophagy in skeletal muscle of newborn goats

Cold exposure is a common stressor for newborn goats. Skeletal muscle plays an important role in maintaining whole-body homeostasis of glucose and lipid metabolism. However, the molecular mechanisms underlying regulation of skeletal muscle of newborn goats by cold exposure remains unclear. In this study, we found a significant increase (P < 0.01) in serum glucagon levels after 24 h of cold exposure (COLD, 6°C), while glucose and insulin concentrations were significantly decreased (P < 0.01) compared to room temperature (RT, 25°C). Additionally, we found that cold exposure reduced glycogen content (P < 0.01) in skeletal muscle. Pathway enrichment analysis revealed that cold exposure activated skeletal muscle glucose metabolism pathways (including insulin resistance and the insulin signaling pathway) and mitophagy-related pathways. Cold exposure up-regulated the expression of genes involved in fatty acid and triglyceride synthesis, promoting skeletal muscle lipid deposition. Notably, cold exposure induced mitophagy in skeletal muscle.

Effects of combined training on nonshivering thermogenic activity of muscles in individuals with overweight and type 2 diabetes

BACKGROUND

Increased thermogenic activity has shown to be a promising target for treating and preventing obesity and type 2 diabetes (T2DM). Little is known about the muscular influence on nonshivering thermogenesis (NST), and it remains unclear whether physical training and potential metabolic improvements could be associated with changes in this type of thermogenic activity.

OBJECTIVE

The present study aimed to assess muscular NST activity in overweight and T2DM before and after a combined training period (strength training followed by aerobic exercise).

METHODS

Nonshivering cold-induced 18-fluoroxyglucose positron emission computed tomography (18F-FDG PET/CT) was performed before and after 16 weeks of combined training in 12 individuals with overweight and T2DM. The standard uptake value (SUV) of 18F-FDG was evaluated in skeletal muscles, the heart and the aorta.

RESULTS

Muscles in the neck region exhibit higher SUV pre- and posttraining. Furthermore, a decrease in glucose uptake by the muscles of the lower and upper extremities and in the aorta was observed after training when adjusted for brown adipose tissue (BAT). These pre-post effects are accompanied by increased cardiac SUV and occur concurrently with heightened energy expenditure and metabolic improvements.

CONCLUSIONS

Muscles in the neck region have greater metabolic activity upon exposure to cold. In addition, combined training appears to induce greater NST, favoring the trunk and neck region compared to limbs based on joint work and adaptations between skeletal muscles and BAT.

Is stimulation of browning of human adipose tissue a relevant therapeutic target?

Brown adipose tissue (BAT) and beige adipose tissues are important contributors to cold-induced whole body thermogenesis in rodents. The documentation in humans of cold- and ß-adrenergic receptor agonist-stimulated BAT glucose uptake using positron emission tomography (PET) and of a decrease of this response in individuals with cardiometabolic disorders led to the suggestion that BAT/beige adipose tissues could be relevant targets for prevention and treatment of these conditions. In this brief review, we will critically assess this question by first describing the basic rationale for this affirmation, second by examining the evidence in human studies, and third by discussing the possible means to activate the thermogenic response of these tissues in humans.

The effects of the β1-adrenergic receptor antagonist bisoprolol administration on mirabegron-stimulated human brown adipose tissue thermogenesis

AIM

Pharmacological stimulation of human brown adipose tissue (BAT) has been hindered by ineffective activation or undesirable off-target effects. Oral administration of the maximal allowable dose of mirabegron (200 mg), a β3-adrenergic receptor (β3-AR) agonist, has been effective in stimulating BAT thermogenesis and whole-body energy expenditure. However, this has been accompanied by undesirable cardiovascular effects. Therefore, we hypothesized that combining mirabegron with a β1-AR antagonist could suppress these unwanted effects and increase the stimulation of the β3-AR and β2-AR in BAT.

METHODS

We performed a randomized crossover trial (NCT04823442) in 8 lean men. Mirabegron (200 mg) was administered orally with or without the β1-AR antagonist bisoprolol (10 mg). Dynamic [11C]-acetate and 2-deoxy-2-[18F]fluoro-d-glucose PET/CT scans were performed sequentially after oral administration of mirabegron ± bisoprolol.

RESULTS

Compared to room temperature, mirabegron alone increased BAT oxidative metabolism (0.84 ± 0.46 vs. 1.79 ± 0.91 min-1, p = 0.0433), but not when combined with bisoprolol. The metabolic rate of glucose in BAT, measured using [18F]FDG PET, was significantly higher with mirabegron than mirabegron with bisoprolol (24 ± 10 vs. 16 ± 8 nmol/g/min, p = 0.0284). Bisoprolol inhibited the mirabegron-induced increase in systolic blood pressure and heart rate.

CONCLUSION

The administration of bisoprolol decreases the adverse cardiovascular effects of mirabegron. However, the provided dose also blunted the mirabegron-stimulated increase in BAT lipolysis, thermogenesis, and glucose uptake. The attenuation in BAT blood flow induced by the large dose of bisoprolol may have limited BAT thermogenesis.

The Different Shades of Thermogenic Adipose Tissue

PURPOSE OF REVIEW

By providing a concise overview of adipose tissue types, elucidating the regulation of adipose thermogenic capacity in both physiological contexts and chronic wasting diseases (a protracted hypermetabolic state that precipitates sustained catabolism and consequent progressive corporeal atrophy), and most importantly, delving into the ongoing discourse regarding the role of adipose tissue thermogenic activation in chronic wasting diseases, this review aims to provide researchers with a comprehensive understanding of the field.

RECENT FINDINGS

Adipose tissue, traditionally classified as white, brown, and beige (brite) based on its thermogenic activity and potential, is intricately regulated by complex mechanisms in response to exercise or cold exposure. This regulation is adipose depot-specific and dependent on the duration of exposure. Excessive thermogenic activation of adipose tissue has been observed in chronic wasting diseases and has been considered a pathological factor that accelerates disease progression. However, this conclusion may be confounded by the detrimental effects of excessive lipolysis. Recent research also suggests that such activation may play a beneficial role in the early stages of chronic wasting disease and provide potential therapeutic effects. A more comprehensive understanding of the changes in adipose tissue thermogenesis under physiological and pathological conditions, as well as the underlying regulatory mechanisms, is essential for the development of novel interventions to improve health and prevent disease.

Changes in temperature in preheated crystalloids at ambient temperatures relevant to a prehospital setting: an experimental simulation study with the application of prehospital treatment of trauma patients suffering from accidental hypothermia

BACKGROUND

Accidental hypothermia is common in all trauma patients and contributes to the lethal diamond, increasing both morbidity and mortality. In hypotensive shock, fluid resuscitation is recommended using fluids with a temperature of 37-42°, as fluid temperature can decrease the patient's body temperature. In Sweden, virtually all prehospital services use preheated fluids. The aim of the present study was to investigate how the temperature of preheated infusion fluids is affected by the ambient temperatures and flow rates relevant for prehospital emergency care.

METHODS

In this experimental simulation study, temperature changes in crystalloids preheated to 39 °C were evaluated. The fluid temperature changes were measured both in the infusion bag and at the patient end of the infusion system. Measurements were conducted in conditions relevant to prehospital emergency care, with ambient temperatures varying between - 4 and 28 °C and flow rates of 1000 ml/h and 6000 ml/h, through an uninsulated infusion set at a length of 175 cm.

RESULTS

The flow rate and ambient temperature affected the temperature in the infusion fluid both in the infusion bag and at the patient end of the system. A lower ambient temperature and lower flow rate were both associated with a greater temperature loss in the infusion fluid.

CONCLUSION

This study shows that both a high infusion rate and a high ambient temperature are needed if an infusion fluid preheated to 39 °C is to remain above 37 °C when it reaches the patient using a 175-cm-long uninsulated infusion set. It is apparent that the lower the ambient temperature, the higher the flow rate needs to be to limit temperature loss of the fluid.

Tracers and Imaging of Fatty Acid and Energy Metabolism of Human Adipose Tissues

White adipose tissue and brown adipose tissue (WAT and BAT) regulate fatty acid metabolism and control lipid fluxes to other organs. Dysfunction of these key metabolic processes contributes to organ insulin resistance and inflammation leading to chronic diseases such as type 2 diabetes, metabolic dysfunction-associated steatohepatitis, and cardiovascular diseases. Metabolic tracers combined with molecular imaging methods are powerful tools for the investigation of these pathogenic mechanisms. Herein, I review some of the positron emission tomography and magnetic resonance imaging methods combined with stable isotopic metabolic tracers to investigate fatty acid and energy metabolism, focusing on human WAT and BAT metabolism. I will discuss the complementary strengths offered by these methods for human investigations and current gaps in the field.

Adipose tissue browning and thermogenesis under physiologically energetic challenges: a remodelled thermogenic system

Metabolic diseases such as obesity and diabetes are often thought to be caused by reduced energy expenditure, which poses a serious threat to human health. Cold exposure, exercise and caloric restriction have been shown to promote adipose tissue browning and thermogenesis. These physiological interventions increase energy expenditure and thus have emerged as promising strategies for mitigating metabolic disorders. However, that increased adipose tissue browning and thermogenesis elevate thermogenic consumption is not a reasonable explanation when humans and animals confront energetic challenges imposed by these interventions. In this review, we collected numerous results on adipose tissue browning and whitening and evaluated this bi-directional conversion of adipocytes from the perspective of energy homeostasis. Here, we propose a new interpretation of the role of adipose tissue browning under energetic challenges: increased adipose tissue browning and thermogenesis under energy challenge is not to enhance energy expenditure, but to reestablish a more economical thermogenic pattern to maintain the core body temperature. This can be achieved by enhancing the contribution of non-shivering thermogenesis (adipose tissue browning and thermogenesis) and lowering shivering thermogenesis and high intensity shivering. Consequently, the proportion of heat production in fat increases and that in skeletal muscle decreases, enabling skeletal muscle to devote more energy reserves to overcoming environmental stress.

The Effects of Intermittent Cold Exposure on Adipose Tissue

Intermittent cold exposure (ICE) has garnered increased attention in popular culture, largely for its proposed effects on mood and immune function, but there are also suggestions that the energy-wasting mechanisms associated with thermogenesis may decrease body weight and fat mass. Considering the continued and worsening prevalence of obesity and type II diabetes, any protocol that can reduce body weight and/or improve metabolic health would be a substantial boon. Here, we present a narrative review exploring the research related to ICE and adipose tissue. Any publicly available original research examining the effects of repeated bouts of ICE on adipose-related outcomes was included. While ICE does not consistently lower body weight or fat mass, there does seem to be evidence for ICE as a positive modulator of the metabolic consequences of obesity, such as glucose tolerance and insulin signaling. Further, ICE consistently increases the activity of brown adipose tissue (BAT) and transitions white adipose tissue to a phenotype more in line with BAT. Lastly, the combined effects of ICE and exercise do not seem to provide any additional benefit, at least when exercising during ICE bouts. The majority of the current literature on ICE is based on rodent models where animals are housed in cold rooms, which does not reflect protocols likely to be implemented in humans such as cold water immersion. Future research could specifically characterize ICE via cold water immersion in combination with controlled calorie intake to clearly determine the effects of ICE as it would be implemented in humans looking to lower their body weight via reductions in fat mass.

Comprehensive quantification of metabolic flux during acute cold stress in mice

Cold-induced thermogenesis (CIT) is widely studied as a potential avenue to treat obesity, but a thorough understanding of the metabolic changes driving CIT is lacking. Here, we present a comprehensive and quantitative analysis of the metabolic response to acute cold exposure, leveraging metabolomic profiling and minimally perturbative isotope tracing studies in unanesthetized mice. During cold exposure, brown adipose tissue (BAT) primarily fueled the tricarboxylic acid (TCA) cycle with fat in fasted mice and glucose in fed mice, underscoring BAT's metabolic flexibility. BAT minimally used branched-chain amino acids or ketones, which were instead avidly consumed by muscle during cold exposure. Surprisingly, isotopic labeling analyses revealed that BAT uses glucose largely for TCA anaplerosis via pyruvate carboxylation. Finally, we find that cold-induced hepatic gluconeogenesis is critical for CIT during fasting, demonstrating a key functional role for glucose metabolism. Together, these findings provide a detailed map of the metabolic rewiring driving acute CIT.

Effect of high-dose glucocorticoid treatment on human brown adipose tissue activity: a randomised, double-blinded, placebo-controlled cross-over trial in healthy men

BACKGROUND

Glucocorticoids (GCs) are widely applied anti-inflammatory drugs that are associated with adverse metabolic effects including insulin resistance and weight gain. Previous research indicates that GCs may negatively impact brown adipose tissue (BAT) activity in rodents and humans.

METHODS

We performed a randomised, double-blinded cross-over trial in 16 healthy men (clinicaltrials.govNCT03269747). Participants received 40 mg of prednisone per day for one week or placebo. After a washout period of four weeks, participants crossed-over to the other treatment arm. Primary endpoint was the increase in resting energy expenditure (EE) in response to a mild-cold stimulus (cold-induced thermogenesis, CIT). Secondary outcomes comprised mean 18F-FDG uptake into supraclavicular BAT (SUVmean) as determined by FDG-PET/CT, volume of the BAT depot as well as fat content determined by MRI. The plasma metabolome and the transcriptome of supraclavicular BAT and of skeletal muscle biopsies after each treatment period were analysed.

FINDINGS

Sixteen participants were recruited to the trial and completed it successfully per protocol. After prednisone treatment resting EE was higher both during warm and cold conditions. However, CIT was similar, 153 kcal/24 h (95% CI 40-266 kcal/24 h) after placebo and 186 kcal/24 h (95% CI 94-277 kcal/24 h, p = 0.38) after prednisone. SUVmean of BAT after cold exposure was not significantly affected by prednisone (3.36 g/ml, 95% CI 2.69-4.02 g/ml, vs 3.07 g/ml, 95% CI 2.52-3.62 g/ml, p = 0.28). Results of plasma metabolomics and BAT transcriptomics corroborated these findings. RNA sequencing of muscle biopsies revealed higher expression of genes involved in calcium cycling. No serious adverse events were reported and adverse events were evenly distributed between the two treatments.

INTERPRETATION

Prednisone increased EE in healthy men possibly by altering skeletal muscle calcium cycling. Cold-induced BAT activity was not affected by GC treatment, which indicates that the unfavourable metabolic effects of GCs are independent from thermogenic adipocytes.

FUNDING

Grants from Swiss National Science Foundation (PZ00P3_167823), Bangerter-Rhyner Foundation and from Nora van der Meeuwen-Häfliger Foundation to MJB. A fellowship-grant from the Swiss National Science Foundation (SNF211053) to WS. Grants from German Research Foundation (project number: 314061271-TRR 205) and Else Kröner-Fresenius (grant support 2012_A103 and 2015_A228) to MR.

The serotonin transporter sustains human brown adipose tissue thermogenesis

Activation of brown adipose tissue (BAT) in humans is a strategy to treat obesity and metabolic disease. Here we show that the serotonin transporter (SERT), encoded by SLC6A4, prevents serotonin-mediated suppression of human BAT function. RNA sequencing of human primary brown and white adipocytes shows that SLC6A4 is highly expressed in human, but not murine, brown adipocytes and BAT. Serotonin decreases uncoupled respiration and reduces uncoupling protein 1 via the 5-HT2B receptor. SERT inhibition by the selective serotonin reuptake inhibitor (SSRI) sertraline prevents uptake of extracellular serotonin, thereby potentiating serotonin's suppressive effect on brown adipocytes. Furthermore, we see that sertraline reduces BAT activation in healthy volunteers, and SSRI-treated patients demonstrate no 18F-fluorodeoxyglucose uptake by BAT at room temperature, unlike matched controls. Inhibition of BAT thermogenesis may contribute to SSRI-induced weight gain and metabolic dysfunction, and reducing peripheral serotonin action may be an approach to treat obesity and metabolic disease.

Practical internal and external cooling methods do not influence rapid recovery from simulated taekwondo performance

Background/Objectives

The influence of post-exercise cooling on recovery has gained much attention in the empirical literature, however, data is limited in regards to optimizing recovery from taekwondo performance when combat is repeated in quick succession within the same day. The aim of this study was therefore to compare the effects of external and internal cooling after simulated taekwondo combat upon intestinal temperature (T_int), psychomotor skills (reaction time, response time, movement time), and neuromuscular function (peak torque, average power, time to reach peak torque).

Methods

Using a randomized counterbalanced crossover design, 10 well-trained male taekwondo athletes completed four recovery methods on separate occasions: passive recovery (CON), a 5-minute thermoneutral water immersion (35°C) (TWI), a 5-min cold water immersion (15°C) (CWI), and ice slurry ingestion (-1°C) (ICE; consumed every 5 min for 30 min). Heart rate (HR), blood lactate (Blac) concentrations, and T_int were determined at rest, immediately after combat, and at selected intervals during a 90-min recovery period. Neuromuscular functional (measured with isokinetic dynamometer) and psychomotor indices were assessed at baseline and after the recovery period.

Results

ICE led to a significantly lower T_int at 30 min (P<0.01) and 45 min (P<0.01) after simulated combat; 15-30 min after cessation of ingesting ice slurry, compared with the CON and TWI conditions, respectively. However, there were no differences in T_int across time points between the other conditions (P>0.05). Psychomotor skills and neuromuscular function indices returned to baseline values after the 90 min recovery period (P>0.05) with no differences observed between conditions (P>0.05).

Conclusion

The present findings suggest that internal (ICE) and external (CWI) recovery methods appear to have little impact on physiological and functional indices over the time course required to influence repeated taekwondo combat performance.

Rats lacking Ucp1 present a novel translational tool for the investigation of thermogenic adaptation during cold challenge

AIM

Valuable studies have tested the role of UCP1 on body temperature maintenance in mice, and we sought to knockout Ucp1 in rats (Ucp1-/- ) to provide insight into thermogenic mechanisms in larger mammals.

METHODS

We used CRISPR/Cas9 technology to create Ucp1-/- rats. Body weight and adiposity were measured, and rats were subjected to indirect calorimetry. Rats were maintained at room temperature or exposed to 4°C for either 24 h or 14 days. Analyses of brown and white adipose tissue and skeletal muscle were conducted via histology, western blot comparison of oxidative phosphorylation proteins, and qPCR to compare mitochondrial DNA levels and mRNA expression profiles. RNA-seq was performed in skeletal muscle.

RESULTS

Ucp1-/- rats withstood 4°C for 14 days, but core temperature steadily declined. All rats lost body weight after 14 days at 4°C, but controls increased food intake more robustly than Ucp1-/- rats. Brown adipose tissue showed signs of decreased activity in Ucp1-/- rats, while mitochondrial lipid metabolism markers in white adipose tissue and skeletal muscle were increased. Ucp1-/- rats displayed more visible shivering and energy expenditure than controls at 4°C. Skeletal muscle transcriptomics showed more differences between genotypes at 23°C than at 4°C.

CONCLUSION

Room temperature presented sufficient cold stress to rats lacking UCP1 to activate compensatory thermogenic mechanisms in skeletal muscle, which were only activated in control rats following exposure to 4°C. These results provide novel insight into thermogenic responses to UCP1 deficiency; and highlight Ucp1-/- rats as an attractive translational model for the study of thermogenesis.

Central circuit controlling thermoregulatory inversion and torpor-like state

To maintain core body temperature in mammals, the CNS thermoregulatory networks respond to cold exposure by increasing brown adipose tissue and shivering thermogenesis. However, in hibernation or torpor, this normal thermoregulatory response is supplanted by "thermoregulatory inversion", an altered homeostatic state in which cold exposure causes inhibition of thermogenesis and warm exposure stimulates thermogenesis. Here we demonstrate the existence of a novel, dynorphinergic thermoregulatory reflex pathway between the dorsolateral parabrachial nucleus and the dorsomedial hypothalamus that bypasses the normal thermoregulatory integrator in the hypothalamic preoptic area to play a critical role in mediating the inhibition of thermogenesis during thermoregulatory inversion. Our results indicate the existence of a neural circuit mechanism for thermoregulatory inversion within the CNS thermoregulatory pathways and support the potential for inducing a homeostatically-regulated, therapeutic hypothermia in non-hibernating species, including humans.

Brown Adipose Tissue-A Translational Perspective

Brown adipose tissue (BAT) displays the unique capacity to generate heat through uncoupled oxidative phosphorylation that makes it a very attractive therapeutic target for cardiometabolic diseases. Here, we review BAT cellular metabolism, its regulation by the central nervous and endocrine systems and circulating metabolites, the plausible roles of this tissue in human thermoregulation, energy balance, and cardiometabolic disorders, and the current knowledge on its pharmacological stimulation in humans. The current definition and measurement of BAT in human studies relies almost exclusively on BAT glucose uptake from positron emission tomography with 18F-fluorodeoxiglucose, which can be dissociated from BAT thermogenic activity, as for example in insulin-resistant states. The most important energy substrate for BAT thermogenesis is its intracellular fatty acid content mobilized from sympathetic stimulation of intracellular triglyceride lipolysis. This lipolytic BAT response is intertwined with that of white adipose (WAT) and other metabolic tissues, and cannot be independently stimulated with the drugs tested thus far. BAT is an interesting and biologically plausible target that has yet to be fully and selectively activated to increase the body's thermogenic response and shift energy balance. The field of human BAT research is in need of methods able to directly, specifically, and reliably measure BAT thermogenic capacity while also tracking the related thermogenic responses in WAT and other tissues. Until this is achieved, uncertainty will remain about the role played by this fascinating tissue in human cardiometabolic diseases.

Stimulation of the beta-2-adrenergic receptor with salbutamol activates human brown adipose tissue

While brown adipose tissue (BAT) is activated by the beta-3-adrenergic receptor (ADRB3) in rodents, in human brown adipocytes, the ADRB2 is dominantly present and responsible for noradrenergic activation. Therefore, we performed a randomized double-blinded crossover trial in young lean men to compare the effects of single intravenous bolus of the ADRB2 agonist salbutamol without and with the ADRB1/2 antagonist propranolol on glucose uptake by BAT, assessed by dynamic 2-[18F]fluoro-2-deoxy-D-glucose positron emission tomography-computed tomography scan (i.e., primary outcome). Salbutamol, compared with salbutamol with propranolol, increases glucose uptake by BAT, without affecting the glucose uptake by skeletal muscle and white adipose tissue. The salbutamol-induced glucose uptake by BAT positively associates with the increase in energy expenditure. Notably, participants with high salbutamol-induced glucose uptake by BAT have lower body fat mass, waist-hip ratio, and serum LDL-cholesterol concentration. In conclusion, specific ADRB2 agonism activates human BAT, which warrants investigation of ADRB2 activation in long-term studies (EudraCT: 2020-004059-34).

Physiological and molecular mechanisms of cold-induced improvements in glucose homeostasis in humans beyond brown adipose tissue

Exposure to low ambient temperatures has previously been demonstrated to markedly improve glucose homeostasis in both rodents and humans. Although the brown adipose tissue is key in mediating these beneficial effects in rodents, its contribution appears more limited in humans. Hence, the exact tissues and underlying mechanisms that mediate cold-induced improvements in glucose homeostasis in humans remain to be fully established. In this review, we evaluated the response of the main organs involved in glucose metabolism (i.e. pancreas, liver, (white) adipose tissue, and skeletal muscle) to cold exposure and discuss their potential contribution to cold-induced improvements in glucose homeostasis in humans. We here show that cold exposure has widespread effects on metabolic organs involved in glucose regulation. Nevertheless, cold-induced improvements in glucose homeostasis appear primarily mediated via adaptations within the skeletal muscle and (presumably) white adipose tissue. Since the underlying mechanisms remain elusive, future studies should be aimed at pinpointing the exact physiological and molecular mechanisms involved in humans. Nonetheless, cold exposure holds great promise as a novel, additive lifestyle approach to improve glucose homeostasis in insulin resistant individuals. Parts of this graphical abstract were created using (modified) images from Servier Medical Art, licensed under the Creative Commons Attribution 3.0 Unported License. TG = thermogenesis, TAG = triacylglycerol, FFA = free fatty acid, SLN = sarcolipin, UCP3 = uncoupling protein 3, β2-AR = beta-2 adrenergic receptor, SNS = sympathetic nervous system.

Reduced contextually induced muscle thermogenesis in rats with calorie restriction and lower aerobic fitness but not monogenic obesity

We have previously identified predator odor as a potent stimulus activating thermogenesis in skeletal muscle in rats. As this may prove relevant for energy balance and weight loss, the current study investigated whether skeletal muscle thermogenesis was altered with negative energy balance, obesity propensity seen in association with low intrinsic aerobic fitness, and monogenic obesity. First, weight loss subsequent to 3 wk of 50% calorie restriction suppressed the muscle thermogenic response to predator odor. Next, we compared rats bred based on artificial selection for intrinsic aerobic fitness - high- and low-capacity runners (HCR, LCR) - that display robust leanness and obesity propensity, respectively. Aerobically fit HCR showed enhanced predator odor-induced muscle thermogenesis relative to the less-fit LCR. This contrasted with the profound monogenic obesity displayed by rats homozygous for a loss of function mutation in Melanocortin 4 receptor (Mc4rK3a,4X/K314X rats), which showed no discernable deficit in thermogenesis. Taken together, these data imply that body size or obesity per se are not associated with deficient muscle thermogenesis. Rather, the physiological phenotype associated with polygenic obesity propensity may encompass pleiotropic mechanisms in the thermogenic pathway. Adaptive thermogenesis associated with weight loss also likely alters muscle thermogenic mechanisms.

Brown adipose tissue is not associated with cachexia or increased mortality in a retrospective study of patients with cancer

Although brown fat is strongly associated with a constellation of cardiometabolic benefits in animal models and humans, it has also been tied to cancer cachexia. In humans, cancer-associated cachexia increases mortality, raising the possibility that brown fat in this context may be associated with increased cancer death. However, the effect of brown fat on cancer-associated cachexia and survival in humans remains unclear. Here, we retrospectively identify patients with and without brown fat on fluorodeoxyglucose (18F-FDG) positron-emission tomography (PET) scans obtained as part of routine cancer care and assemble a cohort to address these questions. We did not find an association between brown fat status and cachexia. Furthermore, we did not observe an association between brown fat and increased mortality in patients with cachexia. Our analyses controlled for confounding factors including age at cancer diagnosis, sex, body mass index, cancer site, cancer stage, outdoor temperature, comorbid conditions (heart failure, type 2 diabetes mellitus, coronary artery disease, hypertension, dyslipidemia, cerebrovascular disease), and β-blocker use. Taken together, our results suggest that brown fat is not linked to cancer-associated cachexia and does not worsen overall survival in patients with cachexia.NEW & NOTEWORTHY This study finds that brown fat is not linked to cancer-associated cachexia. Moreover, this work shows that brown fat does not worsen overall survival in patients with cachexia.

Thermogenic Brown Fat in Humans: Implications in Energy Homeostasis, Obesity and Metabolic Disorders

In mammals including humans, there are two types of adipose tissue, white and brown adipose tissues (BATs). White adipose tissue is the primary site of energy storage, while BAT is a specialized tissue for non-shivering thermogenesis to dissipate energy as heat. Although BAT research has long been limited mostly in small rodents, the rediscovery of metabolically active BAT in adult humans has dramatically promoted the translational studies on BAT in health and diseases. It is now established that BAT, through its thermogenic and energy dissipating activities, plays a role in the regulation of body temperature, whole-body energy expenditure, and body fatness. Moreover, increasing evidence has demonstrated that BAT secretes various paracrine and endocrine factors, which influence other peripheral tissues and control systemic metabolic homeostasis, suggesting BAT as a metabolic regulator, other than for thermogenesis. In fact, clinical studies have revealed an association of BAT not only with metabolic disorders such as insulin resistance, diabetes, dyslipidemia, and fatty liver, but also with cardiovascular diseases including hypertension and atherosclerosis. Thus, BAT is an intriguing tissue combating obesity and related metabolic diseases. In this review, we summarize current knowledge on human BAT, focusing its patho-physiological roles in energy homeostasis, obesity and related metabolic disorders. The effects of aging and sex on BAT are also discussed.

Modulation of cold-induced shivering activity by intermittent and continuous voluntary suppression

This investigation assessed the physiological effects of voluntary suppression of shivering thermogenesis in response to whole body cooling. Eleven healthy volunteers underwent passive air cooling (10°C), across three visits: NO_SUP, where participants allowed their body to freely regulate against the cold; FULL_SUP, where participants constantly suppressed shivering; INT_SUP, where participants intermittently suppressed shivering (5 min phases), interspersed with 5 min free regulation. Shivering was assessed via electromyography (EMG), mechanomyography (MMG), and whole body oxygen uptake (V̇o₂), whereas body temperature and heat exchange were assessed via skin temperature, rectal temperature, and heat flux sensors. A 29% increase was observed in shivering onset time in the FULL_SUP trial compared with NO_SUP (P = 0.032). Assessing shivering intensity, EMG activity decreased by 29% (P = 0.034), MMG activity decreased by 35% (P = 0.031), whereas no difference was observed in V̇o₂ (P = 0.091) in the FULL_SUP trial compared with NO_SUP. Partitioning the no-suppression and suppression phases of the INT_SUP trial, acute voluntary suppression significantly decreased V̇o₂ (P = 0.001), EMG (P < 0.001), and MMG (P = 0.012) activity compared with the no-suppression phases. Shivering activity was restored in the no-suppression phases, equivalent to that in the NO_SUP trial (P > 0.3). No difference was observed in thermal metrics between conditions up to 60 min (P > 0.4). Humans can both constantly and periodically suppress shivering activity, leading to a delay in shivering onset and a reduction in shivering intensity. Following suppression, regular shivering is resumed.

Kinetics of lipid indicators in response to short- and long-duration whole-body, cold-water immersion

Cold exposure-induced secretion of stress hormones activates cold-defense responses and mobilizes substrates for increased energy demands to fuel thermogenesis. However, it is unclear whether acute cold exposure-induced stress hormone response kinetics affect circulating lipid parameter kinetics. Therefore, we aimed to investigate the 2-day kinetics of stress hormones (i.e., cortisol, epinephrine, and norepinephrine) and the lipid profile (i.e., total cholesterol [TC], high-density lipoprotein [HDL] cholesterol, low-density lipoprotein [LDL] cholesterol, and triglycerides) in response to whole-body long- (intermittent 170 min; 170-CWI) or short-duration (10 min; 10-CWI) cold-water immersion (CWI; 14 °C water) in 17 healthy, young, adult men. Both CWI trials induced a marked release of the stress hormones, epinephrine, and norepinephrine, with higher concentrations detected after 170-CWI (p < 0.05) and a disrupted diurnal peak of cortisol lasting for a few hours. 170-CWI increased triglyceride levels from immediately after until 2 h after CWI, thereafter the concentration decreased at 4 h, 6 h, 1 day and 2 days after CWI (p < 0.05). Furthermore, the HDL-cholesterol level increased immediately after and at 6 h after 170-CWI (p < 0.05), while TC and LDL-cholesterol levels were not altered within 2 days. Lipid parameters were not affected within the 2 days after 10-CWI. Although both CWIs decreased deep body temperature and increased stress hormone levels for a few hours, only long-duration CWI induced changes in the circulating lipid profile within 2 days after CWI. This should be considered when discussing therapeutic protocols to improve circulating lipid profiles and ameliorate diseases associated with such profiles.

Cooling down with Entresto. Can sacubitril/valsartan combination enhance browning more than coldness?

BACKGROUND

It is a growing importance to induce a new treatment approach to encourage weight loss but also to improve maintenance of lost weight. It has been shown that promotion of brown adipose tissue (BAT) function or acquisition of BAT characteristics in white adipose tissue (terms referred as "browning") can be protective against obesity.

MAIN TEXT

Amongst numerous established environmental influences on BAT activity, cold exposure is the best interested technique due to its not only effects on of BAT depots in proliferation process but also de novo differentiation of precursor cells via β-adrenergic receptor activation. A novel combination drug, sacubitril/valsartan, has been shown to be more efficient in reducing cardiovascular events and heart failure readmission compared to conventional therapy. Also, this combination of drugs increases the postprandial lipid oxidation contributing to energy expenditure, promotes lipolysis in adipocytes and reduces body weight. To date, there is no research examining potential of combined sacubitril/valsartan use to promote browning or mechanisms in the basis of this thermogenic process.

CONCLUSION

Due to the pronounced effects of cold and sacubitril/valsartan treatment on function and metabolism of BAT, the primary goal of further research should focused on investigation of the synergistic effects of the sacubitril/valsartan treatment at low temperature environmental conditions.

RNA binding motif protein 3 (RBM3) promotes protein kinase B (AKT) activation to enhance glucose metabolism and reduce apoptosis in skeletal muscle of mice under acute cold exposure

The main danger of cold stress to animals in cold regions is systemic metabolic changes and protein synthesis inhibition. RBM3, an exceptional cold shock protein, is rapidly upregulated in response to hypothermia to resist the adverse effects of cold stress. However, the mechanism of the protective effect and the rapid upregulation of RBM3 remains unclear. O-GlcNAcylation, an atypical O-glycosylation, is precisely regulated only by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA) and participates in the signal transduction of multiple cellular stress responses as a "stress and nutrition receptor." Therefore, our study aimed to explore the mechanism of RBM3 regulating glucose metabolism and promoting survival in skeletal muscle under acute cold exposure. Meanwhile, our study verifies whether O-GlcNAcylation mediated by OGT rapidly upregulates RBM3. The blood and skeletal muscle of mice were collected at the end of cold exposure treatment for 0, 2, and 4 h. Changes in levels of RBM3, AKT, glycolysis apoptosis, and OGT were measured. The results show that acute cold exposure upregulated RBM3, OGT, and AKT phosphorylation and increased energy consumption, which enhanced glycolysis and prevent apoptosis. In the 32 °C mild hypothermia model in vitro, overexpression of RBM3 enhanced AKT phosphorylation. Meanwhile, inactivation of AKT by wortmannin resulted in increased apoptosis and decreased glucose metabolism in skeletal muscle under acute cold exposure. In addition, OGT-mediated O-GlcNAcylation of p65 was confirmed in mouse myoblast cell line (C2C12) cells at mild hypothermia. O-GlcNAcylation level affected p65 activity and nuclear translocation. Compared with wild type (WT) mice, RBM3 and p65 phosphorylation were decreased in specific skeletal muscle Ogt (KO) mice, whereas AKT phosphorylation, glycolysis, and apoptosis were increased. Taken together, O-GlcNAcylation of p65 upregulates RBM3 to promote AKT phosphorylation, enhance glucose metabolism, and reduce apoptosis in skeletal muscle of mice under acute cold exposure.

Aging attenuates diurnal lipid uptake by brown adipose tissue

Brown adipose tissue (BAT) contributes to cardiometabolic health by taking up glucose and lipids for oxidation, a process that displays a strong diurnal rhythm. While aging has been shown to reduce thermogenic characteristics of BAT, it is as yet unknown whether this reduction is specific to the time of day. Therefore, we assessed whole-body and BAT energy metabolism in young and middle-aged male and female C57BL/6J mice and studied the consequences for lipid metabolism in humanized APOE*3-Leiden.CETP mice (also on a C57BL/6J background). We demonstrate that in middle-aged versus young mice body temperature is lower in both male and female mice, while uptake of triglyceride (TG)-derived fatty acids (FAs) by BAT, reflecting metabolic activity, is attenuated at its peak at the onset of the dark (wakeful) phase in female mice. This coincided with delayed plasma clearance of TG-rich lipoproteins and TG-depleted lipoprotein core remnants, and elevated plasma TGs at the same time point. Furthermore, middle-aged female mice showed increased adiposity, accompanied by lipid accumulation, increased expression of genes involved in lipogenesis, and reduced expression of genes involved in fat oxidation and the intracellular clock machinery in BAT. Peak abundance of lipoprotein lipase (LPL), a crucial regulator of FA uptake, was attenuated in BAT. Our findings suggest that LPL is a potential therapeutic target for restoring diurnal metabolic BAT activity, and that efficiency of strategies targeting BAT may be improved by including time of day as an important factor.

Cerenkov luminescence imaging of interscapular brown adipose tissue using a TSPO-targeting PET probe in the UCP1 ThermoMouse

Rationale: [¹⁸F]fluorodeoxyglucose-positron emission tomography ([¹⁸F]FDG-PET) has been widely used as an imaging technique to measure interscapular brown adipose tissue (iBAT) activity. However, it is challenging to obtain iBAT-specific images using [¹⁸F]FDG-PET because increased uptake of [¹⁸F]FDG is observed in tumors, muscle, and inflamed tissues. Uncoupling protein 1 (UCP1) in the mitochondrial membrane, a well-known molecular marker of BAT, has been proposed as a useful BAT imaging marker. Recently, the UCP1 ThermoMouse was developed as a reporter mouse for monitoring UCP1 expression and investigating BAT activation. In addition, Translocator protein-18 kDa (TSPO) located in the outer mitochondrial membrane is also overexpressed in BAT, suggesting that TSPO-targeting PET has potential for iBAT imaging. However, there are no studies monitoring BAT using TSPO-targeting PET probes in the UCP1 ThermoMouse. Moreover, the non-invasive Cerenkov luminescence imaging (CLI) using Cerenkov radiation from the PET probe has been proposed as an alternative option for PET as it is less expensive and user-friendly. Therefore, we selected [¹⁸F]fm-PBR28-d₂ as a TSPO-targeting PET probe for iBAT imaging to evaluate the usefulness of CLI in the UCP1 ThermoMouse.

Methods: UCP1 ThermoMouse was used to monitor UCP1 expression. Western blotting and immunohistochemistry were performed to measure the level of protein expression. [¹⁸F]fm-PBR28-d₂ and [¹⁸F]FDG were used as radioactive probes for iBAT imaging. PET images were acquired with SimPET, and optical images were acquired with IVIS 100.

Results: UCP1 ThermoMouse showed that UCP1 and TSPO expressions were correlated in iBAT. In both PET and CLI, the TSPO-targeting probe [¹⁸F]fm-PBR28-d₂ was superior to [¹⁸F]FDG for acquiring iBAT images. The high molar activity of the probe was essential for CLI and PET imaging. We tested the feasibility of TSPO-targeting probe under cold exposure by imaging with TSPO-PET/CLI. Both signals of iBAT were clearly increased after cold stimulation. Under prolonged isoflurane anesthesia, TSPO-targeting images showed higher signals from iBAT in the short-term than in long-term groups.

Conclusion: We demonstrated that TSPO-PET/CLI reflected UCP1 expression in iBAT imaging better than [¹⁸F]FDG-PET/CLI under the various conditions. Considering convenience and cost, TSPO-CLI could be used as an alternative TSPO-PET technique for iBAT imaging.

Regulation of CIRP by genetic factors of SP1 related to cold sensitivity

Cold-inducible RNA-binding-protein (CIRP) is a cold shock protein that plays a protective role in genotoxic stress response. CIRP modulates inflammation in human diseases, inhibits cell proliferation, and protects cells from genotoxic damage during cellular stress. The mild cold responsive element and specificity protein 1 (SP1) play a role in Cirp expression at low temperatures. Although previous studies have provided insights into the immune functions of SP1 or CIRP, the mechanisms by which CIRP and SP1 me diate inflammatory responses remain largely unknown. Therefore, in the current study, we examined whether Cirp expression is affected by genetic factors related to temperature sensitivity as well as under low temperature. We performed a genome-wide association study on cold sensitivity in 2,000 participants. Fifty-six genome-wide significant trait-locus pairs were identified (p&lt;1×10-5, false discovery rate &lt; 0.05). Among these variants, rs1117050 and rs11170510 had a strong linkage disequilibrium (r2 &gt; 0.8) relationship and expression quantitative trait locus-associated signals with the nearest Sp1 gene. We confirmed that the minor alleles of rs11170510 and rs58123204 were associated with increased Sp1 expression. Additionally, Sp1 overexpression led to CIRP translocation from the nucleus to the cytoplasm. CIRP protein levels increased in serum samples that had minor alleles of rs11170510 and rs58123204. Levels of various pro-inflammatory cytokines were also significantly increased in human peripheral blood mononuclear cells with minor alleles of rs11170510 and rs58123204. These results suggest that genetic factors related to cold sensitivity regulate CIRP expression and function and provide valuable insights into prediction of potential diseases through analysis of inherent genetic factors in humans.

High-fructose feeding suppresses cold-stimulated brown adipose tissue glucose uptake independently of changes in thermogenesis and the gut microbiome

Diets rich in added sugars are associated with metabolic diseases, and studies have shown a link between these pathologies and changes in the microbiome. Given the reported associations in animal models between the microbiome and brown adipose tissue (BAT) function, and the alterations in the microbiome induced by high-glucose or high-fructose diets, we investigated the potential causal link between high-glucose or -fructose diets and BAT dysfunction in humans. Primary outcomes are changes in BAT cold-induced thermogenesis and the fecal microbiome (clinicaltrials.gov, NCT03188835). We show that BAT glucose uptake, but not thermogenesis, is impaired by a high-fructose but not high-glucose diet, in the absence of changes in the gastrointestinal microbiome. We conclude that decreased BAT glucose metabolism occurs earlier than other pathophysiological abnormalities during fructose overconsumption in humans. This is a potential confounding factor for studies relying on ¹⁸F-FDG to assess BAT thermogenesis.

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Fructose overfeeding decreases brown adipose tissue glucose metabolism

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These changes occur independently of oxidative metabolism

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No change is observed with glucose overfeeding

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The gut microbiome is not affected by fructose/glucose overfeeding

Xenon-enhanced computed tomography assessment of brown adipose tissue distribution and perfusion in lean, obese, and diabetic primates

OBJECTIVE

This study aimed to validate xenon-enhanced computed tomography (XECT) for the detection of brown adipose tissue (BAT) and to use XECT to assess differences in BAT distribution and perfusion between lean, obese, and diabetic nonhuman primates (NHPs).

METHODS

Whole-body XECT imaging was performed in anesthetized rhesus and vervet monkeys during adrenergic stimulation of BAT thermogenesis. In XECT images, BAT was identified as fat tissue that, during xenon inhalation, underwent significant radiodensity enhancement compared with subcutaneous fat. To measure BAT blood flow, BAT radiodensity enhancement was measured over time on the six computed tomography scans acquired during xenon inhalation. Postmortem immunohistochemical staining was used to confirm imaging findings.

RESULTS

XECT was able to correctly identify all BAT depots that were confirmed at necropsy, enabling construction of the first comprehensive anatomical map of BAT in NHPs. A significant decrease in BAT perfusion was found in diabetic animals compared with obese animals and healthy animals, as well as absence of axillary BAT and significant reduction of supraclavicular BAT in diabetic animals compared with obese and lean animals.

CONCLUSIONS

The use of XECT in NHP models of obesity and diabetes allows the analysis of the impact of metabolic status on BAT mass and perfusion.

Role of thermogenic adipose tissue in lipid metabolism and atherosclerotic cardiovascular disease: lessons from studies in mice and humans

Brown adipocytes within brown adipose tissue (BAT) and beige adipocytes within white adipose tissue dissipate nutritional energy as heat. Studies in mice have shown that activation of thermogenesis in brown and beige adipocytes enhances the lipolytic processing of triglyceride-rich lipoproteins (TRLs) in plasma to supply these adipocytes with fatty acids for oxidation. This process results in formation of TRL remnants that are removed from the circulation through binding of apolipoprotein E (ApoE) on their surface to the low-density lipoprotein receptor (LDLR) on hepatocytes, followed by internalization. Concomitantly, lipolytic processing of circulating TRLs leads to generation of excess surface phospholipids that are transferred to nascent high-density lipoproteins (HDL), increasing their capacity for reverse cholesterol transport. Activation of thermogenic adipocytes thus lowers circulating triglycerides and non-HDL-cholesterol, while it increases HDL-cholesterol. The combined effect is protection from atherosclerosis development, which becomes evident in humanized mouse models with an intact ApoE-LDLR clearance pathway only, and is additive to the effects of classical lipid-lowering drugs including statins and proprotein convertase subtilisin/kexin type 9 inhibitors. A large recent study revealed that the presence of metabolically active BAT in humans is associated with lower triglycerides, higher HDL-cholesterol and lower risk of cardiovascular diseases. This narrative review aims to provide leads for further exploration of thermogenic adipose tissue as a therapeutic target. To this end, we describe the latest knowledge on the role of BAT in lipoprotein metabolism and address, for example, the discovery of the β2-adrenergic receptor as the dominant adrenergic receptor in human thermogenic adipocytes.

Targeting skeletal muscle mitochondrial health in obesity

Metabolic demands of skeletal muscle are substantial and are characterized normally as highly flexible and with a large dynamic range. Skeletal muscle composition (e.g., fiber type and mitochondrial content) and metabolism (e.g., capacity to switch between fatty acid and glucose substrates) are altered in obesity, with some changes proceeding and some following the development of the disease. Nonetheless, there are marked interindividual differences in skeletal muscle composition and metabolism in obesity, some of which have been associated with obesity risk and weight loss capacity. In this review, we discuss related molecular mechanisms and how current and novel treatment strategies may enhance weight loss capacity, particularly in diet-resistant obesity.

Cool-Water Immersion Reduces Postexercise Quadriceps Femoris Muscle Perfusion More Than Cold-Water Immersion

PURPOSE

The muscle perfusion response to postexercise cold-water immersion (CWI) is not well understood. We examined the effects of graded postexercise CWI upon global and regional quadriceps femoris muscle perfusion using positron emission tomography and [15O]H2O.

METHODS

Using a matched-group design, 30 healthy men performed cycle ergometer exercise at 70% V̇O2peak to a core body temperature of 38°C, followed by either 10 min of CWI at 8°C, 22°C, or seated rest (control). Quadriceps muscle perfusion; thigh and calf cutaneous vascular conductance; intestinal, muscle, and local skin temperatures; thermal comfort; mean arterial pressure; and heart rate were assessed at preexercise, postexercise, and after CWI.

RESULTS

Global quadriceps perfusion was reduced beyond the predefined minimal clinically relevant threshold (0.75 mL per 100 g·min-1) in 22°C water versus control (difference (95% confidence interval (CI)), -2.5 (-3.9 to -1.1) mL per 100 g·min-1). Clinically relevant decreases in muscle perfusion were observed in the rectus femoris (-2.0 (-3.0 to -1.0) mL per 100 g·min-1) and vastus lateralis (-3.5 (-4.9 to -2.0) mL per 100 g·min-1) in 8°C water, and in the vastus lateralis (-3.3 (-4.8 to -1.9) mL per 100 g·min-1) in 22°C water versus control. The mean effects for vastus intermedius and vastus medialis perfusion were not clinically relevant. Clinically relevant decreases in thigh and calf cutaneous vascular conductance were observed in both cooling conditions.

CONCLUSIONS

The present findings revealed that less noxious CWI (22°C) promoted clinically relevant postexercise decreases in global quadriceps muscle perfusion, whereas noxious cooling (8°C) elicited no effect.

The involvement of the adrenergic nervous system in activating human brown adipose tissue and browning

Obesity is a chronic condition of multifactorial etiology characterized by excessive body fat due to a calorie intake higher than energy expenditure. Given the intrinsic limitations of surgical interventions and the difficulties associated with lifestyle changes, pharmacological manipulation is currently one of the main therapies for metabolic diseases. Approaches aiming to promote energy expenditure through induction of thermogenesis have been explored and, in this context, brown adipose tissue (BAT) activation and browning have been shown to be promising strategies. Although such processes are physiologically stimulated by the sympathetic nervous system, not all situations that are known to increase adrenergic signaling promote a concomitant increase in BAT activation or browning in humans. Thus, a better understanding of factors involved in the thermogenesis attributed to these tissues is needed to enable the development of future therapies against obesity. Herein we carry out a critical review of original articles in humans under conditions previously known to trigger adrenergic responses-namely, cold, catecholamine-secreting tumor (pheochromocytoma and paraganglioma), burn injury, and adrenergic agonists-and discuss which of them are associated with increased BAT activation and browning. BAT is clearly stimulated in individuals exposed to cold or treated with high doses of the β3-adrenergic agonist mirabegron, whereas browning is certainly induced in patients after burn injury or with pheochromocytoma, as well as in individuals treated with β3-adrenergic agonist mirabegron for at least 10 weeks. Given the potential effect of increasing energy expenditure, adrenergic stimuli are promising strategies in the treatment of metabolic diseases.

Response of blood glucose and GLP-1 to different food temperature in normal subject and patients with type 2 diabetes

Background

Eating behavior is a major factor in type 2 diabetes. We investigated the different responses of glucose-regulating hormones to cold and hot glucose solutions in normal subjects and patients with type 2 diabetes.

Methods

In this crossover, self-controlled study, normal subjects (N = 19) and patients with type 2 diabetes (N = 22) were recruited and randomly assigned to a hot (50 °C) or a cold (8 °C) oral glucose-tolerance test (OGTT). The subsequent day, they were switched to the OGTT at the other temperature. Blood glucose, insulin, GIP, glucagon-like peptide-1 (GLP-1), and cortisol were measured at 0, 5, 10, 30, 60, and 120 min during each OGTT. After the hot OGTT, all subjects ingested hot (>42 °C) food and water for that day, and ingested food and water at room temperature (≤24 °C) for the day after cold OGTT. All participants had continuous glucose monitoring (CGM) throughout the study.

Results

Compared to cold OGTT, blood glucose was significantly higher with hot OGTT in both groups (both P < 0.05). However, insulin and GLP-1 levels were significantly higher in hot OGTT in normal subjects only (both P < 0.05). The GIP and cortisol responses did not differ with temperature in both groups. CGM showed that normal subjects had significantly higher 24-h mean glucose (MBG) (6.11 ± 0.13 vs. 5.84 ± 0.11 mmol/L, P = 0.021), and standard deviation of MBG with hot meals (0.59 ± 0.06 vs. 0.48 ± 0.05 mmol/L, P = 0.043), T2DM patients had higher MBG only (8.46 ± 0.38 vs. 8.88 ± 0.39 mmol/L, P = 0.022).

Conclusions

Food temperature is an important factor in glucose absorption and GLP-1 response. These food temperatures elicited differences are lost in type 2 diabetes.

Cold-Induced Thermogenesis Shows a Diurnal Variation That Unfolds Differently in Males and Females

CONTEXT

Cold exposure mobilizes lipids to feed thermogenic processes in organs, including brown adipose tissue (BAT). In rodents, BAT metabolic activity exhibits a diurnal rhythm, which is highest at the start of the wakeful period.

OBJECTIVE

We investigated whether cold-induced thermogenesis displays diurnal variation in humans and differs between the sexes.

METHODS

This randomized crossover study included 24 young and lean male (n = 12) and female (n = 12) participants who underwent 2.5-hour personalized cooling using water-perfused mattresses in the morning (7:45 am) and evening (7:45 pm), with 1 day in between. We measured energy expenditure (EE) and supraclavicular skin temperature in response to cold exposure.

RESULTS

In males, cold-induced EE was higher in the morning than in the evening (+54% ± 10% vs +30% ± 7%; P = 0.05) but did not differ between morning and evening in females (+37% ± 9% vs +30% ± 10%; P = 0.42). Only in males, supraclavicular skin temperature upon cold increased more in morning than evening (+0.2 ± 0.1 °C vs -0.2 ± 0.2 °C; P = 0.05). In males, circulating free fatty acid (FFA) levels were increased after morning cold exposure, but not evening (+90% ± 18% vs +9% ± 8%; P < 0.001). In females, circulating FFA (+94% ± 21% vs +20% ± 5%; P = 0.006), but also triglycerides (+42% ± 5% vs +29% ± 4%, P = 0.01) and cholesterol levels (+17% ± 2% vs 11% ± 2%; P = 0.05) were more increased after cold exposure in morning than in evening.

CONCLUSION

Cold-induced thermogenesis is higher in morning than evening in males; however, lipid metabolism is more modulated in the morning than the evening in females.

Thermogenic responses to different clamped skin temperatures in cold-exposed men and women

Despite many decades of research examining thermoregulatory responses under varying cold stresses in humans, very little is known about the variability in metabolic heat production and shivering activity. Here, we used a novel closed-loop mean skin temperature clamping technique with a liquid-conditioned suit to isolate the effects of mean skin temperature on the subjective evaluation of thermal sensation, heat production, shivering responses, and oxidative fuel selection in young, lean and healthy men (n = 12) and women (n = 12). Our results showed a skin temperature-dependent increase in metabolic heat production (5.2±1.0 kJ/min, 5.9±1.0 kJ/min and 7.0±1.0 kJ/min with skin temperature maintained at 31°C, 29°C and 27°C, respectively; P< 0.0001) and shivering intensity in both men and women (0.6±0.1 %MVC, 1.1±0.4 %MVC and 2.5±0.7 %MVC, respectively; P<0.0001), including sex-dependent differences in heat production at all three temperatures (P < 0.005). Even when controlling for lean body mass and fat mass, sex differences persisted (P = 0.048 and P = 0.004, respectively), whereas controlling for differences in body surface area eliminated these differences. Interestingly, there were no sex differences in the cold-induced change in thermogenesis. Despite clamping skin temperature, there was tremendous variability in the rate of heat production and shivering intensity. Collectively this data suggests that many of the inter-individual differences in thermogenesis and shivering may be explained by differences in morphology and body composition.

Brown adipose tissue: a protective mechanism in "pre-prediabetes"?

Brown adipose tissue (BAT) is present in a significant number of adult humans and has been postulated to exert beneficial metabolic effects. Lean, non-diabetic patients undergoing clinical positron emission tomography (PET)/computed tomography (CT) imaging are more likely to exhibit incidental BAT activation. The aim of this study was to assess metabolic changes associated with the cold-activation of BAT and to compare baseline blood metabolites in participants with varying amounts of active BAT. Methods: Serum blood samples were collected from healthy adult volunteers (body mass index 18.0-25.0 and age≤35 years) before and after 2 h cold exposure. ¹⁸F-flurodeoxyglucose (FDG) PET/CT imaging was performed immediately following cold exposure. Activated BAT was segmented and fasting glucose, insulin, lipid, and other blood metabolite levels were correlated with volume and intensity of active BAT. Using a median cutoff, subjects were classified as BATHIGH or BATLOW. Results: A higher volume of activated BAT was associated with significantly higher pre-cooling glucose and insulin levels (P<0.001 for each). Pre-cooling thyroid stimulating hormone (TSH) and triglyceride levels were significantly higher in the BATHIGH than in the BATLOW group (P = 0.002 and P<0.001, respectively). Triglyceride levels tended to increase over the cooling period in both BAT groups, but increased significantly more in the BATHIGH group (15.7±13.2 md/dl; P<0.001) than in the BATLOW group (4.5±12.2 mg/dl; P = 0.061). Conclusion: These findings may indicate that BAT is recruited to counteract incipient "pre-prediabetic" states, potentially serving as a first-line protective mechanism against very early metabolic or hormonal variations.

Batokines: Mediators of Inter-Tissue Communication (a Mini-Review)

PURPOSE OF REVIEW

This review highlights aspects of brown adipose tissue (BAT) communication with other organ systems and how BAT-to-tissue cross-talk could help elucidate future obesity treatments.

RECENT FINDINGS

Until recently, research on BAT has focused mainly on its thermogenic activity. New research has identified an endocrine/paracrine function of BAT and determined that many BAT-derived molecules, termed "batokines," affect the physiology of a variety of organ systems and cell types. Batokines encompass a variety of signaling molecules including peptides, metabolites, lipids, or microRNAs. Recent studies have noted significant effects of batokines on physiology as it relates whole-body metabolism and cardiac function. This review will discuss batokines and other BAT processes that affect the liver, cardiovascular system, skeletal muscle, immune cells, and brown and white adipose tissue. Brown adipose tissue has a crucial secretory function that plays a key role in systemic physiology.

Plasma Levels of Bile Acids Are Related to Cardiometabolic Risk Factors in Young Adults

CONTEXT

Bile acids (BA) are known for their role in intestinal lipid absorption and can also play a role as signaling molecules to control energy metabolism. Prior evidence suggests that alterations in circulating BA levels and in the pool of circulating BA are linked to an increased risk of obesity and a higher incidence of type 2 diabetes in middle-aged adults.

OBJECTIVE

We aimed to investigate the association between plasma levels of BA with cardiometabolic risk factors in a cohort of well-phenotyped, relatively healthy young adults.

METHODS

Body composition, brown adipose tissue, serum classical cardiometabolic risk factors, and a set of 8 plasma BA (including glyco-conjugated forms) in 136 young adults (age 22.1 ± 2.2 years, 67% women) were measured.

RESULTS

Plasma levels of chenodeoxycholic acid (CDCA) and glycoursodeoxycholic acid (GUDCA) were higher in men than in women, although these differences disappeared after adjusting for body fat percentage. Furthermore, cholic acid (CA), CDCA, deoxycholic acid (DCA), and glycodeoxycholic acid (GDCA) levels were positively, yet weakly associated, with lean body mass (LBM) levels, while GDCA and glycolithocholic acid (GLCA) levels were negatively associated with 18F-fluorodeoxyglucose uptake by brown adipose tissue. Interestingly, glycocholic acid (GCA), glycochenodeoxycholic acid (GCDCA), and GUDCA were positively associated with glucose and insulin serum levels, HOMA index, low-density lipoprotein cholesterol, tumor necrosis factor alpha, interleukin (IL)-2, and IL-8 levels, but negatively associated with high-density lipoprotein cholesterol, ApoA1, and adiponectin levels, yet these significant correlations partially disappeared after the inclusion of LBM as a confounder.

CONCLUSION

Our findings indicate that plasma levels of BA might be sex dependent and are associated with cardiometabolic and inflammatory risk factors in young and relatively healthy adults.

Resting Energy Expenditure and Cold-induced Thermogenesis in Patients With Overt Hyperthyroidism

CONTEXT

Thyroid hormone (TH) is crucial for the adaptation to cold.

OBJECTIVE

To evaluate the effect of hyperthyroidism on resting energy expenditure (REE), cold-induced thermogenesis (CIT) and changes in body composition and weight.

METHODS

This was a prospective cohort study at the endocrine outpatient clinic of a tertiary referral center. Eighteen patients with overt hyperthyroidism were included. We measured REE during hyperthyroidism, after restoring euthyroid TH levels and after 3 months of normal thyroid function. In 14 of the 18 patients, energy expenditure (EE) was measured before and after a mild cold exposure of 2 hours and CIT was the difference between EEcold and EEwarm. Skin temperatures at 8 positions were recorded during the study visits. Body composition was assessed by dual X-ray absorption.

RESULTS

Free thyroxine (fT4) and free triiodothyronine (fT3) decreased significantly over time (fT4, P = .0003; fT3, P = .0001). REE corrected for lean body mass (LBM) decreased from 42 ± 6.7 kcal/24 hour/kg LBM in the hyperthyroid to 33 ± 4.4 kcal/24 hour/kg LBM (-21%, P < .0001 vs hyperthyroid) in the euthyroid state and 3 months later to 33 ± 5.2 kcal/24 hour/kg LBM (-21%, P = .0022 vs hyperthyroid, overall P < .0001). fT4 (P = .0001) and fT3 (P < 0.0001) were predictors of REE. CIT did not change from the hyperthyroid to the euthyroid state (P = .96). Hyperthyroidism led to increased skin temperature at warm ambient conditions but did not alter core body temperature, nor skin temperature after cold exposure. Weight regain and body composition were not influenced by REE and CIT during the hyperthyroid state.

CONCLUSION

CIT is not increased in patients with overt hyperthyroidism.

Effects of a short-term cold exposure on circulating microRNAs and metabolic parameters in healthy adult subjects

This discovery study investigated in healthy subjects whether a short‐term cold exposure may alter circulating microRNAs and metabolic parameters and if co‐expression networks between these factors could be identified. This open randomized crossover (cold vs no cold exposure) study with blind end‐ point evaluation was conducted at 1 center with 10 healthy adult male volunteers. Wearing a cooling vest perfused at 14°C for 2 h reduced the local skin temperature without triggering shivering, increased norepinephrine and blood pressure while decreasing copeptin, C‐peptide and heart rate. Circulating microRNAs measured before and after wearing the cooling vest twice (4 time points) identified 196 mature microRNAs with excellent reproducibility over 72 h. Significant correlations of microRNA expression with copeptin, norepinephrine and C‐peptide were found. A co‐expression‐based microRNA‐microRNA network, as well as microRNA pairs displaying differential correlation as a function of temperature were also detected. This study demonstrates that circulating miRNAs are differentially expressed and coregulated upon cold exposure in humans, supporting their use as predictive and dynamic biomarkers of cardio‐metabolic disorders.

Metabolic determinants of Alzheimer's disease: A focus on thermoregulation

Alzheimer's disease (AD) is a complex age-related neurodegenerative disease, associated with central and peripheral metabolic anomalies, such as impaired glucose utilization and insulin resistance. These observations led to a considerable interest not only in lifestyle-related interventions, but also in repurposing insulin and other anti-diabetic drugs to prevent or treat dementia. Body temperature is the oldest known metabolic readout and mechanisms underlying its maintenance fail in the elderly, when the incidence of AD rises. This raises the possibility that an age-associated thermoregulatory deficit contributes to energy failure underlying AD pathogenesis. Brown adipose tissue (BAT) plays a central role in thermogenesis and maintenance of body temperature. In recent years, the modulation of BAT activity has been increasingly demonstrated to regulate energy expenditure, insulin sensitivity and glucose utilization, which could also provide benefits for AD. Here, we review the evidence linking thermoregulation, BAT and insulin-related metabolic defects with AD, and we propose mechanisms through which correcting thermoregulatory impairments could slow the progression and delay the onset of AD.

Feasibility of using infrared thermal imaging to examine brown adipose tissue in infants aged 18 to 25 months

BACKGROUND

Recent studies in adults indicate that cold-induced temperature change of supraclavicular skin corresponds with brown adipose tissue (BAT) thermogenesis.

AIM

This study examined the feasibility of using thermography to assess temperature changes in infants aged 18-25 months after mild cooling. Further, this study sought to evaluate whether cold exposure induces a thermal response suggestive of BAT activity underlying the supraclavicular region.

SUBJECTS AND METHODS

Changes in maximum skin temperature at the supraclavicular and interscapular regions were determined using thermal imaging following a mild 5-minute cooling condition (by removal of clothes in a climate-controlled room) in 67 Samoan infants. Temperature changes of the forehead and hand, known BAT-free regions, served as indicators of cooling efficacy.

RESULTS

Infants with increased hand and forehead temperatures after cold exposure were excluded from analysis, reducing the effective sample size to 19 infants. On average, forehead (p < 0.001), hand (p < 0.001) and back (0.029) temperatures dropped significantly while supraclavicular temperatures remained constant. Participants with greater decreases in forehead temperature tended to exhibit greater supraclavicular thermogenesis (p = 0.084), suggesting potential BAT activity in this region.

CONCLUSIONS

While further work is necessary to develop a reliable cooling condition, this study provides proof-of-concept for non-invasive assessment of BAT activity in infants.

OGT upregulates myogenic IL-6 by mediating O-GlcNAcylation of p65 in mouse skeletal muscle under cold exposure

Cold exposure is an unavoidable and severe challenge for people and animals residing in cold regions of the world, and may lead to hypothermia, drastic changes in systemic metabolism, and inhibition of protein synthesis. O-linked-N-acetylglucoseaminylation (O-GlcNAcylation) directly regulates the activity and function of target proteins involved in multiple biological processes by acting as a stress receptor and nutrient sensor. Therefore, our study aimed to examine whether O-GlcNAcylation affected myogenic IL-6 expression, regulation of energy metabolism, and promotion of survival in mouse skeletal muscle under acute cold exposure conditions. Total protein was extracted from C2C12 cells that had been cultured at 32°C for 3, 6, 9, and 12 h. Western blot analysis showed that mild hypothermia enhanced O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA) expression. Furthermore, global OGT-dependent glycosylation and interleukin-6 (IL-6) levels peaked 3 h after induction of mild hypothermia. Enhanced activation of the NF-κB pathway was also observed in response to mild hypothermia. Alloxan and Thiamet G were used to reduce and increase global OGT glycosylation levels in C2C12 cells, respectively. Increased O-GlcNAcylation was associated with significant upregulation of IL-6 expression, as well as enhanced activity and nuclear translocation of p65, while decreased O-GlcNAcylation had the opposite effect. In addition, increased O-GlcNAcylation was associated with significantly increased glucose metabolism, and OGT-mediated O-GlcNAcylation of p65. We generated skeletal muscle-specific OGT knockout mice and exposed them to cold at 4°C for 3 h per day for 1 week. OGT deficiency attenuated the O-GlcNAcylation, activity, and nuclear translocation of p65, resulting in downregulation of IL-6 in mouse skeletal muscle of mice exposed to cold conditions. Taken together, our data suggested that O-GlcNAcylation of p65 enhanced p65 activity and nuclear translocation leading to the upregulation of IL-6, which maintained energy homeostasis and promotes cell survival in mouse skeletal muscle during cold exposure.

Altered brown fat thermoregulation and enhanced cold-induced thermogenesis in young, healthy, winter-swimming men

The Scandinavian winter-swimming culture combines brief dips in cold water with hot sauna sessions, with conceivable effects on body temperature. We study thermogenic brown adipose tissue (BAT) in experienced winter-swimming men performing this activity 2–3 times per week. Our data suggest a lower thermal comfort state in the winter swimmers compared with controls, with a lower core temperature and absence of BAT activity. In response to cold, we observe greater increases in cold-induced thermogenesis and supraclavicular skin temperature in the winter swimmers, whereas BAT glucose uptake and muscle activity increase similarly to those of the controls. All subjects demonstrate nocturnal reduction in supraclavicular skin temperature, whereas a distinct peak occurs at 4:30–5:30 a.m. in the winter swimmers. Our data leverage understanding of BAT in adult human thermoregulation, suggest both heat and cold acclimation in winter swimmers, and propose winter swimming as a potential strategy for increasing energy expenditure.

Winter swimmers have a lower core temperature at a thermal comfort state than controls

Winter swimmers had no BAT glucose uptake at a thermal comfort state

Winter swimmers have higher cold-induced thermogenesis than control subjects

Human supraclavicular skin temperature varies with a diurnal rhythm

Prolonged Treatment with Grains of Paradise (Aframomum melegueta) Extract Recruits Adaptive Thermogenesis and Reduces Body Fat in Humans with Low Brown Fat Activity

Increasing adaptive thermogenesis through the activation of brown adipose tissue (BAT) is a promising practical strategy for preventing obesity and related disorders. Ingestion of a single dose of 40 mg of an extract of Grains of Paradise (GP), a ginger family species, reportedly triggers BAT thermogenesis in individuals with high but not in those with low BAT activity. We hypothesized that prolonged treatment with GP might revive BAT in individuals who have lost active BAT. In the present study, we recruited 9 healthy young male volunteers with reduced BAT that was assessed by fluorodeoxyglucose positron emission tomography and computed tomography (FDG-PET/CT) following 2-h cold exposure at 19ºC. The subjects ingested GP extract (40 mg/d) or placebo every day for 5 wk. Before and after the treatment with either GP or placebo, their body composition and BAT-dependent cold-induced thermogenesis (CIT)-a non-invasive index of BAT-were measured in a single-blinded, randomized, placebo-controlled cross-over design. Their whole-body resting energy expenditure at a thermoneutral condition remained unchanged following GP treatment. However, CIT after treatment was significantly higher in GP-treated individuals than in placebo-treated individuals. Body weight and fat-free mass did not change significantly following GP or placebo treatment. Notably, body fat percentage slightly but significantly decreased after GP treatment but not after placebo treatment. These results suggest that repeated ingestion of GP elevates adaptive thermogenesis through the re-activation of BAT, thereby reducing body fat in individuals with low BAT activity.