Dietary fatty acid metabolism of brown adipose tissue in cold-acclimated men

Adipose-targeted nanohybrid as a browning inducer for synergistic hyperthermia-pharmacotherapy of obesity

Inducing adipose browning to increase energy expenditure has recently emerged as a promising approach for antiobesity treatment. However, its therapeutic efficacy is often limited by poor adipose-targeted drug delivery and suboptimal browning efficiency. To address these challenges, an adipose-targeting aptamer (Apt8) and browning agent resveratrol (Res) were used to construct an Apt-modified and Res-loaded degradable mesoporous silica-coated Au nanorods nanocarriers (NC), termed Res@NC@Apt8, achieving adipose-targeted hyperthermia-pharmacotherapy. Upon internalization by adipocytes, laser irradiation induces mild local hyperthermia (LHT) via Res@NC@Apt8, triggering calcium ion (Ca2+) influx. Simultaneously, the interaction of the nanohybrid with local glutathione (GSH) releases Res. The dual mechanisms activate the adenosine 5'-monophosphate-activated protein kinase (AMPK) pathway, reduce the lipid droplet content, enhance mitochondrial biogenesis, and accelerate metabolism, thereby synergistically promoting adipose browning. Intravenous Res@NC@Apt8 administration in obese mice significantly drives adipose reduction and further achieves excellent antiobesity therapeutic efficacy. This synergistic treatment achieves a superior weight reduction of 17.2% compared with 6.9% and 10.6% achieved using LHT and pharmacotherapy alone, respectively. This study introduces a novel strategy for achieving activatable LHT and drug release for synergetic obesity treatment.

Cold exposure increases circulating fibroblast growth factor 21 in the evening in males and females

Objectives

Cold exposure is linked to cardiometabolic benefits. Cold activates brown adipose tissue (BAT), increases energy expenditure, and induces secretion of the hormones fibroblast growth factor 21 (FGF21) and growth differentiation factor 15 (GDF15). The cold-induced increase in energy expenditure exhibits a diurnal rhythm in men. Therefore, we aimed to investigate the effect of cold exposure on serum FGF21 and GDF15 levels in humans and whether cold-induced changes in FGF21 and GDF15 levels differ between morning and evening in males and females.

Method

In this randomized cross-over study, serum FGF21 and GDF15 levels were measured in healthy lean males (n = 12) and females (n = 12) before, during, and after 90 min of stable cold exposure in the morning (07:45 h) and evening (19:45 h) with a 1-day washout period in between.

Results

Cold exposure increased FGF21 levels in the evening compared to the morning both in males (+61% vs -13%; P < 0.001) and in females (+58% vs +8%; P < 0.001). In contrast, cold exposure did not significantly modify serum GDF15 levels, and no diurnal variation was found. Changes in FGF21 and GDF15 levels did not correlate with changes in cold-induced energy expenditure in the morning and evening.

Conclusion

Cold exposure increased serum FGF21 levels in the evening, but not in the morning, in both males and females. GDF15 levels were not affected by cold exposure. Thus, this study suggests that the timing of cold exposure may influence cold-induced changes in FGF21 levels but not GDF15 levels and seems to be independent of changes in energy expenditure.

New improved radiometabolite analysis method for [18F]FTHA from human plasma: a test-retest study with postprandial and fasting state

BACKGROUND

Fatty acid uptake can be measured using PET and 14-(R,S)-[18F]fluoro-6-thia-heptadecanoic acid ([18F]FTHA). However, the relatively rapid rate of [18F]FTHA metabolism significantly affects kinetic modeling of tissue uptake. Thus, there is a need for accurate chromatographic methods to analyze the unmetabolized [18F]FTHA (parent fraction). Here we present a new radiometabolite analysis (RMA) method, with comparison to a previous method for parent fraction analysis, and its use in a test-retest clinical study under fasting and postprandial conditions. We developed a new thin-layer chromatography (TLC) RMA method for analysis of [18F]FTHA parent fraction and its radiometabolites from plasma, by testing stationary phases and eluent combinations. Next, we analyzed [18F]FTHA, its radiometabolites, and plasma radioactivity from subjects participating in a clinical study. A total of 17 obese or overweight participants were dosed with [18F]FTHA twice under fasting, and twice under postprandial conditions and plasma samples were obtained between 14 min (mean of first sample) and 72 min (mean of last sample) post-injection. Aliquots of 70 plasma samples were analyzed using both methods, enabling head-to-head comparisons. We performed test-retest and group comparisons of the parent fraction and plasma radioactivity.

RESULTS

The new TLC method separated seven [18F]FTHA radiometabolite peaks, while the previous method separated three. The new method revealed at least one radiometabolite that was not previously separable from [18F]FTHA. From the plasma samples, the mean parent fraction value was on average 7.2 percentage points lower with the new method, compared to the previous method. Repeated [18F]FTHA investigations on the same subject revealed reproducible plasma SUV and parent fractions, with different kinetics between the fasted and postprandial conditions.

CONCLUSIONS

The newly developed improved radio-TLC method for [18F]FTHA RMA enables accurate parent fraction correction, which is required to obtain quantitative data for modelling [18F]FTHA PET data. Our test-retest study of fasted and postprandial conditions showed robust reproducibility, and revealed clear differences in the [18F]FTHA metabolic rate under different study settings.

TRIAL REGISTRATION

EudraCT No: 2020-005211-48, 04Feb2021; and Clinical Trials registry NCT05132335, 29Oct2021, URL: https://classic.

CLINICALTRIALS

gov/ct2/show/NCT05132335 .

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.

Brown adipose tissue metabolism in women is dependent on ovarian status

In rodents, loss of estradiol (E2) reduces brown adipose tissue (BAT) metabolic activity. Whether E2 impacts BAT activity in women is not known. BAT oxidative metabolism was measured in premenopausal (n = 27; 35 ± 9 yr; body mass index = 26.0 ± 5.3 kg/m2) and postmenopausal (n = 25; 51 ± 8 yr; body mass index = 28.0 ± 5.0 kg/m2) women at room temperature and during acute cold exposure using [11C]acetate with positron emission tomography coupled with computed tomograph. BAT glucose uptake was also measured during acute cold exposure using 2-deoxy-2-[18F]fluoro-d-glucose. To isolate the effects of ovarian hormones from biological aging, measurements were repeated in a subset of premenopausal women (n = 8; 40 ± 4 yr; BMI = 28.0 ± 7.2 kg/m2) after 6 mo of gonadotropin-releasing hormone agonist therapy to suppress ovarian hormones. At room temperature, there was no difference in BAT oxidative metabolism between premenopausal (0.56 ± 0.31 min-1) and postmenopausal women (0.63 ± 0.28 min-1). During cold exposure, BAT oxidative metabolism (1.28 ± 0.85 vs. 0.91 ± 0.63 min-1, P = 0.03) and net BAT glucose uptake (84.4 ± 82.5 vs. 29.7 ± 31.4 nmol·g-1·min-1, P < 0.01) were higher in premenopausal than postmenopausal women. In premenopausal women who underwent gonadotropin-releasing hormone agonist, cold-stimulated BAT oxidative metabolism was reduced to a similar level (from 1.36 ± 0.66 min-1 to 0.91 ± 0.41 min-1) to that observed in postmenopausal women (0.91 ± 0.63 min-1). These results provide the first evidence in humans that reproductive hormones are associated with BAT oxidative metabolism and suggest that BAT may be a target to attenuate age-related reduction in energy expenditure and maintain metabolic health in postmenopausal women.NEW & NOTEWORTHY In rodents, loss of estrogen reduces brown adipose tissue (BAT) activity. Whether this is true in humans is not known. We found that BAT oxidative metabolism and glucose uptake were lower in postmenopausal compared to premenopausal women. In premenopausal women who underwent ovarian suppression to reduce circulating estrogen, BAT oxidative metabolism was reduced to postmenopausal levels. Thus the loss of ovarian function in women leads to a reduction in BAT metabolic activity independent of age.

Highland deer mice support increased thermogenesis in response to chronic cold hypoxia by shifting uptake of circulating fatty acids from muscles to brown adipose tissue

During maximal cold challenge (cold-induced V̇O2,max) in hypoxia, highland deer mice (Peromyscus maniculatus) show higher rates of circulatory fatty acid delivery compared with lowland deer mice. Fatty acid delivery also increases with acclimation to cold hypoxia (CH) and probably plays a major role in supporting the high rates of thermogenesis observed in highland deer mice. However, it is unknown which tissues take up these fatty acids and their relative contribution to thermogenesis. The goal of this study was to determine the uptake of circulating fatty acids into 24 different tissues during hypoxic cold-induced V̇O2,max, by using [1-14C]2-bromopalmitic acid. To uncover evolved and environment-induced changes in fatty acid uptake, we compared lab-born and -raised highland and lowland deer mice, acclimated to either thermoneutral (30°C, 21 kPa O2) or CH (5°C, 12 kPa O2) conditions. During hypoxic cold-induced V̇O2,max, CH-acclimated highlanders decreased muscle fatty acid uptake and increased uptake into brown adipose tissue (BAT) relative to thermoneutral highlanders, a response that was absent in lowlanders. CH acclimation was also associated with increased activities of enzymes citrate synthase and β-hydroxyacyl-CoA dehydrogenase in the BAT of highlanders, and higher levels of fatty acid translocase CD36 (FAT/CD36) in both populations. This is the first study to show that cold-induced fatty acid uptake is distributed across a wide range of tissues. Highland deer mice show plasticity in this fatty acid distribution in response to chronic cold hypoxia, and combined with higher rates of tissue delivery, this contributes to their survival in the cold high alpine environment.

Healthy Aging in Times of Extreme Temperatures: Biomedical Approaches

Climate extremes and rising energy prices present interconnected global health risks. Technical solutions can be supplemented with biomedical approaches to promote healthy longevity in hot and cold conditions. In summer, reducing basal metabolic rate through mild caloric restriction or CR mimetics, such as resveratrol, can potentially be used to lower body temperature. In winter, activating brown adipose tissue (BAT) for non-shivering thermogenesis and improved metabolic health can help adaptation to colder environments. Catechins found in green tea and in other food could be alternatives to drugs for these purposes. This review examines and discusses the biomedical evidence supporting the use of CR mimetics and BAT activators for health benefits amid increasingly extreme temperatures.

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.

Indication of mixed glucose and fatty acid use by inferred brown adipose tissue activity in Samoans

OBJECTIVES

Despite the growing rates of global obesity and the known positive associations between brown adipose tissue (BAT) and cardiovascular health, little is known about the metabolic effects of BAT activity in Samoans, a population at high risk of obesity and type II diabetes. Here we assessed the potential effects of inferred BAT activity on metabolic health markers in Samoan adults exposed to mild cold.

METHODS

Using point-of-care finger prick technology we measured fasting glucose, total cholesterol, high-density lipoprotein (HDL), and low-density lipoprotein (LDL) levels before and after 30 min of cold exposure among 61 individuals (38 females, 23 males, ages 31-54) from 'Upolu Island, Samoa. Respiratory quotient was measured by indirect calorimetry to determine substrate metabolism at room temperature and cold exposure.

RESULTS

Fasting glucose levels decreased significantly (p < .001) after cold exposure while neither total cholesterol (p = .88), HDL (p = .312), nor LDL (p = .089) changed. Respiratory quotient decreased significantly (p = .009) between exposures, suggesting an increased preference for lipid metabolism as a response to cold.

CONCLUSIONS

The observed effects of inferred BAT activity on biomarkers suggest BAT activity utilizes both glucose and lipid-derived fatty acids as fuel for thermogenesis. Our work provides evidence for the beneficial metabolic effects of BAT and emphasizes the need for the population-specific development of metabolic treatments involving BAT to ensure the successful and equitable minimization of extreme consequences of obesity and metabolic health.

Proanthocyanidins-Based Synbiotics as a Novel Strategy for Nonalcoholic Fatty Liver Disease (NAFLD) Risk Reduction

Nonalcoholic fatty liver disease (NAFLD), the most common liver disease worldwide, is a spectrum of liver abnormalities ranging from steatosis to nonalcoholic steatohepatitis (NASH) characterized by excessive lipid accumulation. The prevalence of NAFLD is predicted to increase rapidly, demanding novel approaches to reduce the global NAFLD burden. Flavonoids, the most abundant dietary polyphenols, can reduce the risk of NAFLD. The majority of dietary flavonoids are proanthocyanidins (PACs), which are oligomers and polymers of the flavonoid sub-group flavan-3-ols. The efficacy of PAC in reducing the NAFLD risk can be significantly hindered by low bioavailability. The development of synbiotics by combining PAC with probiotics may increase effectiveness against NAFLD by biotransforming PAC into bioavailable metabolites. PAC and probiotic bacteria are capable of mitigating steatosis primarily through suppressing de novo lipogenesis and promoting fatty acid β-oxidation. PAC and probiotic bacteria can reduce the progression of steatosis to NASH mainly through ameliorating hepatic damage and inflammation induced by hepatic oxidative stress, endoplasmic reticulum stress, and gut microbiota dysbiosis. Synbiotics of PAC are superior in reducing the risk of NAFLD compared to independent administration of PAC and probiotics. The development of PAC-based synbiotics can be a novel strategy to mitigate the increasing incidence of NAFLD.

Very-low-density lipoprotein triglyceride and free fatty acid plasma kinetics in women with high or low brown adipose tissue volume and overweight/obesity

Although a high amount of brown adipose tissue (BAT) is associated with low plasma triglyceride concentration, the mechanism responsible for this relationship in people is not clear. Here, we evaluate the interrelationships among BAT, very-low-density lipoprotein triglyceride (VLDL-TG), and free fatty acid (FFA) plasma kinetics during thermoneutrality in women with overweight/obesity who had a low (<20 mL) or high (≥20 mL) volume of cold-activated BAT (assessed by using positron emission tomography in conjunction with 2-deoxy-2-[18F]-fluoro-glucose). We find that plasma TG and FFA concentrations are lower and VLDL-TG and FFA plasma clearance rates are faster in women with high BAT than low BAT volume, whereas VLDL-TG and FFA appearance rates in plasma are not different between the two groups. These findings demonstrate that women with high BAT volume have lower plasma TG and FFA concentrations than women with low BAT volumes because of increased VLDL-TG and FFA clearance rates. This study was registered at ClinicalTrials.gov (NCT02786251).

Impact of estrogens on resting energy expenditure: A systematic review

The fear of weight gain is one of the main reasons for women not to initiate or to early discontinue hormonal contraception or menopausal hormone therapy. Resting energy expenditure is by far the largest component and the most important determinant of total energy expenditure. Given that low resting energy expenditure is a confirmed predictive factor for weight gain and consecutively for the development of obesity, research into the influence of sex steroids on resting energy expenditure is a particularly exciting area. The objective of this systematic review was to evaluate the effects of medication with natural and synthetic estrogens on resting energy expenditure in healthy normal weight and overweight women. Through complex systematic literature searches, a total of 10 studies were identified that investigated the effects of medication with estrogens on resting energy expenditure. Our results demonstrate that estrogen administration increases resting energy expenditure by up to +208 kcal per day in the context of contraception and by up to +222 kcal per day in the context of menopausal hormone therapy, suggesting a preventive effect of circulating estrogen levels and estrogen administration on weight gain and obesity development.

Dietary fatty acids activate or deactivate brown and beige fat

Brown adipose tissue (BAT) and beige fat have been documented to rapidly consume fatty acids (FAs) rather than deposit of lipid, and they have high capacity to dissipate energy via nonshivering thermogenesis, making BAT and beige fat potential organs to fight obesity and related chronic diseases. As the main substrate for thermogenesis and the basic constituent unit of triacylglycerol, FAs could modify BAT and remodel white adipose tissue (WAT) to beige fat. However, there are few comprehensive review covering the link between dietary FAs and thermogenic adipocyte..In this review, we described the metabolism of thermogenic adipose upon activation and comprehensively summarized publications on the dietary FAs that activate or deactivate BAT and beige fat. Specifically, eicosapentaenoic acid/docosahexaenoic acid (EPA/DHA), α-linolenic acid (α-ALA), conjugated linoleic acid (CLA), oleic acid (OA), long-chain saturated fatty acid (LC-SFA) and medium-chain fatty acid (MCFA). in addition, the influences on BAT function, WAT remodeling, and lipid metabolism, as well as delineated the possible mechanisms are also reviewed. Characterizing thermogenic or obesogenic dietary FAs may offer novel insight into dietary oil and nutritional treatment.

Adipose Tissue and Metabolic Health

In this review, we provide a brief synopsis of the connections between adipose tissue and metabolic health and highlight some recent developments in understanding and exploiting adipocyte biology. Adipose tissue plays critical roles in the regulation of systemic glucose and lipid metabolism and secretes bioactive molecules possessing endocrine, paracrine, and autocrine functions. Dysfunctional adipose tissue has a detrimental impact on metabolic health and is intimately involved in key aspects of metabolic diseases such as insulin resistance, lipid overload, inflammation, and organelle stress. Differences in the distribution of fat depots and adipose characteristics relate to divergent degrees of metabolic dysfunction found in metabolically healthy and unhealthy obese individuals. Thermogenic adipocytes increase energy expenditure via mitochondrial uncoupling or adenosine triphosphate-consuming futile substrate cycles, while functioning as a metabolic sink and participating in crosstalk with other metabolic organs. Manipulation of adipose tissue provides a wealth of opportunities to intervene and combat the progression of associated metabolic diseases. We discuss current treatment modalities for obesity including incretin hormone analogs and touch upon emerging strategies with therapeutic potential including exosome-based therapy, pharmacological activation of brown and beige adipocyte thermogenesis, and administration or inhibition of adipocyte-derived factors.

Changes in plasma endocannabinoids concentrations correlate with 18F-FDG PET/MR uptake in brown adipocytes in humans

Introduction: Recent data suggest a possible role of endocannabinoids in the regulation of brown adipose tissue (BAT) activity. Those findings indicate potential treatment options for obesity. The aim of this study was to evaluate the relationship between plasma endocannabinoids concentrations and the presence of BAT in humans. Methods: The study group consisted of 25 subjects divided into two groups: BAT positive BAT(+), (n = 17, median age = 25 years) and BAT negative BAT(-), (n = 8, median age = 28 years). BAT was estimated using 18F-FDG PET/MR after 2 h of cold exposure. The level of plasma endocannabinoids was assessed at baseline, 60 min and 120 min of cold exposure. Results: In both groups, BAT(+) and BAT(-), during the cooling, we observed a decrease of the same endocannabinoids: arachidonoylethanolamide (AEA), eicosapentaenoyl ethanolamide (EPEA) and oleoyl ethanolamide (OEA) with a much more profound decline in BAT(+) subjects. Statistically significant fall of PEA (palmitoylethanolamide) and SEA (stearoylethanolamide) concentrations after 60 min (FC = 0.7, p = 0.007 and FC = 0.8, p = 0.03, respectively) and 120 min (FC = 0.81, p = 0.004, and FC = 0.9, p = 0.01, respectively) of cooling was observed only in individuals with BAT. Conclusion: We noticed the profound decline of endocannabinoids concentrations in subjects with increased 18F-FDG PET/MR uptake in BAT. Identification of a new molecules related to BAT activity may create a new target for obesity treatment.

Integrated Analysis of DNA Methylome and Transcriptome Reveals Epigenetic Regulation of Cold Tolerance in Litopenaeus vannamei

DNA methylation is an important epigenetic modification that has been shown to be associated with responses to non-biological stressors. However, there is currently no research on DNA methylation in response to environmental signals in shrimp. In this study, we conducted a comprehensive comparative analysis of DNA methylation profiles and differentially expressed genes between two strains of Litopenaeus vannamei with significantly different cold tolerance through whole genome bisulfite sequencing (WGBS) and transcriptome sequencing. Between Lv-C and Lv-T (constant temperature of 28 °C and low temperatures of 18 °C and 10 °C) under cytosine-guanine (CG) environments, 39,100 differentially methylated regions (DMRs) were identified, corresponding to 9302 DMR-related genes (DMRGs). The DMRs were mainly located in the gene body (exons and introns). Gene Ontology (GO) analysis showed that these DMRGs were significantly enriched in cell parts, catalytic activity, and metabolic processes. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis showed significant enrichment of these DMRGs in pathways such as proteasome (ko03050), oxidative phosphorylation (ko00190), mTOR signaling pathway (ko04150), fatty acid metabolism (ko01212), and fatty acid degradation (ko00071). The comprehensive results suggested that L. vannamei mainly regulates gene expression in response to low temperatures through hypermethylation or demethylation of some genes involved in thermogenesis, glycolysis, the autophagy pathway, the peroxisome, and drug metabolism pathways. These results provide important clues for studying DNA methylation patterns and identifying cold tolerance genes in shrimp.

Multifaceted mitochondrial quality control in brown adipose tissue

Brown adipose tissue (BAT) controls mammalian core body temperature by non-shivering thermogenesis. BAT is extraordinarily rich in mitochondria, which have the peculiarity of generating heat by uncoupled respiration. Since the mitochondrial activity of BAT is subject to cycles of activation and deactivation in response to environmental temperature changes, an integrated mitochondrial quality control (MQC) system is of fundamental importance to ensure BAT physiology. Here, we provide an overview of the conventional and alternative mechanisms through which thermogenic adipocytes selectively remove damaged parts of mitochondria and how macrophages participate in the MQC system by removing extracellular mitochondrial waste to maintain the thermogenic function of BAT.

Human thermogenic adipose tissue

Human thermogenic adipose tissue has long been touted as a promising therapeutic target for obesity and its associated metabolic diseases. Here, we provide a brief overview of the current knowledge of in vivo human thermogenic adipose tissue metabolism. We explore the evidence provided by retrospective and prospective studies describing the association of brown adipose tissue (BAT) [¹⁸F]fluorodeoxyglucose accumulation and various cardiometabolic risk factors. Although these studies have been invaluable in generating hypothesis, it has also raised some questions about the reliability of this method as an indicator of BAT thermogenic capacity. We discuss the evidence in support of human BAT functioning as a local thermogenic organ and energy sink, as an endocrine organ, and as a biomarker of adipose tissue health.

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).

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.

Mitochondrial Energy Metabolism in the Regulation of Thermogenic Brown Fats and Human Metabolic Diseases

Brown fats specialize in thermogenesis by increasing the utilization of circulating blood glucose and fatty acids. Emerging evidence suggests that brown adipose tissue (BAT) prevents the incidence of obesity-associated metabolic diseases and several types of cancers in humans. Mitochondrial energy metabolism in brown/beige adipocytes regulates both uncoupling protein 1 (UCP1)-dependent and -independent thermogenesis for cold adaptation and the utilization of excess nutrients and energy. Many studies on the quantification of human BAT indicate that mass and activity are inversely correlated with the body mass index (BMI) and visceral adiposity. Repression is caused by obesity-associated positive and negative factors that control adipocyte browning, de novo adipogenesis, mitochondrial energy metabolism, UCP1 expression and activity, and noradrenergic response. Systemic and local factors whose levels vary between lean and obese conditions include growth factors, inflammatory cytokines, neurotransmitters, and metal ions such as selenium and iron. Modulation of obesity-associated repression in human brown fats is a promising strategy to counteract obesity and related metabolic diseases through the activation of thermogenic capacity. In this review, we highlight recent advances in mitochondrial metabolism, thermogenic regulation of brown fats, and human metabolic diseases.

VLDL-VLDLR axis facilitates brown fat thermogenesis through replenishment of lipid fuels and PPARβ/δ activation

In mammals, brown adipose tissue (BAT) is specialized to conduct non-shivering thermogenesis for survival under cold acclimation. Although emerging evidence suggests that lipid metabolites are essential for heat generation in cold-activated BAT, the underlying mechanisms of lipid uptake in BAT have not been thoroughly understood. Here, we show that very-low-density lipoprotein (VLDL) uptaken by VLDL receptor (VLDLR) plays important roles in thermogenic execution in BAT. Compared with wild-type mice, VLDLR knockout mice exhibit impaired thermogenic features. Mechanistically, VLDLR-mediated VLDL uptake provides energy sources for mitochondrial oxidation via lysosomal processing, subsequently enhancing thermogenic activity in brown adipocytes. Moreover, the VLDL-VLDLR axis potentiates peroxisome proliferator activated receptor (PPAR)β/δ activity with thermogenic gene expression in BAT. Accordingly, VLDL-induced thermogenic capacity is attenuated in brown-adipocyte-specific PPARβ/δ knockout mice. Collectively, these data suggest that the VLDL-VLDLR axis in brown adipocytes is a key factor for thermogenic execution during cold exposure.

No evidence of brown adipose tissue activation after 24 weeks of supervised exercise training in young sedentary adults in the ACTIBATE randomized controlled trial

Exercise modulates both brown adipose tissue (BAT) metabolism and white adipose tissue (WAT) browning in murine models. Whether this is true in humans, however, has remained unknown. An unblinded randomized controlled trial (ClinicalTrials.gov ID: NCT02365129) was therefore conducted to study the effects of a 24-week supervised exercise intervention, combining endurance and resistance training, on BAT volume and activity (primary outcome). The study was carried out in the Sport and Health University Research Institute and the Virgen de las Nieves University Hospital of the University of Granada (Spain). One hundred and forty-five young sedentary adults were assigned to either (i) a control group (no exercise, n = 54), (ii) a moderate intensity exercise group (MOD-EX, n = 48), or (iii) a vigorous intensity exercise group (VIG-EX n = 43) by unrestricted randomization. No relevant adverse events were recorded. 97 participants (34 men, 63 women) were included in the final analysis (Control; n = 35, MOD-EX; n = 31, and VIG-EX; n = 31). We observed no changes in BAT volume (Δ Control: −22.2 ± 52.6 ml; Δ MOD-EX: −15.5 ± 62.1 ml, Δ VIG-EX: −6.8 ± 66.4 ml; P = 0.771) or ¹⁸F-fluorodeoxyglucose uptake (SUVpeak Δ Control: −2.6 ± 3.1 ml; Δ MOD-EX: −1.2 ± 4.8, Δ VIG-EX: −2.2 ± 5.1; p = 0.476) in either the control or the exercise groups. Thus, we did not find any evidence of an exercise-induced change on BAT volume or activity in young sedentary adults.

Exercise modulates brown adipose tissue (BAT) metabolism in murine models. Here the authors report that there is no evidence that 24 weeks of supervised exercise training modulates BAT volume or function in young sedentary adults in the ACTIBATE randomized controlled trial.

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

Regulatory networks determining substrate utilization in brown adipocytes

Brown adipose tissue (BAT) is often considered as a sink for nutrients to generate heat. However, when the complex hormonal and nervous inputs and intracellular signaling networks regulating substrate utilization are considered, BAT appears much more as a tightly controlled rheostat, regulating body temperature and balancing circulating nutrient levels. Here we provide an overview of key regulatory circuits, including the diurnal rhythm, determining glucose, fatty acid, and amino acid utilization and the interdependency of these nutrients in thermogenesis. Moreover, we discuss additional factors mediating sympathetic BAT activation beyond β-adrenergic signaling and the limitations of glucose-based BAT activity measurements to foster a better understanding and interpretation of BAT activity data.

Effect of Acute Cold Exposure on Energy Metabolism and Activity of Brown Adipose Tissue in Humans: A Systematic Review and Meta-Analysis

Background: The benefit of cold exposure for humans against obesity has brought the energy metabolism and activity of brown adipose tissue (BAT) induced by cold into focus. But the results are inconsistent. This review is aimed to systematically explore the effect of cold exposure on the activity of BAT and energy metabolism in humans.

Methods: We searched relevant papers that were published from 1990 to 2021 and were cited in PubMed Central, Web of science, Embase and Cochrane Library databases to conduct this systematic review and meta-analysis. Energy metabolism, BAT volume, BAT activity and non-esterified fatty acids (NEFA) data reported in eligible researches were extracted. Meta-analysis was applied to combine the mean difference or standard mean difference with their 95% confidence intervals (95%CI). RevMan 5.3 software was used for meta-analysis and evaluating the risk of bias. Stata 16.0 was used for evaluating the publication bias.

Results: Ten randomized controlled trials were included in meta-analysis. Compared with human exposed in room temperature at 24°C, the energy expenditure (EE) was increased after acute cold exposure at 16∼19°C (Z = 7.58, p < 0.05, mean different = 188.43kal/d, 95% CI = 139.73–237.13); BAT volume (Z = 2.62, p < 0.05; standard mean different = 0.41, 95% CI = 0.10–0.73); BAT activity (Z = 2.05, p = 0.04, standard mean difference = 1.61, 95% CI = 0.07–3.14) and the intake of BAT NEFA (Z = 2.85, p < 0.05; standard mean different = 0.53, 95% CI = 0.17–0.90) also increased.

Conclusion: Acute cold exposure could improve the energy expenditure and BAT activity in adults, which is beneficial for human against obesity.

Chronic Fatty Acid Depletion Induces Uncoupling Protein 1 (UCP1) Expression to Coordinate Mitochondrial Inducible Proton Leak in a Human-Brown-Adipocyte Model

Thermogenic brown fat contributes to metabolic health in adult humans. Obese conditions are known to repress adipose-tissue browning and its activity. Herein, we found that chronic fatty acid (FA) depletion induced uncoupling protein 1 (UCP1) expression in the chemical-compound-induced brown adipocytes (ciBAs). The ciBAs, converted from human dermal fibroblasts under FA-free conditions, had low intracellular triglyceride levels and strongly activated UCP1 expression. Prolonged treatment with carnitine also reduced triglyceride accumulation and induced UCP1 expression. Transcriptome analysis revealed that the UCP1 induction was accompanied by the activation of lipid metabolic genes. The FA-depleted conditions repressed mitochondrial proton-leak activity and mitochondrial membrane potential (MMP), despite maintaining a high UCP1 expression. The evidence suggested that UCP1 expression was induced to compensate for the proton-leak activity under low MMP. Our study reports a regulatory mechanism underlying UCP1 expression and mitochondrial-energy status in human brown adipocytes under different nutritional conditions.

Combined effects of voluntary running and liraglutide on glucose homeostasis, fatty acid composition of brown adipose tissue phospholipids, and white adipose tissue browning in db/db mice

There is a potential therapeutic application targeting brown adipose tissue (BAT). Either voluntary running or liraglutide increases the thermogenesis of BAT in type 2 diabetes mellitus, but their combined effect is not yet clarified. Male leptin receptor-deficient db/db diabetic mice (n = 24) were randomly divided into voluntary running, liraglutide, voluntary running + liraglutide, and control groups (n = 6/group). Normal male C57 mice were the negative control (n = 6). Fasting blood glucose was monitored every week, plasma insulin and lipid profiles were analyzed, and thermogenic protein expression in BAT and white adipose tissue (WAT) were analyzed by the western blot. A total of 128 metabolites associated with phosphatidylcholines, phosphatidylethanolamines, sphingomyelins, and ceramides were targeted in BAT. Compared to the control group, voluntary running or liraglutide treatment significantly lowered the blood glucose and increased the insulin level; the combined group showed a better effect than liraglutide alone. Hence, the combined treatment showed an enhanced hypoglycemic effect. Uncoupling protein 1 (UCP1) and OXPHOS protein expression in BAT and UCP1 in WAT were significantly increased after exercise training and liraglutide treatment. However, BAT metabolomics showed that compared to the control mice, nine fatty acids increased in the exercise group, six increased in the liraglutide group, and only three increased in the combined group. These results may suggest a higher hypoglycemic effect and the activation of BAT and WAT browning in the combined group.

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.

PET/MRI-evaluated brown adipose tissue activity may be related to dietary MUFA and omega-6 fatty acids intake

An investigation of new ways to activate brown adipose tissue (BAT) is highly valuable, as it is a possible tool for obesity prevention and treatment. The aim of our study was to evaluate the relationships between dietary intake and BAT activity. The study group comprised 28 healthy non-smoking males aged 21–42 years. All volunteers underwent a physical examination and 75-g OGTT and completed 3-day food intake diaries to evaluate macronutrients and fatty acid intake. Body composition measurements were assessed using DXA scanning. An FDG-18 PET/MR was performed to visualize BAT activity. Brown adipose tissue was detected in 18 subjects (67% normal-weight individuals and 33% overweight/obese). The presence of BAT corresponded with a lower visceral adipose tissue (VAT) content (p = 0.04, after adjustment for age, daily kcal intake, and DXA Lean mass). We noted significantly lower omega-6 fatty acids (p = 0.03) and MUFA (p = 0.02) intake in subjects with detected BAT activity after adjustment for age, daily average kcal intake, and DXA Lean mass, whereas omega-3 fatty acids intake was comparable between the two groups. BAT presence was positively associated with the concentration of serum IL-6 (p = 0.01) during cold exposure. Our results show that BAT activity may be related to daily omega-6 fatty acids intake.

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.

β-Adrenergic Receptors and Adipose Tissue Metabolism: Evolution of an Old Story

The role of β-adrenergic receptors (βARs) in adipose tissue to promote lipolysis and the release of fatty acids and nonshivering thermogenesis in brown fat has been studied for so many decades that one would think there is nothing left to discover. With the rediscovery of brown fat in humans and renewed interest in UCP1 and uncoupled mitochondrial respiration, it seems that a review of adipose tissue as an organ, pivotal observations, and the investigators who made them would be instructive to understanding where the field stands now. The discovery of the β₃-adrenergic receptor was important for accurately defining the pharmacology of the adipocyte, while the clinical targeting of this receptor for obesity and metabolic disease has had its highs and lows. Many questions still remain about how βARs regulate adipocyte metabolism and the signaling molecules through which they do it.

n-3 polyunsaturated fatty acids in the regulation of adipose tissue browning and thermogenesis in obesity: Potential relationship with gut microbiota

BACKGROUND

Obesity is a worldwide public health problem characterized by fat tissue accumulation, favouring adipose tissue and metabolic alterations. Increasing energy expenditure (EE) through brown adipose tissue activation and white adipose tissue (WAT) browning has gained relevance as a therapeutic approach. Different bioactive compounds, such as n-3 polyunsaturated fatty acids (PUFA), have been shown to induce those thermogenic effects. This process is regulated by the gut microbiota as well. Nevertheless, obesity is characterized by gut microbiota dysbiosis, which can be restored by weight loss and n-3 PUFA intake, among other factors. Knowledge gap: However, the role of the gut microbiota on the n-3 PUFA effect in inducing thermogenesis in obesity has not been fully elucidated.

OBJECTIVE

This review aims to elucidate the potential implications of this interrelation on WAT browning adiposw sittue (BAT), BAT activity, and EE regulation in obesity models.

Recent Advances in Adipose Tissue Dysfunction and Its Role in the Pathogenesis of Non-Alcoholic Fatty Liver Disease

Obesity is a serious ongoing health problem that significantly increases the incidence of nonalcoholic fatty liver disease (NAFLD). During obesity, adipose tissue dysfunction is obvious and characterized by increased fat deposition (adiposity) and chronic low-grade inflammation. The latter has been implicated to critically promote the development and progression of NAFLD, whose advanced form non-alcoholic steatohepatitis (NASH) is considered one of the most common causes of terminal liver diseases. This review summarizes the current knowledge on obesity-related adipose dysfunction and its roles in the pathogenesis of hepatic steatosis and inflammation, as well as liver fibrosis. A better understanding of the crosstalk between adipose tissue and liver under obesity is essential for the development of new and improved preventive and/or therapeutic approaches for managing NAFLD.

Lower brown adipose tissue activity is associated with non-alcoholic fatty liver disease but not changes in the gut microbiota

In rodents, lower brown adipose tissue (BAT) activity is associated with greater liver steatosis and changes in the gut microbiome. However, little is known about these relationships in humans. In adults (n = 60), we assessed hepatic fat and cold-stimulated BAT activity using magnetic resonance imaging and the gut microbiota with 16S sequencing. We transplanted gnotobiotic mice with feces from humans to assess the transferability of BAT activity through the microbiota. Individuals with NAFLD (n = 29) have lower BAT activity than those without, and BAT activity is inversely related to hepatic fat content. BAT activity is not related to the characteristics of the fecal microbiota and is not transmissible through fecal transplantation to mice. Thus, low BAT activity is associated with higher hepatic fat accumulation in human adults, but this does not appear to have been mediated through the gut microbiota.

Physiological Responses to Hypoxia on Triglyceride Levels

Hypoxia is a condition during which the body or specific tissues are deprived of oxygen. This phenomenon can occur in response to exposure to hypoxic environmental conditions such as high-altitude, or because of pathophysiological conditions such as obstructive sleep apnea. Circumstances such as these can restrict supply or increase consumption of oxygen, leading to oxyhemoglobin desaturation and tissue hypoxia. In certain cases, hypoxia may lead to severe health consequences such as an increased risk of developing cardiovascular diseases and type 2 diabetes. A potential explanation for the link between hypoxia and an increased risk of developing cardiovascular diseases lies in the disturbing effect of hypoxia on circulating blood lipids, specifically its capacity to increase plasma triglyceride concentrations. Increased circulating triglyceride levels result from the production of triglyceride-rich lipoproteins, such as very-low-density lipoproteins and chylomicrons, exceeding their clearance rate. Considerable research in murine models reports that hypoxia may have detrimental effects on several aspects of triglyceride metabolism. However, in humans, the mechanisms underlying the disturbing effect of hypoxia on triglyceride levels remain unclear. In this mini-review, we outline the available evidence on the physiological responses to hypoxia and their impact on circulating triglyceride levels. We also discuss mechanisms by which hypoxia affects various organs involved in the metabolism of triglyceride-rich lipoproteins. This information will benefit scientists and clinicians interested in the mechanistic of the regulatory cascade responsible for the response to hypoxia and how this response could lead to a deteriorated lipid profile and an increased risk of developing hypoxia-related health consequences.

Metabolic effects of brown fat in transitioning from hyperthyroidism to euthyroidism

OBJECTIVE

Brown adipose tissue (BAT) controls metabolic rate through thermogenesis. As its regulatory factors during the transition from hyperthyroidism to euthyroidism are not well established, our study investigated the relationships between supraclavicular brown adipose tissue (sBAT) activity and physiological/metabolic changes with changes in thyroid status.

DESIGN

Participants with newly diagnosed Graves' disease were recruited. A thionamide antithyroid drug (ATD) such as carbimazole (CMZ) or thiamazole (TMZ) was prescribed in every case. All underwent energy expenditure (EE) measurement and supraclavicular infrared thermography (IRT) within a chamber calorimeter, as well as 18F-fluorodeoxyglucose (18F-FDG) positron-emission tomography/magnetic resonance (PET/MR) imaging scanning, with clinical and biochemical parameters measured during hyperthyroidism and repeated in early euthyroidism. PET sBAT mean/maximum standardized uptake value (SUV mean/max), MR supraclavicular fat fraction (sFF) and mean temperature (Tscv) quantified sBAT activity.

RESULTS

Twenty-one (16 female/5 male) participants aged 39.5 ± 2.5 years completed the study. The average duration to attain euthyroidism was 28.6 ± 2.3 weeks. Eight participants were BAT-positive while 13 were BAT-negative. sFF increased with euthyroidism (72.3 ± 1.4% to 76.8 ± 1.4%; P < 0.01), but no changes were observed in PET SUV mean and Tscv. Significant changes in serum-free triiodothyronine (FT3) levels were related to BAT status (interaction P value = 0.04). FT3 concentration at hyperthyroid state was positively associated with sBAT PET SUV mean (r = 0.58, P = 0.01) and resting metabolic rate (RMR) (P < 0.01).

CONCLUSION

Hyperthyroidism does not consistently lead to a detectable increase in BAT activity. FT3 reduction during the transition to euthyroidism correlated with BAT activity.

Metabolic-associated fatty liver disease and lipoprotein metabolism

BACKGROUND

Non-alcoholic fatty liver disease, or as recently proposed 'metabolic-associated fatty liver disease' (MAFLD), is characterized by pathological accumulation of triglycerides and other lipids in hepatocytes. This common disease can progress from simple steatosis to steatohepatitis, and eventually end-stage liver diseases. MAFLD is closely related to disturbances in systemic energy metabolism, including insulin resistance and atherogenic dyslipidemia.

SCOPE OF REVIEW

The liver is the central organ in lipid metabolism by secreting very low density lipoproteins (VLDL) and, on the other hand, by internalizing fatty acids and lipoproteins. This review article discusses recent research addressing hepatic lipid synthesis, VLDL production, and lipoprotein internalization as well as the lipid exchange between adipose tissue and the liver in the context of MAFLD.

MAJOR CONCLUSIONS

Liver steatosis in MAFLD is triggered by excessive hepatic triglyceride synthesis utilizing fatty acids derived from white adipose tissue (WAT), de novo lipogenesis (DNL) and endocytosed remnants of triglyceride-rich lipoproteins. In consequence of high hepatic lipid content, VLDL secretion is enhanced, which is the primary cause of complex dyslipidemia typical for subjects with MAFLD. Interventions reducing VLDL secretory capacity attenuate dyslipidemia while they exacerbate MAFLD, indicating that the balance of lipid storage versus secretion in hepatocytes is a critical parameter determining disease outcome. Proof of concept studies have shown that promoting lipid storage and energy combustion in adipose tissues reduces hepatic lipid load and thus ameliorates MAFLD. Moreover, hepatocellular triglyceride synthesis from DNL and WAT-derived fatty acids can be targeted to treat MAFLD. However, more research is needed to understand how individual transporters, enzymes, and their isoforms affect steatosis and dyslipidemia in vivo, and whether these two aspects of MAFLD can be selectively treated. Processing of cholesterol-enriched lipoproteins appears less important for steatosis. It may, however, modulate inflammation and consequently MAFLD progression.

Brown adipose tissue is associated with healthier body fat distribution and metabolic benefits independent of regional adiposity

The association of brown adipose tissue (BAT) and body fat distribution and their combined effects on metabolic health in humans remains unknown. Here, we retrospectively identify individuals with and without BAT on ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) positron emission tomography (PET)/computed tomography (CT) and assemble a propensity score-matched study cohort to compare body fat distribution and determine its role in mediating the benefits of brown fat. We find that BAT is associated with lower amounts of visceral adipose tissue and higher amounts of subcutaneous adipose tissue, resulting in less central obesity. In addition, BAT is independently associated with lower blood glucose and white blood cell count, improved lipids, lower prevalence of type 2 diabetes mellitus, and decreased liver fat accumulation. These observations are most prominent in individuals with central obesity. Our results support a role of BAT in protection from visceral adiposity and improved metabolic health.

Brown adipose tissue is associated with more subcutaneous and less visceral fat

Brown adipose tissue is associated with health independent of fat distribution

Brown adipose tissue is associated with less liver fat and type 2 diabetes

Brown adipose tissue is most beneficial in individuals with central obesity

Atrial Natriuretic Peptide Orchestrates a Coordinated Physiological Response to Fuel Non-shivering Thermogenesis

Atrial natriuretic peptide (ANP) is a cardiac hormone controlling blood volume and pressure in mammals. It is still unclear whether ANP controls cold-induced thermogenesis in vivo. Here, we show that acute cold exposure induces cardiac ANP secretion in mice and humans. Genetic inactivation of ANP promotes cold intolerance and suppresses half of cold-induced brown adipose tissue (BAT) activation in mice. While white adipocytes are resistant to ANP-mediated lipolysis at thermoneutral temperature in mice, cold exposure renders white adipocytes fully responsive to ANP to activate lipolysis and a thermogenic program, a physiological response that is dramatically suppressed in ANP null mice. ANP deficiency also blunts liver triglycerides and glycogen metabolism, thus impairing fuel availability for BAT thermogenesis. ANP directly increases mitochondrial uncoupling and thermogenic gene expression in human white and brown adipocytes. Together, these results indicate that ANP is a major physiological trigger of BAT thermogenesis upon cold exposure in mammals.

Polyphenols and their anti-obesity role mediated by the gut microbiota: a comprehensive review

Obesity is a global public health problem that results in chronic pathologies such as diabetes, cardiovascular diseases, and cancer. The treatment approach based on energy restriction and promotion of physical activity is ineffective in the long term. Due to the high prevalence of this pathology, complementary treatments such as brown adipose tissue activation (BAT) and white adipose tissue browning (WAT) have been proposed. Dietary polyphenols are plant secondary metabolites that can stimulate browning and thermogenesis of adipose tissue. They have also been shown to prevent body weight gain, and decrease systemic inflammation produced by high-fat diets. Ingested dietary polyphenols that reach the colon are metabolized by the gut microbiota (GM), regulating its composition and generating a great array of metabolites. GM is involved in the production of short chain fatty acids and secondary bile salts that regulate energetic metabolism. The alteration in the composition of GM observed in metabolic diseases such as obesity and type 2 diabetes can be attenuated by polyphenols. Recent studies support the hypothesis that GM would mediate WAT browning and BAT thermogenesis activation induced by polyphenol administration. Together, these results indicate that GM in the presence of polyphenols plays a fundamental role in the control of obesity possible through BAT activation.

Energy metabolism in brown adipose tissue

Brown adipose tissue (BAT) is well known to burn calories through uncoupled respiration, producing heat to maintain body temperature. This 'calorie wasting' feature makes BAT a special tissue, which can function as an 'energy sink' in mammals. While a combination of high energy intake and low energy expenditure is the leading cause of overweight and obesity in modern society, activating a safe 'energy sink' has been proposed as a promising obesity treatment strategy. Metabolically, lipids and glucose have been viewed as the major energy substrates in BAT, while succinate, lactate, branched-chain amino acids, and other metabolites can also serve as energy substrates for thermogenesis. Since the cataplerotic and anaplerotic reactions of these metabolites interconnect with each other, BAT relies on its dynamic, flexible, and complex metabolism to support its special function. In this review, we summarize how BAT orchestrates the metabolic utilization of various nutrients to support thermogenesis and contributes to whole-body metabolic homeostasis.

Nutritional Regulation of Human Brown Adipose Tissue

The recent identification of brown adipose tissue in adult humans offers a new strategy to increase energy expenditure to treat obesity and associated metabolic disease. While white adipose tissue (WAT) is primarily for energy storage, brown adipose tissue (BAT) is a thermogenic organ that increases energy expenditure to generate heat. BAT is activated upon cold exposure and improves insulin sensitivity and lipid clearance, highlighting its beneficial role in metabolic health in humans. This review provides an overview of BAT physiology in conditions of overnutrition (obesity and associated metabolic disease), undernutrition and in conditions of altered fat distribution such as lipodystrophy. We review the impact of exercise, dietary macronutrients and bioactive compounds on BAT activity. Finally, we discuss the therapeutic potential of dietary manipulations or supplementation to increase energy expenditure and BAT thermogenesis. We conclude that chronic nutritional interventions may represent a useful nonpharmacological means to enhance BAT mass and activity to aid weight loss and/or improve metabolic health.

Role of the Endocannabinoid System in the Adipose Tissue with Focus on Energy Metabolism

The endocannabinoid system is involved in a wide range of processes including the control of energy acquisition and expenditure. Endocannabinoids and their receptors are present in the central nervous system but also in peripheral tissues, notably the adipose tissues. The endocannabinoid system interacts with two main hormones regulating appetite, namely leptin and ghrelin. The inhibitory effect of the cannabinoid receptor 1 (CB₁) antagonist rimonabant on fat mass suggested that the endocannabinoid system can also have a peripheral action in addition to its effect on appetite reduction. Thus, several investigations have focused on the peripheral role of the endocannabinoid system in the regulation of metabolism. The white adipose tissue stores energy as triglycerides while the brown adipose tissue helps to dissipate energy as heat. The endocannabinoid system regulates several functions of the adipose tissues to favor energy accumulation. In this review we will describe the presence of the endocannabinoid system in the adipose tissue. We will survey the role of the endocannabinoid system in the regulation of white and brown adipose tissue metabolism and how the eCB system participates in obesity and metabolic diseases.

The Roles of Dietary, Nutritional and Lifestyle Interventions in Adipose Tissue Adaptation and Obesity

The obesity and the associated non-communicable diseases (NCDs) are globally increasing in their prevalence. While the modern-day lifestyle required less ventilation of metabolic energy through muscular activities, this lifestyle transition also provided the unlimited accession to foods around the clock, which prolong the daily eating period of foods that contained high calorie and high glycemic load. These situations promote the high continuous flux of carbon substrate availability in mitochondria and induce the indecisive bioenergetic switches. The disrupted bioenergetic milieu increases the uncoupling respiration due to the excess flow of the substrate-derived reducing equivalents and reduces ubiquinones into the respiratory chain. The diversion of the uncoupling proton gradient through adipocyte thermogenesis will then alleviate the damaging effects of free radicals to mitochondria and other organelles. The adaptive induction of white adipose tissues (WAT) to beige adipose tissues (beAT) has shown beneficial effects on glucose oxidation, ROS protection and mitochondrial function preservation through the uncoupling protein 1 (UCP1)-independent thermogenesis of beAT. However, the maladaptive stage can eventually initiate with the persistent unhealthy lifestyles. Under this metabolic gridlock, the low oxygen and pro-inflammatory environments promote the adipose breakdown with sequential metabolic dysregulation, including insulin resistance, systemic inflammation and clinical NCDs progression. It is unlikely that a single intervention can reverse all these complex interactions. A comprehensive protocol that includes dietary, nutritional and all modifiable lifestyle interventions, can be the preferable choice to decelerate, stop, or reverse the NCDs pathophysiologic processes.

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.