Differences in the metabolic status of healthy adults with and without active brown adipose tissue

Mechanistic Insights into Propolis in Targeting Type 2 Diabetes Mellitus: A Systematic Review

Type 2 diabetes mellitus (T2DM) is a major global health concern characterized by insulin resistance and impaired glucose metabolism. Growing interest in natural therapies has led to the exploration of propolis, a resinous bee product, for its potential anti-diabetic effects. This review examines the mechanisms by which propolis may aid in T2DM management. A literature search was conducted in SCOPUS and PubMed using the terms (Propolis) AND (diabetes OR "insulin resistance" OR hyperglycemia), focusing on studies published from 2014 onwards. The search yielded 384 and 207 records in SCOPUS and PubMed, respectively. After screening and full-text review, 42 studies met the inclusion criteria. Key variables analyzed included the type and source of propolis, experimental models, dosage, treatment duration, and primary and secondary outcomes. Findings highlight multiple mechanisms through which propolis may benefit T2DM, including enhancing pancreatic β-cell function, improving insulin sensitivity, regulating glucose and lipid metabolism, modulating gut microbiota, and reducing oxidative stress and inflammation. Some studies also reported protective effects on renal and hepatic function. Overall, propolis exhibits promising potential as a complementary therapy for T2DM. However, further well-designed clinical trials are necessary to confirm its efficacy, determine optimal dosing, and identify key bioactive compounds responsible for its therapeutic effects. Future research should focus on optimizing its clinical application for diabetes management.

Serum secreted EMC10 (scEMC10) levels are inversely associated with metabolically active brown adipose tissue in humans

BACKGROUND/OBJECTIVES

Secreted endoplasmic reticulum membrane complex subunit 10 (scEMC10) has been implicated in obesity in mice and humans. In this study, the associations of serum scEMC10 levels with thermoneutrality-modulated brown adipose tissue (BAT) activity and thyroid hormone (TH)-dependent thermogenesis were investigated in humans.

SUBJECTS/METHODS

Serum scEMC10 levels were measured in participants from multiple cohorts using enzyme-linked immunosorbent assay, including participants with or without active BAT determined by PET-CT scanning, participants with positive BAT before and after thermoneutrality, and patients with hyperthyroidism before and after anti-thyroid drug (ATD) treatment. The difference in serum scEMC10 between participants with positive or negative BAT, and the changes of serum scEMC10 in participants with positive BAT before and after thermoneutrality and in patients with Grave's disease-caused hyperthyroidism before and after ATD treatment were determined.

RESULTS

PET-CT scan with 18F-FDG indicated participants with positive BAT were significantly younger and leaner than ones with negative BAT. There was, however, no significant difference in serum scEMC10 between the two groups. Serum scEMC10 levels in participants with positive BAT were significantly elevated by 2-h thermoneutrality (p = 0.0017), concomitant with disappearance of active BAT. No significant association of serum scEMC10 with serum levels of either TSH, FT3, or FT4 was observed in participants from both Chinese and White cohorts. ATD treatment normalized thyroid function and reduced the uptake of 18F-FDG into skeletal muscle of patients with hyperthyroidism. Serum scEMC10 concentration, however, remained unchanged in these patients before and after ATD treatment.

CONCLUSIONS

Serum scEMC10 levels are inversely associated with BAT activity in humans.

Leptin, Leptin Receptor Concentrations and Free Leptin Index (FLI) in Polish Healthy Children and Adolescents

BACKGROUND

Leptin physiology in children is crucial for diagnosing and managing pediatric endocrine and metabolic disorders. The aim of this study was to assess the values of leptin, leptin receptor (sOB-R), and free leptin index (FLI) depending on age and sex in healthy Polish children and adolescents.

MATERIALS AND METHODS

A total of 236 children and adolescents aged 1-18 years were recruited. Leptin and leptin receptor concentrations were determined by immunoenzymatic methods. FLI values were calculated as leptin divided by sOB-R concentrations. In 114 children between the ages of 5 and 10 years, a measurement of fat mass was assessed by dual-energy X-ray absorptiometry.

RESULTS

The studied groups of girls and boys were of similar age and did not differ in terms of weight, height, body mass index (BMI), BMI Z-score values, or leptin receptor concentrations. Leptin concentrations and FLI were higher by about 50% (p=0.006; p=0.051, respectively) in girls than boys. Positive correlations were found between leptin and age, BMI, and BMI Z-score values (r=0.562, r=0.563, p=0.397; p<0.001, respectively), and even stronger between FLI and age, BMI, and BMI Z-score values (r=0.670, r=0.632, p=0.409; p<0.001, respectively).

CONCLUSIONS

The results concerning leptin and leptin receptor concentrations and FLI values in healthy individuals may be useful in clinical practice in early identification of children and adolescents with an unfavorable adipokine profile resulting in a predisposition to the development of obesity and obesity-related complications. These markers may also be helpful in monitoring therapy effectiveness in patients with obesity, diabetes, and metabolic syndrome.

Role of Brown Adipose Tissue in Metabolic Health and Efficacy of Drug Treatment for Obesity

(1) Background: Brown adipose tissue (BAT) is responsible for non-shivering thermogenesis, and its activation has become a new object as both a determinant of metabolic health and a target for therapy. This study aimed to identify the relationships between the presence of BAT, parameters that characterize metabolic health (glucose, lipids, blood pressure (BP)), and the dynamics of body mass index (BMI) during weight-reducing therapy. (2) Methods: The study included 72 patients with obesity. We investigated metabolic parameters, anthropometric parameters, and BP. Dual-energy X-ray absorptiometry (DXA) and positron emission tomography and computed tomography (PET/CT) imaging with 18F-fluorodeoxyglucose (18F-FDG) were performed. (3) Results: Before weight-reducing therapy, BAT was revealed only in 19% patients with obesity. The presence of BAT was associated with a lower risk of metabolic deviations that characterize metabolic syndrome: shorter waist circumference (WC) (p = 0.02) and lower levels of glucose (p = 0.03) and triglycerides (p = 0.03). Thereafter, patients were divided into four groups according to the type of therapy (only lifestyle modification or with Liraglutide or Reduxin or Reduxin Forte). We did not find a relationship between the presence of BAT and response to therapy: percent weight reduction was 10.4% in patients with BAT and 8.5% in patients without BAT (p = 0.78) during six months of therapy. But we noted a significant positive correlation between the volume of BAT and the effectiveness of weight loss at 3 months (r = 0.52, p = 0.016). The dynamic analysis of BAT after 6 months of therapy showed a significant increase in the volume of cold-induced metabolically active BAT, as determined by PET/CT with 18F-FDG in the Liraglutide group (p = 0.04) and an increase in the activity of BAT standardized uptake value (SUV mean and SUV max) in the Reduxin (p = 0.02; p = 0.01, respectively) and Liraglutide groups (p = 0.02 in both settings). (4) Conclusions: The presence of brown adipose tissue is associated with a lower risk of metabolic abnormalities. In general, our study demonstrated that well-established drugs in the treatment of obesity (Liraglutide and Reduxin) have one more mechanism for implementing their effects. These drugs have the ability to increase the activity of BAT. A significant positive relationship between the total volume of BAT and the percentage of weight loss may further determine the priority mechanism of the weight-reducing effect of these medicaments.

Obesity and the risk of cardiometabolic diseases

The prevalence of obesity has reached pandemic proportions, and now approximately 25% of adults in Westernized countries have obesity. Recognized as a major health concern, obesity is associated with multiple comorbidities, particularly cardiometabolic disorders. In this Review, we present obesity as an evolutionarily novel condition, summarize the epidemiological evidence on its detrimental cardiometabolic consequences and discuss the major mechanisms involved in the association between obesity and the risk of cardiometabolic diseases. We also examine the role of potential moderators of this association, with evidence for and against the so-called 'metabolically healthy obesity phenotype', the 'fatness but fitness' paradox or the 'obesity paradox'. Although maintenance of optimal cardiometabolic status should be a primary goal in individuals with obesity, losing body weight and, particularly, excess visceral adiposity seems to be necessary to minimize the risk of cardiometabolic diseases.

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.

Estimating the effect size of a hidden causal factor between SNPs and a continuous trait: a mediation model approach

BACKGROUND

Observational studies and Mendelian randomization experiments have been used to identify many causal factors for complex traits in humans. Given a set of causal factors, it is important to understand the extent to which these causal factors explain some, all, or none of the genetic heritability, as measured by single-nucleotide polymorphisms (SNPs) that are associated with the trait. Using the mediation model framework with SNPs as the exposure, a trait of interest as the outcome, and the known causal factors as the mediators, we hypothesize that any unexplained association between the SNPs and the outcome trait is mediated by an additional unobserved, hidden causal factor.

RESULTS

We propose a method to infer the effect size of this hidden mediating causal factor on the outcome trait by utilizing the estimated associations between a continuous outcome trait, the known causal factors, and the SNPs. The proposed method consists of three steps and, in the end, implements Markov chain Monte Carlo to obtain a posterior distribution for the effect size of the hidden mediator. We evaluate our proposed method via extensive simulations and show that when model assumptions hold, our method estimates the effect size of the hidden mediator well and controls type I error rate if the hidden mediator does not exist. In addition, we apply the method to the UK Biobank data and estimate parameters for a potential hidden mediator for waist-hip ratio beyond body mass index (BMI), and find that the hidden mediator has a large effect size relatively to the effect size of the known mediator BMI.

CONCLUSIONS

We develop a framework to infer the effect of potential, hidden mediators influencing complex traits. This framework can be used to place boundaries on unexplained risk factors contributing to complex traits.

The Role of Thermogenic Fat Tissue in Energy Consumption

Mammalian adipose tissues are broadly divided into white adipose tissue (WAT) and thermogenic fat tissue (brown adipose tissue and beige adipose tissue). Uncoupling protein 1 (UCP1) is the central protein in thermogenesis, and cells that exhibit induced UCP1 expression and appear scattered throughout WAT are called beige adipocytes, and their induction in WAT is referred to as “beiging”. Beige adipocytes can differentiate from preadipocytes or convert from mature adipocytes. UCP1 was thought to contribute to non-shivering thermogenesis; however, recent studies demonstrated the presence of UCP1-independent thermogenic mechanisms. There is evidence that thermogenic fat tissue contributes to systemic energy expenditure even in human beings. This review discusses the roles that thermogenic fat tissue plays in energy consumption and offers insight into the possibility and challenges associated with its application in the treatment of obesity and type 2 diabetes.

Brown Fat as a Regulator of Systemic Metabolism beyond Thermogenesis

Brown adipose tissue (BAT) is a specialized tissue for nonshivering 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. Moreover, several remarkable advancements have been made in brown fat biology over the past decade: The molecular and functional analyses of inducible thermogenic adipocytes (socalled beige adipocytes) arising from a developmentally different lineage from classical brown adipocytes have been accelerated. In addition to a well-established thermogenic activity of uncoupling protein 1 (UCP1), several alternative thermogenic mechanisms have been discovered, particularly in beige adipocytes. It has become clear that BAT influences other peripheral tissues and controls their functions and systemic homeostasis of energy and metabolic substrates, suggesting BAT as a metabolic regulator, other than for thermogenesis. This notion is supported by discovering that various paracrine and endocrine factors are secreted from BAT. We review the current understanding of BAT pathophysiology, particularly focusing on its role as a metabolic regulator in small rodents and also in humans.

The chemerin-CMKLR1 axis limits thermogenesis by controlling a beige adipocyte/IL-33/type 2 innate immunity circuit

IL-33-associated type 2 innate immunity has been shown to support beige fat formation and thermogenesis in subcutaneous inguinal white adipose tissue (iWAT), but little is known about how it is regulated in iWAT. Chemerin, as a newly identified adipokine, is clinically associated with obesity and metabolic disorders. We here show that cold exposure specifically reduces chemerin and its receptor chemerin chemokine-like receptor 1 (CMKLR1) expression in iWAT. Lack of chemerin or adipocytic CMKLR1 enhances cold-induced thermogenic beige fat via potentiating type 2 innate immune responses. Mechanistically, we identify adipocytes, particularly beige adipocytes, as the main source for cold-induced IL-33, which is restricted by the chemerin-CMKLR1 axis via dampening cAMP-PKA signaling, thereby interrupting a feed-forward circuit between beige adipocytes and type 2 innate immunity that is required for cold-induced beige fat and thermogenesis. Moreover, specific deletion of adipocytic IL-33 inhibits cold-induced beige fat and type 2 innate immune responses. Last, genetic blockade of adipocytic CMKLR1 protects against diet-induced obesity and enhances the metabolic benefits of cold stimulation in preestablished obese mice. Thus, our study identifies the chemerin-CMKLR1 axis as a physiological negative regulator of thermogenic beige fat via interrupting adipose-immune communication and suggests targeting adipose CMKLR1 as a potential therapeutic strategy for obesity-related metabolic disorders.

Aerobic exercise promotes the functions of brown adipose tissue in obese mice via a mechanism involving COX2 in the VEGF signaling pathway

Background

High-fat diet (HFD)-induced obesity causes immune cells to infiltrate adipose tissue, leading to chronic inflammation and metabolic syndrome. Brown adipose tissue (BAT) can dissipate the energy produced by lipid oxidation as heat, thereby counteracting obesity. Aerobic exercise activates BAT, but the specific underlying mechanism is still unclear.

Methods

Male C57BL/6 J mice were divided into a normal diet control group (NC group) and HFD group (H group). After becoming obese, the animals in the H group were subdivided into a control group (HC group) and an exercise group (HE group, with treadmill training). After 4 weeks, the mRNA profile of BAT was determined, and then differentially expressed key genes and pathways were verified in vitro.

Results

Relative to the NC group, the genes upregulated in the HC group coded mainly for proteins involved in immune system progression and inflammatory and immune responses, while the downregulated genes regulated lipid metabolism and oxidation–reduction. Relative to the HC group, the genes upregulated in the HE group coded for glycolipid metabolism, while those that were downregulated were involved in cell death and apoptosis. VEGF and other signaling pathways were enhanced by aerobic exercise. Interaction analysis revealed that the gene encoding cyclooxygenase 2 (COX2) of the VEGF signaling pathway is central to this process, which was verified by a sympathetic activator (isoprenaline hydrochloride) and COX2 inhibitor (NS-398).

Conclusions

In mice with HFD-induced obesity, four weeks of aerobic exercise elevated BAT mass and increased the expression of genes related to glycolipid metabolism and anti-inflammatory processes. Several pathways are involved, with COX2 in the VEGF signaling pathway playing a key role.

Brown Adipose Tissue and Its Role in Insulin and Glucose Homeostasis

The increased worldwide prevalence of obesity, insulin resistance, and their related metabolic complications have prompted the scientific world to search for new possibilities to combat obesity. Brown adipose tissue (BAT), due to its unique protein uncoupling protein 1 (UPC1) in the inner membrane of the mitochondria, has been acknowledged as a promising approach to increase energy expenditure. Activated brown adipocytes dissipate energy, resulting in heat production. In other words, BAT burns fat and increases the metabolic rate, promoting a negative energy balance. Moreover, BAT alleviates metabolic complications like dyslipidemia, impaired insulin secretion, and insulin resistance in type 2 diabetes. The aim of this review is to explore the role of BAT in total energy expenditure, as well as lipid and glucose homeostasis, and to discuss new possible activators of brown adipose tissue in humans to treat obesity and metabolic disorders.

Neuropeptide Y Plays an Important Role in the Relationship Between Brain Glucose Metabolism and Brown Adipose Tissue Activity in Healthy Adults: A PET/CT Study

INTRODUCTION

Brown adipose tissue (BAT) becomes the favorite target for preventing and treating metabolic diseases because the activated BAT can produce heat and consume energy. The brain, especially the hypothalamus, which secretes Neuropeptide Y (NPY), is speculated to regulate BAT activity. However, whether NPY is involved in BAT activity's central regulation in humans remains unclear. Thus, it's essential to explore the relationship between brain glucose metabolism and human BAT activity.

METHODS

A controlled study with a large sample of healthy adults used Positron emission tomography/computed tomography (PET/CT) to noninvasively investigate BAT's activity and brain glucose metabolism in vivo. Eighty healthy adults with activated BAT according to the PET/CT scan volunteered to be the BAT positive group, while 80 healthy adults without activated BAT but with the same gender, similar age, and BMI, scanning on the same day, were recruited as the control (BAT negative). We use Statistical parametric mapping (SPM) to analyze the brain image data, Picture Archiving & Communication System (PACS), and PET/CT Viewer software to calculate the semi-quantitative values of brain glucose metabolism and BAT activity. ELISA tested the levels of fasting plasma NPY. The multiple linear regression models were used to analyze the correlation between brain glucose metabolism, the level of NPY, and the BAT activity in the BAT positive group.

RESULTS

(1) Compared with controls, BAT positive group showed significant metabolic decreases mainly in the right Insula (BA13a, BA13b) and the right claustrum (uncorrected P <0.01, adjusted BMI). (2) The three brain regions' semi-quantitative values in the BAT positive group were significantly lower than the negative group (all P values < 0.05). (3) After adjusting for age, gender, BMI, and outside temperature, there was a negative correlation between brain metabolic values and BAT activity (all P values < 0.05). However, after further adjusting for NPY level, there were no significant differences between the BA13b metabolic values and BAT activity (P>0.05), while the correlation between the BA13a metabolic values and BAT activity still was significant (P< 0.05).

CONCLUSIONS

Regional brain glucose metabolism is closely related to healthy adults' BAT activity, which may be mediated by NPY.

Brown Adipose Tissue, Diet-Induced Thermogenesis, and Thermogenic Food Ingredients: From Mice to Men

Since the recent rediscovery of brown adipose tissue (BAT) in adult humans, this thermogenic tissue has been attracting increasing interest. The inverse relationship between BAT activity and body fatness suggests that BAT, because of its energy dissipating activity, is protective against body fat accumulation. Cold exposure activates and recruits BAT, resulting in increased energy expenditure and decreased body fatness. The stimulatory effects of cold exposure are mediated through transient receptor potential (TRP) channels and the sympathetic nervous system (SNS). Most TRP members also function as chemesthetic receptors for various food ingredients, and indeed, agonists of TRP vanilloid 1 such as capsaicin and its analog capsinoids mimic the effects of cold exposure to decrease body fatness through the activation and recruitment of BAT. The antiobesity effect of other food ingredients including tea catechins may be attributable, at least in part, to the activation of the TRP-SNS-BAT axis. BAT is also involved in the facultative thermogenesis induced by meal intake, referred to as diet-induced thermogenesis (DIT), which is a significant component of the total energy expenditure in our daily lives. Emerging evidence suggests a crucial role for the SNS in BAT-associated DIT, particularly during the early phase, but several gut-derived humoral factors may also participate in meal-induced BAT activation. One intriguing factor is bile acids, which activate BAT directly through Takeda G-protein receptor 5 (TGR5) in brown adipocytes. Given the apparent beneficial effects of some TRP agonists and bile acids on whole-body substrate and energy metabolism, the TRP/TGR5-BAT axis represents a promising target for combating obesity and related metabolic disorders in humans.

Decreased insulin-stimulated brown adipose tissue glucose uptake after short-term exercise training in healthy middle-aged men

AIMS

To test the hypothesis that high-intensity interval training (HIIT) and moderate-intensity continuous training (MICT) improve brown adipose tissue (BAT) insulin sensitivity.

PARTICIPANTS AND METHODS

Healthy middle-aged men (n = 18, age 47 years [95% confidence interval {CI} 49, 43], body mass index 25.3 kg/m² [95% CI 24.1-26.3], peak oxygen uptake (VO_2peak ) 34.8 mL/kg/min [95% CI 32.1, 37.4] ) were recruited and randomized into six HIIT or MICT sessions within 2 weeks. Insulin-stimulated glucose uptake was measured using 2-[¹⁸ F]flouro-2-deoxy-D-glucose positron-emission tomography in BAT, skeletal muscle, and abdominal and femoral subcutaneous and visceral white adipose tissue (WAT) depots before and after the training interventions.

RESULTS

Training improved VO_2peak (P = .0005), insulin-stimulated glucose uptake into the quadriceps femoris muscle (P = .0009) and femoral subcutaneous WAT (P = .02) but not into BAT, with no difference between the training modes. Using pre-intervention BAT glucose uptake, we next stratified subjects into high BAT (>2.9 µmol/100 g/min; n = 6) or low BAT (<2.9 µmol/100 g/min; n = 12) groups. Interestingly, training decreased insulin-stimulated BAT glucose uptake in the high BAT group (4.0 [2.8, 5.5] vs 2.5 [1.7, 3.6]; training*BAT, P = .02), whereas there was no effect of training in the low BAT group (1.5 [1.2, 1.9] vs 1.6 [1.2, 2.0] µmol/100 g/min). Participants in the high BAT group had lower levels of inflammatory markers compared with those in the low BAT group.

CONCLUSIONS

Participants with functionally active BAT have an improved metabolic profile compared with those with low BAT activity. Short-term exercise training decreased insulin-stimulated BAT glucose uptake in participants with active BAT, suggesting that training does not work as a potent stimulus for BAT activation.

A new method of infrared thermography for quantification of brown adipose tissue activation in healthy adults (TACTICAL): a randomized trial

The ability to alter the amount and activity of brown adipose tissue (BAT) in human adults is a potential strategy to manage obesity and related metabolic disorders associated with food, drug, and environmental stimuli with BAT activating/recruiting capacity. Infrared thermography (IRT) provides a non-invasive and inexpensive alternative to the current methods (e.g. 18F-FDG PET) used to assess BAT. We have quantified BAT activation in the cervical-supraclavicular (C-SCV) region using IRT video imaging and a novel image computational algorithm by studying C-SCV heat production in healthy young men after cold stimulation and the ingestion of capsinoids in a prospective double-blind placebo-controlled randomized trial. Subjects were divided into low-BAT and high-BAT groups based on changes in IR emissions in the C-SCV region induced by cold. The high-BAT group showed significant increases in energy expenditure, fat oxidation, and heat output in the C-SCV region post-capsinoid ingestion compared to post-placebo ingestion, but the low-BAT group did not. Based on these results, we conclude that IRT is a promising tool for quantifying BAT activity.

Contributors to Metabolic Disease Risk Following Spinal Cord Injury

Spinal cord injury (SCI) induced changes in neurological function have significant impact on the metabolism and subsequent metabolic-related disease risk in injured individuals. This metabolic-related disease risk relationship is differential depending on the anatomic level and severity of the injury, with high level anatomic injuries contributing a greater risk of glucose and lipid dysregulation resulting in type 2 diabetes and cardiovascular disease risk elevation. Although alterations in body composition, particularly excess adiposity and its anatomical distribution in the visceral depot or ectopic location in non-adipose organs, is known to significantly contribute to metabolic disease risk, changes in fat mass and fat-free mass do not fully account for this elevated disease risk in subjects with SCI. There are other negative adaptations in body composition including reductions in skeletal muscle mass and alterations in muscle fiber type, in addition to significant reduction in physical activity, that contribute to a decline in metabolic rate and increased metabolic disease risk following SCI. Recent studies in adult humans suggest cold- and diet-induced thermogenesis through brown adipose tissue metabolism may be important for energy balance and substrate metabolism, and particularly sensitive to sympathetic nervous signaling. Considering the alterations that occur in the autonomic nervous system (SNS) (sympathetic and parasympathetic) following a SCI, significant dysfunction of brown adipose function is expected. This review will highlight metabolic alterations following SCI and integrate findings from brown adipose tissue studies as potential new areas of research to pursue.

The role of active brown adipose tissue (aBAT) in lipid metabolism in healthy Chinese adults

Background

The prevalence of dyslipidemia in China was increased over the last several years. Studies have shown that the activity of aBAT is related to the lipid metabolism. In this study, we analyzed blood lipid level in tumor-free healthy Chinese adults in order to determine the role of aBAT in lipid metabolism.

Methods

We retrospectively analyzed the factors that affect the blood lipid level in 717 tumor-free healthy adults who received blood lipid measurement and PET/CT scan by multivariate regression analysis. We also determined the role of aBAT on lipid profile by case–control study.

Results

(1) Our results showed that 411 (57.3 %) subjects had dyslipidemia. The prevalence of the subjects with hypercholesteremia, hypertriglyceridemia, low high-density lipoprotein cholesterol and high low-density lipoprotein cholesterol was 9.5 %, 44.4 %, 30.8 % and 1.4 %, respectively. Multivariate logistic regression analysis with dyslipidemia as the dependent variable showed that body mass index (BMI) and smoking are independent risk factors for dyslipidemia (OR > 1, P < 0.05), while the presence of aBAT is the independent protective factor for dyslipidemia (OR < 1, P < 0.05). (2) The incidence of aBAT was 1.81 %. Subjects with aBAT had significantly lower serum triglyceride and higher serum high-density lipoprotein cholesterol than the subjects without aBAT. The serum total cholesterol and low-density lipoprotein cholesterol were not significantly different between the subjects with aBAT and those without aBAT.

Conclusions

Dyslipidemia is caused by multiple factors and the presence of aBAT is a protective factor for dyslipidemia in healthy Chinese adults.

Activation and recruitment of brown adipose tissue by cold exposure and food ingredients in humans

Since the recent re-discovery of brown adipose tissue (BAT) in adult humans, this thermogenic tissue has attracted increasing interest. The inverse relationship between the BAT activity and body fatness suggests that BAT, because of its energy dissipating activity, is protective against body fat accumulation. Cold exposure activates and recruits BAT in association with increased energy expenditure and decreased body fatness. The stimulatory effects of cold are mediated through transient receptor potential channels (TRP), most of which are also chemesthetic receptors for various food ingredients. In fact, capsaicin and its analog capsinoids, representative agonists of TRPV1, mimic the effects of cold to decrease body fatness through the activation and recruitment of BAT. The anti-obesity effect of some other food ingredients including tea catechins may also be attributable to the activation of the TRP-BAT axis. Thus, BAT is a promising target for combating obesity and related metabolic disorders in humans.

Capsaicin and Related Food Ingredients Reducing Body Fat Through the Activation of TRP and Brown Fat Thermogenesis

Brown adipose tissue (BAT) is a site of sympathetically activated adaptive nonshivering thermogenesis, thereby being involved in the regulation of energy balance and body fatness. Recent radionuclide imaging studies have revealed the existence of metabolically active BAT in adult humans. Human BAT is activated by acute cold exposure and contributes to cold-induced increase in whole-body energy expenditure. The metabolic activity of BAT is lower in older and obese individuals. The inverse relationship between the BAT activity and body fatness suggests that BAT, because of its energy dissipating activity, is protective against body fat accumulation. In fact, repeated cold exposure recruits BAT in association with increased energy expenditure and decreased body fatness. The stimulatory effects of cold are mediated through the activation of transient receptor potential (TRP) channels, most of which are also chemesthetic receptors for various naturally occurring substances including herbal plants and food ingredients. Capsaicin and its analog capsinoids, representative agonists of TRPV1, mimic the effects of cold to decrease body fatness through the activation and recruitment of BAT. The well-known antiobesity effect of green tea catechins is also attributable to the activation of the sympathetic nerve and BAT system. Thus, BAT is a promising target for combating obesity and related metabolic disorders in humans.

Brown adipose tissue activation is inversely related to central obesity and metabolic parameters in adult human

Background

Recent studies have shown that adult human possess active brown adipose tissue (BAT), which might be important in affecting obesity. However, the supporting evidence on the relationship between BAT and central obesity and metabolic profile in large population based studies is sparse.

Methodology/Principal Findings

We studied 4011 (2688 males and 1323 females) tumor-free Chinese adults aged 18-89 for BAT activities, visceral/subcutaneous fat areas (VFA/SFA), waist circumferences (WC) and metabolic parameters. We found that the prevalence of BAT was around 2.7% in our study participants, with a significant sexual difference (5.5% in the females vs. 1.3% in the males; p<0.0001). BAT detection was increased in low temperature and declined in elderly subjects. The BAT positive subjects had lower BMI (P<0.0001), less SFA (P<0.01), VFA (P<0.0001), WC (P<0.0001), lower fasting glucose and triglyceride levels (both P<0.01) and increased HDL cholesterol concentrations (P<0.0001), compared with the BAT negative subjects. Robust logistic regression revealed that after adjustment for covariates (including age, sex, BMI, VFA, SFA and WC), age and BMI in the males (0.92 [95%CI, 0.88-0.96] and 0.84 [95% CI, 0.75-0.96], both P ≤0.008) while age and VFA in the females (0.87 [95%CI, 0.83-0.91] and 0.98 [95%CI, 0.97-0.99], respectively, P<0.05) were independently associated with detectable BAT.

Conclusions/Significance

Our data suggest that decreased amount of active BAT might be associated with accumulation of visceral fat content and unfavorable metabolic outcomes.

Activation and recruitment of brown adipose tissue as anti-obesity regimens in humans

Brown adipose tissue (BAT) is the site of sympathetically activated adaptive thermogenesis during cold exposure and after hyperphagia, thereby controlling whole-body energy expenditure (EE) and body fat. BAT thermogenesis is primarily dependent on the energy-dissipating activity of uncoupling protein 1 (UCP1). There are two types of UCP1-expressing adipocyte, classical brown and beige/brite adipocytes. Recent radionuclide studies have demonstrated the existence of metabolically active BAT composed of mainly beige/brite adipocytes in adult humans. Human BAT is activated by acute cold exposure, being positively correlated to cold-induced increases in EE. The inverse relationship between the BAT activity and body fatness suggests that BAT, because of its energy-dissipating activity, is protective against body fat accumulation. In fact, either repeated cold exposure or daily ingestion of some food ingredients acting on transient receptor potential channels recruited BAT in association with increased EE and decreased body fat. Moreover, possible contribution of BAT to glucose tolerance has been suggested. In addition to the sympathetic nervous system, some endocrine factors also have potential for activation/recruitment of BAT. Thus, BAT is a promising therapeutic target for combating human obesity and related metabolic disorders.

Brown adipose tissue during puberty and with aging

It was previously assumed that brown adipose tissue (BAT) is present in humans only for a short period following birth, the time in which mechanisms of generating heat by way of shivering are not yet developed. Although BAT is maximally recruited in early infancy, findings in recent years have led to a new consensus that metabolically active BAT remains present in most children and many adult humans. Evidence to date supports a slow and steady decline in BAT activity throughout life, with the exception of an intriguing spike in the prevalence and volume of BAT around the time of puberty that remains poorly understood. Because BAT activity is more commonly observed in individuals with a lower body mass index, an association seen in both adult and pediatric populations, there is the exciting possibility that BAT is protective against childhood and adult obesity. Indeed, the function and metabolic relevance of human BAT is currently an area of vigorous research. The goal of this review is to summarize what is currently known about changes that occur in BAT during various stages of life, with a particular emphasis on puberty and aging.

Brown adipose tissue activity as a target for the treatment of obesity/insulin resistance

Presence of brown adipose tissue (BAT), characterized by the expression of the thermogenic uncoupling protein 1 (UCP1), has recently been described in adult humans. UCP1 is expressed in classical brown adipocytes, as well as in “beige cells” in white adipose tissue (WAT). The thermogenic activity of BAT is mainly controlled by the sympathetic nervous system. Endocrine factors, such as fibroblast growth factor 21 (FGF21) and bone morphogenic protein factor-9 (BMP-9), predominantly produced in the liver, were shown to lead to activation of BAT thermogenesis, as well as to “browning” of WAT. This was also observed in response to irisin, a hormone secreted by skeletal muscles. Different approaches were used to delineate the impact of UCP1 on insulin sensitivity. When studied under thermoneutral conditions, UCP1 knockout mice exhibited markedly increased metabolic efficiency due to impaired thermogenesis. The impact of UCP1 deletion on insulin sensitivity in these mice was not reported. Conversely, several studies in both rodents and humans have shown that BAT activation (by cold exposure, β3-agonist treatment, transplantation and others) improves glucose tolerance and insulin sensitivity. Interestingly, similar results were obtained by adipose tissue-specific overexpression of PR-domain-containing 16 (PRDM16) or BMP4 in mice. The mediators of such beneficial effects seem to include FGF21, interleukin-6, BMP8B and prostaglandin D2 synthase. Interestingly, some of these molecules can be secreted by BAT itself, indicating the occurrence of autocrine effects. Stimulation of BAT activity and/or recruitment of UCP1-positive cells are therefore relevant targets for the treatment of obesity/type 2 diabetes in humans.

Human brown adipose tissue: regulation and anti-obesity potential

Brown adipose tissue (BAT) is the site of sympathetically activated adaptive thermognenesis during cold exposure and after hyperphagia, thereby controlling whole-body energy expenditure (EE) and body fat. Radionuclide imaging studies have demonstrated that adult humans have metabolically active BAT composed of mainly beige/brite adipocytes, recently identified brown-like adipocytes. The inverse relationship between the BAT activity and body fatness suggests that BAT is, because of its energy dissipating activity, protective against body fat accumulation in humans as it is in small rodents. In fact, either repeated cold exposure or daily ingestion of some food ingredients acting on transient receptor potential channels recruits BAT in parallel with increased EE and decreased body fat. In addition to the sympathetic nervous system, several endocrine factors are also shown to recruit BAT. Thus, BAT is a promising therapeutic target for combating human obesity and related metabolic disorders.