The thermoneutral zone in women takes an "arctic" shift compared to men.

Conventionally, women are perceived to feel colder than men, but controlled comparisons are sparse. We measured the response of healthy, lean, young women and men to a range of ambient temperatures typical of the daily environment (17 to 31 °C). The Scholander model of thermoregulation defines the lower critical temperature as threshold of the thermoneutral zone, below which additional heat production is required to defend core body temperature. This parameter can be used to characterize the thermoregulatory phenotypes of endotherms on a spectrum from "arctic" to "tropical." We found that women had a cooler lower critical temperature (mean ± SD: 21.9 ± 1.3 °C vs. 22.9 ± 1.2 °C, P = 0.047), resembling an "arctic" shift compared to men. The more arctic profile of women was predominantly driven by higher insulation associated with more body fat compared to men, countering the lower basal metabolic rate associated with their smaller body size, which typically favors a "tropical" shift. We did not detect sex-based differences in secondary measures of thermoregulation including brown adipose tissue glucose uptake, muscle electrical activity, skin temperatures, cold-induced thermogenesis, or self-reported thermal comfort. In conclusion, the principal contributors to individual differences in human thermoregulation are physical attributes, including body size and composition, which may be partly mediated by sex.

Opportunities and challenges in the therapeutic activation of human energy expenditure and thermogenesis to manage obesity.

The current obesity pandemic results from a physiological imbalance in which energy intake chronically exceeds energy expenditure (EE), and prevention and treatment strategies remain generally ineffective. Approaches designed to increase EE have been informed by decades of experiments in rodent models designed to stimulate adaptive thermogenesis, a long-term increase in metabolism, primarily induced by chronic cold exposure. At the cellular level, thermogenesis is achieved through increased rates of futile cycling, which are observed in several systems, most notably the regulated uncoupling of oxidative phosphorylation from ATP generation by uncoupling protein 1, a tissue-specific protein present in mitochondria of brown adipose tissue (BAT). Physiological activation of BAT and other organ thermogenesis occurs through ß-adrenergic receptors (AR), and considerable effort over the past 5 decades has been directed toward developing AR agonists capable of safely achieving a net negative energy balance while avoiding unwanted cardiovascular side effects. Recent discoveries of other BAT futile cycles based on creatine and succinate have provided additional targets. Complicating the current and developing pharmacological-, cold-, and exercise-based methods to increase EE is the emerging evidence for strong physiological drives toward restoring lost weight over the long term. Future studies will need to address technical challenges such as how to accurately measure individual tissue thermogenesis in humans; how to safely activate BAT and other organ thermogenesis; and how to sustain a negative energy balance over many years of treatment.

Activating Human Adipose Tissue with the ß3-Adrenergic Agonist Mirabegron.

An appealing strategy for treatment of metabolic disease in humans is activation of brown adipose tissue (BAT), a thermogenic organ best visualized through 18F-FDG PET/CT. BAT has been activated to varying degrees by mild cold exposure. However, this approach can cause undesirable stress, and there remains no consensus protocol. Here, we describe standardized methods for both acute and chronic activation of BAT using the orally administered ß3-adrenergic receptor (AR) agonist, mirabegron. Acute pharmacological stimulation has enabled quantification of whole-body BAT volume and metabolic activity using PET/CT imaging, and chronic stimulation increases these properties of BAT over time.

Combining a ß3 adrenergic receptor agonist with alpha-lipoic acid reduces inflammation in male mice with diet-induced obesity.

OBJECTIVES: Beta-3 adrenergic receptors (ß3-AR) stimulate lipolysis and thermogenesis in white and brown adipose tissue (WAT and BAT). Obesity increases oxidative stress and inflammation that attenuate AT ß3-AR signaling. The objective of this study was to test the hypothesis that the combination of the ß3-AR agonist CL-316,243 (CL) and the antioxidant alpha-lipoic acid (ALA) would lower inflammation in diet-induced obesity (DIO) and improve ß3-AR function. METHODS: A total of 40 DIO mice were separated into four groups: Control (per os and intraperitoneal [IP] vehicle); CL alone (0.01 mg/kg IP daily); ALA alone (250 mg/kg in drinking water); or ALA+CL combination, all for 5 weeks. RESULTS: Food intake was similar in all groups; however, mice receiving ALA+CL showed improved body composition and inflammation as well as lower body weight (+1.7 g Control vs. -2.5 g ALA+CL [-7%]; p < 0.01) and percentage of body fat (-9%, p < 0.001). Systemic and epididymal WAT inflammation was lower with ALA+CL than all other groups, with enhanced recruitment of epididymal WAT anti-inflammatory CD206+ M2 macrophages. ß3-AR signaling in WAT was enhanced in the combination-treatment group, with higher mRNA and protein levels of thermogenic uncoupling protein 1 and AT lipases. CONCLUSIONS: Chronic treatment with ALA and a ß3-AR agonist reduces DIO-induced inflammation. AT immune modulation could be a therapeutic target in patients with obesity.

ß3-Adrenergic receptors regulate human brown/beige adipocyte lipolysis and thermogenesis.

ß3-Adrenergic receptors (ß3-ARs) are the predominant regulators of rodent brown adipose tissue (BAT) thermogenesis. However, in humans, the physiological relevance of BAT and ß3-AR remains controversial. Herein, using primary human adipocytes from supraclavicular neck fat and immortalized brown/beige adipocytes from deep neck fat from 2 subjects, we demonstrate that the ß3-AR plays a critical role in regulating lipolysis, glycolysis, and thermogenesis. Silencing of the ß3-AR compromised genes essential for thermogenesis, fatty acid metabolism, and mitochondrial mass. Functionally, reduction of ß3-AR lowered agonist-mediated increases in intracellular cAMP, lipolysis, and lipolysis-activated, uncoupling protein 1-mediated thermogenic capacity. Furthermore, mirabegron, a selective human ß3-AR agonist, stimulated BAT lipolysis and thermogenesis, and both processes were lost after silencing ß3-AR expression. This study highlights that ß3-ARs in human brown/beige adipocytes are required to maintain multiple components of the lipolytic and thermogenic cellular machinery and that ß3-AR agonists could be used to achieve metabolic benefit in humans.