Endocrine Abstracts

In 2009 three independent research groups confirmed the existence of metabolically active brown adipose tissue in human adults. There is increasing evidence that enhanced BAT function can improve systemic health in humans by utilizing glucose and lipids from the circulation and by increasing metabolic rate. Therefore BAT activation could provide a strategy to combat the increasing epidemic of obesity and type 2 diabetes. The function of BAT is controlled by various endogenous factors, and hormones including the thyroid hormones. In states of disease or abnormal metabolism, such as hyperthyroidism or obesity, the regulatory systems may become impaired. Positron emission tomography (PET) has provided a non-invasive way to investigate BAT function in humans in vivo. BAT glucose uptake increases in lean men up to 10-fold during cold exposure. The potential of this imaging technique has only partly utilized and focused on glucose metabolism only. Although there is growing amount of reports on BAT activation in humans, evidence regarding the browning of WAT is limited. We and others have shown that BAT activity in morbidly obesity increases after bariatric surgery. Chronic cold exposure for 6 weeks at 17°C during 2-hour per day resulted increase in BAT activity and cold-induced thermogenesis in on-obese individuals with low BAT activity in a study of Yoneshiro et al. The stimulation of BAT during the cold is mediated via NE-dependent pathway. Mirabegron, a beta3-adrenergic receptor agonist used for the treatment of overactive bladder was promising in humans in a short but not in a longer intervention. Irisin is one of the promising ones release e.g. by exercise. The fibroblast growth factor-21 (FGF-21) also interacts with the FGF receptors on the brown adipocytes and they have been shown to stimulate glucose oxidation and thermogenic mechanism in the BAT. Studies about effects of exercise to induce browning have given controversial results. This might be partly due altered substrate metabolism towards utilization of more fatty acids, which is more difficult to measure directly in human BAT. Even the changes in outdoor temperature seems to be blunt the exercise induced effects. The physiological role of BAT in adult humans is still largely open. Totally new areas are coming up constantly. Recent studies have suggested a link between BAT volume and total and spine bone mineral density mainly in women. Numerous efforts are ongoing to find drugs for browning. The heat is still on.