Abstract
Brown adipose tissue (BAT) possesses a unique uncoupling protein (UCP1) which, when activated, enables the rapid generation of heat and the oxidation of lipids or glucose or both. It is present in small amounts (~15–350 mL) in adult humans. UCP1 is rapidly activated at birth and is essential in preventing hypothermia in newborns, who rapidly generate large amounts of heat through non-shivering thermogenesis. Since the “re-discovery” of BAT in adult humans about 10 years ago, there has been an exceptional amount of research interest. This has been accompanied by the establishment of beige fat, characterised as discrete areas of UCP1-containing cells dispersed within white adipocytes. Typically, the amount of UCP1 in these depots is around 10% of the amount found in classic BAT. The abundance of brown/beige fat is reduced with obesity, and the challenge is to prevent its loss with ageing or to reactivate existing depots or both. This is difficult, as the current gold standard for assessing BAT function in humans measures radio-labelled glucose uptake in the fasted state and is usually dependent on cold exposure and the same subject can be found to exhibit both positive and negative scans with repeated scanning. Rodent studies have identified multiple pathways that may modulate brown/beige fat function, but their direct relevance to humans is constrained, as these studies typically are undertaken in cool-adapted animals. BAT remains a challenging organ to study in humans and is able to swiftly adapt to changes in the thermal environment and thus enable rapid changes in heat production and glucose oxidation.
Highlights
The subject of brown adipose tissue (BAT) has become increasingly topical and controversial since its re-discovery in adult humans in 20071
Brown fat is important because, though present in relatively small amounts in the body, it has the potential to rapidly produce large amounts of heat and impact on both energy balance and glucose and lipid homeostasis[6,7]. This is exemplified in the rapid activation of brown fat around the time of birth and the critical role it plays in the prevention of hypothermia[8]
Lineage-tracing experiments in mice indicated that classic brown adipocytes, characterised as possessing the unique mitochondrial uncoupling protein 1 (UCP1), have a common lineage with skeletal muscle and are very different from the cellular origins of beige and white adipocytes[8,11]
Summary
The subject of brown adipose tissue (BAT) has become increasingly topical and controversial since its re-discovery in adult humans in 20071. The full extent to which changes in brown fat function can contribute to inadequate glucose homeostasis remains to be fully explored It is noteworthy, that two recent studies have indicated that raised temperature is associated with increased risk of diabetes, either during pregnancy in Canada[34] or in all adults across the United States[35]. The link among inflammation, obesity-induced insulin resistance, and atherosclerosis has been clear for decades[48] It is likely, that these results in rodents have been confounded by chronic mild-cold stress and that better modelling of human physiology, especially with regard to the role of brown fat in metabolic disease, will be needed in future. The critical role of temperature has been highlighted in vitro[49], under which conditions the appearance of UCP1 appears to be dependent on reduced ambient temperature[28]
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