Adipose tissue has generally been characterized by its role in storing and dissipating energy as well as releasing endocrine factors into circulation. Although energy storage is a feature often attributed to white adipose tissue (WAT) and energy dissipation to brown adipose tissue (BAT), it has become increasingly evident that both tissues can fulfill these roles and serve important endocrine functions. Clonal and histological analyses, lineage tracing, and now more recently, single cell/nucleus RNA sequencing experiments have highlighted the heterogeneity in the cellular composition within these adipose tissue depots [1]. This may explain the functional diversity, versatility, and plasticity of these adipose tissue depots.

BAT is a highly vascularized and innervated organ that contains adipocytes with thermogenic properties (brown and beige adipocytes) [2]. These adipocytes contain multiple small lipid droplets and high amounts of mitochondria containing uncoupling protein 1 (UCP1), the protein responsible for their tremendous thermogenic capacity [2]. Until recently, it was thought to only be present in small mammals, such as rodents, and in human infants [3]. Significant advancements in human BAT research have since led to the characterization in the anatomical distribution, function, and plasticity of human BAT in response to physiological and pathophysiological conditions [4]. WAT, in contrast, is poorly vascularized and contains adipocytes with a large unilocular lipid droplet and few mitochondria. Recent evidence shows important common and divergent features between WAT depots, the dynamic nature of the different cell populations contained within them and the thermogenic potential of WAT 5, 6. Despite some of the apparent differences between BAT and WAT, there are several overlapping features that raise important questions about their interaction and respective contributions to whole-body energy homeostasis.

This short review aims to highlight our current understanding of thermogenic adipose tissue in humans. First, we discuss our current knowledge of human thermogenic adipose tissue, particularly clinical BAT investigations. We then discuss how the function of human BAT may be to provide a local heat source and metabolic sink to vital organs, serve a paracrine or endocrine role, and/or represents a biomarker of adipose tissue health. Finally, we finish by offering our perspective on the future directions in the field.