Burning fast & slow: a review of the differential metabolic routes of brown adipose tissue activity during cold exposure and their implications for populational variation

Brown adipose tissue (BAT) has received renewed attention for its ability to increase metabolic rate and heat production, via non-shivering thermogenesis, in response to mild cold exposure. When activated, BAT also appears to improve circulating glucose and fatty acid levels, highlighting the potential for this mitochondria-dense tissue to be used as an antiobesogenic tool. A great deal of populational and individual variation is seen in BAT metabolic responses as well as substrate metabolism even between populations from similar climates, though we do not yet understand the underlying reasons for this variation. Here, using an anthropological and biological lens, we review what is currently known about BAT physiology and its variation across human populations. We then review evidence that suggests BAT may use aerobic and anaerobic metabolism in a way that mirrors skeletal muscle. We then argue that the many and often redundant pathways leading to BAT thermogenesis may be what explains the observed population variation. We end the review by briefly suggesting a potential way to test whether aerobic and anaerobic metabolism are occurring within human BAT in hopes of more fully understanding and explaining BAT physiological variation.