From where does the fuel for heat production come?
Brown adipose tissue has lipid stored in the form of triglyceride droplets (this being the reason that brown adipose tissue has traditionally been classified as being an adipose tissue, rather than being a close relative of muscle, as we think of it today; refs. 11, 12). Consistent with this, data in the paper by Ouellet et al. (2) indicate that during the 3 hours of cold exposure, the main energy came from combustion of lipids stored in the brown adipocytes themselves.
Had the experiment been extended, these stores would have been rapidly depleted, and continued heat production would have been dependent on circulating sources of energy. This could be in the form of circulating free fatty acids from lipid breakdown in white fat. Ouellet et al. demonstrated such an uptake of fatty acids into brown adipose tissue, but only at a low level. In brown adipose tissue, prolonged heat production is maintained to an even higher degree by uptake by the brown fat cells of circulating triglycerides mediated by the action of lipoprotein lipase, the gene expression of which is much enhanced in the cold (13). Thus, a major fraction of the substrate for combustion in prolonged cold even in humans would probably come from chylomicrons from the gut and from lipoproteins from the liver (14). Additional energy would come from circulating glucose, as can be appreciated from the prominent glucose uptake that led to the original identification of active adult human brown adipose tissue.