Fasting and Refeeding Cycles Dramatically Reshape Fat Metabolism in Brown Adipose Tissue

Brown adipose tissue (BAT) is a thermogenic organ that burns fatty acids, glucose, and amino acids to generate heat, making it a powerful regulator of metabolic health. Despite its importance, the molecular mechanisms by which BAT adapts its lipid metabolism during cycles of fasting and refeeding have remained poorly characterized—a gap this study directly addresses.

Using liquid chromatography (LC-MS), capillary electrophoresis (CE-MS), and spatially resolved mass spectrometry imaging in male C57BL/6 mice, the researchers constructed a detailed atlas of BAT's free fatty acid (FFA) and lipid profiles. A striking baseline finding was that BAT is uniquely enriched in very long-chain polyunsaturated fatty acids (VLC-PUFAs) and medium-chain C13–C14 fatty acids compared to white adipose tissue (WAT), suggesting a distinct preference for unsaturated fats under normal fed conditions.

When mice were subjected to alternate-day fasting (ADF), BAT underwent dynamic and selective metabolic reprogramming. Free fatty acid profiles shifted substantially, with accompanying changes in upper glycolysis metabolites, glyceroneogenesis intermediates, and triglyceride synthesis pathways. Crucially, upon refeeding, multiple lipid classes in BAT—including glycerolipids, glycerophospholipids, and sphingolipids—transitioned from highly unsaturated toward more saturated species. This saturation shift was considerably less pronounced in WAT, highlighting a BAT-specific adaptive response. Spatially resolved imaging further revealed that lipid species redistribute within BAT tissue architecture during fasting-refeeding cycles, indicating dynamic spatial as well as compositional reprogramming.

Mechanistically, ADF cycles activated mTORC1 signaling in BAT. Genetic inactivation of mTORC1 specifically in BAT cells attenuated the ADF-induced increases in lipid saturation, lipid storage, and spatial redistribution, firmly establishing mTORC1 as a central orchestrator of these adaptive responses. This places BAT's lipid reprogramming downstream of a nutrient-sensing pathway known to regulate cell growth, anabolism, and autophagy.

These findings offer a new mechanistic framework for understanding how BAT maintains metabolic flexibility. While BAT's baseline preference for unsaturated lipids likely supports its thermogenic function, the fasting-refeeding-induced shift toward saturation and increased lipid storage may represent an adaptive strategy to build fuel reserves during refeeding anticipating future fasting bouts. The mTORC1 axis could represent a therapeutic target to modulate BAT activity in metabolic disease.