Periodic fasting and refeeding re-shapes lipid saturation, storage, and distribution in brown adipose tissue.

Brown adipose tissue (BAT) functions as a metabolic sink, efficiently processing fatty acids (FAs), glucose, and amino acids, playing a pivotal role in metabolic regulation and energy homeostasis. However, the metabolic adaptations enabling BAT to respond to fasting and refeeding cycles are not well understood. Using mass spectrometry techniques-Liquid Chromatography (LC), Capillary Electrophoresis (CE), and Spatially Resolved Imaging-we demonstrate that BAT exhibits a unique free fatty acid (FFA) and lipid-bound FA profile, with enrichment of very long-chain polyunsaturated fatty acids (VLC-PUFAs) and C13-C14 FAs compared to white adipose tissue (WAT) in male C57BL/6 mice. Alternate-day fasting (ADF) triggered a dynamic change of these FFAs in BAT, accompanied by selective alterations of upper glycolysis, glyceroneogenesis, and triglyceride synthesis, a shift less pronounced in WAT. Additionally, several BAT lipid species, including glycerolipids, glycerophospholipids, and sphingolipids, transitioned from highly unsaturated to more saturated lipids upon refeeding, alongside significant spatial and dynamic reprogramming. Mechanistically, periodic fasting and refeeding activated mTORC1, and genetic inactivation of mTORC1 in BAT diminished ADF-induced lipid saturation, storage, and redistribution in the C57BL/6 background. These findings reveal that while BAT generally prefers unsaturated fats, it undergoes substantial lipid saturation and spatially dynamic reprogramming in response to fasting and refeeding, offering new insights into BAT's adaptive role in metabolic homeostasis.