Impaired mitochondrial flexibility hinders fat pad remodeling in obese mice after 24 h of fasting.

Obesity involves positive energy balance and mitochondrial dysfunction. Mitochondria play a vital role in reshaping tissues by oxidizing substrates; however, tissue-specific organellar plasticity during energy deprivation remains poorly characterized in obesity. We investigated the effects of 24-h fasting on mitochondrial dynamics in adipose tissue, liver, and muscle of lean and diet-induced obese (DIO) male C57BL/6 mice. Fasting reduced body weight by 13% and subcutaneous adipose tissue (SAT) by 40% in lean mice, but only 6.4% in DIO mice, which maintained SAT mass. Indirect calorimetry revealed an attenuated reduction in the respiratory exchange ratio (RER) in DIO mice during the fed-to-fasting transition. In lean mice, fasting triggered tissue-specific mitochondrial morphological adaptations, characterized by increased mitochondrial size in the SAT, liver, and muscle, alongside higher mitochondrial density in the liver. In contrast, DIO mice displayed blunted mitochondrial morphological plasticity. Interestingly, fasting increased endoplasmic reticulum (ER)-mitochondria proximity (MAMs) and modulated mitochondrial chaperone and protease expression in specific tissues of DIO mice. Furthermore, fasted DIO mice showed a downregulation of mtDNA-encoded genes and selected mitochondrial unfolded protein response (UPRmt) markers. In brown adipose tissue (BAT), mitochondrial architecture remained largely unaltered after fasting in both groups. These findings demonstrate that obesity impairs tissue-specific mitochondrial structural adaptations and blunts systemic metabolic flexibility during fasting. Conversely, acute fasting partially promotes ER-mitochondria ultrastructural proximity in obese mice in a tissue-specific manner, establishing a baseline for future functional interventions.