Novel assessment of postprandial metabolism reveals sex-specific metabolic flexibility and lipid remodeling following volumetric muscle loss

Localized skeletal muscle injury [e.g., volumetric muscle loss (VML)] can disrupt diurnal metabolic flexibility. However, it remains unclear how the evaluation of whole body metabolism and physical activity following a prandial stimuli may reveal differences in metabolic flexibility between injured and uninjured states. This study aimed to develop a novel tool to examine whole body metabolic impairments following VML injury, with consideration of lipid-related mechanisms. Adult C57BL/6J mice (<i>n</i> = 50; equal males and females) were randomized to a methodology development cohort to undergo VML injury, intramuscular glycerol injection, or remain as controls. The developed tool using an intraperitoneal glucose injection and indirect calorimetry was used to characterize the dynamic nature of whole body metabolism. Whole body metabolism, in vivo muscle function, and markers of lipid and glycemic regulation were evaluated 6 wk following injury. Females, regardless of injury, exhibited greater daily energy expenditure alongside increases in activity. Females exhibit lower whole body lipid oxidation during the inactive period, despite higher in active period, suggesting more coordinated substrate utilization. Biological sex differences in postprandial substrate utilization reveal that males fail to suppress whole body lipid oxidation, exhibiting marked impairments in postprandial metabolic flexibility. VML injury increases protein expression of perilipin 2 and SIRT1 in the muscle remaining, while inducing sex-specific changes, with adipose triglyceride lipase (ATGL) expression markedly increased and with perilipin 5 expression markedly reduced in females. The remaining muscle following VML accumulates neutral lipids and perilipin 1-positive adipocytes. This work highlights sex-specific mechanisms of metabolic disruption following traumatic skeletal muscle injuries, such as VML.<b>NEW & NOTEWORTHY</b> We developed a novel, freely ambulatory preclinical tool to assess dynamic postprandial metabolic flexibility. Using this tool, we identified sex differences in whole body energy expenditure and substrate utilization. The use of a single intramuscular glycerol injection model is not sufficient as a comparative model for evaluating chronic ectopic lipid accumulation and metabolic disruptions. In the context of volumetric muscle loss injury, disruptions in metabolic flexibility occur alongside alterations in lipid handling and ectopic lipid accumulation.