Changes in IMCL content and storage characteristics during an acute bout of high intensity interval exercise are influenced by sex and muscle fibre type

Introduction: Recently it was reported that intramyocellular lipids (IMCL) do not contribute to energy production during high intensity interval exercise (HIIE) in males. However, females rely to a greater extent on lipid during moderate intensity exercise compared with males, thus perhaps IMCL contribute to energy production during HIIE in females. Purpose: Determine whether sex influences IMCL use or IMCL storage characteristics during an acute bout of HIIE. Methods: Young, healthy, recreationally active males (n=12) and females (n=12) performed an acute bout of HIIE (10x1min @90%HRmax) with biopsies taken from vastus lateralis prior to and following exercise for electron microscopic determination of IMCL storage and western blot analysis of proteins related to lipid and aerobic metabolism. Total energy expenditure and relative contributions of fat and carbohydrate oxidation were determined using indirect calorimetry. Results: Though carbohydrate oxidation was greater in males (males: 2.49g/min, females: 1.69g/min, p<0.0001) during exercise, there were no sex differences in fat oxidation (males: 0.045g/min, females: 0.049g/min, p=0.99). Sex influenced changes in the peripheral IMF region of type II fibres with IMCL content decreasing by 28% in females but increasing by 26% in males (p=0.049), while the proportion of IMCL-mitochondria apposition increased by 26% in females compared to only 3% in males (p=0.03). We also report differences in the change in IMCL morphology that are consistent with altered IMCL handling and mitochondrial interaction in females. Conclusion: We identify novel, sex-specific changes in IMCL content, apposition with mitochondria and morphology during an acute bout of HIIE.