No net utilization of intramuscular lipid droplets during repeated high-intensity intermittent exercise

Intramuscular lipids are stored as subsarcolemmal or intramyofibrillar droplets with potential diverse roles in energy metabolism. We examined intramuscular lipid utilization through transmission electron microscopy during repeated high-intensity intermittent exercise, an aspect that is hitherto unexplored. Seventeen moderately to well-trained males underwent three periods (EX1-EX3) of 10 x 45-s high-intensity cycling [similar to 100%-120% Watt(max) (W-max)] combined with maximal repeated sprints (similar to 250%-300% W-max). M. vastus lateralis biopsies were obtained at baseline, after EX1, and EX3. During the complete exercise session, no net decline in either subsarcolemmal or intermyofibrillar lipid volume density occurred. However, a temporal relationship emerged for subsarcolemmal lipids with an similar to 11% increase in droplet size after EX1 (P = 0.024), which reverted to baseline levels after EX3 accompanied by an similar to 30% reduction in the numerical density of subsarcolemmal lipid droplets compared with both baseline (P = 0.019) and after EX1 (P = 0.018). Baseline distinctions were demonstrated with an approximately twofold higher intermyofibrillar lipid volume in type 1 versus type 2 fibers (P = 0.008), mediated solely by a higher number rather than the size of lipid droplets (P < 0.001). No fiber-type-specific differences were observed in subsarcolemmal lipid volume although type 2 fibers exhibited similar to 17% larger droplets (P = 0.034) but a lower numerical density (main effect; P = 0.010) including 3% less droplets at baseline. Collectively, these findings suggest that intramuscular lipids do not serve as an important substrate during high-intensity intermittent exercise; however, the repeated exercise pattern mediated a temporal remodeling of the subsarcolemmal lipid pool. Furthermore, fiber-type- and compartment-specific differences were found at baseline underscoring the heterogeneity in lipid droplet deposition.