Kinetics of intramuscular triglyceride fatty acids in exercising humans

A pulse ([¹⁴C]palmitate)-chase ([³H]palmitate) approach was used to study intramuscular triglyceride (imTG) fatty acid and plasma free fatty acid (FFA) kinetics during exercise at ~45% peak O₂ consumption in 12 adults. Vastus lateralis muscle was biopsied before and after 90 min of bicycle exercise; ³H₂O production, breath ¹⁴CO₂ excretion and lipid oxidation (indirect calorimetry) rates were measured during exercise. Results: during exercise, 8.2 ± 1.2 and 8.4 ± 0.7 μmol·kg^-1·min^-1 of imTG fatty acids and plasma FFA, respectively, were oxidized according to isotopic measurements. The sum of these two values was not different (P =0.6) from lipid oxidation by indirect calorimetry (15.4 ± 1.6 μmol·kg^-1·min^-1); the isotopic and indirect calorimetry values were correlated (r = 0.79, P < 0.005). During exercise, imTG turnover rate was 0.32 ± 0.07%/min (6.0 ± 2.0 μmol of imTG·kg wet muscle^-1·min^-1) and plasma FFA were incorporated into imTG at a rate of 0.7 ± 0.1 μmol·kg wet muscle^-1·min^-1. The imTG pool size did not change during exercise. This pulse-chase, dual tracer appears to be a reasonable approach to measure oxidation and synthesis kinetics of imTG.