Intramuscular triacylglycerol utilization in human skeletal muscle during exercise: is there a controversy?

Intramuscular triacylglyerols (IMTGs) represent a potentially important energy source for contracting human skeletal muscle. Although the majority of evidence from isotope tracer and H-1-magnetic resonance spectroscopy (MRS) studies demonstrate IMTG utilization during exercise, controversy regarding the importance of IMTG as a metabolic substrate persists. The controversy stems from studies that measure IMTG in skeletal muscle biopsy samples and report no significant net IMTG degradation during prolonged moderate-intensity (55-70% maximal O-2 consumption) exercise lasting 90-120 min. Although postexercise decrements in IMTG levels are often reported from direct muscle measurements, the marked between-biopsy variability (similar to23%) that has been reported with this technique in untrained subjects is larger than the expected decrease in IMTG content, effectively precluding significant findings. In contrast, recent data obtained in endurance-trained subjects demonstrated reduced variability between duplicate biopsies (similar to12%), and significant changes in IMTG were detected after 120 min of moderate-intensity exercise. Therefore, it is our contention that the muscle biopsy, isotope tracer, and H-1-MRS techniques report significant and energetically important oxidation of free fatty acids derived from IMTGs during prolonged moderate exercise.