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 (1)H-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 (approximately 23%) 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 (approximately 12%), 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 (1)H-MRS techniques report significant and energetically important oxidation of free fatty acids derived from IMTGs during prolonged moderate exercise.
Watt
Matthew J
MJ
Department of Human Biology and Nutritional Sciences, University of Guelph, Guelph, Ontario N1G 2W1, Canada. mwatt@uoguelph.ca
Heigenhauser
George J F
GJ
Spriet
Lawrence L
LL
D004798:Enzymes / Q000502:physiology; D015444:Exercise / Q000502:physiology*; D006801:Humans; D009682:Magnetic Resonance Spectroscopy; D018482:Muscle, Skeletal / Q000378:metabolism*; D014280:Triglycerides / Q000378:metabolism*
D004798:Enzymes; D014280:Triglycerides