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1 Radiology, University Hospital Maastricht, Maastricht, The Netherlands; Nutrition Research Institute Maastricht (Nutrim), University Maastricht, Maastricht, The Netherlands; Department of Biomedical Engineering, Eindhoven University of Technology, Eindhoven, The Netherlands
2 Nutrition Research Institute Maastricht (Nutrim), University Maastricht, Maastricht, The Netherlands
3 Department of Biomedical Engineering, Eindhoven University of Technology, Eindhoven, The Netherlands
4 Radiology, University Hospital Maastricht, Maastricht, The Netherlands
* To whom correspondence should be addressed. E-mail: vhi{at}rdia.azm.nl.
Intramyocellular lipid (IMCL) content has been reported to decrease following prolonged submaximal exercise in active muscle and therefore seems to form an important local substrate source. Since exercise leads to a substantial increase in plasma free fatty acid (FFA) availability with a concomitant increase in FFA uptake by muscle tissue, we aimed to investigate potential differences in the net changes in IMCL content between contracting and non-contracting skeletal muscle following prolonged endurance exercise. IMCL content was quantified by magnetic resonance spectroscopy in 8 trained cyclists before and after a 3h cycling protocol (55% Wmax) in the exercising vastus lateralis and the non-exercising biceps brachii muscle. Blood samples were taken before and after exercise to determine plasma FFA, glycerol and triglyceride concentrations and substrate oxidation was measured with indirect calorimetry. Prolonged endurance exercise resulted in a 20.4±2.8% (P<0.001) decrease in IMCL content in the vastus lateralis muscle. In contrast, we observed a substantial (37.9±9.7%; P<0.01) increase in IMCL content in the less active biceps brachii muscle. Plasma FFA and glycerol concentrations were substantially increased following exercise (from 85±6 to 1450±55µM and 57±11 to 474±54µM, respectively; P<0.001), whereas plasma triglyceride concentrations were decreased (from 1498±39 to 703±7µM: P<0.001). IMCL is an important substrate source during prolonged moderate intensity exercise, and is substantially decreased in the active vastus lateralis muscle. However, prolonged endurance exercise with its concomitant increase in plasma FFA concentration results in a net increase in IMCL content in less active muscle.
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