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The Increase in Intramyocellular Lipid Content Is a Very Early Response to Training

Department of Radiology (V.B.S.-H., J.M.A.v.E., M.E.K.), University Hospital Maastricht, 6229 HX Maastricht, The Netherlands; Nutrition and Toxicology Research Institute Maastricht (V.B.S.-H., P.S., M.K.C.H., W.H.M.S.), and Departments of Human Biology (P.S., W.H.M.S.) and Movement Sciences (M.K.C.H.), Maastricht University, 6200 MD Maastricht, The Netherlands; Department of Biomedical Engineering (V.B.S.-H., K.N.), Eindhoven University of Technology, 5612 AZ Eindhoven, The Netherlands; and Department of Clinical Epidemiology and Medical Technology Assessment (A.G.H.K.), University Hospital Maastricht, 6229 HX Maastricht, The Netherlands

Address all correspondence and requests for reprints to: Vera B. Schrauwen-Hinderling, Department of Radiology, University Hospital Maastricht, P.O. Box 5800, 6202 AZ Maastricht, The Netherlands. E-mail: vhi{at}rdia.azm.nl.

The present study investigated the influences of a 2-wk training program on intramyocellular lipid (IMCL) content, IMCL decrease during exercise, fat oxidation, and insulin sensitivity.

Nine untrained men (age, 23.3 ± 3.2 yr; body mass index, 22.6 ± 2.6 kg/m²; maximal power output, 3.8 ± 0.6 W/kg body weight) trained for 2 wk. Before and after training, subjects cycled for 3 h while substrate oxidation was measured. IMCL content in the vastus lateralis muscle was determined before and after cycling by proton magnetic resonance spectroscopy. Before and after training, insulin sensitivity was assessed by an insulin tolerance test.

The training period resulted in a significant increase in IMCL content by 42 ± 14%. IMCL content decreased significantly during cycling. However, 2 wk of training were not sufficient to achieve increases in fat oxidation and/or use of IMCL during exercise. All markers used to test insulin sensitivity point toward improved insulin sensitivity, albeit not significant. We conclude that the increase in IMCL content is a very early response to training, preceding significant changes in insulin sensitivity. The results suggest that the presence of triglycerides alone does not necessarily have detrimental effects on insulin sensitivity. We confirm earlier reports that IMCL contributes to the energy used during prolonged submaximal exercise.

P.S. was supported by a grant from the Netherlands Organization for Scientific Research.

Abbreviations: EMCL, Extramyocellular lipids; FFA, free fatty acids; ¹H-MRS, proton magnetic resonance spectroscopy; IMCL, intramyocellular lipid; ITT, insulin tolerance test; TMA, trimethylammonium compounds; Wmax, maximal power output.

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