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1 Department of Human Biology, Maastricht University, Nutrition Research Institute Maastricht, Maastricht, The Netherlands
2 Department of Human Biology, Maastricht University, Nutrition Research Institute Maastricht, Maastricht, The Netherlands; Department of Radiology, University Hospital Maastricht, Maastricht, The Netherlands
3 Department of Movement Sciences, Maastricht University, Nutrition Research Institute Maastricht, Maastricht, The Netherlands
4 Department of Radiology, University Hospital Maastricht, Maastricht, The Netherlands
* To whom correspondence should be addressed. E-mail: L.vanLoon{at}HB.unimaas.nl.
Intramuscular triglycerides (IMTG) are assumed to form an important substrate source during prolonged endurance exercise in trained males. This study investigated the effects of endurance exercise and recovery diet on IMTG content in vastus lateralis muscle. Nine male cyclists were provided with a standardized diet for 3d after which they performed a 3h exercise trial at a 55% Wmax workload. Before, immediately after exercise and after 24 and 48h of recovery, magnetic resonance spectroscopy (MRS) was performed to quantitate IMTG content. Muscle biopsies were taken after 48h of recovery to determine IMTG content by using quantitative fluorescence microscopy. The entire procedure was performed twice; in one trial a normal diet containing 39 En% fat was provided (NF), in the other a typical carbohydrate-rich athletes' diet (LF: 24 En% fat). During exercise, IMTG content decreased by 21.4±3.1%. During recovery, IMTG content significantly increased in the NF trial only, reaching pre-exercise levels within 48h. In accordance with MRS, fluorescence microscopy showed significantly higher IMTG content in the NF compared to the LF trial, with differences restricted to the type I muscle fibres (2.1±0.2 versus 1.4±0.2 %area lipid staining, respectively). In conclusion, IMTG content in the vastus lateralis muscle declines significantly during prolonged endurance exercise in male cyclists. When using a normal diet, IMTG contents are subsequently repleted within 48h of post-exercise recovery. In contrast, IMTG repletion is substantially impaired when using a typical, carbohydrate-rich athletes' diet. Data obtained by quantitative fluorescence microscopy correspond well with MRS results, implying that both are valid methods to quantify IMTG content.
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