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1 Department of Human Biology, University of Cape Town, Cape Town, South Africa
2 School of Biomedical Sciences, University of Nottingham Medical School, Nottingham, United Kingdom
* To whom correspondence should be addressed. E-mail: juliag{at}sports.uct.ac.za.
The aim of this study was to investigate the effect of a high-fat diet (HFD) followed by 1-d of carbohydrate (CHO) loading on substrate utilization, heart rate variability (HRV), effort perception (RPE), muscle recruitment (EMG) and performance during a 100-km cycling time-trial. In this randomized single-blind crossover study, 8 well-trained cyclists completed two trials, ingesting either a high-CHO diet (HCD) (68% CHO energy) or an iso-energetic high-fat diet (HFD) (68% fat energy) for 6 days, followed by 1-d of CHO-loading (8-10g CHO/kg). Subjects completed a 100-km time-trial on day 1 and a 1-hr cycle at 70% of VO2peak on days 3, 5 and 7, during which resting HRV and resting and exercising respiratory exchange ratio (RER) were measured. On day 8, subjects completed a 100-km performance time-trial, during which blood samples were drawn and EMG was recorded. Ingestion of the HFD reduced RER at rest (p<0.005) and during exercise (p<0.01) and increased plasma FFA levels (p<0.01), indicating increased fat utilization. There was a tendency for the low frequency power component of HRV to be greater for HFD-CHO (p=0.056), suggestive of increased sympathetic activation. Overall 100-km time-trial performance was not different between diets, however 1-km sprint power output following HFD-CHO was lower (p<0.05) compared to HCD-CHO. Despite a reduced power output with HFD-CHO, RPE, heart rate and EMG were not different between trials. In conclusion, the HFD-CHO dietary strategy increased fat oxidation, but compromised high intensity sprint performance, possibly by increased sympathetic activation or altered contractile function.
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