Metabolic adaptations to a high-fat diet in endurance cyclists.
We examined the time course of metabolic adaptations to 15 days of a high-fat diet (HFD). Sixteen endurance-trained cyclists were assigned randomly to a control (CON) group, who consumed their habitual diet (30% +/- 8% mJ fat), or a HFD group, who consumed a high-fat isocaloric diet (69% +/- 1% mJ fat). At 5-day intervals, the subjects underwent an oral glucose tolerance test (OGTT); on the next day, they performed a 2.5-hour constant-load ride at 70% peak oxygen consumption (VO2peak), followed by a simulated 40-km cycling time-trial while ingesting a 10% 14C-glucose + 3.44% medium-chain triglyceride (MCT) emulsion at a rate of 600 mL/h. In the OGTT, plasma glucose concentrations at 30 minutes increased significantly after 5 days of the HFD and remained elevated at days 10 and 15 versus the levels measured prior to the HFD (P < .05). The activity of carnitine acyltransferase (CAT) in biopsies of the vastus lateralis muscle also increased from 0.45 to 0.54 micromol/g/min over days 0 to 10 of the HFD (P < .01) without any change in citrate synthase (CS) or 3-hydroxyacyl-coenzyme A dehydrogenase (3-HAD) activities. Changes in glucose tolerance and CAT activity were associated with a shift from carbohydrate (CHO) to fat oxidation during exercise (P < .001), which occurred within 5 to 10 days of the HFD. During the constant-load ride, the calculated oxidation of muscle glycogen was reduced from 1.5 to 1.0 g/min (P < .001) after 15 days of the HFD. Ingestion of a HFD for as little as 5 to 10 days significantly altered substrate utilization during submaximal exercise but did not attenuate the 40-km time-trial performance.
Goedecke
J H
JH
Medical Research Council/University of Capetown Bioenergetics of Exercise Research Unit, University of Cape Town Medical School, Newlands, South Africa.
Christie
C
C
Wilson
G
G
Dennis
S C
SC
Noakes
T D
TD
Hopkins
W G
WG
Lambert
E V
EV
D000222:Adaptation, Physiological; D000328:Adult; D001642:Bicycling / Q000502:physiology*; D001769:Blood / Q000378:metabolism; D004040:Dietary Carbohydrates / Q000378:metabolism; D004041:Dietary Fats / Q000008:administration & dosage* / Q000378:metabolism / Q000494:pharmacology; D005951:Glucose Tolerance Test; D006801:Humans; D008297:Male; D018482:Muscle, Skeletal / Q000201:enzymology; D009994:Osmolar Concentration; D010807:Physical Endurance / Q000502:physiology*
D004040:Dietary Carbohydrates; D004041:Dietary Fats