Rowlands, D. S., & Hopkins, W. G. (1999). Effects of two-week changes in diet on metabolism and performance in cycling. Medicine and Science in Sports and Exercise, 31(5), Supplement abstract 295.

Trained cyclists (N = 7) performed cycle ergometer tests before and after three diets:

• 13 days of high carbohydrate (69 + 11% CHO; 16 + 5% fat);
• 13 days of high fat (15 + 4% CHO; 66 + 10% fat); and
• 11 days of high fat, followed by 2 days of high CHO.

Each set of tests was preceded by two weeks of standardized, normal diet. Testing consisted of a 15-minute warm-up; 15-minute time-trial; 45 minutes of riding at 50% of peak power; six 10-minute incremental loads; and a 100-km time trial.

Up to intensities of 70% VO2max, average energy from fat was a relatively constant for each diet: 25% after CHO, 62% after fat, 46% after the mixed, and 36-45% after the normal. At higher intensities, energy from fat declined. In the 100-km time trial, fat oxidation in the CHO diet condition changed from 28% at 10 km to 49% at 90 km. There was very little change for the other diets. Diets had no significant effect on performance in either the 15-minute or 100-km time trials. However, individual differences in the effects of the diets were noted.

Implication. High-CHO, high-fat, mixed, and normal diets could have differential effects on performance depending upon the individual. Before electing a dietary regimen, it would be wise to carefully evaluate the effects of each upon the individual's performance.

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