A common belief is that high intensity exercise (>60%VO_2max) is best sustained by high rates of carbohydrate oxidation. The belief is based, in part, on an idea developed by Krogh and Lindhard in 1920. In the 100 years since, few studies have tested its validity. We tested the null hypothesis that performance in competitive recreational athletes exercising at >80% VO_2max, during simulated 5-km running time trials (5KTT) would be impaired during a 6-week period of adaption to a low-carbohydrate, high-fat (LCHF) diet, compared to their performances when they ate a diet higher in carbohydrate and lower in fat (HCLF). Seven male athletes (age 35.6 ± 8.4 years, height 178.7 ± 4.1 cm, weight 68.6 ± 1.6 kg) completed two maximal exercise (VO_2max) tests (Day 1 and 39) and four 5KTT (Day 4, 14, 28, and 42) in a fasted state during two 6-week periods when they ate either a HCLF or a LCHF diet, in a randomized counterbalanced, crossover design. Exercise performance during the VO_2max tests was unchanged on either diet (p = 0.251). Performance in the initial 5KTT was significantly slower on the LCHF diet (p = 0.011). There were no diet-related performance differences in the remaining three 5KTT (p > 0.22). Subjects exercised at ~82%VO_2max. Carbohydrate oxidation provided 94% of energy on the HCLF diet, but only 65% on the LCHF diet. 5KTT performance at ~82%VO_2max was independent of the runners’ habitual diet. The HCLF diet offered no advantage over a diet with a high-fat content. Since these athletes run faster than 88% of recreational distance runners in the United States (U.S.), this finding may have wide general application.

• We found that athletes running 5km time trials at >80%VO_2max performed equally well when eating diets high in either carbohydrate or fat for 14 days or more.

• But performance during the initial maximal exercise test performed within 4 days of adopting the low carbohydrate diet was impaired.

• Athletes involved in higher intensity exercise (>60% VO_2max) are usually advised to eat high carbohydrate diets (7-12g carbohydrate/kg body weight/day). Our study shows that the vast majority of recreational athletes (best marathon times slower than 3hrs 20min) may not need to eat high carbohydrate diets to optimize performance in both racing and training.