EXERCISE PERFORMANCE RESPONSES TO A KETOGENIC DIET

Abstract

High-fat diets have been hypothesized to increase endurance performance by increasing fat oxidation and sparing muscle glycogen. The effects of high-fat diets on endurance performance are mixed and there is virtually no information on higher-intensity exercise performance. The primary purpose of this study was to examine the effects of a six-week ketogenic diet on maximal oxygen consumption, muscular power, and endurance capacity. Twelve healthy normally-active men switched from their habitual diet (48% carbohydrate) to a ketogenic diet (8% carbohydrate) for six weeks and eight men served as contrls consuming their normal diet. Peak oxygen consumption was assessed on a cycle ergometer. On a separate day, peak and mean power were assessed during two 30-second Wingate sprints with two minutes rest on a cycle ergometer. After a 30-minute rest, subjects performed a 45-minute timed ride on a cycle ergometer. Subjects in the ketogenic group lost 2.2 kg while control subjects gained 0.1 kg. No change was noted in relative peak VO2 (ml/kg/min) in either group. However, a significant decrease was observed in absolute peak VO2 (L/min) from week 0 to week 6 in the ketogenic group (−6%). Peak and mean power output during bout 1 decreased significantly from week 0 to week 6 in the ketogenic group (−9 and −7/%, respectively) while the control group remained unchanged. When analyzed relative to body weight (W/kg) only peak power output was significantly decreased. No changes was noted in peak and mean power in either group during bout 2. The respiratory exchange ratio was lower during the 45 min timed-ride in the ketogenic group indicating greater utilization of fat; however, total work output was significantly lower (−18%). These data indicate that a ketogenic diet results in moderate decreases in absolute muscular power and endurance that may be partially due to decreased body weight. This study was supported by a grant from the Atkins Foundation, New York, NY. %