Maximal Fat Oxidation In High-level Ultra-marathon Runners Habitually Consuming Very Low-carbohydrate And High-carbohydrate Diets: 1799 Board #144 May 28, 2: 00 PM - 3: 30 PM

Previous research has indicated that maximal rates of fat oxidation (FATox) are less than 1 g/min, but this has not been tested in high-level athletes habitually following a diet restricted in carbohydrates. A growing number of elite ultra-endurance athletes have switched to a high-fat, low-carbohydrate diet to maximize FATox and enhance performance. PURPOSE: To examine maximal fat oxidation rates and the intensity at which such fat oxidation occurs (i.e., %VO2max), between high-level ultra-marathoners consuming either low-carbohydrate/high-fat (LCD) or high-carbohydrate/low-fat diets (HCD). METHODS: 20 elite level ultra-running men (age 33.5 ± 6.4 yr, weight 67.1 ± 7.7 kg, BMI 22.6 ± 3.3 kg/m2) habitually consuming a HCD (n=10; 58% CHO, 15% PRO, 28% FAT) or LCD (n=10; 11% CHO, 19% PRO, 71% FAT) for at least 6 months were matched for age and competition performance. They performed a maximal capacity graded exercise test on a high-speed treadmill to measure maximal aerobic capacity (VO2max). Stages were two minutes in duration and continual indirect calorimetry was analyzed via a TrueOne 2400 metabolic cart, fitted with a Hans Rudolph 7450 V2 facemask. Participants ran until volitional fatigue. RESULTS: There were no significant differences in VO2max between groups (HCD 64.3 ± 6.2; LCD 64.7 ± 3.7 mL/kg/min) (P = 0.850). There was a significant difference in maximal FATox (HCD 0.67 g/min; LCD 1.54 g/min) (P<0.0001) and %VO2max @ maximal FATox (HCD 54.89%; LCD 70.25%) (P<0.0001). There was a significant difference in maximal CHOox (HCD 7.83 g/min; LCD 5.65 g/min) (P = 0.002). CONCLUSION: Adaptation to a LCD in elite ultrarunners results in profound increases in maximal fat oxidation at least 50% greater than the highest rates ever reported. These findings underscore the robust effect very low-carbohydrate diets have on accelerating fat oxidation beyond that achieved with chronic training in high-level athletes.