PURPOSE: While the brain is usually wholly dependent on glucose for fuel, prolonged carbohydrate deprivation results in adaptations that allow the brain to access fat via ketone bodies. Given the depletion of carbohydrate that takes place during prolonged exercise, a shift toward ketone bodies may maintain central nervous system function, preventing the central fatigue observed during long-duration exercise. In addition, enhancing peripheral muscle’s use of free fatty acids and intra-muscular lipids might prolong contractions. Thus, high fat/low carb diets have been proposed to delay fatigue during endurance exercise. However, studies utilizing periods of 1 to 5 weeks of adaptation to high fat diets have shown equivocal changes in performance at moderate or high intensities. We compare endurance and time trial performance of a keto-adapted male runner (KETO) to 8 subjects (NORM) on whom we have previously reported.
METHODS: We measured peripheral and central fatigue in 8 men runners (38±2 yrs; VO2peak 59±3 ml/kg/min) who habitually ate a carbohydrate-liberal diet (NORM) and in a runner (41 yrs, 70 ml/kg/min) who followed a high fat/low carb diet for >2 years. Water was provided at 1% of body mass/hr, during a 2-hr run at ventilatory threshold (~65% VO2peak), followed by a self-paced 2-km time trial (TT). RPE and respiratory measures were determined every 20 minutes. Strength was tested in a semi-reclined position [75° hip flexion, to facilitate femoral nerve stimulation] pre-exercise, after the 2-hr run and post-time trial as follows: voluntary isometric quad strength was measured on the Biodex with the knee flexed 60°, and with superimposed peripheral magnetic stimulation of the femoral nerve to determine central activation (CAR).
RESULTS: Respiratory exchange ratio indicated that KETO (0.78) used less carbohydrate than NORM (0.86±0.01) during the 2-hr run. While we measured no fatigue of any kind in KETO, NORM declined in voluntary strength (-16±5%) and experienced loss of central drive to the muscle (CAR decreased from 0.85±0.04 to 0.76±0.05). KETO ran the TT at a mean pace of 16.4 km/hr compared to 14.8±0.7 km/hr in NORM.
CONCLUSIONS: Our KETO runner did not display strength loss and may have had better TT performance after exhausting exercise as compared to our NORM runners who exhibited central fatigue.