The aim of the present study was to investigate the relationship between maximal fat oxidation rate (MFO) measured during a progressive exercise test on a cycle ergometer and ultra-endurance performance. 61 male ironman athletes (age: 35±1 yrs. [23-47 yrs.], with a BMI of 23.6±0.3 kg/m2 [20.0-30.1 kg/m2], a body fat percentage of 16.7±0.7% [8.4-30.7%] and a VO2peak of 58.7±0.7 ml/min/kg [43.9-72.5 ml/min/kg] SEM [Range]) were tested in the laboratory between 25 and 4 days prior to the ultra-endurance event, 2016 Ironman Copenhagen. Simple bivariate analyses revealed significant negative correlations between race time and MFO (r2=0.12, p<0.005) and VO2peak (r2=0.45, p<0.0001) and a positive correlation between race time and body fat percentage (r2=0.27, p<0.0001). MFO and VO2peak were not correlated. When the significant variables from the bivariate regression analyses were entered into the multiple regression models, VO2peak and MFO together explained 50% of the variation observed in race time among the 61 Ironman athletes (adj R2=0.50, p<0.001). These results suggests that maximal fat oxidation rate exert an independent influence on ultra-endurance performance (>9 h). Furthermore, we demonstrate that 50% of the variation in Ironman triathlon race time can be explained by peak oxygen uptake and maximal fat oxidation.
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