The Reliability of Fatmax

The exercise intensity eliciting maximal fat oxidation (Fatmax) is typically determined during an incremental test. Its reproducibility, however, has not been thoroughly investigated so far. To address this issue, 21 healthy subjects (23.5±1.7 years; BMI 22.4±1.8 kg/m2; VO2peak 47.4±11.3 mL/min/kg) carried out two identical cycling tests to determine Fatmax after an initial incremental baseline test. The duration of each of five stages during the Fatmax tests was 6 min. The first stage equalled the first increase in blood lactate during the baseline test; the highest stage corresponded to a respiratory exchange ratio of 1.00. Between these intensities the other three stages were distributed evenly. Fatmax was 28.0±8.6 L/min (59.2±18.1% VO2peak) in the first test and 29.8±10.5 L/min (62.9±22.2% VO2peak) in the second one. There was no significant difference between both Fatmax determinations [number of stage: P=0.31; total VO2: P=0.20; VO2 utilized for fat oxidation (VO2Fat): P=0.33]. Linear correlation coefficients between tests were r=0.84 (total VO2; P<0.001) and r=0.83 (VO2Fat; P<0.001). However, Bland–Altman plots revealed wide 95% limits of agreement of 0.91 L/min (total VO2) and 0.32 L/min (VO2Fat). In conclusion, spontaneous intraindividual variability in Fatmax appears too large to recommend the use of this parameter for the prescription of training intensity.