The day-to-day reliability of peak fat oxidation and FATMAX

Purpose Prior studies exploring the reliability of peak fat oxidation (PFO) and the intensity that elicits PFO (FAT MAX ) are often limited by small samples. This study characterised the reliability of PFO and FAT MAX in a large cohort of healthy men and women. Methods Ninety-nine adults [49 women; age: 35 (11) years; V ˙ O 2 peak: 42.2 (10.3) mL·kg BM −1 ·min −1 ; mean (SD)] completed two identical exercise tests (7–28 days apart) to determine PFO (g·min −1 ) and FAT MAX (% V ˙ O 2 peak) by indirect calorimetry. Systematic bias and the absolute and relative reliability of PFO and FAT MAX were explored in the whole sample and sub-categories of: cardiorespiratory fitness, biological sex, objectively measured physical activity levels, fat mass index (derived by dual-energy X-ray absorptiometry) and menstrual cycle status. Results No systematic bias in PFO or FAT MAX was found between exercise tests in the entire sample (− 0.01 g·min −1 and 0% V ˙ O 2 peak, respectively; p > 0.05). Absolute reliability was poor [within-subject coefficient of variation: 21% and 26%; typical errors: ± 0.06 g·min −1 and × / ÷ 1.26% V ˙ O 2 peak; 95% limits of agreement: ± 0.17 g·min −1 and × / ÷ 1.90% V ˙ O 2 peak, respectively), despite high ( r = 0.75) and moderate ( r = 0.45) relative reliability for PFO and FAT MAX, respectively. These findings were consistent across all sub-groups. Conclusion Repeated assessments are required to more accurately determine PFO and FAT MAX .