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

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.

Ninety-nine adults [49 women; age: 35 (11) years; [Formula: see text]O₂peak: 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 (%[Formula: see text]O₂peak) 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.

No systematic bias in PFO or FAT_MAX was found between exercise tests in the entire sample (- 0.01 g·min^-1 and 0%[Formula: see text]O₂peak, 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%[Formula: see text]O₂peak; 95% limits of agreement: ± 0.17 g·min^-1 and × / ÷ 1.90%[Formula: see text]O₂peak, 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.

Repeated assessments are required to more accurately determine PFO and FAT_MAX.