Author(s): HOGAN, C., CROSS, T.J., PRATHEEPAN, P., MACHAN, E., TIONG, M., ROONEY, K., Institution: THE UNIVERSITY OF SYDNEY, Country: AUSTRALIA, Abstract-ID: 1350

INTRODUCTION:
Fat oxidation during exercise is often described as a parabolic relationship, from which the maximal rate of fat oxidation (MFO) and its corresponding intensity (Fatmax) are derived. However, evidence indicates substantial variability in both MFO and the shape of the fat oxidation–intensity relationship across individuals with differing fitness and metabolic health. Deviations from the assumed parabolic profile are observed in less trained individuals, raising questions about the validity of polynomial curve fitting to identify MFO and Fatmax. It also remains unclear whether Fatmax coincides with the first lactate threshold (LT1), based on the hypothesis that lactate accumulation suppresses fat oxidation. The primary aim of this study was to determine whether distinct fat oxidation curve phenotypes exist across a broad range of fitness levels. The secondary aim was to examine whether Fatmax occurs at the same exercise intensity as LT1 and whether this relationship differs by fitness and phenotype.
METHODS:
110 healthy adults (62 males, 48 females) completed two laboratory visits involving a maximal ramp cycling test and a multi-stage Fatmax protocol. Gas exchange was measured to determine peak oxygen consumption (V̇O₂peak), ventilatory thresholds, and substrate oxidation rates, with blood lactate sampled to identify lactate thresholds. Substrate oxidation was calculated during steady-state stages, and MFO and Fatmax were derived via fitting a third-order polynomial. Individual fat oxidation–intensity profiles were qualitatively classified into phenotypes by independent researchers using a blinded consensus approach. Differences in physiological and substrate oxidation responses were analysed across V̇O₂peak percentile–defined fitness groups, and the relationship between Fatmax and LT1 was assessed using ANCOVA, linear mixed-effects models, and Pearson’s correlation.
RESULTS:
Three primary fat oxidation phenotypes were identified, showing substantial inter-individual variability in curve shape. MFO and fat oxidation rates differed by fitness level, with the ≤60th V̇O₂peak percentile exhibiting lower fat oxidation across exercise stages (P<0.05) and a lower MFO compared with the 90th percentile group (P=0.003). Across the full sample, Fatmax and LT1 were moderately correlated (r =0.55–0.59, P<0.0001); however, the strength of this relationship increased with fitness and differed between phenotypes, with the strongest associations in the increasing phenotypes (r =0.91–0.92, P<0.0001).
CONCLUSION:
Fat oxidation responses show substantial inter-individual variability influenced by fitness and phenotype. Group-level parabolic models do not adequately capture these differences. The relationship between Fatmax and LT1 varies with fitness and phenotype, indicating lactate does not universally explain fat oxidation patterns. Results emphasize the need for personalised approaches to Fatmax assessment and interpretation, tailored to an individual’s metabolic profile and fitness status.