The influence of cardiorespiratory fitness level on the relationship between work rates at the aerobic threshold (AerT) and the point of maximal fat oxidation (Fatmax) in untrained adults

Abstract

Introduction: In this study, we investigated the impact of cardiorespiratory fitness (CRF), quantified as peak oxygen consumption (VO_2peak), on the relationship between work rates (WR) at the aerobic threshold (AerT) and the point of maximal fat oxidation rate (Fat_max).

Methods: A total of 761 untrained adults aged 41-68 completed a one-minute incremental exercise test on a cycle ergometer, using breath-by-breath gas analysis to determine VO_2peak, AerT, and Fat_max. AerT was determined using automatic and visual detection methods, and Fat_max was determined using indirect calorimetry. Participants were categorized into CRF-groups: low (<25th percentile), medium (≥25th percentile and <75th percentile), and high (≥75th percentile).

Results: Fat_max was found at 43 ± 7% WR_peak, 37% ± 6% WR_peak and 35% ± 7% WR_peak in the low, medium, and high CRF-groups, respectively. In contrast, AerT was located at significantly higher relative work rates: 51% ± 8% WR_peak, 47% ± 10% WR_peak, and 47% ± 11% WR_peak in the respective CRF-groups. There was a weak agreement between Fat_max and AerT [intraclass correlation coefficient (ICC) = .19, p < .001], and the ICC decreased from .35 to .12 to .13, while the mean bias ±95% limits of agreement increased from 8% ± 14% WR_peak to 8% ± 19% WR_peak to 12% ± 44% WR_peak from CRF-low to CRF-medium to CRF-high. The mean difference between Fat_max and AerT was significantly different among the CRF subgroups: 8% ± 7% WR_peak vs. 10% ± 10% WR_peak vs. 12% ± 11% WR_peak in low, medium, and high CRF-groups, respectively. Nonetheless, multiple regression analysis revealed only a weak positive correlation between the difference in relative work rates (% WR_peak) between Fat_max and AerT (dependent variable) and the predictor variables CRF and sex, both identified as significant (R = .19, p < .001).

Conclusion: Our study confirms substantial differences in exercise intensities between Fat_max and AerT in untrained adults (10% ± 19% WR_peak, ranging from -14% to 53% WR_peak). Importantly, this difference remains relatively consistent across varying CRF levels, emphasizing the distinct nature of Fat_max and AerT, with CRF playing a limited role in influencing their relationship in our study's untrained adults.

Keywords: aerobic capacity; aerobic threshold (AerT); agreement; exercise physiology; exercise testing (CPET); fat oxidation capacity; point of maximal fat oxidation (Fatmax).