INTRODUCTION: Aligned with the idea of a vagal cardiometabolic-autonomic link, nonlinear heart rate variability has been proposed as an early marker of metabolic alterations (Zamora-Justo et al., 2025), and resting vagal activity has been associated with maximal fat oxidation (MFO) in postmenopausal women (Monferrer-Marín et al., 2024), This population is often characterized by age- and oestrogen-related metabolic and autonomic impairments, including reduced fat oxidation (Abildgaard et al., 2013) and lower vagal HRV indices (Giunta et al., 2024). However, the dynamic interaction between autonomic regulation and substrate utilization during exercise remains poorly characterized in this population.
AIM: This study aimed to examine the interaction between vagal and sympathetic activity and energy substrate utilization during an incremental exercise test in active postmenopausal women.
METHODS: Seventy-eight active postmenopausal women (68.2±5.4 years; 66.3±11.2 kg) completed a two-day cross-sectional protocol involving a graded FATmax test with indirect calorimetry and heart rate variability (HRV) monitoring. Primary outcomes focused on FATox and its autonomic correlates, defined as heart rate variability indices reflecting short-term fractal correlation properties (DFAα1) and vagal modulation (RMSSD), while secondary analyses assessed the coupling between CHOox and linear autonomic indices, including a geometric HRV-derived metric reflecting sympathetic predominance as Stress Index and also RMSSD. Time-series were normalised to the percentage of test duration. Dynamic cross-correlation matrices, and exponentially weighted moving-average rolling correlations (|r|), a time-resolved method to assess dynamic associations between variables.
RESULTS: RMSSD displayed the strongest temporal association with FATox (|r| = 0.62), DFAa1 showed moderate dynamic correlation (|r| = 0.40), and SampEn exhibited weak and unstable correlations. In contrast, CHOox demonstrated stronger and more stable associations with autonomic indices, being inversely associated with RMSSD (|r| = 0.87) and positively linked to the Stress Index (|r| = 0.84).
CONCLUSIONS: Data reveal an intensity-dependent association between autonomic dynamics and substrate utilization during exercise in active postmenopausal women. Fat oxidation shows a consistent relationship with vagal indexes, whereas carbohydrate oxidation is associated with vagal withdrawal and sympathetic activation markers. CHOox associations appear stronger, likely reflecting its faster kinetics compared with FATox and closer temporal alignment with autonomic fluctuations. Overall, these findings support the presence of coordinated, yet intensity-dependent, autonomic–metabolic interactions shaping exercise responses in this population.