Menopausal duration, relative skeletal muscle index, and V˙O 2 kinetics predict substrate oxidation changes during walking in postmenopausal women with overweight/obesity

Menopausal duration is often accompanied by increased adiposity, declining aerobic fitness, and reduced skeletal muscle mass / function. Together, these shifts predispose postmenopausal women to metabolic inflexibility – an impaired ability to adjust substrate oxidation in response to physiological demands. Since metabolic flexibility directly relates to cardiometabolic disease risk, this study aimed to 1) assess changes in fatty acid (ΔFAox) and carbohydrate (ΔCHOox) oxidation during walking, and 2) identify key physiological factors related to these responses. We hypothesized that chronological age, exercise capacity, and body mass index (BMI) would predict ΔFAox and ΔCHOox during walking in postmenopausal women with overweight/obesity. Methods: Thirty-six postmenopausal women (64 ± 7 y; BMI: 30.0 ± 3.3 kg/m 2 ) completed a standardized (0.9 m/s at 0% grade) treadmill task coupled with indirect calorimetry to calculate oxygen (Formula: see textO 2 ­) uptake kinetics and estimate ΔFAox and ΔCHOox. A 6-minute walk test (6MWT) was used to assess exercise capacity. Dual-energy X-ray absorptiometry was used to quantify body composition, including relative skeletal muscle index (RSMI). Pearson correlations were used to explore associations of interest. Multiple linear regression was used to evaluate the level of significance and relative contributions of key predictors to substrate use variability. Results: ΔFAox was associated with age (r = -0.465, P = 0.004), menopausal duration (r = -0.588, P 0.065). Menopausal duration (partial r = -0.567, P < 0.001), RSMI (partial r = 0.514, P = 0.002), and recovery Formula: see textO 2 mean response time (partial r = 0.277, P = 0.188) were able to explain the most variance in ΔFAox (adjusted r2 = 0.534, P < 0.001). Menopausal duration (partial r = 0.530, P = 0.001), visceral adipose tissue volume (partial r = -0.475, P =0.004), and O 2 deficit (partial r = 0.408, P = 0.017) were able to explain the most variance in ΔCHOox (adjusted r2 = 0.371, P < 0.001). Conclusion: Age-related changes in metabolic flexibility were detected in postmenopausal women with overweight/obesity during walking. Menopausal duration, rather than chronological age, was the strongest predictor of changes in substrate utilization. RSMI was a better predictor of ΔFAox during submaximal walking than exercise capacity or Formula: see textO 2 uptake kinetics. Future work should test whether high-intensity resistance exercise can improve metabolic flexibility and support activities of daily living in postmenopausal women. Funding: Supported by the Indiana Clinical and Translational Sciences Institute, funded in part by grant number UL1TR002529 from the NIH-NCATS. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.