Vagal activity and fat oxidation correlates during basal metabolic rate in active postmenopausal women

Background Heart Rate Variability at rest has been recently associated with metabolic outcomes along exercise, which in turn have been traditionally associated with energy expenditure, muscle power and fat mass. This study aimed to analyse the relationship at rest between autonomic function and metabolic outcomes, in physically active postmenopausal women. We hypothesised that, more than basal metabolic rate, autonomic function is more strongly associated with fat oxidation, similarly to muscle power and fat mass. Sixty-one active postmenopausal women (67.9±5.3 years; 40.3±4.3 kg muscle mass) completed basal metabolic rate and Heart Rate Variability analysis, recorded simultaneously for 30 minutes with participants resting supine under standardised activity and diet conditions. Results Root Mean Square of Successive Differences of heart beats (RMSSD) and Total Power showed a predictive value for resting fat oxidation (β = 0.46; β = 0.26), explaining 30% of the variance. Including traditional predictors as energy expenditure increased explained variance to 57.5%. In this model, RMSSD association disappeared, Total Power (β = 0.88) became the strongest predictor, and together with energy expenditure (β = 0.53), showed significant associations with fat Oxidation Rates. Respiratory exchange ratio only correlated with RMSSD (β = –0.54) in the isolated Heart Rate Variability model, without basal metabolic rate associations. Box plots of RMSSD quartiles revealed a difference in fat-oxidation between the highest and lowest quartiles, a pattern not seen for Total Power. Conclusio n Baroreflex activity and fat oxidation associate at rest in active postmenopausal women with preserved cardiovascular function. Total Power emerges as the strongest Heart Rate Variability predictor of baseline fat oxidation, as does Root Mean Square of Successive Differences categorization, with fat oxidation rates varying by quartile. Trail registration: NCT06336070, registered 04 april 2024, (https://classic.clinicaltrials.gov/ct2/show/NCT06336070).