Association between autonomic regulation parameters and gut microbiota composition in women with metabolic syndrome

Metabolic syndrome is defined as a multicomponent cardiometabolic disorder that combines abdominal obesity, insulin resistance, atherogenic dyslipidemia, and arterial hypertension, is associated with an increased risk of cardiovascular events and type 2 diabetes mellitus, and demonstrates a growing prevalence among women in the second period of adulthood. The aim of the study was to evaluate the relationship between heart rate variability (HRV) indices and the taxonomic structure of the gut microbiome in women with metabolic syndrome. Materials and methods. Sixty-eight women in the second period of adulthood were examined (50 with clinically confirmed metabolic syndrome and 18 apparently healthy controls). Anthropometric and biochemical assessments were performed. Autonomic regulation was evaluated using time- and frequency-domain HRV parameters derived from 5-minute ECG recordings. The taxonomic profile of the gut microbiota was determined by 16S rRNA sequencing with analysis of the relative abundance of Firmicutes, Bacteroidetes, Actinobacteria, and the Firmicutes/Bacteroidetes (F/B) ratio. Statistical analysis included nonparametric tests and Pearson correlation analysis. Results. In women with metabolic syndrome, HR was 8 % higher (p = 0.0018) and mRR was 6 % lower (p = 0.0149). SDNN decreased by 23 % (p = 0.0019), RMSSD by 27 % (p = 0.0067), and pNN50 by 47 % (p = 0.0056). LF norm was increased (p = 0.0256), LF/HF rose 1.49-fold (p = 0.0130), HRV Ti decreased by 20 % (p = 0.0049), IARS increased by 28 % (p = 0.0166), and VLF/HF increased 1.67-fold (p = 0.0119). The relative abundance of Bacteroidetes was 6.8 % lower (p = 0.0385) and Actinobacteria 24.8 % lower (p = 0.0167), whereas the «Other» category was 3.34-fold higher (p < 0.0001). Differences in Firmicutes (p = 0.3949) and F/B ratio (p = 0.5023) were not statistically significant. The relative abundance of Firmicutes positively correlated with indices of total and vagal-mediated HRV (SDNN, RMSSD, pNN50, CV) as well as with spectral parameters (TP, VLF, LF, HF). Bacteroidetes showed negative associations with CV, LF, and TP. The F/B ratio was associated with SDNN, pNN50, CV, TP, LF, and HF. No significant associations were found between Actinobacteria and most HRV parameters, except for negative relationships with IC and VLF/HF. Conclusions. Women with metabolic syndrome demonstrated significant alterations in time- and frequency-domain HRV parameters, including increased HR, shortened mRR, reduced SDNN, RMSSD, and pNN50, as well as elevated LF norm and LF/HF accompanied by decreased HRV Ti and increased IARS, reflecting reduced overall variability, diminished vagal modulation, a shift in autonomic balance, and increased regulatory strain with decreased adaptive reserve. The gut microbiome profile in metabolic syndrome was characterized by reduced relative abundance of Bacteroidetes and Actinobacteria and a significant increase in the «Other» category without significant changes in the F/B ratio, indicating qualitative microbial remodeling rather than an isolated Firmicutes/Bacteroidetes shift. Significant associations were identified between Firmicutes and HRV parameters (SDNN, RMSSD, pNN50, TP, LF, HF), whereas Bacteroidetes demonstrated negative associations with selected spectral indices. The F/B ratio showed positive associations with several time- and frequency-domain HRV indices. Overall, the findings demonstrate an interrelationship between autonomic HRV parameters and phylum-level characteristics of the gut microbiome in women with metabolic syndrome, reflecting a link between autonomic dysfunction and gut microbial profile within the pathophysiological framework of metabolic syndrome.