Introduction
There is extensive evidence that gut microbiota has important functions in host metabolism and immunity. Gut microbiota and its metabolites are known to play a role in the pathogenesis of metabolic disease [
1] and chronic inflammatory disease [
2], and their association with other diseases has been discovered, including the association between gut microbiota and cardiovascular diseases [
3,
4]. We previously reported that amounts of
Bacteroides vulgatus and
Bacteroides dorei—predominant gram-negative gut microbes—were lower in patients with coronary artery disease (CAD) compared to control subjects [
5]. Administration of these two
Bacteroides species inhibited atherogenesis in apoe
−/− mice via decreasing inflammation. The gut microbiota-derived metabolites of dietary choline or carnitine, trimethylamine (TMA), and trimethylamine-
N-oxide (TMAO) are related to cardiovascular disease, including atrial fibrillation (AF) [
6‐
8]. In an earlier study, we investigated gut microbial composition and host plasma metabolites in heart failure and demonstrated the alteration of gut microbial composition and gain of plasma TMAO [
9].
AF is the most common arrhythmia that increases the risk of heart failure, cerebral infarct, and arterial embolism. In the current aging society, patients with AF are increasing [
10]. Obesity, type 2 diabetes mellitus, and hypertension are reportedly associated with gut microbial dysbiosis [
1,
11] and are independent risk factors of AF. The gut microbial metabolites lipopolysaccharide (LPS), TMAO, and indoxyl sulfate (IS) increase the instability of atrial electrophysiology [
8,
12,
13] and are associated with AF [
14‐
16].
Several factors, including age, ethnicity, residential location, and dietary habits, affect gut microbial composition. Diet also influences host metabolism, e.g., high-fat diet induces insulin resistance via alteration of gut microbial composition and metabolic endotoxemia [
17]. The Prevención con Dieta Mediterránea (PREDIMED) trial demonstrated that extra-virgin olive oil in a Mediterranean diet may reduce AF risk [
18]. Low-carbohydrate diets were associated with increased risk of AF [
19]. In AF patients, plasma LPS levels predicted major cardiovascular events and were negatively affected by adherence to a Mediterranean diet [
14].
A recent investigation [
20] of gut microbial composition in AF patients revealed differences in gut microbiota relative to control subjects with different comorbidities and medications. Alteration of gut microbiota in AF patients remains poorly understood, much less the association between gut microbiota and dietary habits. The present study reports an analysis of gut microbiota and dietary habits in AF patients.
Discussion
The relationship of CAD and HF to gut microbiota and its metabolites has been reported in cardiovascular diseases [
3‐
7,
9]. AF has also been associated with gut microbial metabolites [
8,
12,
13], although the alteration of gut microbial composition in AF is unclear.
Here, we measured the gut microbial composition in control subjects and AF patients. Unlike a previous report [
20], gut microbial richness was lower in AF patients. Gut microbial composition changes with age, and gut microbial richness is no exception, increasing with age [
31]. The difference in age between control and AF groups could have been a confounding factor in the previous report. To our knowledge, low bacterial richness is associated with insulin resistance, dyslipidemia, and inflammation, which are risk factors of AF [
32]. Thus, a reduction in gut microbial richness may be associated with metabolic and chronic inflammatory diseases, including AF.
Enterotype 2, which is characterized by an abundance of the genus
Prevotella, was decreased, while enterotype 3, which is characterized by an abundance of the genus
Ruminococcus, was increased in AF. Previously [
20], when participants were divided into two clusters, the AF group was less in enterotype 2, (dominated by
Prevotella) but had more of enterotype 1 (dominated by
Bacteroides). A lower
Prevotella-prominent enterotype in AF patients was obtained in both studies. The increase in the
Ruminococcus-predominant enterotype is characteristic of AF patients, and similar results were obtained from studies of CAD [
3], symptomatic atherosclerosis [
33], and obstructive sleep apnea–hypopnea syndrome (OSAHS) [
34]. Patients with atherosclerosis and OSAHS have risk factors similar to those of AF patients. In OSAHS, hypoxia during sleep stimulates sympathetic nerve activity, which possibly triggers AF. OSAHS itself is a risk factor of AF [
35]. Alteration of gut microbial enterotypes might be involved in the pathogenesis of these diseases.
In a previous report [
20], the proportions of
Eubacterium,
Roseburia,
Ruminococcus,
Blautia,
Streptococcus,
Dorea,
Veillonella, and
Enterococcus (phylum
Firmicutes) were much higher, while the proportions of
Prevotella and
Alistipes (phylum
Bacteroidetes) and
Sutterella and
Bilophila (phylum
Proteobacteria) were depleted in AF. The preponderance of genera belonging to
Proteobacteria in control subjects and of the genera
Streptococcus and
Dorea in AF were similar to our results. It is noteworthy that patients with AF shared the enrichment of numerous microbial flora, such as
Streptococcus and
Dorea, demonstrated in hypertension [
36], heart failure [
37], and CAD [
4]. An excess of these genera agrees with a recent study showing that overgrowth of genera is related to plasma and fecal indole [
20]. Dietary tryptophan is converted to indole by the gut microbial enzyme tryptophanase. Indole is subsequently oxidized and sulfated in the host liver to form IS [
9]. IS induces oxidative stress and consequently increases pulmonary vein and atrial arrhythmogenesis [
13]. Koike H et al. demonstrated that the maintenance of sinus rhythm after AF ablation decreased serum IS levels in patients with high IS [
38]. This suggests that serum IS may not only induce the onset of AF but may also be affected by the presence of AF. Another gut microbial metabolite, TMAO, could increase the instability of atrial electrophysiology [
8].
Lachnoclostridium,
Parabacteroides, and
Dorea, which were increased in AF patients, produce high amounts of TMAO in the human gut [
39]. Conversely,
Enterobacter, which was depleted in AF patients, can consume TMAO [
40]. The alteration of these gut microbiota might affect gut microbial metabolites and consequently, the pathogenesis of AF.
Hypertension is reported to have an association with gut microbiota [
11]; increased abundance of
Prevotella species (sp.),
Klebsiella sp., and
Enterobacter sp. has been observed in hypertension patients. Li et al. demonstrated elevation of blood pressure after fecal transplantation from hypertensive human donors to germ-free mice, compared with that after transplantation from control donors with normal blood pressure [
11]. This suggested that the alteration of gut microbiota during hypertension was not the result of hypertension, but may be one of the causes of hypertension. Although the prevalence of HT was matched, the blood pressure was lower in AF patients than in control subjects. In this study, we demonstrated that the abundance of
Prevotella-enterotype and
genus Enterobacter was reduced in AF patients. These changes in gut microbiota were opposite to those observed in hypertension patients [
11], and may account for the lowering of blood pressure in AF patients. Alteration of hemodynamics and reduction in cardiac output due to AF may affect the gut microbiota. However, there is no evidence to date linking the relationship between hemodynamics and gut microbiota. The relationship between the gut microbiota and heart failure, in which cardiac output may be reduced, has been reported [
9,
37]. However, the results of those studies were contrasting, and the specific gut microbial phenotype of heart failure remains controversial. In this study, we were unable to clarify the relationship between hemodynamics and gut microbiota due to lack of data. Future studies should elucidate this association.
Contrary to a previous report on the correlation between dietary fat and gut microbial composition [
41], there was a weak positive correlation between fat intake and
Firmicutes, but not
Bacteroidetes. We previously reported that
Bacteroides vulgatus and
Bacteroides dorei reduce plasma LPS activities and show anti-inflammatory functions. High-fat diet was found to induce metabolic endotoxemia and inflammation by increasing intestinal permeability [
5]. An animal-based diet increased the abundance of
Alistipes,
Parabacteroides, and
Odoribacter, which were increased with AF [
42]. Fat intake may trigger AF via metabolic endotoxemia and chronic inflammation.
The significantly higher n-3 PUFA intake in AF patients does not contradict the report that the supplementation of fish oil (enriched in n-3 PUFAs) depleted members of the family
Enterobacteriaceae [
43]. Multiple randomized controlled trials have assessed the effects of n-3 PUFA supplementation on cardiovascular events [
44], which have shown no benefits of n-3 PUFA supplementation for AF. A Danish cohort study showed a U-shaped association between the consumption of marine n-3 PUFA and AF risk, with the lowest risk close to the median intake of n-3 PUFA [
45]. Although only the anti-inflammatory effects of n-3 PUFAs are attracting attention, the effect on gut microbiota should also be investigated. A high intake of n-6 PUFA is associated with inflammatory bowel disease, because most n-6 PUFAs are metabolized to AA, which is a precursor to potent pro-inflammatory mediators [
46]. In AF patients, the intake of eicosadienoic acid, an n-6 PUFA, was increased. Eicosadienoic acid is converted to AA, modulates the production of pro-inflammatory modulators in macrophages, and is associated with prolonged inflammation [
47]. Dietary composition is important to host immunity with or without alteration of gut microbial composition.
We should consider the impact of medication on the alteration of gut microbiota, especially PPI. Since most AF patients took PPI for the prevention of esophageal ulcers before ablation, we could not match PPI administration between control subjects and AF patients. Oral administration of PPI alters gut microbiota [
30] and reduces its richness and diversity. PPI use reduces the intragastric acid concentration and impairs the bactericidal effect of pharyngeal bacteria. Consequently, the
Lactobacillales order, and particularly genus
Streptococcus, is more abundant in PPI users, as is
Parabacteroides. It is unclear whether
Streptococcus and
Parabacteroides abundance in AF patients was affected by PPI.
The present study had several limitations. First, the number of patients was small; therefore, larger studies are warranted to verify our observations. Second, the residential locations of the AF patients differed from those of control subjects, and we cannot exclude the influence of regional differences. In addition, the absence of AF in control subjects was determined only through self-assessment questionnaires of anamnesis and drug uses; therefore, we could not exclude subclinical AF in control subjects. Finally, medication should be considered a confounding factor; however, we could not completely match the medication between the two groups.
Nevertheless, our results indicated that gut microbial composition was altered in AF patients. In addition, alteration of gut microbiota was possibly related to dietary composition. A prospective cohort study is needed to identify whether the alteration of gut microbiota involves the etiology of AF.
Publisher's Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.