Researchers explored how much variance within the gut microbiome could explain the association between intake of flavonoid-rich foods and blood pressure (BP) in a study published in the journal Hypertension. The study, which involved 904 participants (age range, 25 to 82 years, 57% males), investigated the correlation between consumption of flavonoid-rich foods and BP, as well as the sequenced gut microbiome composition, in all participants.

Participants' intake of flavonoid-rich foods during the previous year was determined using a self-reported food questionnaire detailing the frequency and quantity eaten for112 foods. Flavonoid values were assigned to foods according to U.S. Department of Agriculture data on flavonoid content in foods. Each participant's gut microbiome was assessed in fecal bacterial DNA extracted from stool samples. After an overnight fast, participants' BP levels were measured three times in 3-minute intervals after an initial 5-minute rest period.

The researchers also collected participants' lifestyle information, including gender, age, smoking status, medication use, and physical activity, as well as family history of coronary artery disease, number of daily calories and fiber consumed, and BMI. It was found that participants who had the highest intake of flavonoid-rich foods, including berries, red wine, apples, and pears, had lower systolic BP levels as well as greater diversity in their gut microbiome compared with participants who consumed the lowest levels of flavonoid-rich foods.

As much as 15.2% of the association between flavonoid-rich foods and systolic BP could be explained by the diversity of participants' gut microbiome. Eating 1.6 servings of berries per day (one serving = 80 g, or 1 cup) was associated with an average reduction in systolic BP levels of 4.1 mmHg, and about 12% of the association was explained by gut-microbiome factors. Drinking 2.8 glasses of red wine (125 mL wine per glass) per week was associated with an average of 3.7 mmHg lower systolic BP level, of which 15% could be explained by the gut microbiome.

The researchers wrote that, in this evaluation, higher intakes of flavonoid-rich foods, including berries, red wine, apples, and pears, are associated with lower systolic BP and pulse pressure, greater microbial diversity, and greater abundance of Parabacteroides and unassigned genera belonging to the family Ruminococcaceae.

Lead study investigator Aed’n Cassidy, PhD, chair and professor in nutrition and preventive medicine at the Institute for Global Food Security at Queen's University in Belfast, Northern Ireland, stated, "Our gut microbiome plays a key role in metabolizing flavonoids to enhance their cardioprotective effects, and this study provides evidence to suggest these blood pressure-lowering effects are achievable with simple changes to the daily diet." 

Dr. Cassidy also remarked, "Our findings indicate future trials should look at participants according to metabolic profile in order to more accurately study the roles of metabolism and the gut microbiome in regulating the effects of flavonoids on blood pressure. A better understanding of the highly individual variability of flavonoid metabolism could very well explain why some people have greater cardiovascular protection benefits from flavonoid-rich foods than others."

In addition, the authors stated that study participants were from the general population and were unaware of the hypothesis. However, residual or unmeasured confounding factors (such as other health conditions or genetics) can lead to bias; therefore these findings cannot prove a direct cause and effect (although the researchers did conduct a detailed adjustment in their investigation for an extensive range of diet and lifestyle factors). The authors noted that the focus of this study was on specific foods rich in flavonoids, not all food and beverages containing flavonoids.

The researchers concluded that their data highlight the key role of the gut microbiome in the links between flavonoid-rich foods and BP, with up to 15% of the observed association explained by the gut microbiome. Lastly, the authors noted that further study is warranted.

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