The success of the DASH (Dietary Action to Stop Hypertension) diet , which is widely recognised for reducing blood pressure, may be due to the actions of bacteria located in the mouth, suggests Professor Amrita Ahluwalia, co-director at the William Harvey Research Institute at the Queen Mary University of London.
The diet, which is rich in fruit and vegetables and low in saturated fat, has also recognised as having benefits in reducing cardiovascular disease and strokes.
Speaking at the recent European Microbiome Conference in London Ahluwalia explained that oral bacteria, mainly located at the back of the tongue, transform dietary nitrate into nitrite, which in turn leads to lower blood pressure via the production of nitric oxide (NO).
“The magnitude of the blood pressure reduction was found to be directly proportional to the rise in circulating nitrite levels,” said Ahluwalia.
The finding is significant because loss of NO has been proven to be involved in the early stages of cardiovascular (CV) disease and the development of hypertension.
NO is important to the CV system. NO is produced in a healthy individual by the endothelium of all of our blood vessels. It exerts a range of beneficial, protective effects. It endows the blood vessels with an anti-hypertensive and anti-atherosclerotic phenotype,” explained Ahluwalia.
Endothelial malfunction is an early characteristic of CV disease, and this arises from a deficit in nitric oxide, Ahluwalia elaborated.
“So strategies that might replace that lost NO and recover the bioavailable levels of NO in CV disease have clear therapeutic potential,” she suggested.
When researchers fed healthy volunteers with dietary doses of nitrate, they found not only that blood concentrations of nitrate increased, but also that there were also huge increases in the nitrate concentration of saliva.
Good sources of dietary nitrate are beetroot and green leafy vegetables.
“We see 10 times higher concentrations of nitrate in the saliva in comparison to circulating levels and it is here that that the nitrate comes into close contact with the bacteria,” explained Ahluwalia.
“There are bacteria which express nitrate reductase in the oral cavity they convert nitrate to nitrite. You can't help swallow your saliva and that's how it re-enters the body the body,” she continued.
The bacteria reduce nitrate to nitrite in the mouth. The nitrite-rich saliva is subsequently re-ingested into the body where it is converted to the anti-hypertensive nitric oxide (NO). The researchers also confirmed that the blood pressure reduction effect was derived from the nitrite in the saliva, as patients who were made to spit out (rather than swallow) their saliva, did not exhibit lower blood pressure.
Research has indicated that the nitrate-reductase activity in the mouth is probably performed predominantly by Veillonella species. However, since the experiment that identified this required culturing of the bacteria, the result might possibly have been skewed, explained Ahluwalia.
A subsequent study, in which daily nitrate was given to patients with high cholesterol, demonstrated a significant shift in oral microbiome composition over the six-week trial period. Rothia mucilaginosa and a species of Neisseria were the two main types of bacteria found to be driving this shift.
“The microbiome community structure was substantially altered by nitrate. We had changed the profile of the microbiome to one that was possibly favouring nitrate reduction,” commented Ahluwalia.
Mouthwash inhibits effect
Ahluwalia also reiterated findings of a small trial in which, after using chlorhexidine mouthwash for a week, the numbers of nitrate-reducing bacteria present in the mouth were found to be substantially reduced. The lower level of nitrate reduction activity was directly correlated with a marginal rise in blood pressure (around 2.5 -3.5 millimetres of mercury (mmHg)).
The result highlights the effects of how an everyday hygiene product can have an adverse health effect to its impact on the microbiome.
Whereas this blood pressure rise may be not critical at an individual level, “Perhaps at a population level it is an important rise because 2mmHg increase in blood pressure has been associated with a 7% increase in heart attacked and 11% increase in stroke,” said Ahluwalia.
Ahluwalia hypothesised that the oral nitrate reducing microbiome might be critical to sustaining cardiovascular health.
“Increasing consumption of fruit and vegetables provides sustained blood pressure lowering and I would like to suggest that it may be the nitrate content within that fruit and vegetable diet that is responsible for this.
“There have been a number of assessments showing the DASH diet contains between 6 and 20 millimoles per day of nitrate and we know absolutely that these kind of doses will reduce blood pressure,” she concluded.