The blood pressure-lowering effect of exercise is significantly reduced when people rinse their mouths with antibacterial mouthwash rather than water, according to a study which shows the importance of oral bacteria in cardiovascular health. The study, published in the journal Free Radical Biology and Medicine, suggests that health professionals should pay attention to the oral environment when recommending interventions involving physical activity for high blood pressure.
"Scientists already know that blood vessels open up during exercise, as the production of nitric oxide increases the diameter of the blood vessels (known as vasodilation), increasing blood flow circulation to active muscles," said Raul Bescos, from the University of Plymouth in the UK.
"What has remained a mystery is how blood circulation remains higher after exercise, in turn triggering a blood-pressure lowering response known as post-exercise hypotension," Bescos said.
Previous research has suggested that nitric oxide was not involved in this post-exercise response -- and only involved during exercise -- but the new study challenges these views. "It's all to do with nitric oxide degrading into a compound called nitrate, which for years was thought to have no function in the body. But research over the last decade has shown that nitrate can be absorbed in the salivary glands and excreted with saliva in the mouth," said Bescos.
Twenty-three healthy adults were asked to run on a treadmill for a total of 30 minutes on two separate occasions, after which they were monitored for two hours.
On each occasion at one, 30, 60 and 90 minutes after exercise they were asked to rinse their mouths with a liquid -- either antibacterial mouthwash (0.2 per cent chlorhexidine) or a placebo of mint-flavoured water.
Their blood pressure was measured and saliva and blood samples were taken before exercise and at 120 minutes after exercise. The study found that when participants rinsed with the placebo, the average reduction in systolic blood pressure was 5.2 milimetres of mercury (mmHg) at one hour after exercise.
However when participants rinsed with the antibacterial mouthwash, the average systolic blood pressure was 2.0 mmHg at the same time point.
Systolic blood pressure refers to the highest blood pressure level when the heart is squeezing and pushing the blood round the body. These results show that the blood pressure-lowering effect of exercise was diminished by more than 60 per cent over the first hour of recovery, and totally abolished two hours after exercise when participants were given the antibacterial mouthwash.
Previous views also suggested that the main source of nitrite in the circulation after exercise was nitric oxide formed during exercise in the endothelial cells (cells that line the blood vessels).
However, the new study challenges this. When antibacterial mouthwash was given to the participants, their blood nitrite levels did not increase after exercise. It was only when participants used the placebo that nitrite levels in blood raised, indicating that oral bacteria are a key source of this molecule in the circulation at least over the first period of recovery after exercise.
"These findings show that nitrite synthesis by oral bacteria is hugely important in kick-starting how our bodies react to exercise over the first period of recovery, promoting lower blood pressure and greater muscle oxygenation," said Craig Cutler, who conducted the research as part of his PhD at the University of Plymouth.
"In effect, it's like oral bacteria are the 'key' to opening up the blood vessels. If they are removed, nitrite can't be produced and the vessels remain in their current state," he said.