“This talk tells the story of how a simple, unexpected observation helped uncover a new way that vegetables can benefit our health. It turns out that the bacteria living in our mouths play a crucial role, highlighting a fascinating symbiosis between our bodies and our microbiome. They help convert substances in leafy greens into nitric oxide—a tiny signalling molecule that helps our blood vessels relax, lowers blood pressure, and improves how our bodies use oxygen,” said Jon Lundberg of the Department of Physiology and Pharmacology at the Karolinska Institute, Sweden.
We can all probably remember from childhood being told that vegetables are good for us. We know they contain antioxidants, vitamins, polyphenols, fibre and minerals. And there have been huge epidemiological studies showing the health effects especially of leafy green vegetables on cardiovascular health. But how does this work? What is the mechanism?
Lundberg’s work since the 1990s has shown the importance of nitrate (NO3) which is found in leafy green vegetables (like spinach and arugula or rocket) and beetroot, and which the body is able to convert to nitric oxide (NO) which has hugely beneficial effects on cardiovascular health and can be used to enhance athletic performance by reducing oxygen consumption in muscles. But how did this all come about?
Part of the answer lies in the simple act of a student burping.
“We have to go back to 1994,” explained Lundberg. “I was new in the laboratory, and the hottest signalling molecule was nitric oxide – NO – which naturally occurs as gas and is also found in cigarette smoke and car-exhaust fumes.”
“Probably about 250 000 papers have been published on its vasodilation, inflammation inhibiting, prevention of blood clotting and positive effects on metabolism. We know it’s good for cardiovascular health and for treating cardiovascular disorders like hypertension and atherosclerosis. The 1998 Nobel Prize in Physiology or Medicine was awarded to Robert Furchgott, Louis Ignarro and Ferid Murad for their discoveries concerning nitric oxide as a signalling molecule in the cardiovascular system."
“We were doing experiments measuring exhaled NO which comes out in exhaled breath in tiny amounts when we found a sudden high peak – 1000 times higher than usual in one student. The student admitted he had burped. But what was the source?”
Lundberg explained that a burp is the delivery of gas from the stomach to the exhaled breath. They knew that nitrite (NO2) is present in human saliva and that the stomach is extremely acidic. They realised that mixing nitrite from the saliva with the stomach acid generated NO.
But where is the nitrite coming from? Further studies confirmed that the nitrite is generated in the mouth by bacteria in the oral cavity. Saliva is naturally rich in nitrate and the bacteria reduce this nitrate to form nitrite (“Our own cells can’t make nitrite from nitrate,” said Lundberg, “so we need the bacteria to make it.”) And also that adding leafy vegetables known to be naturally high in nitrate (NO3) to the mix led to increased generation of nitric oxide per burp.
So, does nitrate have any biological effects when ingested?
Lundberg and his colleagues went on to do further human studies to see the effect of consuming nitrate and found that, like NO, it significantly lowered blood pressure. In the experimental group that consumed beetroot juice there was a considerable drop in blood pressure which was comparable to existing medication and was still in effect 24 hours later.
“Our studies in rats showed that giving nitrate in water lowered blood pressure but if you kill the oral bacteria with mouthwash the blood pressure effect is gone.” (He added here that the mouthwashes used are very strong antibacterial mouthwashes – and that weaker, more generally used versions would still allow some activity − so we shouldn’t immediately all throw out our mouthwashes!)
Studies on the effect of nitrate on oxygen use during physical exercise in mice models followed and found that nitrate makes energy use more efficient with less oxygen needed for the same workload. “Based on this and other studies the International Olympic Committee allows five dietary supplements of which one is nitrate. So, it’s now used by Olympians.”
“It’s proposed but not proven that the mechanism is that nitrate reduces the proton leak making energy use more effective,” said Lundberg.
But can we get enough cardio protection from vegetables alone? Lundberg explained that there have been two very recent phase II, double-blind, placebo-controlled studies comparing nitrate given as a pill versus beetroot juice and involving 800 patients who had had heart attacks or chest pain (angina) being followed for two years. “The studies showed that blood pressure was lower, the arteries less narrow and kidneys less damaged in those receiving the dietary nitrate,” he said. “There was also a remarkable 50% reduction in serious heart events in the dietary nitrate group at one year follow-up.”
Whilst at STIAS he is trying to plan a phase III trial. “It’s a challenge because no drug companies will pay because there are no patents,” he said.
“Our work has shown two important pathways for the generation of nitric oxide and that symbiotic oral bacteria play a central role,” he said. “We have also shown that dietary nitrate enhances exercise performance and is cardioprotective.”
“This ‘nitrate–nitrite–nitric oxide pathway’ shows that our health depends not only on our own cells, but also on the microbes we live with. It challenges how we think about diet, oral health, and the role of bacteria in the body, and may even open the door to new treatments for cardiovascular disease.”