Aberdeen University research on the benefits of eating your greens could help to save Britain's lettuce industry, writes Olga Wojtas.

For many years, scientists have thought that nitrate, found in foods such as green vegetables, may be harmful. Since it can be converted to nitrite in the body, it can theoretically form N-nitrosamines, which have been linked with stomach cancer.

The European Union is now threatening to impose much harsher limits on the amount of nitrate allowed in green vegetables, which would put most of Britain's lettuce industry, worth Pounds 60 million a year, out of business.

Nitrate is concentrated in saliva and converted to nitrite by a special type of bacteria that lives on the tongue's surface. This nitrite is then swallowed and reacts chemically in the mouth and stomach to form large amounts of nitric oxide.

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Nigel Benjamin, senior lecturer in Aberdeen's department of medicine and therapeutics, speculated that from the evolutionary point of view, it made little sense for nitrate to be concentrated in saliva if this caused harm. His research on nitric oxide has shown that it is generated by a variety of cells in the body in order to fight infection.

After a two-year project involving the neighbouring Rowett agricultural research institute and Aberdeen's department of health sciences, the research team has published its findings in Nature Medicine, suggesting the bacteria on the tongue is part of a deliberate process to produce nitric oxide in the mouth and stomach which will kill harmful bacteria and stop gastroenteritis and possibly tooth decay. "The irony is that we're forming these bacteria to help protect us against other germs we don't want," Dr Benjamin said.

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There is no evidence dietary nitrate causes cancer. The Aberdeen team believes legislation to reduce nitrate content in vegetables and drinking water may be premature, and could discourage people from eating vegetables despite the known health benefits.