PROPOSALS in Europe to limit the level of nitrates in lettuce may be based on
outdated theories about the chemicals鈥� physiological role. A researcher at St
Bartholomew鈥檚 Hospital in London has found that far from being dangerous to
health, dietary nitrates are actually an essential part of our mechanism for
neutralising toxic bacteria in the stomach.
Nitrates occur naturally in all green plants, especially lettuce and spinach.
Since the 1970s there has been concern about the level of nitrates in our diet
because of links to a rare condition in babies called methaemoglobinaemia, or
鈥渂lue baby syndrome鈥�.
Another potential hazard is that nitrites, derived from nitrates, might react
to form N-nitrosamines, which cause stomach cancer in rats. The
European Commission plans to introduce strict curbs on the nitrate contents of
lettuces.
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Ben Benjamin, then a clinical pathologist working at the University of
Aberdeen, discovered that nitrates in human saliva are converted to nitrites by
specialised bacteria that live on the tongue. The acid conditions of the stomach
break the nitrites down into nitric oxide gas. Hydrochloric acid in the stomach
is a well-known part of the body鈥檚 defence against the bacteria that can cause
food poisoning, says Benjamin. But though the acid alone may stop bacteria from
multiplying, it cannot kill them.
Benjamin has now discovered that the combination of nitric oxide and acid is
highly effective in destroying harmful bacteria such as salmonella and shigella
(Antimicrobial Agents and Chemotherapy, vol 40, p 1422). The nitrate
found in saliva may therefore be part of the body鈥檚 defence against disease.
鈥淭he mechanism is a rare example of a symbiotic relationship between bacteria
and higher organisms,鈥� says Benjamin. 鈥淚t鈥檚 the first time that bacteria on the
body surface (in this case the tongue) have been shown to be
beneficial鈥攈elping to destroy other microbes in a different part of the
产辞诲测.鈥�
Although some nitrates are synthesised naturally in the body, those in saliva
come mostly from the diet. 鈥淪o it would be unwise to restrict the levels found
in food until researchers have worked out whether nitrate supplements are useful
in protecting against infection,鈥� says Benjamin.
Ron Walker, professor of food science at the University of Surrey, agrees.
鈥淭here is no evidence of nitrates in vegetables having any adverse effects, and
in the light of Benjamin鈥檚 work, there may even be advantages.鈥�