A new classification of dietary substance must be invented to describe chemicals in food that were previously regarded as toxic but are increasingly implicated in protecting the body against diseases such as cancer, say scientists.

The substances are widespread in fruit and vegetables and often impart flavour. But they do not come under the list of essential nutrients because a lack of them does not cause a specific disease.

Yet scientists fear that just as the importance of these "nutrients" is being recognised, they are being tampered with by genetic plant manipulation. And no one knows what the consequences of this might be for human health.

Scientists at the Institute of Food Research in Norwich are studying chemicals produced by plants as natural pesticides to protect them against attack.

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The fascination with these chemicals stems from a clash between big-scale studies of food and cancer and biochemical findings about food. There is now overwhelming epidemiological evidence that a high intake of fruit and vegetables protects against many cancers. Yet the same foods contain natural toxins which, when added to human cells in a test tube, cause them damage and often appear to be carcinogenic.

Ian Johnson has been exploring this paradox. "Brussels sprouts and broccoli are full of chemicals which are potentially toxic," he said. "We can see damage to cells when you take the isolated chemicals from plants." Yet the vegetables themselves are not toxic.

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Potatoes contain glycoalkaloids: "These are very toxic. Probably the safety level is only five to ten times above natural intake," says Dr Johnson. Such low safety marginswould never be permitted for industrial food additives.

Dr Johnson and his team have tested allyl isothiocyanate, which is found in vegetables like broccoli and brussels sprouts and gives the taste to mustard. They added the chemical to two types of human cancer cell in a test tube. One type had been specially treated so that it had lost its capacity for uncontrolled growth and so was more "normal" than the other type.

The "normal'' cells were far less sensitive to the toxic effects of allyl isothiocyanate. Perhaps it is a potential anticarcinogen, targeting cancer cells in preference to normal cells.

"It's a highly significant difference in vitro. What we haven't done yet is confirm that it works in vivo," said Dr Johnson. "The substances are probably present in the diet at low concentrations which are sub-toxic but the fact that cells are being exposed to them suggests that they might be protective."

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It is possible that the chemicals trigger the body's natural de-fences in the liver and other tissues. "Continuous exposure to these things in the diet may perk up these defence mechanisms which are necessary for dealing with drugs and environmental contaminants," he said.

He has coined the term "phytoprotectants" for the substances. "The growing realisation that such compounds exert biochemical and physiological effects in humans raises important theoretical and practical issues." The research has implications for cancer and for genetic manipulation of plants.

"It will clearly be of interest to determine whether other compounds with demonstrated cytotoxic activity might also be selective. And If so, whether such compounds might have a practical role to play as food-borne protective factors in chemoprevention, or as chemotherapeutic agents.' he writes in a forthcoming edition of Nutrition Research Reviews.

The level of human intake of these toxins may turn out to be crucial, yet their concentrations are being altered by genetic manipulation of plants.

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"There is a desire to obtain crops that have natural defences against pests," said Dr Johnson. "A lot of these 'phytoprotectants' are natural pesticides. We have to be sure that we're not increasing the levels of toxic ones. It is unclear whether any of the compounds identified experimentally as potential naticarcinogens would represent a real hazard if their intake were increased."