Common foods from burnt toast to low-fat salad dressing have been linked to increased cancer risk.

But US scientists have now warned that many reports connecting familiar ingredients with increased cancer risk have little statistical significance and should be treated with caution.

"When we examined the reports, we found many had borderline or no statistical significance," the Guardian quoted Dr Jonathan Schoenfeld of the Harvard School of Public Health in Boston, as saying.

Writing in a paper in the American Journal of Clinical Nutrition, Schoenfeld and his co-author, John Ioannidis of Stanford University, said that trials have repeatedly failed to find effects for observational studies, which had initially linked various foods to cancer.

Recent reports have linked colouring in fizzy drinks, low-fat salad dressing, burnt toast and tea to elevated cancer risk. In the past, red meat, hot dogs, doughnuts and bacon have also been highlighted.

To examine the implications of these reports, Schoenfeld and Ioannidis selected ingredients at random from the Boston Cooking-School Cook Book.

"We used random numbers to select recipes and collected the ingredients from these" said Schoenfeld.

"This gave us a good range of common and a few not so common foods.

Then we put each of those ingredients into a search engine to find out their associations with cancer risks in medical literature. We found that 40 out of the 50 ingredients we had selected had been studied as having possible links with cancer. The 10 that had not been studied were less common ingredients," he added.

Among the 40 foods that had been linked to cancer risks were flour, coffee, butter, olives, sugar, bread and salt, as well as peas, duck, tomatoes, lemon, onion, celery, carrot, parsley and lamb, together with more unusual ingredients, including lobster, tripe, veal, mace, cinnamon and mustard.

Schoenfeld and Ioannidis then analysed the scientific papers produced after initial investigations into these foods. They also looked at how many times an ingredient was supposed to increase cancer risk and the statistical significance of the studies.

Their work suggested that many reports linking foodstuffs to cancer revealed no valid medical pattern at all.

"We found that, if we took one individual study that finds a link with cancer, it was very often difficult to repeat that in other studies. People need to know whether a study linking a food to cancer risk is backed up before jumping to conclusions," said Schoenfeld.