New doubts about what constitutes a healthy diet are raised today by two studies that question the value of fibre in preventing cancer.

New doubts about what constitutes a healthy diet are raised today by two studies that question the value of fibre in preventing cancer.

The studies, by American researchers, found no evidence that adding bran to breakfast cereals or eating extra fruit and vegetables reduced the recurrence of growths in the intestine that could lead to bowel cancer. The finding, published in the New England Journal of Medicine, is a blow to the low-fat, high-fibre dietary enthusiasts who claimed that a change in eating habits could halt the advance of chronic diseases in the West, such as cancer.

But scientists said yesterday that people should not throw out their packs of All Bran or cancel their fruit and vegetable orders. The studies were looking at patients with a specific condition - growths, called polyps, in their intestine - and did not necessarily apply to the general population.

Dr Mary Berrington, of the Cancer Research Campaign, said: "The people in the studies were already at higher risk of bowel cancer because they had had a polyp. Although the finding may be disappointing, it is not the same as saying that a high-fibre diet won't reduce bowel cancer in the population."

Fibre, the raison d'être of most breakfast cereals, has been a totem of the health establishment since Denis Burkitt observed in 1972 that bowel cancer was almost unknown among Africans whose staple diet was grains and vegetables. Dietary fibre has been thought to act as a kind of colonic broom, sweeping food quickly through the gut and diluting toxic chemicals that build up there. The lack of fibre in the average Western diet - which is low in vegetables, cereals and fruit - was thought to be the key to many chronic diseases.

The theory received a blow a year ago when one of the world's widest studies, of more than 88,000 nurses who were followed for 16 years, found no link between bowel cancer and the consumption of fibre. However, researchers pointed out then that fibre has other beneficial effects, such as protecting against heart disease, diabetes and other bowel disorders.

The latest studies examined the effect of modifying the diets of people who had already had polyps removed. Polyps can develop into cancer. In one study of 2,000 patients, by researchers from the US National Cancer Institute, half were put on a low-fat, high-fibre diet and half told to follow their usual diet. In the second study of more than 1,400 patients, led by researchers from the Arizona Cancer Centre, Tucson, half were given a high-fibre supplement. Neither group showed any reduction in the recurrence of polyps over three to four years but the ones taking fibre had some unpleasant gastro-intestinal side-effects.