US scientists have questioned whether a high fibre diet can reduce the risk of bowel cancer, but UK experts said that there was research which shows a benefit, the BBC reported.

An analysis of data from 13 studies published in the Journal of the American Medical Association (JAMA) found there was no added protection.

Studies which have examined whether eating fibre can reduce the risk of cancer have reached conflicting results, largely depending on how they have been carried out, the researchers from Harvard Medical School suggest.

They say studies which asked cancer patients and healthy people to look back and describe their diet have found a link between low fibre intake and cancer risk. But prospective studies, which looked at people's diet and then followed them to see if they developed cancer, have not found a link.

The Harvard team looked at 13 prospective studies which looked at a total of 725,628 men and women whose progress was monitored for six to 20 years. Over 8,000 developed bowel cancer.

People in Europe tended to get most of their fibre from cereals, while in North America, fruit and vegetables are the main sources. On first analysis, those who ate most fibre looked to have a 16% lower risk of bowel cancer.

But when the researchers also considered factors such as folic acid and milk intake (which research suggests reduce risk), and red meat and alcohol intake, (which can increase risk), no association between fibre intake and reduced cancer risk was seen.

So people may eat fibre, but they may also eat other foods which could explain their cancer risk "status".

Fibre intake from cereals, fruits, and vegetables was not associated with any effect on colorectal cancer risk.

But writing in JAMA, the team led by Dr Yikyung Park, said: "Although high dietary fibre intake may not have a major effect on the risk of colorectal [bowel] cancer, a diet high in dietary fibre from whole plant foods can be advised because this has been related to lower risks of other chronic conditions such as heart disease and diabetes."

In an editorial in the same journal, Dr John Baron, of Dartmouth Medical School in New Hampshire, said short-term studies appeared to suggest there was no effect from high dietary fibre intake on bowel cancer risk.

"Understanding longer-term relationships with any type of fibre will require more work," he said. "Studies like that of Park et al provide valuable help, but unfortunately there is more to do."

"We are convinced of the value of a high fibre diet in the prevention of bowel cancer," said a spokeswoman for Bowel Cancer UK. "Two large-scale international studies - one involving more than 500,000 people - in 2003, both strongly supported the link. In the largest study, consuming 35g of fibre daily reduced the risk by up to 40%."

Professor Sheila Bingham of the European Prospective Investigation into Cancer and Nutrition consortium (EPIC), which conducted one of the studies, said: "People with the most reduction were eating seven portions of fruit and vegetables per day, similar to the amount eaten by Mediterranean populations, and the equivalent of five slices of wholemeal bread."

She said the JAMA research was "a reanalysis of old studies" which had been overtaken by other studies, including EPIC.