By Patricia Reaney

LONDON (Reuters) - Scientists said on Tuesday they may have found a reason why eating too much red meat increases the risk of colorectal cancer.

By studying cells from volunteers eating different diets, they discovered that red meat raises levels of compounds in the large bowel which can alter DNA and increase the likelihood of cancer.

"It is the first definite link between red meat and the very first stage in cancer," said Professor Sheila Bingham, of the Medical Research Council Dunn Nutrition Unit in Cambridge, England.

In earlier research, Bingham and her team showed there was a strong correlation between eating red and processed meat and the risk of colon cancer.

The chance of developing colorectal cancer was a third higher in people who regularly ate more than two portions of red or processed meat a day compared to someone who ate less than one portion a week.

In their latest study, published in the journal Cancer Research, the scientists studied cells from the lining of the colon from people who consumed red meat, vegetarian, high red meat or high fiber diets for 15 days.

"We looked at whether eating red meat alters the DNA of these cells," Bingham told Reuters.

They found that red meat consumption was linked to increased levels of substances called N-nitrosocompounds, which are formed in the large bowel. The compounds may stick to DNA, making it more likely to undergo mutations that increase the odds of cancer.

The DNA damage may be repaired naturally in the body, and fiber in the diet may help the process. But if it isn't, cancer can develop, Bingham said.

The scientists said the findings could help to develop a screening test for very early changes related to the disease.

Colorectal is one of the most common cancers in developed countries. More than 940,000 cases are diagnosed each year and about 492,000 people die from the illness, according to the International Agency for Cancer Research (IARC) in Lyon, France.

A diet rich in fat, animal protein and refined carbohydrates and lack of exercise are risk factors for the illness. Most cases are in people over 60 years old and about 5 percent of them are inherited.

Health experts estimate that about 70 percent of colorectal cancers could be prevented by changes in diet and nutrition. Diarrhea, constipation and rectal bleeding can be symptoms.