Will eating more fibre cut your risk of bowel cancer?

By Lilian Anekwe

BMJ Group News

What do we know already?

Diet is believed to be an important part of whether people get colon and rectal cancers. The effect of reducing the amount of red and processed meat and alcohol in the diet has been studied in depth. But we are less sure about the effect of fibre and grains. We also don’t know what types or source of fibre, if any, are best to eat to prevent these cancers.

Experts aren’t yet sure what role fibre could play in preventing colon and rectal cancers. It’s possible that diets that are high in fibre and grains could dilute the cancer-causing chemicals in the digestive system. They could also reduce the time it takes food to be digested, meaning cancer-causing chemicals spend less time in the digestive system.

Researchers looked at all the evidence from 25 studies of nearly 2 million people, 14,000 of whom got colon or rectal cancer during the study period. The researchers collected information on what types of fibre people ate and how much, and then compared how likely people who included fibre in their diet were to have the cancers than people who did not include fibre in their diet.

What does the new study say?

People who included fibre in their diet were 12 percent less likely to have colorectal cancer than those who did not.

The risk of colorectal cancer fell by 10 percent for every 10 grams of fibre - about the same as five slices of wholemeal bread - that people included in their daily diet.

The risk of colorectal cancer also fell by 10 percent for every 10 grams of cereal fibre people included in their diets. This is roughly the same as two bowls of bran flakes.

People who had diets with large amounts of whole grain - such as brown rice and quinoa - were 17 percent less likely to have colorectal cancer than people with a diet with a lower intake of whole grain. But these benefits were seen in people who ate 90 grams of whole grain every day - the equivalent of 10 servings of wholemeal pasta.

How reliable is the research?

Large studies like this are a good way to find out if there is a link between diet and cancers. Studies that ask people about their diets and then count how many of those people then go on to get cancer are also an accurate source of information.

But we don’t know how much of an effect other differences in people’s diets, that weren’t accounted for in these studies, had on their risk of cancer. People who eat lots of fibre and whole grains may also be likely to have a healthy overall diet, and to exercise and follow a generally healthy lifestyle, which may also have contributed to a lower risk of colon and rectal cancer.

We’re also not able to tell how people’s diets were recorded in the different studies. If different foods were counted as fibre in different studies, or the amount of fibre was measured differently in different studies, this could have affected the reliability of the results.