A diet high in fibre-rich foods such as porridge, brown rice and cereal cuts the risk of bowel cancer, according to an analysis of 25 studies.
Experts said cereal fibre and whole grains in particular cut the risk but found "no significant evidence" of a reduction for fibre in fruit, vegetables and legumes such as lentils and beans.
The study, published online in the British Medical Journal (BMJ), found that for every 10g a day increase in fibre intake, there was a 10% drop in risk of bowel cancer.
So people who already had a diet containing 5g of fibre a day had a 10% reduction in risk if they ate 15g a day.
There was also a 20% reduction in risk for every three servings a day (90g a day) of whole grains.
Whole grain foods include whole grain breads and cereals, brown rice and porridge.
The researchers, including experts from Imperial College London and the University of Leeds, said: "Our results indicate a 10% reduction in risk of colorectal cancer for each 10 g/day intake of total dietary fibre and cereal fibre and about a 20% reduction for each three servings (90 g/day) of whole grain daily, and further reductions with higher intake."
The lifetime risk of developing bowel cancer for men is around one in 18. For women, it is around one in 20.
Previous studies have produced mixed results on whether fibre reduces the risk of bowel cancer.
The researchers said more studies were needed on different types of fibre and on people with different lifestyles and diets.
The latest combined research involved almost two million participants.
Around 38,500 cases of bowel cancer are diagnosed every year in the UK and the disease kills more than 16,000 people.
Dr Rachel Thompson, deputy head of science for the World Cancer Research Fund (WCRF), said: "Taken together with other evidence, the findings of this study provides strong evidence that foods containing fibre reduce risk of bowel cancer.
"This new study, which is part of our Continuous Update Project, is one of the main reasons that our expert panel now judges the evidence on the link stronger than before.
"This adds further weight to our recommendation that people who want to reduce their risk of cancer should eat more of a variety of vegetables, fruits, whole grains and pulses."
Deborah Alsina, chief executive of Bowel Cancer UK, said: "This valuable study provides further evidence that it is possible to stack the odds against bowel cancer.
"Simple changes to your diet and lifestyle can make a huge difference. Changes such as increasing your intake of fibre can genuinely help you feel healthier and reduce your risk of the disease.
"This is great news because it's achievable by most people and could have a big impact on the nation's health."