Scientists examined what happened when American and African volunteers swapped diets for just two weeks - Western diets are typically high in protein and fat but low in fibre, while African diets are high in fibre and low in fat and protein.
The research, published in the journal Nature Communications, confirmed that a high fibre diet can substantially reduce risk, and also revealed that bacteria living in the gut can play an important role in this effect.
An international team of scientists from the University of Pittsburgh and Imperial College London carried out a study with a group of 20 African American volunteers and another group of 20 participants from rural South Africa.
The two groups swapped diets under tightly controlled conditions for two weeks.
The volunteers had colonoscopy examinations before and after the diet swap. The researchers also measured biological markers that indicate colon cancer risk and studied samples of bacteria taken from the colon.
Intriguingly, at the beginning of the study almost half of the American subjects had polyps – abnormal growths in the bowel lining that may be harmless but can progress to cancer.
Meanwhile none of the African volunteers had these abnormalities.
But after two weeks on the African diet, the American group had significantly less inflammation in the colon and reduced cancer risk. While in the African group, measurements indicating cancer risk dramatically increased.
Colon cancer is the fourth most common cause of death from cancer worldwide.
It accounts for more than 600,000 deaths per year and in the West, colon cancer rates are far higher than in Africa or the Far East.
Professor Jeremy Nicholson, the team leader from the Department of Surgery and Cancer at Imperial College London, said: “We can’t definitively tell from these measurements that the change in their diet would have led to more cancer in the African group or less in the American group, but there is good evidence from other studies that the changes we observed are signs of cancer risk.
“The findings suggest that people can substantially lower their risk of colon cancer by eating more fibre."
He added that the finding is not new in itself. But what surprised by how "quickly and dramatically the risk markers can switch in both groups following diet change".
"These findings also raise serious concerns that the progressive westernisation of African communities may lead to the emergence of colon cancer as a major health issue,” he added.
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Previous studies on Japanese migrants in Hawaii found that it only takes one generation of westernisation to alter their risk of colon cancer.
Meanwhile Professor Stephen O’Keefe at the University of Pittsburgh, who directed the study, added: "Our study suggests that westernisation of the diet induces changes in biomarkers of colon cancer risk in the colonic mucosa within two weeks.
"Perhaps even more importantly, a change in diet from a westernised composition to a ‘traditional African’ high fibre low fat diet reduced these biomarkers of cancer risk within two weeks, indicating that it is likely never too late to change your diet to change your risk of colon cancer.”
The study was funded by the National Institutes of Health in the US and the National Institute for Health Research Imperial Biomedical.