A high fibre diet can substantially reduce the risk of colon cancer according to a study published in Nature Communications by Imperial College London and the University of Pittsburgh.

Colon cancer is the fourth most common cause of death from cancer worldwide, accounting for more than 600,000 deaths per year. Colon cancer rates are much higher in the western world than in Africa or the far east, yet in the US African Americans suffer the most from the disease.

An international study of 20 African American volunteers and another group of 20 rural South Africans swapped diets under tightly controlled conditions for two weeks. The volunteers had colonoscopy examinations before and after the diet swap. Biological markers that indicate colon cancer risk were also measured, and samples of bacteria taken from the colon were studied.

At the start, when each group had been eating a regular diet, almost half of the American subjects had polyps – abnormal growths in the bowel lining that may be harmless but can progress to cancer. None of the South Africans had these abnormalities. The American diet is classified as high in protein and fat but low in fibre.

After two weeks on the African diet – high in fibre and low in fat and protein – the American group showed significantly less inflammation in the colon and reduced biomarkers of cancer risk. The South Africans, however, showed a dramatic increase in cancer risk after two weeks on the western diet.

While the study provided further evidence for the fact that people can lower their risk of colon cancer by eating more fibre, the rapid and dramatic time it takes for risk markers to switch surprised the researchers.

“It is likely never too late to change your diet to change your risk of colon cancer,” said Professor Stephen O'Keefe of the University of Pittsburgh.

The study found that a major reason for the changes in cancer risk was the way in which the bacteria in the gut – known as microbiome – altered their metabolism to adapt to the new diets. In the American group, the researchers found that the South African diet they tried for two weeks led to an increase in the production of butyrate, a byproduct of fibre metabolism that has important anti-cancer effects.

This research emphasises that therapies and remedies targeting gut bacteria are the way forward for the prevention and treatment of cancer.