Diet swap study suggests gut bacteria play a role in bowel cancer risk

In collaboration with the Press Association

The benefits of a high-fibre diet in reducing bowel cancer risk could be due to its effects on gut bacteria, according to a preliminary international study.

The findings, published in Nature Communications, add to growing evidence that certain gut bacteria affect the disease’s development, although experts cautioned that longer and larger studies were needed to confirm this. 

"This study found that swapping to a high fibre and low animal fat diet affected the levels of some biological markers of bowel cancer risk" - Tom Stansfeld, Cancer Research UK

Bowel cancer is known to be less common among people who eat a high-fibre diet. But the reason why has remained elusive. 

To find out more, the researchers measured changes in gut chemistry and biology among 40 people fed different diets for two weeks.

20 of them were rural South Africans who switched from a high-fibre, low-protein diet to a more ‘Western’ diet, low in fibre but high in protein and animal fat.

A further 20 African-Americans did the opposite, swapping their Western habits for a diet containing at least 50g fibre a day, and low in protein and animal fats.

Two weeks later, the Americans had lower levels of inflammation in their guts, and reduced levels of chemicals associated with cancer risk. 

The African group, on the other hand, had a dramatic increase in measurements linked to cancer. 

Further analysis revealed the chemical changes were mirrored by changes in the type of bacteria in the gut – particularly those that produce a chemical called butyrate.

Butyrate, produced by fibre-metabolising bacteria, has been shown in laboratory studies to suppress uncontrolled cell growth in bowel cells.

"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,” said co-author Professor Jeremy Nicholson, from Imperial College London, “but there is good evidence from other studies that the changes we observed are signs of cancer risk.”

“What is really surprising is how quickly and dramatically the risk markers can switch in both groups following diet change,” he added. 

The findings also raise “serious concerns” that the adoption of Western diets by African communities could lead to the emergence of bowel cancer as a major health issue on the continent, he said.

Tom Stansfeld, Cancer Research UK’s health information officer, said the study’s findings needed confirming in “larger and longer-term studies”.  

Making small changes over a long term period of time is “far more effective” than big changes in maintaining a healthier lifestyle, Stansfeld said, pointing out that the change in diet on the study was “fairly drastic”. 

A typical Western diet contains less than 15g of fibre per day.

As well as diet changes, there were other things people could do to reduce their risk, Stansfeld said. 

"Half of bowel cancers could be prevented through lifestyle changes - such as being a non-smoker, keeping  a healthy weight, drinking less alcohol, being physically active, as well as eating a diet low in red and processed meat and high in fibre.”


  • O'Keefe, SDJ et al, Fat, fibre and cancer risk in African Americans and rural Africans. Nature Communications (2015) DOI: 10.1038/ncomms7342