Researchers asked people to switch diets for two weeks – 20 US volunteers moved to a low-fat, high-fibre diet while 20 volunteers from rural Africa were asked to eat more “junk” food.
Although the swap was brief, its impact was visible, Nature Communication says.
The Americans benefited from less bowel inflammation, while the African volunteers’ bowel health deteriorated.
It is not possible to make any firm conclusions based on such a small study, say experts.
But the findings do support the belief that modern Western diets – which are high in fat and sugar and low in fibre – are bad for us.
Other studies with Japanese migrants to Hawaii have shown that it takes only one generation of Westernisation to change their low incidence of colon cancer to the high rates seen in native Hawaiians.
And research shows a high intake of dietary fibre, particularly cereal and whole grains, reduces bowel cancer risk, while eating red and processed meat increases the risk.
In the diet swap study, the Western-style diet given to the native African volunteers was typical junk or fast food – burgers and fries.
The US volunteers, meanwhile, where switched to a diet containing lots of pulses and beans.
All the participants had a barrage of medical tests before and after the diet change.
The dietary swaps appeared to cause significant changes to the cells lining the gut as well as the bacteria that live in the bowel – with the US volunteers faring better.
Lead researcher Dr Stephen O’Keefe, from the University of Pittsburgh, said: “In just two weeks, a change in diet from a Westernised composition to a traditional African high-fibre, low-fat diet reduced these biomarkers of cancer risk, indicating that it is likely never too late to modify the risk of colon cancer.”
Experts estimate that up to a third of bowel cancer cases could be avoided by eating more healthily.
A spokesman for Cancer Research UK said larger and longer term studies were still needed.
“The diet swap was also fairly drastic whereas we know that making small changes you can stick with long-term is far more effective to maintain a healthier lifestyle.”