The researchers say increasing the amount of dietary fiber likely led to the reductions in colon cancer biomarkers.
The international team reports the findings in Nature Communications.
Colon or colorectal cancer is the third most common cancer in the world, with nearly 1.4 million new cases diagnosed in 2012. It is the fourth most common cause of cancer death, accounting for over 600,000 deaths a year.
Rates of colon cancer are much higher in the western world than in Africa and the Far East, yet in the US, the highest rates occur among African-Americans.
For the study, 20 African-American and 20 rural South African volunteers aged 50-65 spent 2 weeks under controlled conditions where they ate only each other's diets.
The research team examined fecal and colon content sampled from each participant at the start and the end of the diet swap. The participants also underwent colonoscopy exams at the start and end of the period.
Before the diet swap, the researchers had spent time with the participants in their own surroundings to learn about their diets and the ingredients.
They then prepared and gave the participants meals using cooking ingredients and methods typical of the other group.
The study took place at a university site in the US and a lodging facility in South Africa, allowing the researchers to control for the influence of smoking and other environmental factors on the cancer risk measurements.
African diet contains more soluble fiber, less animal protein and fat
Lead researcher Stephen O'Keefe, professor of medicine at the University of Pittsburgh's Division of Gastroenterology, Hepatology and Nutrition, says:
"The African-American diet, which contains more animal protein and fat, and less soluble fiber than the African diet, is thought to increase colon cancer risk."
He notes that research on Japanese migrants to Hawaii shows after only one generation, Westernization can change a low rate of cancer to the higher one seen in native Hawaiians.
The results showed that despite the brief period of the diet swap, each group took on the other group's indicators for colon cancer risk. These included levels of fiber fermentation, turnover of cells in the lining of the gut, markers of metabolic activity in gut microbes and inflammation.
Particularly marked was the increase in butyrate production in the gut of the African-Americans after 2 weeks on the African diet. Butyrate is a byproduct of fiber metabolism with important anticancer properties.
This finding suggests the diet swap had a significant effect on the bacteria in the gut - the gut microbiome. The bacteria's metabolism changed to adapt to the new diet.
The researchers also note that in the colonoscopy exams, they found and removed polyps in nine of the African-American volunteers but found none in the Africans.
Concern that 'progressive Westernization' may lead to emergence of colon cancer
Co-author Jeremy Nicholson, a professor in the Faculty of Medicine at Imperial College London in the UK, says the results cannot prove that the dietary changes would have resulted in more cancer in the African group and less in the American group, but there is good evidence from other studies that the changes they found are signs of cancer risk.
Prof. O'Keefe says increasing the amount of dietary fiber - from around 10 grams to over 50 grams for the African-American group - likely led to the biomarker changes. But eating less animal fat and proteins probably also had an effect. He concludes:
"In just 2 weeks, a change in diet from a Westernized composition to a traditional African high-fiber, low-fat diet reduced these biomarkers of cancer risk, indicating that it is likely never too late to modify the risk of colon cancer."
Prof. Nicholson adds:
"These findings also raise serious concerns that the progressive Westernization of African communities may lead to the emergence of colon cancer as a major health issue."
Funds for the study came from various countries including the US National Institutes of Health, the National Institute for Health Research Imperial Biomedical Research Centre in the UK and the European Research Council.
Meanwhile, Medical News Today recently reported how one researcher is warning that taking too many dietary supplements may raise risk of cancer.
Addressing the 2015 annual meeting of the American Association for Cancer Research, Dr. Tim Byers, of the University of Colorado Cancer Center, said while dietary supplements can offer health benefits, "there is no substitute for good, nutritional food."