By Kate Kelland LONDON (Reuters) – Black Americans who switched to a high-fiber African diet for just two weeks saw a dramatic drop in risk factors for colon cancer, a study published on Tuesday found. A group of Africans who went the other way and started eating American food rich in animal proteins and fats saw their risks rise over the same short period, according to the paper in the journal Nature Communications. Researchers said they were not surprised that eating more fiber appeared to lower colon cancer risk, but were struck by how quickly and dramatically the effects showed. The findings raised concerns about Western diet and about how the increasing “Westernization” of diets in Africa could turn colon cancer into a major health issue there, said Jeremy Nicholson from Imperial College London who co-led the study.