What is interesting is that the background to our studies is the well-recognised variation in colon cancer risk around the world. I had the privilege of working as a gastroenterologist in Africa and in the USA and in rural Africa we do colonoscopies and we never find polyps or cancer. In African Americans nearly 50% of them would have polyps at the time of first colonoscopy. So we knew there was a dramatic difference and we also knew that diet, certain dietary items, are associated with high risk and low risk. So therefore we went into the community and studied differences in the diet.
We confirmed that the low risk in Africans was associated with the intake of high fibre foods, a large quantity of fibre, about 50g a day and that the high risk in African Americans was associated with the reverse, with a high meat, high fat and low fibre diet. What we didn’t know was the way that diet affects colon cancer risk. We worked on the hypothesis that the diet affects the microbiota, the colonic microbiota, to produce metabolites which might be good and healthy for the colon or might produce inflammation and increase neoplastic change. So therefore we managed to get funding to go out to Africa and perform these studies.
The NIH supported the pivotal study in which we switched the dietary intakes of a group of high risk African Americans and low risk rural Africans. We housed everybody under tight conditions so that we actually cooked all the food and we served the food so we knew exactly what was eaten. So we knew exactly that we had done the big dietary change that we wished to do. The results were astounding. Within that two weeks biomarkers of cancer risk measured by biopsy and by colonoscopy showed suppression of cancer risk markers in African Americans given the African diet and the reverse in Africans given the Westernised diet. In fitting with our hypothesis that dietary effects are mediated through the microbiota we found, indeed, that the microbiota changed reversibly at the same time with an increase in butyrate producers in African Americans given the high fibre diet and a reduction in Africans given the Westernised diet.
We also found that colonic bile acids, which are also associated with colon cancer because if you have a high fat diet your liver produces a lot of bile acids which go into the colon and are changed, deconjugated, by the microbes to secondary bile acids which are highly carcinogenic. So reciprocally we found that Africans given the African American diet had a dramatic increase in secondary bile acids in the colon and the reverse in African Americans given the high fibre diet.
What would you recommend from seeing these results?
We’re really concerned that the USDA requirements for fibre are way below what they should be. At the moment it’s 22g of fibre per day for women and 35g for men and our studies, together with a whole lot of experimental studies that we haven’t been over, indicate that the ideal intake level should be at least 50g of fibre a day. So therefore in general the way that you achieve that is to increase your intake of fibre rich foods such as beans, fruits and vegetables. If we do this we’ll get back to what we evolved to take, long before the industrial revolution removed fibre from our diets.
What can you tell us about the diets of native Alaskans?
We’re equally concerned, or even more concerned, that Eskimos, now termed native Alaskans, have got the highest risk of colon cancer in the world. We previously thought it was very low because they have a high fish intake, lots of omega-3 fatty acids which are anti-inflammatory. But the data shows that they’ve got the highest risk of colon cancer worldwide, over 100 per 100,000. We’re working on the hypothesis that although their intake of high biological value meats and fish oils is good, they have a chunk missing from their diet which is fibre-rich foods. With Westernisation they continue to eat a lot of meat and fat but they don’t eat the fresh berries and things that they used to before. So we’re doing intervention studies to see if we can increase fibre supplementation to reduce cancer risk and prevent colon cancer deaths.