Colon cancer remains the #1 cancer killer for nonsmokers in the USA. Among Americans, the highest prevalence of colon cancer occurs among those of African descent. However, this is unlikely to be due largely to genes as the prevalence of colon cancer is about 13 times higher in African Americans than it is in Black rural South Africans. The far higher incidence of colon cancer for African Americans is likely associated with their far higher consumption of animal protein and fat, coupled with a much lower fiber consumption than found in the rural South African diet. Diets high in meat and fat and low in fiber lead to increased levels of colonic secondary bile acids, lower amounts of colonic short-chain fatty acids, and higher mucosal proliferative biomarkers of increased cancer risk in otherwise healthy middle-aged volunteers. To shed more light on how a typical American diet promotes the changes that are believed to lead to the development of colon polyps and colon cancer, a research team led by Dr. Brian O’Keefe at the University of Pittsburg performed a study that involved 2-week food exchanges in subjects from the same populations. In this study, a group of 20 African Americans were fed a high-fiber, low-fat African-style diet, while another group of 20 rural South Africans were fed a high-fat, low-fiber American-style diet. The researchers closely supervised what both groups ate for the next two weeks to assure good compliance with the two very different diet plans. The subjects of this study all underwent a colonoscopy exam before and then again two weeks after the diet swap. At the start of the study, nearly half the American subjects had polyps in their colons, but none of the South Africans had polyps. The majority of colon cancers arise from these precancerous polyps.
When the rural South Africans and African Americans adopted each other’s typical dietary habits, these dietary changes resulted in remarkable reciprocal changes in mucosal biomarkers of cancer risk. After just two weeks on the African diet, the American subjects’ colons showed a reduction in inflammation. In addition, the researchers observed that changes in the cells lining the colon were accompanied by major changes of gut bacteria or microbiota. The changes in these gut microbes are believed to have a major impact on the risk of developing inflammation and polyps, and eventually colon cancer. For example, the increase in saccharolytic fermentation and butyrogenesis observed when the Americans adopted the Africans’ diet resulted in suppressed secondary bile acid synthesis. These changes were observed in the African Americans within just two weeks after adopting the rural South African diet that was low in fat and animal products but high in whole plant foods. The reverse changes occurred in the rural South Africans who adopted the typical American diet for two weeks (1).
The results of Dr. O’Keefe’s recent study confirm and extend the findings of a study done decades ago by Dr. R. James Barnard at UCLA. Dr. Barnard and colleagues looked at changes in the amount of secondary bile acids in the stools of people who came to the Pritikin Longevity Center. Prior to attending Pritikin, 11 female subjects kept a 3-day food diary. One year after attending Pritikin, they filled out another 3-day food diary.The diaries showed that these women did a pretty good job of sticking with the largely unprocessed plant-based diet. According to their diet diaries, they maintained calorie intake at a slightly higher level (7% more on average) than while staying at Pritikin Center, but still significantly lower (by 34%) than their pre-Pritikin diaries. Their diets prior to their stay at the Pritikin Center contained 34% fat and 360mg of cholesterol or close to typical American diet norms at that time. Post-Pritikin, at one-year follow-up, calories from fat averaged only 14% of energy intake and daily dietary cholesterol intake dropped to only 88 mg. Fecal secondary bile acids were also significantly lower after attending Pritikin and these lower levels were also observed a year later. Not surprisingly, serum cholesterol levels were significantly lower at end of their 3 weeks at the Pritikin Longevity Center. But perhaps more importantly, their serum cholesterol levels were also lower a year later while following the diet at home, confirming that the subjects were likely still largely complying with a largely plant-based diet far lower in saturated fat and cholesterol than a typical American diet. These results suggested that switching from a typical American diet high in animal fat and cholesterol and low in fiber to a low-fat, high-fiber diet largely plant-based diet can reduce the excretion of secondary bile acids which are increasingly thought to be involved in the promotion of colon cancer (2).
Bottom Line: The results of these two studies show that if Americans switch from their typical modern diets and stick with a diet that is far lower in fatty animal products and cholesterol and composed largely of whole grains, vegetables, and fruits, then they will dramatically alter the microbiota of their colons and reduce the inflammation and other chemical changes that promote colon cancer.
- O’Keefe SJ, Li JV, Lahti L, et.al. Fat, Fibre and cancer risk in African Americans and rural Africans. Nature Communications April 2015. doi:10.1038/ncomms7342.
- Reddy, B. S., A. Engle, B. Simi, L.T. O’Brien, R.J. Barnard, N. Pritikin, E. L. Wynder. Effect of low-fat, high carbohydrate, high-fiber diet on fecal bile acids and neutral sterols. Preventive Medicine.17:432-439, 1988.
Stephanie Ronco has been editing for Food and Health Communications since 2011. She graduated from Colorado College magna cum laude with distinction in Comparative Literature. She was elected a member of Phi Beta Kappa in 2008.