Despite having similar genetic backgrounds, African Americans are thirteen times more likely to develop colon cancer than rural South Africans. Indeed, environmental factors, rather than genetics, are thought to be the major factor in developing colon cancer, because recent immigrants’ children’s risk is more similar to where they are living than to their parents’ homeland. This environmental risk could be primarily caused by a number of factors, such as antibiotic use or drug use, but many scientists believe that diet, and its influence on the microbiome, is primarily responsible. As it turns out, rural Africans eat much more fiber (almost 5x more) and much less fat (almost 3x less) than African Americans, and these differences have drastic effects on the microbiomes of their hosts. Not only are the most abundant bacterial species different, but the major metabolites vary greatly as well. Scientists from the University of Pittsburgh came up with the clever idea of swapping the foods of rural South Africans and African Americans, to investigate how this dietary intervention would affect each group’s microbiomes and risk for colon cancer. They published the results of their study in Nature Communications last week.
The researchers studied 20 middle aged African American men and 20 middle aged rural South African men. They each had their microbiomes and colons studied for two weeks while eating their normal diets, and then again for two weeks after swapping diets. Initially, the Americans had microbiomes dominated by Bacteroides and the Africans by Prevotella. After the diet though, they noticed a rapid shift in these populations, and it corresponded to an increase in colonic inflammation for the Africans and decrease in the Americans. In addition, an increase in butyrate, the short chained fatty acid (SCFA) that is thought to be beneficial to health, followed the fiber diet as well, and a decrease was associated with eating the high fat diet; this makes sense, as butyrate is produced as a metabolite of fiber fermentation by the microbiome. Interestingly, prior to the diet change a top-level analysis of all the metabolic end products of the microbiome showed that Africans produced more of every single one studied except for choline, which is related to heart disease. Many of the metabolites studied, including choline, followed their diet switch, and were produced according to the food eaten, rather than the person eating it. Perhaps most importantly, secondary bile acids, which are produced by the microbiome and may be carcinogenic and an important cause of colon cancer, followed the diet as well. Africans, who produced much fewer secondary bile acids than Americans while consuming their regular diet, had a 400% increase in production after the diet switch, and vice versa for the Americans, who had a 70% decrease.
This study really illustrates the importance of diet on the output of the microbiome. These metabolites can directly influence our health, and may be more important to our well-being than the bacteria that produce them. According to this study, it appears that eating more veggies and less fat, something that parents have been saying for a long time, fits in with our understanding of the microbiome. As Erica Sonnenburg said in our podcast 3 weeks ago, “Feed your microbiome at every meal!”