Epidemiology is the study of the way that disease relates to particular populations. This is a useful tool in nutrition because although cause and effect cannot be ascribed from observational epidemiological studies, they give a broad understanding of disease risk in specific populations. For example, it has been shown that African Americans living in the united states have a significantly higher risk of colorectal cancer than native Africans living in rural areas of Africa. Looking at the reasons for this discrepancy it becomes apparent that it might relate to diet or other lifestyle factors. In terms of nutritional content, the fibre content of the diet has been highlighted as a possible reason for this difference in disease rates. While those living in rural Africa consume a more traditional diet, those living in the united states generally consume a typical Western diet. The rural traditional diet of native Africans can vary, but estimates suggest that the fibre content is significantly higher than is present in the typical Western diet (perhaps up to 100 grams per day).
Fibre was once thought to simply provide roughage to the diet and in this way aided the transit of food through the gastrointestinal tract. This is essentially true, but more recent studies have shown that fibre plays a far more important role in health than was once considered and may actually be an essential nutrient for human health. In particular, fibre is now known to be able to influence the composition of the microbiota of the gut. This is possible because dietary fibre is used as a fermentative energy source by some bacterial populations that inhabit the colon and this can stimulate their growth. In addition, the products of this saccharolytic fermentation are short-chain fatty acids, such as acetate, propionate and butyrate. These short chain fatty acids are thought to lower the pH of the gut and this in turn alters the conditions of the gut, stimulating the and inhibiting the growth of various bacterial species. Diets high in meat by contrast may cause proteolytic fermentation that results in the formation of ammonia compounds and branched chain fatty acids that are inflammatory and enhance colon cancer risk.
Researchers have assessed the effects of colonic microbes and their metabolites in African Americans and native Africans in relation to colon cancer risk1. Faecal samples from 12 African Americans and 12 native African controls were collected and assessed for their short chain fatty acid and bacterial content. The results of the study showed that the bacterial composition of the faecal samples were fundamentally different between the two groups. Native Africans tended to have high concentrations of Prevotella, while African Americans had a predominance of Bacteroides. Native Africans also had significantly higher concentrations of the major butyrate producing groups of bacteria. The microbial gene coding for secondary bile acids were significantly higher in African Americans compared to native Africans while those genes coding for methanogenesis and hydrogen sulphide were significantly higher in native Africans. The colon cancer risk for these two populations might therefore be explained by differences in gut microbiota.
Not long ago the microbiota in the gut were considered of interest, but their influence to human health was not considered important. However, recent discoveries of how these bacterial colonies can influence health has opened up new areas of research. Under normal circumstances the balance between the health promoting gram positive bacteria and the disease causing gram negative bacteria is maintained. This balance is to a large extent reliant on the conditions in the gut which are in turn reliant on the composition of the diet. Eating a typical Western diet is known to shift this balance in favour of branched chain fatty acid and ammonia producing bacteria and this leads to inflammatory conditions within the colon. In addition, the gram negative bacteria colonies can produce toxins that further heighten inflammation because of the lipopolysaccharides they contain in their cell walls. More traditional diets supply high concentrations of fibre, and this fibre stimulated the saccharolytic fermentative species which increase production of short chain fatty acids, and the lower pH conditions that accompany this are inhibitive of disease causing species.
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