The human oro-gastrointestinal (GI) tract is a complex system, consisting of oral cavity, pharynx, oesophagus, stomach, small intestine, large intestine, rectum and anus, which all together with the accessory digestive organs constitute the digestive system. The function of the digestive system is to break down dietary constituents into small molecules and then absorb these for subsequent distribution throughout the body. Besides digestion and carbohydrate metabolism, the indigenous microbiota has an important influence on host physiological, nutritional and immunological processes, and commensal bacteria are able to modulate the expression of host genes that regulate diverse and fundamental physiological functions. The main external factors that can affect the composition of the microbial community in generally healthy adults include major dietary changes and antibiotic therapy. Changes in some selected bacterial groups have been observed due to controlled changes to the normal diet. The interactions between dietary factors, gut microbiota and host metabolism are increasingly demonstrated to be important for maintaining homeostasis and health. Therefore the aim of the review of Maukonen and Saarela (2014) was to summarise the effect of diet, and especially dietary interventions, on the human gut microbiota.
The answer to the question posed in the title: "Does diet matter in regard to human microbiota?" is: "Yes, it does". From the host's perspective, there are numerous activities of the commensal gut microbiota that are of great importance to health. Moreover, by choosing what we eat, we can decide which bacteria we feed. However, even though diet matters, the results from dietary interventions are not always straight forward. It should be remembered that the detected effect is dependent on the study subjects, study protocols, used DNA-extraction techniques, used methodologies, and in case of next-generation sequencing also the algorithms used for cleaning and analysing the data. In addition, individual variation in human intestinal microbiota is so wide that the subtle changes may not be detected if the study cohort is not large enough. Therefore, studies in which different habitual diets have been compared with each other, usually get clearer correlations with nutrients v. bacteria than those observed in dietary interventions.
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Reference: Maukonen J., Saarela M. Human gut microbiota: does diet matter? Proc Nutr Soc. 2014 Aug 26:1-14. [Epub ahead of print]
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