<qa>  What is the role of gut microbiota in determining host metabolism?The role of gut microbiota in determining host metabolism is a topical area of interest, following discoveries that lean and obese phenotypes can be transferred by gut microbial transplantation in rodent models. Despite the excitement surrounding this area, it remains a highly controversial research field, and the extent to which the human microbiome influences energy metabolism is hotly debated. A variety of microbial metabolites, including short chain fatty acids and indole, however, modulate gut hormone secretion, and likely contribute to basal gut hormone release in the fasting state. An interesting question that has not previously been addressed is whether gut hormone secretion becomes more dependent on microbial metabolism after bypass surgery, either because resident colonic bacteria are supplied with an increased load of fermentable nutrients, or because of increased bacterial growth in the proximal intestine small intestinal bacterial overgrowth. It is, however, well established that microbial metabolism is altered in some post-surgical patients, as assessed non-invasively by the appearance of raised hydrogen or methane levels in exhaled breath. It is also possible that the differing pool of bacteria between people may contribute to a varied metabolic phenotype - it is therefore important to characterise the microbiome to assess changes arising due to surgery or antibiotic treatment. 