Aim: To examine change in the gut community of rats fed high amylose maize starch (HAMS). Methods and Results: Rats were fed AIN93G diets containing HAMS (5% resistant starch type 2) or alphacell (control). HAMS increased faecal shortchain fatty acid output, faecal propionate and total bacteria output but reduced gut pH and blood urea concentrations compared with rats ingesting the control diet. Feeding HAMS resulted in a gut community dominated by four phylotypes homologous with Ruminococcus bromii, Bacteroides uniformis and with yet to be cultivated organisms aligning into the Family Porphyromonadaceae. Enrichment of phylotypes aligning within the Bacteroidetes occurred primarily in the caecum, whereas those homologous with R. bromii were found primarily in the faeces. HAMS altered community structure such that the phylum Bacteroidetes represented the dominant gut lineage and progressively reduced faecal community phylotype richness over the duration of feeding. Conclusions: Feeding HAMS resulted in a caecal and faecal community dominated by organisms that require ammonia as a primary nitrogen source. Gut ammonia derived from endogenous urea represents an important factor contributing to caecal community composition in addition to the ability to utilize HAMS. Increases in faecal propionate, rather than butyrate as is often observed following resistant starch feeding, reflected a gut community dominated by the Bacteroidetes. Significance: Diet-mediated change is often viewed strictly in terms of available carbohydrate. Here, we have shown that ammonia derived from endogenous urea is an important factor contributing to gut community composition and structure in rats fed this substrate. IntroductionResistant starches (RS) are starches or products of starch degradation, which are not digested or absorbed in the small intestine and pass into the colon where they become available as substrate for the resident microbiota (Asp et al. 1992). Resistant starches are currently classified into four categories according to their physical and chemical properties (Topping and Clifton 2001). RS type 1 (RS 1 ) is physically inaccessible to the enzymes of the small intestine, RS type 2 (RS 2 ) is native granular starch that is difficult to hydrate, RS type 3 (RS 3 ) is retrograded amylose, and RS type 4 (RS 4 ) is resistant due to chemical modification. The fermentation of RS, as with other carbohydrates, has been associated with various physiological benefits including increased faecal short-chain fatty acid

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