lean individuals and the impact ofdietary intervention on microbiota, obesity and diet, showed differences in faecal microbiota. Furthermore obese individuals had Molecular characterisation of microbiota and fermentation end-products of lean and obese humans on open recorded diet, and obese humans on saturated fat-rich The human microbiota has recently been implicated in the new obesity metabolic syndrome (MS) biomarkers. epidemic sweeping the world. This thesis explored the gut microbiota of obese and higher faecal short chain fatty acid (SCFA) concentrations (p<0.01, n=13). Additionally, the three groups presented distinct faecal and urine IH-NMR metabolite profiles. C57b16/J mice fed a high fat-diet (HF) for 4 weeks had fewer ceacal Eubacterium rectale/Clostridium coccoides group, Bacteroides-like Mouse Intestinal Bacteria, and Bifidobacterium spp. (p<0.001) compared to standard chow-fed mice. This altered microbiota was concomitant with increased plasma pro-inflammatory cytokine levels, endotoxernia, insulin resistance and obesity. Prebiotic supplementation (oligofructose, 10% wt/wt, 14 weeks) significantly increased caecal bifidobacteria (p<0.001) in HF-fed mice. This was positively correlated with improved MS risk factors. In humans at risk ofMS (n=88), modulating the type and quantity of dietary fat (i.e. saturated vs monounsaturated fat) and carbohydrate (Le. high vs low glycaemic index (GI)) impacted on the gut microbiota. Twenty-four weeks on test diets with increased carbohydrate and reduced fat content, irrespective of dietary GI, increased faecal bifidobacteria (p=O.OOS, p=O.OS2) and decreased fasting glucose (p==O.OlS, p==O.034), total-cholesterol (p=O.032, p==O.023) and LDL-cholesterol (p==O.043, p=O.043). In the high GI group fasting insulin concentrati