Forty-eight prediabetic men and women (50-75 years of age) will participate in a 6-week feeding study in which they will randomized to receive either 10 g/day of inulin or placebo. All subjects will be fed an isocaloric diet (50% carbohydrate, 35% fat, 15% protein,) controlled for micronutrient content for 6 weeks to avoid the potential confound of individual differences in diet on gut microbiota. Measurements of intestinal permeability, insulin sensitivity, and skeletal muscle metabolic flexibility will be made prior to and following the controlled feeding period. Stool samples will be collected to assess gut microbial communities.