The prevalence of obesity has risen to epidemic proportions in the world, resulting from both excessive energy intake and low levels of energy expenditure. The effect of nutrient absorption on energy balance, that is, the relative amount of nutrients consumed vs. the amount excreted in stool, has been reported only in small studies in which energy waste in feces and urine between lean and obese individuals was not found to be different. New studies have shown that bacteria in the gut may play an important role in calorie absorption. We have recently shown that leaner individuals absorbed more calories when overfed compared to when they were given a diet with just enough calories to maintain their own weight. Our studies have also found that overfeeding also changes the kinds of bacteria found in the gut. In lean individuals, these changes in gut bacterial communities with overfeeding were associated with changes in how many calories were absorbed. Our results are similar to those seen in other studies in animals and humans that suggest a role for gut bacteria in weight gain and obesity. To try to better understand the role of gut bacteria in absorbing food, we propose to investigate 1) whether energy loss (as measured in stool and urine) changes following over- and underfeeding relative to body size and 2) whether changes in the gut bacteria, induced by an antibiotic medication, affect nutrient absorption and glucose tolerance. We plan to study 24 healthy non-smoking volunteers age 18 45 years old, not taking any medications (including medications for weight loss, antibiotics or probiotics) for the examination. All participants will be admitted to the Clinical Research Unit for 31 days. During their stay, subjects will be fed a weight maintaining diet for 3 days, followed by two experimental diets (150% and 50% of weight maintaining calories) in a random order. After this, volunteers will be randomly assigned to one of two groups: group 1 will take oral antibiotic medication; group 2 will receive pills that look the same but will not contain any active medication (placebos). Feces (stool) will be collected throughout the study. Additionally, twenty four-hour urine collections will take place each day of the experimental diet period and when stool is collected on the antibiotics. The energy content of these waste products as well as that of the diet (using duplicate plate analysis) will be measured by bomb calorimetry. Bacterial components in feces will be extracted by repeated fractional centrifugation to obtain bacterial mass and by using 16S rDNA-based oligonucleotide probes to obtain data on gut bacteria. Primary results will examine how many calories remain in stool during relative over- and underfeeding and whether changes in gut bacteria, induced by an antibiotic medication, affect nutrient absorption and glucose tolerance....