Summary: Changes in fecal energy loss can result in differences in net energy balance and feed efficiency, and may result from, e.g., alterations in bile acid production, secretion or re-uptake, and changes in activities of enzymes involved in macronutrient digestion or uptake (e.g., pancreatic lipase). This assay will be coupled to food intake determinations and will involve fecal collection and feces weight measures for 48h, which can be coordinated with the meal pattern studies (catalog item D4003) or indirect calorimetry (catalog item D4007). Diet and fecal energy content will be measured by bomb calorimetry and the following reported: fecal energy density, total fecal energy loss\/day, and fecal energy loss as a percent of energy intake. Alternatively, fecal samples and diet samples can be examined without food intake measurements. In this case, only diet and fecal energy density will be reported. Additional tests can be done through proximate analysis (D4005- Digestible Energy by Proximate Analysis) to determine protein or fat absorption.