1. Biological Chemistry, Biomedical Sciences Division, Faculty of Natural Sciences, Imperial College London, Sir Alexander Fleming Building, South Kensington, London SW7 2AZ, United Kingdom, Intestinal Disease Research Program, McMaster University, Hamilton, Canada, and Departments of Nutrition and Health and Bioanalytical Sciences, Nestlé Research Center, P.O. Box 44, Vers-chez-les-Blanc, CH-1000 Lausanne 26, Switzerland