1. Department of Biomolecular Medicine, Division of Surgery, Oncology, Reproductive Biology and Anaesthetics, Faculty of Medicine, Imperial College London, Sir Alexander Fleming Building, South Kensington, London SW7 2AZ, UK, and Department of Nutrition and Health and Bioanalytical Sciences, Nestlé Research Center, P. O. Box 44, Vers-chez-les-Blanc, CH-1000 Lausanne 26, Switzerland