Pediatric Gastroenterology/Hepatology, Section of Nutrition and Metabolism, Research Laboratory of Pediatrics, Department of Pediatrics, Beatrix Children's Hospital/University Medical Center Groningen, P.O. Box 30.001, 9700 RB, Groningen, The Netherlands. Department of Genetics, University of Groningen, University Medical Center Groningen, Hanzeplein 1, 9713 GZ, Groningen, The Netherlands. Pediatric Gastroenterology/Hepatology, Section of Nutrition and Metabolism, Research Laboratory of Pediatrics, Department of Pediatrics, Beatrix Children's Hospital/University Medical Center Groningen, P.O. Box 30.001, 9700 RB, Groningen, The Netherlands. h.j.verkade@umcg.nl.