1. Institute for Diabetes Research and Metabolic Diseases of the Helmholtz Center Munich at the University of Tübingen, 72076 Tübingen, Germany
2. German Center for Diabetes Research (DZD), 85764 Neuherberg, Germany
3. Department of Internal Medicine, Division of Endocrinology, Diabetology and Nephrology, Eberhard Karls University Tübingen, 72076 Tübingen, Germany
4. Institute for Clinical Chemistry and Pathobiochemistry, Department for Diagnostic Laboratory Medicine, University Hospital Tübingen, 72076 Tübingen, Germany
5. German Institute of Human Nutrition, 14558 Potsdam-Rehbrücke, Germany