1. Diabetes Centre, Isala Clinics, Zwolle, Overijssel, the Netherlands;
2. Department of Internal Medicine, Isala Clinics, Zwolle, Overijssel, the Netherlands;
3. Medical Research Group, Langerhans, Zwolle, Overijssel, the Netherlands;
4. Department of General Practice, University Medical Centre Groningen, Groningen, the Netherlands;
5. Department of Internal Medicine, University Medical Centre Groningen, Groningen, the Netherlands.