Insufficient Vitamin D Response to Solar Radiation in German Patients with Type 2 Diabetes or Gestational Diabetes.

Vitamin D deficiency is highly prevalent in all forms of diabetes mellitus. Recently, we reported how ultraviolet B (UVB) radiation affected vitamin D [25(OH)D3] concentrations in patients with type 1 diabetes. Our aim was to analyze whether patients with non-autoimmune diabetes, such as type 2 diabetes mellitus (T2DM) and gestational diabetes mellitus (GDM) also show the same vitamin D profile in relation to environmental factors including ambient temperature as an indirect parameter for outdoor activities. We analyzed 25(OH)D3 concentrations of T2DM (n=349) and GDM patients (n=327) at the University Hospital Frankfurt from 2005 to 2007. Additionally, daily UVB and monthly outside air temperature measurements for Frankfurt/Germany were obtained. We detected a positive correlation between UVB irradiation and 25(OH)D3 concentrations of T2DM and GDM patients (rho=0.50 and rho=0.63, p=0.003 and p<0.0001, respectively). UVB irradiation was in summer (April-October) higher than in winter (November-March) (5.6 kJ/m² vs. 0.5 kJ/m², p<0.0001). However, the prevalence of vitamin D deficiency in summer remained high with 76% in T2DM and 59% in GDM. In a stepwise regression analysis for the 25(OH)D3 concentration, significant predictors were outdoor temperature (estimate=0.02, p<0.0001), UVB radiation (estimate=-0.0015, p=0.02), year (2006 vs. 2005 estimate=-0.06, p>0.05, 2007 vs. 2005: estimate=-0.13, p<0.0001) and diabetes type (estimate=0.06, p=0.03). In conclusion, the strong correlation between UVB radiation and 25(OH)D3 concentrations in T2DM and GDM patients determines the seasonal variation. Additional determinants for the 25(OH)D3 concentrations were outdoor temperature, year, and diabetes type. Despite the effects of solar radiation both patients groups remain largely vitamin D deficient during summers.