Studies about vitamin D and type 1 diabetes:
Vitamin D Supplementation in Early Childhood and Risk of Type 1 Diabetes: A Systematic Review and Meta-Analysis (2008)
Conclusion: A meta-analysis of five different studies investigating the link between vitamin D supplementation during infancy and early childhood and later development of type 1 diabetes found that children who take supplemental vitamin D are at a lower risk for type 1 diabetes. The meta-analysis involved 1,528 children with type 1 diabetes and 5,156 healthy controls.
The Association Between Ultraviolet B Irradiance, Vitamin D Status and Incidence Rates of Type 1 Diabetes in 51 regions Worldwide (2008)
Conclusion: Incidence rates of Type-1 diabetes are generally higher at latitudes farther away from the equator where there is less UVB radiation necessary for vitamin D production in humans. Type-1 diabetes incidences decrease in regions closer to the equator.
Conclusion: A 30-year Finland birth-cohort study involving 10,366 children found that those who regularly took supplemental vitamin D had a lower risk of developing type 1 diabetes.