"Vitamin D status and ill health: a systematic review"
Lancet
http://www.thelancet.com/journals/landia/article/PIIS2213-8587(13)70165-7/fulltext?_eventId=login 
"Conclusions

Many prospective studies have shown associations between low 25(OH)D concentrations and a wide range of acute and chronic health disorders. However, an equally similar number of randomised trials have not confirmed that raising of 25(OH)D concentrations can modify the occurrence or clinical course of these disorders. Hence, associations between 25(OH)D and health disorders reported by investigators of observational studies are not causal. Low 25(OH)D could be the result of inflammatory processes involved in the occurrence and progression of disease. An exception would be slight gains in survival after the restoration of vitamin D deficits due to lifestyle changes induced by ageing and ill health. Five trials including 2150—20 000 patients aged 50 years or older are in progress, testing whether vitamin D supplementation at 40—80 μg per day can reduce the risk of cancer, cardiovascular diseases, diabetes, infections, declining cognitive functions, and fractures.63 The first results are not expected before 2017, but these studies have the potential to test our hypotheses."