Does vitamin D have a role in the prevention of non-skeletal diseases?

During the last decades, a large number of studies have suggested an inverse association between plasma 25-hydroxyvitamin D levels and risk of skeletal and non-skeletal diseases. More recently, a number of randomized clinical trials have not been able to document beneficial effects of vitamin D supplementation. However, people included in the trials have not been suffering from vitamin D insufficiency. This raises the question of whether these trials actually have tested the hypothesis suggested by observational studies on adverse health effects of low vitamin D levels.