Interesting studies show that taking Vitamin D supplements reduces total mortality, that Vitamin D might improve or prevent heart disease, that it plays a role in the metabolic syndrome and kidney disease, and even in the well being of our eyes. The authors of the last study believe:

“it is the GPs responsibility to promote ‘sensible sunlight’ exposure, eye and skin protection along with Vitamin D3 supplementation”.

Some researchers have interpreted the variable outcomes of studies, where some did show benefits of giving Vitamin D supplements, but others did not, as being due to “reverse causality”, meaning that it is not the lack of Vitamin D which makes people ill, but rather the illness which reduces the blood Vitamin D level. This has been criticized convincingly by pointing out that Vitamin D deficiency in early life, might lead to irreversible disease in later life. For instance if rickets caused bone deformities, then no trial giving Vitamin D in later life would rectify those established deformities. This press release from November 2014 summarized it.