"Doubt cast on vitamin D's role against disease," reports BBC News. The news come from a study summarising a large body of evidence from the best kind of trials – randomised controlled trials (RCTs).
The results of these trials showed that vitamin D supplements didn't appear to help prevent many diseases, including cancer and cardiovascular disease. Importantly, these trials did not cover – so do not apply to – diseases affecting the bones.
The media reporting was generally balanced and included both the study's conclusions and comments on some of its limitations.
Prospective studies can't prove cause and effect, but well-designed randomised control trials can, so the two study design types were included to ensure all the best available evidence was considered and to see if the findings were similar.
The researchers concluded that, "The discrepancy between observational and intervention studies suggests that low 25(OH)D [vitamin D] is a marker of ill health.
"Inflammatory processes involved in disease occurrence and clinical course would reduce 25(OH)D [vitamin D], which would explain why low vitamin D status is reported in a wide range of disorders.
"In elderly people, restoration of vitamin D deficits due to ageing and lifestyle changes induced by ill health could explain why low-dose supplementation leads to slight gains in survival."
The effect of vitamin D on bone health was not addressed, so readers shouldn't conclude that this research casts doubt on the usefulness of taking vitamin D supplements for good bone health – this study only relates to the effects on diseases not affecting the bones.