However, when examined by type of supplementation, the researchers did find that vitamin D3 alone reduced mortality by 11 per cent. The scientists stress that additional research is needed.

The new research, published today in the British Medical Journal (BMJ), was conducted by an international team of researchers and co-led by Dr Rajiv Chowdhury at the University of Cambridge’s Department of Public Health and Primary Care. He said: “With so many people touting vitamin D as a possible panacea, our study provides useful insight into the limitations of the current evidence base of vitamin D supplements and highlights where additional research is needed.”

Previous research has shown that lower levels of vitamin D in the blood are associated with a number of different diseases (to include multiple sclerosis, cancer and cardiovascular disease). However, it was not clear whether vitamin D supplementation when given alone (that is, not co-adminstered with other interventions such as calcium) would reduce the risk of deaths from various causes. In order to help clarify these associations, researchers analysed the results of 73 observational cohort studies and 22 randomised trials of both naturally circulating vitamin D and supplements (either D2 or D3).

In total, these studies involved nearly a million individuals from 26 countries and included patients with established cardiovascular disease as well as lower risk people without the disease. Additionally, differences in study quality were taken into account to identify and minimise bias.

Their analysis reinforced the observational associations of lower levels of circulating vitamin D concentrations with deaths from cardiovascular, cancer, as well as other causes, but they also found that vitamin D supplements in the trials, overall, did not reduce the risk of all-cause mortality significantly.

However, when stratified by type of supplements, vitamin D3 taken alone reduced mortality significantly by 11% among older adults. By contrast, supplementation with vitamin D2 singly had no overall impact on mortality.

Remarking on the research, Dr Chowdhury added: “Before any recommendation of widespread supplementations with vitamin D3, it is, however, essential that further clinical investigations are conducted into the optimal dosage and safety. It will also be important to examine whether D2 or D3 alone may indeed have different effects on the risk of death in different populations since the current trials were essentially based in elderly high risk populations with a variety of baseline diseases and any beneficial effects on healthy general populations are not yet known.”