Researchers have said that many trials on vitamin D inducing health benefits are either inconclusive or insufficient to draw any firm conclusions.
The first, by researchers based in the UK, Europe and USA, examined data from observational studies and clinical trials (an umbrella review) to summarise health outcomes associated with vitamin D levels, both naturally circulating and as a result of taking supplements.
Of a remarkable 137 different health outcomes reportedly linked to vitamin D, they found that only 10 had been thoroughly tested in trials, and only one (birth weight - linked to a mother's vitamin D levels in late pregnancy) had apparently concordant evidence of "benefit."
In other words, the researchers failed to find any convincing evidence of a clear role of vitamin D for any of the outcomes.
Based on this review, they suggest a "probable" association between vitamin D levels and birth weight, dental caries in children, maternal vitamin D levels at term and parathyroid hormone levels in chronic kidney disease patients requiring dialysis, but "further studies and better designed trials are needed to draw further conclusions."
In the second paper, an international team led by researchers at the University of Cambridge and the Erasmus Medical Centre, analysed the extent to which vitamin D is associated with death from cardiovascular disease, cancer, or other conditions, under various circumstances.
They analysed the results of observational cohort studies and randomised controlled trials of both naturally circulating vitamin D and supplements (given singly as either vitamin D2 or D3 supplements).
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They found that low circulating vitamin D levels in blood were associated with increased mortality risks from cardiovascular disease, cancer, and other causes.
The two papers have been published on bmj.com.