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Historically, vitamin D has been linked to skeletal disease such as osteoporosis and fractures, but it has become popular for the prevention of a wide range of diseases.

Vitamin D supplements taken to prevent cancer, multiple sclerosis, cardiovascular and other diseases appear to be ineffective, a new review finds.

Vitamin D deficiency has been linked to 137 different diseases  and outcomes. The popularity of supplements for the "sunshine vitamin" has taken off, though few trials demonstrate its value.

In Wednesday’s British Medical Journal, researchers took a closer look at previous reviews of the research, raising doubts on how well the supplements work.

"Highly convincing evidence of a clear role of vitamin D does not exist for any outcome, but associations with a selection of outcomes are probable," Evropi Theodoratou of the University of Edinburgh and co-authors concluded.

The only "probable" associations that held up were vitamin D and birth weight, dental caries or cavities in children, maternal vitamin D levels at term and parathyroid hormone levels in chronic kidney disease patients needing dialysis, all of which need to be studied in more rigorous trials, Theodoratou's team said.

The remarkably long list of chronic diseases linked to vitamin D deficiency have even been facetiously suggested as an explanation for why good triumphs in J.R.R. Tolkien’s The Hobbit, said Naveed Sattar and Paul Welsh of the British Heart Foundation Research Centre at Glasgow University, in a journal editorial accompanying the research.

The apparent benefits of vitamin D supplements in observational research fail to hold up when tested in gold-standard randomized control trials that pit the pills against placebo, however, and that should ring alarm bells, the editorial says.

"Some may argue that supplementing those who are apparently "deficient" is cheap, but patients may gain false reassurance from prescription of a "protective" tablet," Sattar and Welsh concluded.

The pair suggested that health-care professionals be cautious about lower-quality observational data where the associations could be a consequence of low blood levels of a form of vitamin D, rather than a cause.

Instead of ordering costly blood tests of vitamin D levels in people showing no symptoms of bone-related conditions, the editorial suggested sticking with what’s proven, such as encouraging better lifestyles and targeting risk factors such as smoking, diabetes, blood pressure and high cholesterol.

In a second review of 14 trials on vitamin D3, the supplements showed an apparent benefit of 11 per cent lower risk of death during followup, which seems remarkable, the editorial noted. Those studies included relatively few patients, however, and many of the studies had a high risk of bias.