The idea that we can make up for the nutrients we lack by popping a pill is undeniably seductive, but is it valid?

In the case of vitamin D supplements, recent research from New Zealand finds the answer is probably not.

Vitamin D – also known by its cheery moniker: the "sunshine vitamin" – is important for regulating the absorption of calcium and phosphorous in our bones, among other functions. And while the body makes vitamin D when skin is directly exposed to sunlight, we can also get it from foods like eggs and fatty fish such as salmon, herring and mackerel.

For years now we have been hearing about the health benefits of taking vitamin D supplements, with observational studies suggesting a link between low levels of vitamin D and greater risks of many acute and chronic diseases – from cancer, heart attack and diabetes to dementia and autoimmune and inflammatory diseases – and even early death.

But according to the new study, led by Dr. Mark Bolland of the University of Auckland in New Zealand, the previous studies lack the evidence to back up their claims and future studies are unlikely to change this outlook.

The team found that many of the associations found in studies of vitamin D are not causal; that is, there is not enough evidence to say vitamin D supplementation has any health benefits, reports Medical News Today.

The researchers looked at 40 randomized controlled trials that examined the use of vitamin D supplements, with or without calcium (many vitamin D supplements include calcium as well). They were unable to conclude that vitamin D supplementation reduces mortality in the general population by more than 5 percent. They found that vitamin D supplements are unlikely to reduce the incidence of heart attack, heart disease, stroke, cancer and bone fractures. And whether the supplements also included calcium or not made no difference.

"Lots of observational studies that measure vitamin D levels at baseline and compare health outcomes over time between groups with high levels and low levels have reported associations between low vitamin D levels and poor health outcomes," Bolland said. "These studies are not able to determine causality because of their design.” 

"The problem with those studies is that you can't determine whether there's a cause and effect," he said. Groups with low levels of vitamin D "tend to be older, heavier, tend to exercise less and spend less time outside," which could explain both their low levels and poor health, he added.

The New Zealand study is not the first to question the benefits of vitamin D. Among other studies, a French review last year of nearly 500 studies found vitamin D’s benefits in reducing the risk of diseases were mostly unconfirmed.

"Associations between vitamin D and health disorders reported by investigators of observational studies are not causal," said lead author of that study, Philippe Autier. "Low vitamin D could be the result of inflammatory processes involved in the occurrence and progression of disease."

And while vitamin D is indeed important for bone health and avoiding things like rickets, Bolland concludes that, “if you are otherwise healthy and active, you are likely to receive enough sunshine to have adequate vitamin D levels and don't need to take vitamin D supplements."

That said, if you do elect to continue taking vitamin D, choose your supplements wisely.  In an analysis of 12 supplements combining vitamin D with calcium, Consumer Reports found levels of lead in nine of them that would have triggered warnings for reproductive risk under California's toxic limits law, Proposition 65

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