The role of vitamin D on health is a cloudy subject. Although many studies have hailed the vitamin for its beneficial effects on medical conditions, such as autoimmune disorders, heart disease and even cancer, other research has suggested otherwise. Now, two new studies published in the BMJ indicate there is no clear evidence that vitamin D is beneficial for certain health problems.

There is no doubt that our bodies need vitamin D. It is essential for regulating the absorption of calcium and phosphorus in our bones and aiding communication between cells. But does vitamin D provide any additional health benefits?

Medical News Today has reported on numerous studies suggesting it does. Earlier this year, UK researchers indicated that high intake of vitamin D during pregnancy may improve offspring muscle strength, while more recent research suggests that high vitamin D levels may increase breast cancer survival.

But many health professionals have their reservations about the potential health benefits of vitamin D. In January this year, we reported on research led by Dr. Mark Bolland of the University of Auckland in New Zealand, which suggested that vitamin D supplements are unlikely to reduce incidence of certain conditions, such as heart attack, heart disease, stroke, cancer and bone fractures.

Another study also cast doubts over vitamin D supplementation; it could find no evidence of additional health benefits. Now, two more studies have reached similar conclusions.

Pattern of findings 'should ring alarm bells'

The first study was conducted by researchers from the US, UK and Europe. The team analyzed data from observational studies and clinical trials that looked at health outcomes linked to vitamin D levels.

The data included trials that looked at vitamin D levels as a result of both naturally occurring vitamin D and supplements.

Although the researchers identified 137 different health outcomes associated with vitamin D levels, they found that only 10 of these outcomes had been comprehensively tested in trials, and only one outcome - that an infant's birth weight is linked to a mother's vitamin D levels in late pregnancy - had enough evidence to deem it a "benefit."

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Researchers say that contrary to previous research, there is no evidence that vitamin D supplements help prevent osteoporosis.

In an editorial linked to the studies, Naveed Sattar and Paul Welsh of the British Heart Foundation Research Centre in the UK say this pattern of findings "should ring alarm bells."

They note that other studies have shown that supplements of other vitamins, such as C and E, may have potentially harmful effects on health outcomes when intake is increased.

"This highlights the often underestimated problems of confounding and reverse causality that can lead to premature causal inferences in observational studies," they add.

In addition, the research team found no evidence that vitamin D supplementation had any benefits for osteoporosis or helped prevent falls - a finding that contradicts previous research. The investigators say this suggests that the vitamin "might not be as essential as previously thought in maintaining bone mineral density."

This is not the only study to reach this conclusion. Last year, Medical News Today reported on a study published in The Lancet suggesting that vitamin D supplements do not prevent osteoporosis and that widespread use of the vitamin for this condition is "inappropriate."

The researchers say that overall, their analysis suggests there may be a "probable" link between vitamin D concentrations and birth weight, dental cavities in children, maternal vitamin D levels at term and hormone levels in patients with chronic kidney disease who require dialysis. But they note that "further studies and better designed trials are needed to draw firmer conclusions."

Vitamin D3 may reduce mortality, but further research is warranted

For the second study, researchers led by the University of Cambridge in the UK and the Erasmus Medical Centre in the Netherlands analyzed observational cohorts and randomized controlled trials that looked at the association between vitamin D and death from cardiovascular disease and cancer, among other conditions.

Trials included in the review assessed the effects of both naturally occurring vitamin D and vitamin D2 and D3 supplements.

In the randomized controlled trials, follow-up varied from 3 to 7 years. The research team found that of the individuals who took vitamin D supplements, 2,527 deaths occurred, compared with 2,587 deaths in control groups.

Looking at the results in more detail, the researchers found that when vitamin D2 supplements were taken alone, there was no reduction in participants' risk of death.

But when vitamin D3 supplements were taken alone, participants had an 11% reduction in the risk of death.

However, the research team notes that since the trials in this review involved elderly populations, in which death is commonly a result of coexisting conditions, further research is warranted to determine the correct dosage of vitamin D supplements that should be taken, duration and safety of the supplements, and whether vitamin D2 or D3 pose different effects on the risk of mortality.

'We should stick to what is proven'

Commenting on the take home messages of both studies, Sattar and Welsh say:

"Firstly, health care professionals should treat all observational data cautiously, as existing disease and associated risk factors may cause, rather than be a consequence of, low circulating 25-hydroxyvitamin D.

Secondly, before widespread supplementation can be considered, new trial data are needed with a focus on potential risks as well as benefits."

Until concrete evidence on the heath benefits of vitamin D is provided, Sattar and Welsh say doctors should avoid recommending the use of vitamin D supplements for patients who do not have bone disease related conditions.

They conclude:

"To improve health and prevent chronic disease, we should stick to what is proven; encourage better lifestyles in general and target established risk factors in people at elevated risk."