Research casts doubt on the value of taking vitamin D supplements
Researchers have cast doubt on the belief that vitamin D supplements can prevent conditions including cancer, diabetes and heart disease, saying low vitamin D may be a consequence, not a cause, of ill health.
The findings could have implications for millions of people who take vitamin D pills and other supplements to ward off illness, People in the United States spend an estimated US$600 million a year on them alone.
Vitamin D, sometimes known as the "sunshine vitamin", is made in the body when the skin is exposed to sunlight and is found in foods such as fish liver oil, eggs and fatty fish such as salmon, herring and mackerel.
It is known to raise absorption of calcium and some observational studies have suggested a link between low levels of vitamin D and greater risks of many acute and chronic diseases.
But it is not clear whether this is a cause-and-effect relationship, so various large trials have been conducted to test whether vitamin D supplementation can reduce the risk of developing disease.
Researchers led by Philippe Autier, of the International Prevention Research Institute in Lyons, France, analysed data from hundreds of observational studies and clinical trials examining the effects of vitamin D levels on non-bone health, including links to illness such as cancer, diabetes and cardiovascular disease.
They found that the benefits of high vitamin D levels seen in observational studies, including reduced risk of cardiovascular events, diabetes and colorectal cancer, were not replicated in trials where participants were given vitamin D to check if it would protect against illness.
"What this discrepancy suggests is that decreases in vitamin D levels are a marker of deteriorating health," Autier said.
He explained that serious illness such as cancer and diabetes may reduce vitamin D concentrations, but that did not necessarily mean that raising vitamin D levels would prevent the illness from occurring. Yet experts not involved in Autier's review said its conclusions were not definitive.
They cautioned against reading it as a reason to dissuade people from taking vitamin D.
"This paper is very useful because it highlights the need for more long-term intervention studies specifically looking at the effect of proper vitamin D supplementation on disease risk," said Nigel Belshaw, at Britain's Institute of Food Research.
"However, it does not suggest that taking vitamin D supplements can not be useful in some cases. Neither does it rule out a health advantage of increasing vitamin D levels in the blood for those who are deficient."