Low vitamin D may be a consequence, not a cause, of ill health, researchers said on Friday, casting doubt on the prevailing wisdom that vitamin D supplements can prevent conditions like cancer, diabetes and heart disease.

The findings, published in The Lancet Diabetes & Endocrinology, could have implications for millions of people who take vitamin D pills and other supplements to ward off illness. Americans spend an estimated $600 million a year on them alone.

The human body makes its own supply of Vitamin D, but it needs to receive the "ingredients" - namely sunlight and foods like fish liver oil, eggs and fatty fish such as salmon, herring and mackerel.

Vitamin D is known to boost the uptake of calcium and bone formation, and some observational studies have also 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.

Benefits not replicated

Researchers led by Philippe Autier of France's International Prevention Research Institute in Lyon analysed data from several hundred observational studies and clinical trials examining the effects of vitamin D levels on so-called 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 randomized trials where participants were given vitamin D to see if it would protect against illness.

"What this discrepancy suggests is that decreases in vitamin D levels are a marker of deteriorating health," said Autier.

In other words, he explained, serious illness like cancer and diabetes may reduce vitamin D concentrations, but that does 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, and 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, research leader at Britain's Institute of Food Research.

"However, it does not suggest that taking vitamin D supplements can not be useful in some cases for some purposes. Neither does it rule out a health advantage of increasing vitamin D levels in the blood for those who are deficient," Belshaw said.

Helen Macdonald, a professor of nutrition and musculoskeletal health at Britain's University of Aberdeen, stressed that vitamin D was important for bone health.

"And we already know that people who are at risk of vitamin D deficiency, like older people, pregnant and breastfeeding women, young children and people with darker skin, need to take a supplement because it is difficult to boost vitamin D levels from food sources alone," she said.

She added that Autier's study did, however, appear to confirm what many nutrition experts have suspected for a while - "that healthy people probably don't need to take a high dose supplement and that the best source of vitamin D for most people is sunlight in the summer, always taking care not to burn."