LONDON: Researchers cast doubt on the prevailing wisdom that vitamin D supplements can prevent conditions like cancer, diabetes and heart disease, saying that low vitamin D may be a consequence, not a cause, of ill health.
The findings, published last week in The Lancet Diabetes & Endocrinology journal, 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 (R6billion) a year on them alone.
Vitamin D, sometimes called the "sunshine vitamin", is made in the body when the skin is exposed to sunlight and is found in foods like fish liver oil, eggs and fatty fish such as salmon, herring and mackerel. It is known to boost the uptake of calcium and bone formation, and some 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, so various large trials have been conducted to try to test whether vitamin D supplementation can reduce the risk of developing disease.
Researchers 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 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 randomised trials where participants were given vitamin D.
"What this discrepancy suggests is that decreases in vitamin D levels are a marker of deteriorating health," according to the study.
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.
Other experts have warned that the conclusions were not definitive and have 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 lead at Britain's Institute of Food Research.
"However, it does not suggest that taking vitamin D supplements cannot 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." - Reuters
Original headline: Sunshine vitamin may not be doing you much good
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