People who take vitamin D supplements in the belief that they protect against heart disease and cancer may be wasting their time, a study suggests.
Scientists who conducted a review of hundreds of studies argued that some of the health benefits attributed to vitamin D were the result of a "chicken and egg" misunderstanding.
Low vitamin D levels were not a cause of chronic conditions such as heart disease, diabetes and cancer but a result of them, they concluded.
The finding challenges the view of many experts - including conventional doctors as well as alternative practitioners - that the vitamin plays a major role in disease prevention.
It is well known that vitamin D promotes the uptake of calcium and bone formation, and children deficient in the vitamin can develop rickets.
What is less clear is the extent to which vitamin D reduces the risk of other major causes of ill-health.
The scientists found that findings from prospective observational studies, which draw inferences from observed phenomena over time, and those from randomly controlled trials did not tally.
Lead researcher Professor Philippe Autier, from the International Prevention Research Institute in Lyon, France, said: "If the health benefits of high vitamin D concentrations shown by data from observational studies are not reproduced in randomised trials (the gold standard method for assessing a causal relation between an exposure and an outcome) then the relation between vitamin D status and disorders are probably the result of confounding or physiological events involved in these disorders.
"What this discrepancy suggests is that decreases in vitamin D levels are a marker of deteriorating health. Ageing and inflammatory processes involved in disease occurrence and clinical course reduce vitamin D concentrations, which would explain why vitamin D deficiency is reported in a wide range of disorders."
Vitamin D supplements are taken by many thousands of people in the UK, accounting for a sizeable fraction of the £674.6 million worth of vitamins and dietary pills sold in 2009.
In the US, annual sales of vitamin D alone are estimated to be worth 600 million dollars (£367 million).
The new study, published in the journal The Lancet Diabetes & Endocrinology, analysed the findings of 290 prospective observational studies and 172 randomised trials up to December last year.
A randomised trial is one in which the effects of a drug or supplement are studied in different groups, one of which is unknowingly given a "dummy" placebo.
The observational studies indicated significant benefits from vitamin D, including an up to 58% reduced risk of cardiovascular events such as heart attacks, a 38% reduced risk of diabetes, and a 34% risk of bowel cancer.
However, these improvements were not confirmed by the randomised trials. The pooled analysis of randomised study results failed to identify any effect on non-bone disease of raising vitamin D levels with supplements.
A linked editorial in the journal called for large-scale clinical trials to assess the impact of vitamin D on non-skeletal health outcomes.
The editorial added: "It would be a real boon to patients if the results are positive, but unless effect sizes for clinically important outcomes are large, the results will only confirm the neutral effect reported by most clinical trials thus far."
Professor Peter Selby, an expert in metabolic bone disease at Manchester Royal Infirmary, said: " It is important not to let the apparently negative finding of the study take anything away from the importance of adequate vitamin D nutrition in the management of bone disease. Recent guidelines relating to vitamin D deficiency were published by the National Osteoporosis Society together with several other professional organisations; these provide widely accepted guidance for the diagnosis and treatment of vitamin D deficiency."
Dr Colin Michie, consultant senior lecturer in paediatrics at Ealing Hospital NHS Trust, said: " From the perspective of a paediatrician, this paper is a valuable contribution in the field of nutrition, but it has little to contribute to our problem in the UK, where low levels of vitamin D result in hypocalcaemic (low calcium) seizures in infants and bony disorders such as rickets.
" The suggestion that vitamin D levels are low because of co-morbidity (illness) is not surprising or novel. This has been proposed in many clinical situations in paediatrics. However it leads to the obvious corollary that such co-morbidities, by reducing vitamin D levels, may compromise skeletal health. From the patient's perspective it therefore becomes important to ensure this does not happen by providing appropriate supplements, including vitamin D."
Dr Benjamin Jacobs, consultant paediatrician and director of children's services at the Royal National Orthopaedic Hospital, said: " The problems of vitamin D deficiency had been thought to be solved in the 1950s but this paper shows there is still much to be learned. A hundred years ago rickets was a major problem in the UK. Then in the 1950s it was thought that vitamin D deficiency was solved, which is why we do not fortify foods or give babies vitamin supplements as much as most of Europe and North America.
"But now we are seeing many children again with rickets, lack of calcium and other bone and muscle diseases. England's Chief Medical Officer recently announced her intention to remedy this deficiency, and the publication of this paper today shows the need for better research to evaluate the role of Vitamin D in many diseases.
"We do not have enough data and may have to wait another five years before making recommendations for the whole UK population. Meanwhile we must continue the current guidance: all pregnant and breastfeeding mothers in this country should take a daily vitamin D supplement, and so should all children from the age of six months to five years."