Low vitamin D levels are a consequence of ill health, not a cause, say researchers from Lyon, France. This casts strong doubts on the benefit of vitamin D supplements as a preventative measure against disease.

Lack of vitamin D has been linked to an array of medical conditions, from anemia, depression and pain, to brain damage. It has also been heralded as playing a role in disease prevention, supposedly offering protection agains cancer, cardiovascular disease and Parkinson's.

However, researchers from the International Prevention Research Institute in Lyon, writing in The Lancet Diabetes & Endocrinology, challenge these findings, claiming they have been unable to replicate the results.

Lead author Prof. Philippe Autier explains:

"If the health benefits of high vitamin D concentrations shown by data from observational studies are not reproduced in randomized 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."

Challenging observational evidence

Vitamin D is known to promote the uptake of calcium and plays a vital role in bone formation, but observational evidence suggests that low levels of the vitamin increases the risk of contracting certain acute or chronic diseases.

Many trials have tested whether vitamin D supplementation can reduce these risks, but the "chicken or egg" question remains - which is the cause and which the consequence.

In an accompanying editorial, The Lancet Diabetes & Endocrinology comments that up to 50% of adult Americans take vitamin D supplements and these may not be safe, particularly among people with liver, kidney or vascular problems.

Vitamin D supplements are big business in the US, with Americans spending $600 million overall each year.

For this research, Prof. Autier and colleagues analyzed data from 290 prospective observational studies and 172 randomized trials completed before December 2012.

They found that observational studies supported the benefits of high vitamin D concentrations - with up to 58% claiming it reduced the risk of cardiovascular episodes and up to 34% stating it was beneficial in reducing risks of colorectal cancer. However, randomized trials did not confirm these findings.

Prof. Autier explains:

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

The research also shows that supplementation among elderly people did show a slight reduction in overall mortality, but it suggests this might be more to do with restoring vitamin deficiency than proof of vitamin D's omnipotence.