Scientists have cast doubt on the value of vitamin D supplements to protect against diseases such as cancers, diabetes and dementia.

Writing in The Lancet Diabetes and Endocrinology, French researchers suggest low vitamin D levels do not cause ill health, although they did not look at bone diseases.

More clinical trials on non-skeletal diseases are needed, they say.

Vitamin D supplements are recommended for certain groups.

Recent evidence has shown it may also have a role to play in preventing non-bone-related diseases such as Parkinson's, dementia, cancers and inflammatory diseases.

Prof Philippe Autier, from the International Prevention Research Institute in Lyon, carried out a review of data from 290 prospective observational studies and 172 randomised trials looking at the effects of vitamin D levels on health outcomes, excluding bone health, up to December 2012.

'Discrepancy'

A large number of the observational studies suggested that there were benefits from high vitamin D - that it could reduce the risk of cardiovascular events by up to 58%, diabetes by up to 38% and colorectal cancer by up to 33%.

But the results of the clinical trials found no reduction in risk, even in people with low vitamin D levels.

And a further analysis of recent randomised trials found no positive effect of vitamin D supplements on diseases occurring.

Prof Autier said: "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... reduce vitamin D concentrations, which would explain why vitamin D deficiency is reported in a wide range of disorders."

High risk

In the UK, vitamin D supplements are recommended for groups at higher risk of deficiency, including all pregnant and breastfeeding women, children under five years old, people aged over 65, and people at risk of not getting enough exposure to sunlight.

People with dark skin, such as people of African-Caribbean and South Asian origin, and people who wear full-body coverings, as well as pale-skinned people are also known to be at higher risk.

In recent years, there has been a four-fold increase in admissions to UK hospital with rickets - a disease that causes bones to become soft and deformed.

Dr Colin Michie, consultant senior lecturer in paediatrics and chair of the nutrition committee at the Royal College of Paediatrics and Child Health, said the review had little to contribute to the problem in the UK because it excluded the measurement of bone health.

"It has been known for almost a century that vitamin D supplements given to those with deficient vitamin D levels results in improved bone health, preventing hypocalcemic seizure and rickets."

He added that it was important to provide appropriate supplements, such as vitamin D, to improve bone health.

More research

Peter Selby, consultant physician and honorary professor of metabolic bone disease at Manchester Royal Infirmary, said the French review was limited.

"It could very well be that the apparent negative results of this study have been obtained simply because they have not been looking at people with sufficient degree of vitamin D insufficiency to have any meaningful biological effect."

But he said the authors were right to say that more interventional research looking at disease outcomes was necessary.

The Scientific Advisory Committee on Nutrition (SACN), an independent group of scientific experts who advise the government on nutrition, is currently reviewing the dietary recommendations for vitamin D for all population groups in the UK.

Their report on vitamin D is expected to go out for public consultation in 2014.