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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.
Start Quote
What this suggests is that decreases in vitamin D levels are a marker of
deteriorating health.
Prof Philippe Autier International Prevention Research Institute
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 - where participants were given vitamin
D supplements - found no reduction in risk, even in people who started out 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.
Continue reading the main story
What is a vitamin D deficiency?
A vitamin D level less than 25nmol/L in the blood is a deficiency, but experts
increasingly believe that lower than 60nmol/L can also be damaging to health.
Most people get enough vitamin D by being exposed to the sun for 10 to 15
minutes a day.
A small amount of vitamin D also comes from foods such as oily fish and dairy
products.
Recently England's chief medical officer said free vitamins should be given to
all young children because more and more of them were being diagnosed with the
bone disease rickets, lack of calcium and other bone and muscle diseases.
"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.