Lack of 'convincing' evidence for vitamin D supplementation, conclude researchers
Researchers have concluded that there is no ‘highly convincing’ evidence that vitamin D supplementation improves outcomes after conducting an analysis of the evidence.
The two large meta-analyses published in the BMJ today cast doubt on Department of Health advice to offer vitamin D supplements to certain groups of people considered at risk of vitamin D deficiency.
However, researchers did say that benefits from vitamin D supplementation were ‘probable’ in some at-risk groups.
Experts said the findings suggest GPs should be cautious about prescribing vitamin D supplements widely in people without symptoms, until the risks as well as the benefits have been properly evaluated.
The DH currently recommends vitamin D supplements should be given to several at-risk groups including pregnant and breastfeeding women, infants and children under five years and older people over 65, as well as middle-aged people who get little sun exposure and those with darker skin.
However, one of the new papers found evidence from meta-analyses of observational and randomised data only consistently supported an association for one health outcome and vitamin D levels. This was birthweight, which was linked with maternal vitamin D levels in late pregnancy.
The authors said the evidence also suggested vitamin D supplementation ‘probably’ reduces the risk of dental caries in children and decreases parathyoid hormone levels in patients with chronic kidney disease, as well as increasing maternal vitamin D levels at term. But they said there was no evidence to support vitamin D only supplementation to increase bone mineral density or to prevent fractures or falls in older people.
The team concluded: ‘Despite a few hundred systematic reviews and meta-analyses, highly convincing evidence of a clear role of vitamin D does not exist for any outcome, but associations with a selection of outcomes are probable.’
A second paper looked at studies evaluating the impact of vitamin D on mortality, and found that vitamin D2 supplements were not associated with a reduction in deaths from any cause, whereas vitamin D3 supplements were associated with an 11% reduction in overall mortality in older people.
However, Dr Paul Welsh and Dr Naveed Sattar, from University of Glasgow, argued in an accompanying editorial that while this reduction in mortality ‘seems remarkable’, most of the effect was driven by studies in older people and so it would not necessarily apply to middle-aged people, while nearly half the studies were poor quality.
They wrote: ‘Before widespread supplementation can be considered, new trial data are needed with a focus on potential risks as well as benefits.’
They added: ‘Some may argue that supplementing those who are apparently “deficient” is cheap, but patients may gain false reassurance from prescription of a “protective” tablet.’
Dr Pam Brown, a GP who specialises in management of osteoporosis, said the overall findings should not detract from current guidance, however, in particular advice on vitamin D treatments for the frail elderly.
Dr Brown said: ‘[The first study] did confirm that there is “suggestive evidence” for a correlation between high vitamin D blood concentrations and low risk of non-vertebral fractures, reduced levels of balance sway, [high] bone mineral density in the femoral neck and muscle strength.’
She added that vitamin D3 is already the form of vitamin D supplied in oral vitamin D or calcium and vitamin D supplements for treatment of osteoporosis and prevention of hip fractures in the frail elderly.
‘Therefore we do not need to make any change to our vitamin D3 use in these groups of patients,’ she said.
NICE is in the process of developing public health guidance on how to implement prevention strategies on vitamin D deficiency.