While we've long known that vitamin D deficiency can increase risk of bone health issues such as rickets in children or osteomalacia in adults, a recent study has revealed it may also lead to increased risk of Alzheimer's disease.
According to findings, a severe lack of vitamin D in older adults can dramatically increase the risk of developing the neurodegenerative disease.
Even a moderate deficiency was found to increase the chances of developing the disease by 69%.
Scientists were surprised by the dramatic findings, which suggest that upping vitamin D intake might delay or prevent Alzheimer's.
But more research has to be done before experts can know if there is a causal link between levels of the vitamin and dementia risk.
The study looked at 1,658 American adults aged 65 and over who were free from dementia, heart disease and stroke at the outset.
Lead researcher Dr David Llewellyn, from the University of Exeter, said: "We expected to find an association between low Vitamin D levels and the risk of dementia and Alzheimer's disease, but the results were surprising - we actually found that the association was twice as strong as we anticipated.
"Clinical trials are now needed to establish whether eating foods such as oily fish or taking vitamin D supplements can delay or even prevent the onset of Alzheimer's disease and dementia.
"We need to be cautious at this early stage and our latest results do not demonstrate that low vitamin D levels cause dementia. That said, our findings are very encouraging, and even if a small number of people could benefit, this would have enormous public health implications given the devastating and costly nature of dementia."
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The research showed that over an average six years, study participants who were moderately deficient in vitamin D had a 53% increased risk of developing any kind of dementia. For those who were severely deficient, the risk increased to 125%.
Similar striking results were seen for Alzheimer's, the most common form of dementia. The chances of people lacking vitamin D developing the disease were raised by 69% to 122% depending on how deficient they were.
Of the 171 study participants who developed some form of dementia, 102 were diagnosed with Alzheimer's.
All were recruited from the Cardiovascular Health Study, a US investigation of heart disease and stroke risk factors.
Previous research had already shown that people with low vitamin D were more likely to develop mental problems. But the new findings, published online in the journal Neurology, are the first to confirm a strong link with Alzheimer's.
Vitamin D can be obtained from the action of sunlight on the skin, foods such as oily fish, and taking supplements.
Sunshine is the most important source, but older people's skin is less efficient at harnessing ultra violet light to manufacture the vitamin.
In addition, winter sun in many northern countries, including the UK, is too weak to generate adequate levels of vitamin D.
For these reasons large numbers of people are deficient in the vitamin.
According to the NHS, groups of the population at risk of not getting enough vitamin D are:
- all pregnant and breastfeeding women
- babies and young children under the age of five
- older people aged 65 years and over
- people who are not exposed to much sun, such as people who cover up their skin when outdoors, or those who are housebound or confined indoors for long periods
- people who have darker skin such as people of African, African-Caribbean and South Asian origin
The study also identified a blood concentration threshold of 50 nanomoles per litre above which vitamin D levels are associated with good brain health.
This is just under the "normal" range for vitamin D levels in the UK which lies between 75 to 185 nanomoles per litre.
Dr Simon Ridley, head of science at the charity Alzheimer's Research UK, said: "This interesting study has strengthened the evidence for an association between vitamin D levels and risk of dementia, but it's not possible to know from this research whether low vitamin D levels could cause dementia.
"To be certain whether increasing vitamin D could help protect against dementia, we would need to see large-scale clinical trials.
"Vitamin D is essential for keeping our bodies healthy, and although these results don't suggest people should start dosing up on supplements, it's important for people to ensure they are getting enough vitamin D. Anyone who is considering changing their diet to include vitamin supplements should speak to a doctor."
Dr Doug Brown, director of research and development at Alzheimer's Society, said: "Shedding light on risk factors for dementia is one of the most important tasks facing today's health researchers. While earlier studies have suggested that a lack of the sunshine vitamin is linked to an increased risk of Alzheimer's disease, this study found that people with very low vitamin D levels were more than twice as likely to develop any kind of dementia.
"During this hottest of summers, hitting the beach for just 15 minutes of sunshine is enough to boost your vitamin D levels. However, we're not quite ready to say that sunlight or vitamin D supplements will reduce your risk of dementia. Large scale clinical trials are needed to determine whether increasing vitamin D levels in those with deficiencies can help prevent dementia from developing."
Currently around 800,000 people in the UK have dementia, 62% of whom suffer from Alzheimer's. Their numbers are expected to swell to a million by 2021.