Older people who have a severe vitamin D deficiency have an increased risk of developing dementia, a study has suggested.
UK researchers, writing in Neurology, looked at about 1,650 people aged over 65.
This is not the first study to suggest a link - but its authors say it is the largest and most robust.
However, experts say it is still too early to say elderly people should take vitamin D as a preventative treatment.
There are 800,000 people with dementia in the UK with numbers set to rise to more than one million by 2021.
Vitamin D comes from foods - such as oily fish, supplements and exposing skin to sunlight.
However older people's skin can be less efficient at converting sunlight into Vitamin D, making them more likely to be deficient and reliant on other sources.
The international team of researchers, led by Dr David Llewellyn at the University of Exeter Medical School, followed people for six years.
All were free from dementia, cardiovascular disease and stroke at the start of the study.
At the end of the study they found the 1,169 with good levels of vitamin D had a one in 10 chance of developing dementia. Seventy were severely deficient - and they had around a one in five risk of dementia.
'Delay or even prevent'
Dr Llewellyn 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."
He said further research was needed to establish if eating vitamin D rich foods such as oily fish - or taking vitamin D supplements - could "delay or even prevent" the onset of Alzheimer's disease and dementia.
But Dr Llewellyn added: "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."
Dr Clare Walton, research communications manager at Alzheimer's Society, said: "A study like this can't tell us whether being deficient in vitamin D can cause dementia.
"At the moment we are still unclear how the two might be linked and there is even a possibility another unknown factor could cause someone to have both dementia and low vitamin D levels.
"If this were the case, using supplements or sun exposure to raise vitamin D levels might have no effect on the development of dementia or Alzheimer's disease."
She added: "We need to see large clinical trials to test directly whether increasing vitamin D levels could be a good way to reduce dementia in the over 65s."