A severe lack of vitamin D can double the risk of Alzheimer's in older people, a study has found.
Even a moderate deficiency was found to increase the chances of developing the disease by 69pc.
Scientists were surprised by the 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.
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 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 age and our latest results do not demonstrate that low vitamin D levels cause dementia.
"Our findings are very encouraging, and even if a small number of people could benefit, this would have enormous public health implications."