Not getting enough vitamin D will double the risk of developing dementia and Alzheimer's in older people, according to a recent study.

Researchers looked at blood levels of vitamin D, which includes vitamin D from food, supplements and sun exposure. Dietary vitamin D is found in fatty fish such as salmon, tuna or mackerel and milk, eggs and cheese.

"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," said study author David J. Llewellyn of the University of Exeter Medical School in the United Kingdom.

For the study, more than 1,600 people older than 65 who were dementia-free had their vitamin D blood levels tested. After an average of six years, 171 participants developed dementia and 102 had Alzheimer's disease.

The study found that people with low levels of vitamin D had a 53 percent increased risk of developing dementia and those who were severely deficient had a 125 percent increased risk compared to participants with normal levels of vitamin D.

The results remained the same after researchers adjusted for other factors that could affect risk of dementia, such as education, smoking and alcohol consumption.

Llewellyn said clinical trials are 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, he said. "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."

The findings were recently published in the online issue of Neurology.