A new study has found that Vitamin D deficiency may lead to faster decline of mental function.

For the study, the researchers followed over 380 people or an average of five years and found that those with dementia had the lowest levels of vitamin D.

"It is unclear what vitamin D might be doing," said study author Joshua Miller, chair of the department of nutritional sciences at Rutgers University School of Environmental and Biological Sciences in New Brunswick, N.J.

"There is good evidence that vitamin D gets into all cells of the body, including the brain," Miller said, so it's possible that vitamin D protects the brain from developing the plaques and tangles that are associated with Alzheimer's disease.

Unfortunately, "there's a good chance that most people over 75 in the United States are vitamin D-deficient," he noted.

Miller, however, warned that it is yet to be proved that taking vitamin D supplements will slow mental decline, as the just established an association between the two.

"All we can say is that supplements might be helpful to you," he said. "And the downside of taking supplements is very small."

The findings of the study were published online on September 14 in the journal JAMA Neurology.

According to the US National Institutes of Health, the recommended daily intake of vitamin D for older adults is 600 to 800 IU. Vitamin D is found in fortified foods, such as milk, orange juice, cereals and yogurt. Fish, egg yolks and liver also contain the vitamin.

Miller and his colleagues defined four levels of vitamin D in blood: deficiency as less than 12 nanograms per milliliter (ng/mL); insufficient as 12 to less than 20 ng/mL; adequate as 20 to less than 50 ng/mL; and high as 50 ng/mL or more.