Two new studies suggest that vitamins D and E might each play important roles in the risk of Alzheimer's disease and dementia.

The first study looked at how a vitamin D deficiency can affect the risk of cognitive decline and is published in the Archives of Internal Medicine. 

For the study, researchers examined the blood levels of vitamin D in 3,325 Americans 65 and older. They defined adequate blood levels as anything about 75 nanomoles per litre of blood, and severely deficient as levels less than 25 nmol/L.

They found the odds of cognitive impairment were about 42 per cent higher in those who were deficient in vitamin D (25 to 50 nmol/L), and a startling 394 per cent higher in people who were severely deficient.

"This is the first study to identify a clear link between low vitamin D levels and cognitive decline," said lead author Dr. David J. Llewellyn of the Peninsula Medical School, University of Exeter, in a new release.

"Previous research has been cross-sectional but we have now been able to demonstrate a connection between having low levels of vitamin D and going on to develop cognitive problems."

Llewellyn noted the precise role of vitamin D in brain health is not known, but the vitamin does seem to play a role in vascular health and the clearance of amyloid from the brain.

In an accompanying editorial, Dr. Andrew Grey and Mark Bolland from the Department of Medicine at the University of Auckland, New Zealand, sad they were cautious about any link between vitamin D and cognitive decline.

They said "it seems intuitively unlikely that a single hormone could play a substantial role in preventing or ameliorating the diverse range of diseases that have been linked to low levels of vitamin D."

Instead, they said, a more likely explanation is that low vitamin D is a marker of overall poor health — low sunlight exposure, low physical activity, high adiposity — not the cause of it.

A second study, this time published in the July issue of the Archives of Neurology suggests that eating foods rich in vitamin E might help lower risks of developing dementia.

The study of about 5,400 people aged 55 and older found that those who reported they had the most vitamin E in their diet -- 18.5 milligrams per day, on average -- were 25 per cent less likely to develop dementia than their counterparts who got the least vitamin E on their diet, about 9 milligrams per day.

Elizabeth R. Devore, of Erasmus Medical Center in Rotterdam, Netherlands, and colleagues also looked at how much vitamin C, beta-carotene and flavonoids the participants consumed, but only dietary vitamin E seemed to be related to dementia risk.

Vitamin E can be found in whole grains, leafy green vegetables, nuts and seeds, but most participants in the study got their vitamin E from margarine, oils, and mayonnaise.

Vitamin E is an antioxidant that might help protect the body from damage caused by harmful molecules called free radicals. These free radicals cause oxidative stress, which is thought to play a role in the degeneration of nervous system cells and the development of Alzheimer's disease.

"The brain is a site of high metabolic activity, which makes it vulnerable to oxidative damage, and slow accumulation of such damage over a lifetime may contribute to the development of dementia," the authors write.

"In particular, when beta-amyloid (a hallmark of pathologic Alzheimer's disease) accumulates in the brain, an inflammatory response is likely evoked that produces nitric oxide radicals and downstream neurodegenerative effects. Vitamin E is a powerful fat-soluble antioxidant that may help to inhibit the pathogenesis of dementia."

Future studies are needed to evaluate how dietary intake of antioxidants might reduce the risk of developing dementia, the authors conclude.

Both studies were presented at the Alzheimer's Association International Conference in Honolulu.