As with so many other perplexing questions about Alzheimer's disease and dementia, researchers are unsure why there seems to be a link between low vitamin D levels and a higher risk of developing these brain diseases.
Now one of the largest studies yet finds that link is even more worrisome: Too little of the vitamin may, in fact, double the risk of developing the devastating conditions.
The new research, which followed nearly 1,700 adults over the age of 65 for about six years, found that those with low blood levels of vitamin D at the beginning of the study had a 53 percent increased risk of developing dementia and a nearly 70 percent higher risk of developing Alzheimer's.
Those whose blood tests indicated they were severely deficient in the vitamin were even worse off, with a 120 to 125 percent increased risk of the two brain conditions - more than double the risk of those with normal vitamin D levels, researchers reported.
Although the scientists had expected to find a link, they were still surprised "that the association was twice as strong as we anticipated," study author David J. Llewellyn, of the University of Exeter Medical School in the United Kingdom, said in a statement. The findings were published online Aug. 6 in the journal Neurology.
While the findings sound dramatic, it's necessary to look at the actual numbers of subjects who suffered cognitive decline.
Of the 1,658 adults in the study, only about 1 in 10 - 171 people - developed dementia or Alzheimer's. Compared to this, the ones who were severely deficient in vitamin D had a 1 in 5 chance of developing the disease, while those who had a low level had a 1 in 6 to 1 in 7 chance of serious cognitive impairment.
The researchers also cautioned that their findings don't prove that low vitamin D causes dementia. It could be that low levels of the vitamin are caused by brain changes, not vice versa, or that too little D is a symptom of poor health that then contributes to declining brain health.
In the body, vitamin D is created when the skin is exposed to sunshine. It's also found in oily fish - such as swordfish, salmon and tuna - and in vitamin D-fortified milk and orange juice, as well as in eggs. The vitamin is needed to maintain strong bones, as well as help the immune system and reduce inflammation.
Vitamin D's role in brain health is less clear. Animal studies have indicated that vitamin D may help clear plaques in the brain associated with Alzheimer's disease, but the process may be different in humans, Llewellyn said.
The findings also don't support older adults taking vitamin D supplements or getting routine blood tests to measure vitamin levels. "Vitamin D testing is expensive, and our results do not establish that routine testing would be useful or cost-effective in primary care" without further research, Llewellyn said in an email.
Until a definite cause can be established, Llewellyn recommended that people boost their vitamin D levels through diet and by spending time outdoors.
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