Older adults who harbor more vitamin D in their brains may stay mentally sharper, a new study suggests.
Researchers found that when older adults had higher levels of vitamin D in their brain tissue, they tended to perform better on standard tests of memory and thinking. They were also less likely to have dementia or milder cognitive impairments.
Experts stressed that the study does not prove that vitamin D, itself, protects against dementia — a complex brain disease that has many contributors. And no one should start downing supplements based on the findings, they said.
For one, too much vitamin D can be harmful. And the study did not assess how much vitamin D participants were actually getting day to day.
"We have no evidence that getting more than the recommended amount of vitamin D is better for the brain," said senior researcher Sarah Booth, who directs the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University in Boston.
Vitamin D is known to have critical roles such as keeping bones and muscles healthy, as well as supporting immune defenses. But whether it helps shield the aging brain is unclear.
The new study — published Dec. 7 in the journal Alzheimer's & Dementia — adds to a mixed bag of research on vitamin D and the aging brain.
Some studies have found a correlation between low vitamin D levels in the blood and a higher risk of dementia in older adults. Others have not.
Meanwhile, a few trials have tested the effects of vitamin D supplements on older adults' memory and thinking. And so far, there is no clear proof of benefits.
According to Booth, her team wanted to take a step back and ask a basic question: Does vitamin D even get to the brain?
To do that, they studied autopsied brain tissue from older adults who had participated in the Rush Memory and Aging Project before their deaths. That project, begun in the 1990s, is a long-term study looking to better understand normal and abnormal brain aging.
Participants undergo yearly cognitive tests and consent to have their brain tissue donated for study after their deaths.
Booth's team analyzed brain tissue from 290 study participants, who were an average age of 92 when they died.
It turned out that vitamin D was, in fact, present in all of the brain regions the researchers analyzed — including two where Alzheimer's-related abnormalities are known to manifest.
And overall, older adults whose brains harbored greater amounts of vitamin D had typically performed better on the study's cognitive tests. For every doubling in vitamin D concentrations, participants were 25% to 33% less likely to have had dementia or mild cognitive impairment at their last study visit.
The findings show an "interesting possible connection" between brain vitamin D and dementia risk, said Claire Sexton, senior director of scientific programs and outreach for the Alzheimer's Association.
But that does not prove cause-and-effect, said Sexton, who was not involved in the study.
She also pointed to another finding: Brain concentrations of vitamin D did not correlate with any of the Alzheimer's-related brain abnormalities that the researchers assessed -- including the protein clumps known as "plaques." So if the vitamin does protect against dementia, it's not clear how.
Booth said one possibility is that good nutrition, including adequate vitamin D, helps "buffer" the brain against the pathologic changes that mark dementia.
In fact, researchers theorize that various environmental factors -- including education, exercise and mental stimulation — may help older adults maintain their cognitive function for a longer time, even when those brain changes set in.
Sexton said the Alzheimer's Association is leading a study, called U.S. POINTER, which is testing whether a combination of lifestyle measures — including healthy eating and exercise — can preserve cognitive function in older adults who are at increased risk of decline.
As for vitamin D, the body naturally synthesizes it when sunlight hits the skin, but few foods naturally contain it: Fatty fish are one source, while foods such as milk, orange juice and breakfast cereals are fortified with vitamin D.
The recommended intake for adults up to age 70 is 600 IU per day; older people should get 800 IU.
It's not actually clear why some people have more vitamin D in the brain than others. In this study, Booth said, there was only a "modest" correlation between vitamin D levels in the blood and those in the brain. And blood levels of vitamin D were not related to older adults' cognitive test performance.
More research, including those of racially diverse groups, is needed to understand what is going on, Booth said. Most people in this study were white, and few had low blood levels of vitamin D. People with darker skin are at increased risk of being deficient in the vitamin.