Can vitamin D lower dementia risk?
Quite possibly, a team of British and Canadian researchers report.
In their study, investigators spent roughly a decade tracking more than 12,000 older people. None had dementia at the start of the study period. In the end, the team determined that those who had been taking vitamin D supplements during that time appeared to face a 40% lower risk for dementia, compared with those who had never taken the supplements.
Even so, Claire Sexton, senior director of scientific programs and outreach at the Alzheimer's Association, cautioned that much more research is needed to better understand a possible link between vitamin D and dementia risk.
For one thing, she noted that the study team did not track how much vitamin D supplementation any of the participants took, nor how long they had been taking them. Similarly, overall patient vitamin D levels were never assessed — either at the study launch or conclusion.
In addition, the study was observational, Sexton added, meaning at no point were patients told to take, or not to take, vitamin D. That means the study cannot prove that vitamin D actually causes dementia risk to fall.
Still, study author Dr. Zahinoor Ismail said that fresh evidence of vitamin D's power against dementia has “great biological plausibility.”
For example, prior research indicates that people with genetic mutations that render them low in vitamin D “have much greater development of Alzheimer's than those who don't,” said Ismail, a professor of psychiatry, neurology, epidemiology and pathology with the Hotchkiss Brain Institute and O'Brien Institute for Public Health at the University of Calgary in Alberta, Canada.
Vitamin D, he added, has also been shown to be anti-inflammatory and key to preventing the build-up of both abnormal protein “tangles” and amyloid beta plaque in the brain. Both are hallmarks of Alzheimer's, with the latter often cited as a likely root cause.
“So, I'm not at all surprised that we found something,” said Ismail.
The study team noted that an estimated 50 million people worldwide now live with some degree of dementia; that figure is projected to triple by 2050.
At the same time, vitamin D deficiency is thought to be a concern among roughly 1 billion people.
To explore the potential protective benefit of vitamin D supplementation among older Americans, the study team focused on thousands of patients who had previously enrolled in one of 40 Alzheimer's Disease Research Centers across the United States.
On average, participants were 71 years of age, and 80% were white. The patient pool included both those with no signs of mental impairment as well as those struggling with so-called mild cognitive impairment (MCI).
Out of the entire pool of patients, about 2,700 developed dementia. But the risk appeared to differ in relation to vitamin D habits.
For example, the team found that over the course of the study timeframe, a little less than 4 in 10 had taken vitamin D, and dementia risk in this pool of patients was nearly 15%.
But among the roughly 60% who had never taken vitamin D supplements, that figure shot up to 26%.
And after taking into account a variety of factors — including age, gender, race, depression status and educational background — the team concluded that vitamin D supplementation was linked to a 40% lower risk for dementia, compared with no exposure.
Taking vitamin D was also linked to notably better survival rates, the investigators found.
For example, about 84% of those who had taken vitamin D supplements were still alive five years out from the study launch; that figure dropped to just over 68% among those who had never taken vitamin D.
The findings were published online March 1 in Alzheimer's & Dementia: Diagnosis, Assessment & Disease Monitoring, a publication of the Alzheimer's Association.
The investigators noted that the apparent protective effect of vitamin D was notably stronger among those with no prior mental impairment, compared with those who already had MCI. That suggests that vitamin D supplementation might be most impactful in warding off dementia when started earlier rather than later.
But Ismail said that one of the most important and surprising findings was a sizeable gender gap in vitamin D's impact on dementia risk. While vitamin D supplementation was linked to a 50% lower dementia risk among women, that figure fell to just 25% among men.
Why? Ismail said it could have to do with plummeting estrogen levels during perimenopause and menopause, given that “estrogen helps to activate vitamin D. So, it could be that women have relatively greater vitamin D deficiency than men, so supplements have a greater effect.”
What are seniors to think? Should vitamin D supplements now figure into everyone's daily health regimen? No, said Sexton.
Given the “significant limitation[s]” of the latest research, “it is not recommended to start vitamin D supplementation to reduce dementia risk,” she said, though Sexton advised anyone contemplating such a move to “always talk to your health care provider before starting supplements or other dietary interventions, and let them know which ones you are already taking."
Ismail, who takes vitamin D himself, is more equivocal.
“While vitamin D can do good, it can also do harm,” he acknowledged, highlighting the adverse impact that mega-dosing can have on bone health. “But I would say that if people are going to take it anyway they should do so cautiously,” and always within the recommended dosing limits of a patient's respective country.
Vitamin D is often referred to as “the sunshine vitamin” because sunlight is one source of this nutrient. In addition, certain types of food, including eggs, fatty fish and seafood can be natural dietary sources of vitamin D. The vitamin is also often added to food staples — such as milk and breakfast cereals.
There's more on dementia at the Alzheimer's Association.
SOURCES: Zahinoor Ismail, MD, professor, psychiatry, neurology, epidemiology and pathology, Hotchkiss Brain Institute and O'Brien Institute for Public Health, University of Calgary, Alberta, Canada; Claire Sexton, DPhil, senior director, scientific programs & outreach, Alzheimer's Association, Chicago; Alzheimer's & Dementia: Diagnosis, Assessment & Disease Monitoring, March 1, 2023, online