Taking vitamin D supplements appeared to ward off dementia in a new, large-scale study

• There were 40% fewer dementia diagnoses among people who took vitamin D supplements in a study.
• Of the nearly 2,670 people who developed dementia in the study, 75% didn't take vitamin D.
• Past research linked low vitamin D levels with dementia and stroke.

People who take vitamin D supplements may have a lower risk of developing dementia, a new, large-scale study suggests.

Taking vitamin D supplements was associated with living dementia-free longer in the study published in Alzheimer's & Dementia: Diagnosis, Assessment & Disease Monitoring on Wednesday. 

The study, conducted by researchers in the UK and Canada, involved more than 12,000 participants with an average at of 71, who were all dementia-free at the beginning of the trial. About 4,600 (or 37%) of the participants reported taking vitamin D supplements. These included calcium-vitamin D, cholecalciferol, and ergocalciferol. 

Ten years after the start of the trial, nearly 2,670 participants developed dementia. Of the dementia patients, just 679 (or 25%) took vitamin D supplements, while about 2,000 (or 75%) did not. 

Researchers found the group of participants who reported taking vitamin D supplements had 40% fewer dementia diagnoses than the rest of the cohort. Women and people with normal cognition, as opposed to those showing signs of memory and congitive ability loss known as mild cognitive impairment, were especially less likely to develop dementia with vitamin D supplementation.

The group of participants who took vitamin D supplements was more educated, less likely to be depressed, included more women, and fewer Black people. Having dark-colored skin reduces the amount of vitamin D your skin makes, according to the National Institutes of Health. 

"Overall, we found evidence to suggest that earlier supplementation might be particularly beneficial, before the onset of cognitive decline," said Zahinoor Ismail, a professor at the University of Calgary and the study's lead author, in a press release.

The paper is limited because researchers only observed the link between supplements and dementia using self-reported data, not through randomly assigning participants to take either vitamin D or a placebo, said Byron Creese, a lecturer of neuroscience at the University of Exeter and the study's co-author, in the press release. Trials that randomly assign placebos or treatments without participants knowing which they take are considered the "gold standard" in science research. 

Prior research has indicated a link between low vitamin D and dementia. An Australian analysis from last year, which used data from 300,000 UK Biobank participants, found having low levels of vitamin D was associated with increased risk for dementia and stroke, and the risk was highest in people who were deficient in the vitamin.

The study can't prove vitamin D reduces the risk of dementia 

Experts in the field who did not work on the study highlighted its limitations. Susan Fairweather-Tait, professor of mineral metabolism at the University of East Anglia, UK, said that the findings are not strong enough to draw firm conclusions on the link between vitamin D and dementia, in part because it only looked into vitamin D supplementation, not how much a person got in other ways such as sunlight and food.

Gill Livingston, professor in psychiatry of older people at UCL, UK, said the groups who did and didn't take vitamin D were "strikingly different." She said it's important to acknowledge that some of the participants had MCI, with research suggesting around 40% of such people eventually developing dementia, and it would be expected that dementia rates in those with MCI would be higher in the study. And the findings are further complicated by the fact that those who took the supplements were more likely to be white and less likely to be depressed.

Professor Tara Spires-Jones of the UK Dementia Research Institute at The University of Edinburgh said the study cannot prove that taking vitamin D supplements lowers the risk of dementia as they "could have healthier lifestyles in general and something else could be causing the lower dementia risk."

Vitamin D supplements can help when exposure to sunlight is limited

People typically get vitamin D through the sun, but foods like fatty fish and beef liver contain high levels of vitamin D. Federal dietary guidelines recommend healthy adults get about 600 international units of vitamin D a day in total. 

In some countries where people get limited exposure to sunlight in the winter months, such as the UK, vitamin D supplements are recommended. The only way to know whether you are deficient in vitamin D is through a blood test.

Nutrition experts previously told Insider to consult a doctor before starting vitamin D supplements, as taking too many can result in constipation, nausea, vomiting, confusion, and muscle cramps.