The connection between Alzheimer's disease (AD) and vitamin D status has been known for some time. A 2015 review of the literature concluded that being deficient in vitamin D presented a 21% greater chance of developing Alzheimer's compared to those with adequate levels of vitamin D above 50 nmol/L. 1 Vitamin D is involved in the clearance of amyloid in the brain. Alzheimer's disease is associated with the decreased presence of amyloid. Studies have also found that vitamin D may help protect the brain against the build-up of tau, another protein associated with the development of dementia. 

The Data

The National Alzheimer's Coordinating Center has one of the largest, oldest (20+ years), and most powerful Alzheimer's datasets, with more than 42 Alzheimer's Disease Research Centers in the US. This data was used to determine whether there is a connection between vitamin D supplementation and decreasing the risk of Alzheimer's. The study was well designed and concluded there is a significant advantage to those who take vitamin D in preventing AD. You can read the study here. Although the benefits are being reported in relative risk reduction, the norm, which I will explain later, the results are nonetheless compelling.

The Study

Researchers at the University of Calgary's Hotchkiss Brain Institute in Canada and the University of Exeter in England explored the relationship between vitamin D supplementation and dementia in more than 12,388 subjects. Two thousand six hundred ninety-six participants with a median age of just over 71 progressed to dementia from 2005 to 2021, representing 21.7% of the total. Nationally, 11% of people 65 and older in the United States have some form of dementia. 

Vitamin D Supplementation Status and Dementia

Participants with previous exposure to any vitamin D supplements were considered the vitamin D-exposed group (D+), while those without any exposure throughout all visits before dementia diagnosis were considered non-exposed (D−). In follow-up visits, participants with no baseline exposure but were exposed to vitamin D were excluded. The type of vitamin D was also considered: calcium–vitamin D, cholecalciferol, and ergocalciferol.

The Numbers Game and Confounding Factors

Based on the raw data, 78.3 percent of the study participants remained dementia-free. The final data calculations for risk presented from the study ignore the healthy patients, a practice called relative risk reduction, which exaggerates outcomes. I will talk more about this later. Nonetheless, the researchers dutifully adjusted for known confounding factors like age, sex, education, race, cognitive diagnosis, depression, and APOE4 (a gene associated with a higher risk for AD), which clarifies their findings. 

The Main Reported Finding

The researchers concluded that a history of vitamin D supplement exposure significantly increased dementia-free survival and decreased the incidence of developing dementia by 40%. These results are stunning. I mentioned that the researchers used relative risk reduction, a normal practice, to enhance the outcome to 40%. Before addressing this, I want to cover some other findings. 

Other Findings

Vitamin D effects were significantly greater in females. Additionally, effects were greater in people with normal cognition compared to those who reported signs of mild cognitive impairment (MCI). The effects of vitamin D were significantly higher in participants who did not possess the APOE4 gene, which causes a greater risk for Alzheimer's dementia. 

Relative Risk

Now let's talk about relative risk reduction and why I'm not too fond of it. Including all 12,388 people in the study, 16 percent of the people that got dementia never took vitamin D, whereas only 5% developed it that had a history of vitamin D supplementation, which is significant proof of the correlation between taking vitamin D and its association to lowering the risk of developing dementia. However, this makes the absolute risk reduction only about 11%, not 40%, because the risk decreased from 16% to 5%. The reduction is far from 40% because that number represents the relative risk reduction which will always represent an over-exaggeration of effectiveness. 

Here's why: relative risk reduction ignores healthy people and skews the results to sound more impressive. Analyses only consider the people who developed illness and compare them to each other as a distinct population separate from those in the study who did not get sick. What we get is the control group equals 100%, but we don't have a 100% risk of developing dementia. If an experimental intervention like giving vitamin D works, the researchers assume it lowers our risk from 100%, which is incorrect and exaggerates the effectiveness. 5 is 31% of 11, but perhaps the adjustments for the confounding factors, such as age, made it 40%. The researchers don't make it clear.

Absolute Risk Reduction

But 11% is still very compelling. Perhaps factoring in the confounding factors could raise it to 15% or more. The researchers have yet to elucidate why vitamin D lowers our risk, but they are designing a study that may uncover the mechanism of action. As mentioned, it may be related to the vitamin D/amyloid/tau connection.

Vitamin D Supplementation

Based on the evidence of the research discussed today, supplementing with vitamin D is an attractive concept. The researchers had no data on the amount or type of vitamin D supplements participants took, but the vitamin D3 variety is the most recommended. 

At the beginning of this post, I mentioned a study demonstrating a 21% lower risk of developing AD with adequate vitamin D status. Supplementing alone only improves risk by around 11%, although testing status while supplementing to ensure you have adequate levels has merit. 

Conclusion

Achieving adequate vitamin D through diet and sun exposure is best. We have been getting it that way for millions of years. Diets with inadequate vitamin D are a recent phenomenon. Dementia and Alzheimer's are new phenomena. Low vitamin D is a new phenomenon. Supplements hold promise for those who don't care or don't want to learn how to eat a nutrient-dense diet that supplies all the needed nutrients. I am not judging anyone. I advise only administering vitamin D with the guidance of a medical practitioner who can test your level, as vitamin D can be toxic.