Researchers combined results from 22 studies involving over 53,000 people to understand if vitamin D affects dementia risk. They found that people with low vitamin D levels had about 49% higher chances of developing dementia compared to those with high levels. The connection appears to be dose-dependent, meaning more vitamin D was linked to lower dementia risk. However, the researchers emphasize this doesn’t prove vitamin D prevents dementia—it just shows a relationship. They’re calling for actual clinical trials where some people take vitamin D supplements and others don’t to determine if it truly makes a difference.

The Quick Take

  • What they studied: Whether people with higher vitamin D levels in their blood have lower chances of developing dementia (memory loss and thinking problems)
  • Who participated: Over 53,000 people from 22 different research studies around the world. The studies tracked people over time to see who developed dementia and compared their vitamin D levels
  • Key finding: People with the lowest vitamin D levels were 49% more likely to develop dementia than those with the highest levels. For every small increase in vitamin D (10 units), dementia risk dropped by about 1.2%
  • What it means for you: While low vitamin D appears connected to higher dementia risk, this doesn’t mean taking vitamin D supplements will prevent memory loss. The connection is real but small. If you’re concerned about dementia risk, talk to your doctor about vitamin D testing and whether supplementation makes sense for you

The Research Details

This is a meta-analysis, which means researchers looked at 22 existing studies and combined their results to see the bigger picture. They searched medical databases (Cochrane Library, PubMed, and Embase) for all studies published through October 2024 that measured vitamin D levels and tracked whether people developed dementia.

The researchers used a special statistical method to combine results from all these different studies. They looked at whether there was a straight-line relationship between vitamin D levels and dementia risk (the more vitamin D, the lower the risk in a consistent pattern). They also checked if the relationship might be curved or have a threshold effect.

This approach is powerful because it combines information from thousands of people, making patterns clearer than any single study could show. However, because all the included studies were observational (watching what happens naturally rather than testing an intervention), the researchers couldn’t prove that vitamin D actually prevents dementia.

Meta-analyses are important because individual studies can have conflicting results or small sample sizes that make findings uncertain. By combining 22 studies with over 53,000 participants, this research provides a much clearer picture of the vitamin D-dementia relationship. The dose-response analysis (showing how much vitamin D relates to how much risk reduction) is particularly valuable because it suggests a real biological relationship rather than just a coincidence

The studies showed moderate consistency (I² = 37.8%), meaning results were fairly similar across different research groups. The findings held up when researchers looked at different subgroups separately. However, all included studies were observational, meaning they watched people naturally rather than randomly assigning them to take vitamin D or a placebo. This limits our ability to say vitamin D causes lower dementia risk—there could be other explanations for the connection

What the Results Show

The main finding was striking: people in the lowest vitamin D category had a 49% higher risk of dementia compared to those in the highest category. To put this in perspective, if 100 people with high vitamin D had 10 cases of dementia, you’d expect about 15 cases in 100 people with low vitamin D.

The dose-response analysis revealed a linear relationship, meaning the connection was consistent across the range of vitamin D levels studied. For every 10-unit increase in vitamin D (measured in nmol/L), dementia risk decreased by about 1.2%. This small but consistent effect across the entire range suggests a real relationship rather than just a threshold effect where only very low levels matter.

The researchers found no evidence of a curved relationship or threshold, meaning there wasn’t a point where more vitamin D stopped helping. The relationship appeared straight and consistent across all vitamin D levels studied.

When researchers looked at different groups separately (by age, sex, geographic location, and study quality), the findings remained consistent. This suggests the vitamin D-dementia connection isn’t limited to specific populations but appears across different groups of people. The consistency across subgroups strengthens confidence in the overall finding

This meta-analysis adds to growing evidence that vitamin D may play a role in brain health and dementia prevention. Previous individual studies have suggested this connection, but results were mixed. By combining all available evidence, this analysis provides stronger support for the relationship. However, the researchers note that while the association is statistically significant, the actual size of the effect is modest

The biggest limitation is that all included studies were observational—researchers watched what happened naturally rather than randomly assigning people to take vitamin D or a placebo. This means we can’t prove vitamin D prevents dementia. People with higher vitamin D might differ in other ways (exercise, diet, overall health) that actually protect against dementia. Additionally, vitamin D levels were measured at one point in time in most studies, but dementia develops over years, so we don’t know if vitamin D levels stayed the same. Finally, different studies measured vitamin D differently and used different definitions of dementia, which could affect results

The Bottom Line

Based on this research alone, we cannot recommend vitamin D supplements specifically to prevent dementia. However, maintaining adequate vitamin D levels is important for overall bone and immune health. If you’re concerned about dementia risk or have low vitamin D, discuss testing and supplementation with your doctor. The evidence suggests a connection, but it’s not yet strong enough to be a primary dementia prevention strategy. Confidence level: Moderate for the association, Low for recommending supplementation based on this evidence alone

This research is relevant to anyone concerned about dementia prevention, especially those with known vitamin D deficiency. It’s particularly important for older adults, people with limited sun exposure, and those with conditions affecting vitamin D absorption. However, people shouldn’t start high-dose vitamin D supplements based solely on this research. Those already taking vitamin D for bone health or other reasons can feel reassured that maintaining adequate levels may have additional brain health benefits

Dementia develops over many years, so any protective effect from vitamin D would take years to become apparent. You wouldn’t expect to notice changes in memory or thinking in weeks or months. If you start vitamin D supplementation, think of it as a long-term health investment rather than something with immediate effects

Want to Apply This Research?

  • Track vitamin D intake (through sun exposure, food sources like fatty fish and fortified milk, and supplements if taking them) weekly. Also track any cognitive changes or memory concerns monthly to establish a baseline and monitor over time
  • If your doctor recommends vitamin D supplementation, set up a daily reminder to take it consistently. Log your supplement intake in the app and note any dietary sources of vitamin D (fatty fish, egg yolks, fortified milk). Consider tracking sun exposure time as a natural vitamin D source
  • Establish a baseline by noting current memory and thinking abilities. Check in quarterly to assess any changes. If you get blood work done, log your vitamin D levels when available. Track other dementia risk factors (exercise, sleep quality, cognitive activities) to see the complete picture of your brain health habits

This research shows an association between vitamin D levels and dementia risk but does not prove that vitamin D supplements prevent dementia. These findings come from observational studies, not clinical trials. Before starting any vitamin D supplementation, especially in high doses, consult with your healthcare provider. This information is for educational purposes and should not replace professional medical advice. Individual vitamin D needs vary based on age, health status, and other factors that only your doctor can assess. If you have concerns about dementia risk or cognitive changes, speak with a healthcare professional for personalized evaluation and recommendations.