Researchers in Japan studied whether vitamin D protects the brain from dementia (severe memory loss). They followed nearly 14,000 adults for about 10 years, measuring their vitamin D levels and tracking who developed dementia. Surprisingly, they found that people with the highest vitamin D levels actually had a higher risk of developing dementia, especially older adults. This challenges the common belief that more vitamin D always means better brain health. The findings suggest that the relationship between vitamin D and dementia is more complicated than previously thought, and more research is needed to understand why.

The Quick Take

  • What they studied: Whether vitamin D levels in the blood protect people from developing dementia (serious memory loss that affects daily life)
  • Who participated: Nearly 14,000 Japanese adults aged 45-74 years old. Researchers measured their vitamin D levels between 1995-1998 and then tracked 1,858 people who developed dementia over the next 10-20 years, comparing them to 2,223 people who stayed healthy
  • Key finding: People with the highest vitamin D levels had a 29% higher risk of developing dementia compared to those with the lowest levels. For older adults (65-74 years), the risk was even higher at 73% more likely to develop dementia
  • What it means for you: Taking high-dose vitamin D supplements to prevent dementia may not be helpful and could potentially be harmful, especially for older adults. This doesn’t mean vitamin D is bad—your body needs it—but more isn’t always better. Talk to your doctor about appropriate vitamin D levels for you rather than assuming higher is better

The Research Details

This was a case-cohort study, which is a special type of research design that’s efficient for studying rare diseases like dementia. Researchers started with a large group of nearly 14,000 Japanese adults and measured their vitamin D levels in blood samples taken between 1995-1998. They then followed these people for 10-20 years (until 2016) to see who developed disabling dementia—the kind that requires long-term care. Instead of tracking everyone, they focused on the 1,858 people who developed dementia and compared them to a randomly selected group of 2,223 people who stayed healthy. This approach saves time and money compared to following everyone in the original group.

The researchers divided people into four groups based on their vitamin D levels, with adjustments made for sex and season (since vitamin D levels naturally vary by season). They used statistical methods to calculate the risk of dementia in each group, adjusting for other factors that might affect dementia risk like age, diet, physical activity, and other health conditions. They also looked separately at men versus women and at younger (45-64 years) versus older (65-74 years) adults to see if vitamin D affected different groups differently.

This research design is important because dementia is relatively rare, so following a huge group of people for many years would be very expensive and time-consuming. By focusing on people who actually developed dementia and comparing them to a sample of healthy people, researchers can get reliable answers much faster. The long follow-up period (10-20 years) is also crucial because dementia develops slowly, so short-term studies wouldn’t capture the real relationship. Additionally, measuring vitamin D at the beginning of the study and then tracking dementia years later helps establish whether vitamin D came before dementia, which is important for understanding cause and effect.

This study has several strengths: it’s based on a large, well-established Japanese health study with careful record-keeping, vitamin D was measured in a lab (not estimated), and dementia diagnoses came from official long-term care insurance records (very reliable). However, there are limitations: vitamin D was only measured once at the beginning, so researchers couldn’t see if levels changed over time; the study only included Japanese adults, so results might not apply to other populations; and the study couldn’t determine which type of dementia people developed (Alzheimer’s versus other types). The unexpected finding that high vitamin D increased dementia risk is surprising and needs confirmation in other studies before drawing firm conclusions.

What the Results Show

The main finding was unexpected and somewhat counterintuitive: people with the highest vitamin D levels had a 29% increased risk of developing dementia compared to those with the lowest levels. This association held true for both men and women. However, when researchers looked at different age groups, the pattern became clearer. Older adults (ages 65-74) with the highest vitamin D levels had a 73% higher risk of developing dementia, which is a substantial increase. In contrast, middle-aged adults (ages 45-64) with high vitamin D levels showed no increased dementia risk. This age difference is important because it suggests that the relationship between vitamin D and dementia may change as people get older.

The researchers also noted that this finding contradicts previous studies suggesting that vitamin D protects the brain. Several earlier studies had shown that vitamin D has properties that might protect nerve cells from damage, which led many people to believe that higher vitamin D levels would prevent dementia. This study suggests that relationship is either not true or much more complicated than previously thought.

It’s important to note that this is an observational study, meaning researchers observed what happened naturally rather than randomly assigning people to take vitamin D or not. This means they can show that high vitamin D and dementia are associated, but they cannot prove that high vitamin D causes dementia. There could be other explanations for why people with high vitamin D developed more dementia.

The study found no significant differences in the vitamin D-dementia relationship between men and women, suggesting that sex doesn’t change how vitamin D affects dementia risk. The researchers also noted that the association was strongest in older adults, which raises questions about whether vitamin D’s effects on the brain change with age. The study used official long-term care insurance records to identify dementia cases, which is a very reliable method but means they could only study dementia severe enough to require care assistance—milder cases might have been missed.

This finding contradicts several earlier studies that suggested vitamin D protects the brain and prevents dementia. Those studies were often based on laboratory research showing that vitamin D can reduce inflammation and protect nerve cells, or on smaller observational studies. However, other recent large studies have also failed to find a protective effect of vitamin D against dementia, so this research adds to growing evidence that the simple story of ‘more vitamin D equals better brain health’ is not accurate. The findings suggest that the relationship between vitamin D and brain health is more nuanced and may depend on factors like age, baseline vitamin D status, and how vitamin D is measured.

Several important limitations should be considered: First, vitamin D was measured only once at the beginning of the study, so researchers couldn’t see if vitamin D levels changed over time or if people took supplements later. Second, the study only included Japanese adults, and vitamin D needs and effects might differ in other populations with different genetics, diets, and sun exposure. Third, the study couldn’t distinguish between different types of dementia (like Alzheimer’s disease versus vascular dementia), which might respond differently to vitamin D. Fourth, the unexpected finding that high vitamin D increased dementia risk is surprising and hasn’t been fully explained, so it needs to be confirmed in other studies before drawing firm conclusions. Finally, the study is observational, so while it shows an association between high vitamin D and dementia, it cannot prove that vitamin D causes dementia—there could be other explanations for the connection.

The Bottom Line

Based on this study alone, there is no evidence that taking high-dose vitamin D supplements prevents dementia. In fact, this study suggests that very high vitamin D levels might be associated with increased dementia risk, particularly in older adults. However, this doesn’t mean you should avoid vitamin D entirely—your body needs adequate vitamin D for bone health, immune function, and other important processes. The recommendation is to maintain normal, healthy vitamin D levels (typically 20-50 ng/mL) rather than pursuing very high levels. Talk to your doctor about what vitamin D level is appropriate for you based on your age, health status, and sun exposure. Don’t take high-dose vitamin D supplements without medical supervision, especially if you’re older.

This research is most relevant to older adults (65 years and older) who are considering taking high-dose vitamin D supplements to prevent dementia. It’s also important for people who have been told they have low vitamin D and are considering supplementation. Healthcare providers should be aware of these findings when counseling patients about vitamin D supplementation. People with normal vitamin D levels don’t need to change anything based on this study. Younger adults (under 65) showed no increased dementia risk with high vitamin D in this study, though the findings still suggest that pursuing very high levels isn’t necessary.

Dementia develops slowly over many years, so any effects of vitamin D would take a long time to appear. In this study, researchers followed people for 10-20 years before seeing dementia develop. This means you shouldn’t expect to see changes in dementia risk in weeks or months—these are long-term processes. If you’re concerned about dementia prevention, focus on evidence-based strategies like staying mentally active, exercising regularly, maintaining social connections, eating a healthy diet, and managing heart health, which have stronger evidence for dementia prevention than vitamin D supplementation.

Want to Apply This Research?

  • Track your vitamin D supplementation (dose and frequency) alongside cognitive function assessments. Users can log daily vitamin D intake and monthly note any changes in memory, concentration, or mental clarity. This helps identify personal patterns and provides data to discuss with healthcare providers.
  • Instead of focusing on maximizing vitamin D intake, use the app to maintain consistent, moderate vitamin D levels through a combination of safe sun exposure, dietary sources (fatty fish, fortified milk), and appropriate supplementation as recommended by your doctor. Set reminders to take prescribed vitamin D at consistent times rather than taking extra doses.
  • Schedule annual vitamin D blood tests and log the results in the app to track whether your levels stay in the healthy range (20-50 ng/mL). Also track cognitive health through regular memory check-ins, mood assessments, and noting any concerns about memory or thinking. Share this data with your healthcare provider during annual check-ups to ensure your vitamin D strategy supports overall health without excessive supplementation.

This study suggests that very high vitamin D levels may be associated with increased dementia risk, particularly in older adults, which contradicts common assumptions about vitamin D supplementation. However, this is observational research and cannot prove that vitamin D causes dementia. Vitamin D is essential for health, and this study does not recommend avoiding vitamin D entirely. Before making any changes to your vitamin D supplementation, diet, or health routine based on this research, consult with your healthcare provider. This information is for educational purposes and should not replace professional medical advice. If you have concerns about dementia risk or vitamin D levels, speak with your doctor about appropriate testing and supplementation for your individual situation.