What Your Vitamin D Levels in Your 30s and 40s Could Mean for Your Brain Decades Later

A study published in April 2026 in Neurology Open Access, a journal of the American Academy of Neurology, followed nearly 800 adults over sixteen years to examine how their vitamin D levels in early midlife correlated with signs of dementia risk later on. Participants had their blood vitamin D measured at an average age of 39. None showed any signs of cognitive decline at that point. About a third had what researchers considered low vitamin D — below 30 nanograms per milliliter.

Sixteen years later, when participants were in their mid-50s, they underwent PET scans to measure tau protein in the brain. Tau is a key biomarker for Alzheimer’s disease; when it misfolds and accumulates, it forms the toxic tangles that choke off healthy brain cells and accelerate cognitive decline.

The results were notable: those with higher vitamin D levels in their late 30s and 40s had meaningfully lower tau deposits in their brains years later. “Higher vitamin D levels in midlife may offer protection against developing these tau deposits in the brain,” said study author Dr. Martin David Mulligan of the University of Galway, “and low vitamin D levels could potentially be a risk factor that could be modified and treated to reduce the risk of dementia.”

Importantly, the researchers were careful to note that this study shows a relationship, not proof of direct cause and effect. More research is needed.

This latest study didn’t emerge in a vacuum. It adds to a steadily accumulating body of evidence linking vitamin D status to dementia risk.

A 2025 meta-analysis published in Frontiers in Neurology pooled data from 22 observational studies encompassing more than 53,000 participants. The findings were striking: people with the lowest vitamin D levels faced a “49% higher risk of dementia” compared to those with the highest levels. The analysis also found a consistent dose-response pattern — for every 10 nmol/L increase in vitamin D, dementia risk fell by about 1.2%.

A separate large study of over 12,000 adults found that vitamin D supplementation was associated with a “40% lower risk of dementia over a decade”. After five years, 84% of supplement users were dementia-free compared to 68% of non-users — a meaningful gap. Notably, the protection extended even to those carrying the APOE e4 gene, a key genetic risk factor for Alzheimer’s disease. This is significant data!