A 2026 study of nearly 800 adults found that higher vitamin D levels at an average age of 39 were associated with lower levels of tau, a brain protein linked to Alzheimer's disease, on brain scans taken 16 years later.*
793 adults. Average age 39 at baseline.
PET brain scans 16 years later.
That's a long view on vitamin D and brain aging. So I went and read the paper.
WHAT THE STUDY ACTUALLY WAS
The Mulligan et al. paper was published in Neurology Open Access, an official journal of the American Academy of Neurology, on April 1, 2026. It was led by Dr. Martin Mulligan and Professor Emer McGrath at the University of Galway, in collaboration with Boston University, the Framingham Heart Study, and UT Health San Antonio.
The participants came from the Framingham Heart Study Generation 3 cohort, the same long-running Massachusetts dataset that has been tracking heart and brain health since 1948.
Vitamin D levels in the blood (serum 25-hydroxyvitamin D) were measured between 2002 and 2005, when participants were on average 39 years old. PET brain scans came an average of 16 years later, between 2016 and 2019. The scans measured two Alzheimer's-related proteins: tau and amyloid beta. None of the participants had dementia at baseline.
The analyses were adjusted for age, sex, time between blood draw and scan, depressive symptoms, season, smoking, blood pressure, and antihypertensive use. The findings also held in sensitivity models that excluded participants who were taking vitamin D supplements.
WHAT THE DATA SHOWED
Higher baseline serum 25(OH)D was significantly associated with lower tau deposition on brain PET 16 years later.* The effect held both for global tau across the cortex and for the "composite tau" regions known to be hit earliest in Alzheimer's disease: the entorhinal cortex, parahippocampal gyrus, fusiform gyrus, amygdala, and inferior and middle temporal cortices.
One thing stood out. Higher vitamin D was linked to less tau, but not to amyloid beta, the other classic Alzheimer's marker. Of the two proteins, tau lines up more directly with cognitive symptoms in many studies. So the signal showed up in the marker most tied to how the disease unfolds, and not in the one that often shows up in healthy brains too.
WHY TAU
Tau is a protein neurons use to maintain their internal structure. When it misfolds and clumps into "tangles," it damages those neurons. Tau buildup is one of the defining brain pathologies of Alzheimer's disease, and where tau spreads in the brain often tracks closely with which functions go first.
Higher vitamin D at age 39 was associated with less of it 16 years later. That's what this study found.*
WHAT IT DOESN'T MEAN
This is an observational study. Association, not causation.
It doesn't prove that vitamin D directly lowers tau in the brain. It doesn't prove that taking a vitamin D supplement prevents Alzheimer's. The authors say so themselves and call for clinical trials to test the question properly. People with higher vitamin D earlier in life may also be doing other things differently: more sun, more activity, better diet, better sleep. The researchers controlled for a lot of those, but they cannot control for everything.
The blood level was also measured once, at baseline. A single snapshot at age 39 doesn't tell you what someone's vitamin D status looked like across the next decade and a half.
The authors describe vitamin D as "a potentially modifiable risk factor." Not a treatment. Not a cure. A factor.
What the study does do is add another data point to a question that keeps coming back: does the foundational stuff you do earlier matter more than we have been giving it credit for?
WHY THE TIMING MATTERS
From Dr. Mulligan in the press release: "Mid-life is a time where risk factor modification can have a greater impact."
That's the part worth chewing on. This isn't a study of people in their 70s with cognitive complaints. The blood draws happened at age 39. The brains in question belong to people now in their mid-50s. That window (your 30s and 40s) is where the brain seems to set up its later trajectory.
The cutoff the study used for "high" vitamin D was 30 ng/mL. Roughly 70% of Americans sit below it.
WHERE THIS FITS
Vitamin D acts more like a hormone than a vitamin.* Receptors for it sit throughout the body, including in brain regions affected by tau pathology.* This one study doesn't prove anything about preventing dementia. But it lines up with a broader picture: vitamin D keeps showing up in research on brain aging that's hard to ignore, and the foundational stuff you do earlier seems to compound.
If 70% of Americans are short on it, we are already running the experiment on ourselves. Worth thinking about whether you want to keep being part of the 70%.
Sources
Mulligan MD, Scott MR, Yang Q, et al. "Association of Circulating Vitamin D in Midlife With Increased Tau-PET Burden in Dementia-Free Adults." Neurology Open Access, April 2026.
American Academy of Neurology. "Is vitamin D associated with lower levels of Alzheimer's biomarkers?" Press release, April 2026.
University of Galway. "Higher vitamin D levels may be linked to lower levels of Alzheimer's biomarkers." April 2026.
Forrest KYZ, Stuhldreher WL. "Prevalence and correlates of vitamin D deficiency in US adults." Nutrition Research, 2011.
Frequently Asked
Does vitamin D protect the brain?
A 2026 study from the Framingham Heart Study followed 793 adults from an average age of 39 and found that higher vitamin D levels at baseline were associated with lower tau, a brain protein linked to Alzheimer's disease, on brain PET scans 16 years later.* The study showed an association, not cause and effect, and the authors call for clinical trials to test whether vitamin D supplementation earlier in life affects long-term brain health.
What did the 2026 Framingham vitamin D and brain aging study find?
The study found that higher serum 25-hydroxyvitamin D at an average age of 39 was significantly associated with lower tau deposition on brain PET 16 years later.* The association was strongest in brain regions known to be affected earliest in Alzheimer's disease. No association was seen with amyloid beta, the other major Alzheimer's biomarker. Published in Neurology Open Access, April 2026.
What is tau and why does it matter for Alzheimer's?
Tau is a protein neurons use to maintain their internal structure. When it misfolds and clumps into tangles, it damages neurons and is one of the defining brain pathologies of Alzheimer's disease. Tau spread in the brain often tracks with which cognitive functions decline first, which is why researchers use it as a key biomarker.
When does vitamin D matter most for brain aging?
The 2026 Framingham study measured vitamin D at an average age of 39 and found that level was associated with brain biomarkers 16 years later.* The authors describe earlier adulthood as a window where modifying risk factors may have a larger long-term impact, since brain changes seen later in life often take decades to develop. More research is needed to confirm whether changing vitamin D status earlier in life alters outcomes.
How much vitamin D is considered enough?
The 2026 Framingham study used a threshold of 30 ng/mL serum 25-hydroxyvitamin D to define high vitamin D status, which aligns with one of the common clinical cutoffs. Roughly 70% of Americans sit below 30 ng/mL. Routine testing isn't part of most standard physicals, so most people don't know where they stand.