05/12/2026
Vitamin D isn't really a vitamin. It's a steroid hormone that crosses the blood-brain barrier and acts on receptors throughout the brain, including the hippocampus and entorhinal cortex, where Alzheimer's tau accumulates first.
A Framingham Heart Study analysis tracked this for 16 years.
793 adults. Mean baseline age 39. Vitamin D measured 2002-2005. Brain PET imaging 2016-2019.
Higher midlife 25-hydroxyvitamin D was associated with less tau in the brain regions where Alzheimer's pathology starts: entorhinal cortex, parahippocampal gyrus, fusiform gyrus. The relationship was continuous. More D, less tau, smoothly across the range.
The finding is narrow and specific. Vitamin D was associated with tau (the tangle protein) only. There was no association with amyloid (the plaque protein).
The mechanism that fits: vitamin D modulates GSK-3β, the kinase that phosphorylates tau. It's also anti-inflammatory in brain immune cells. These mechanisms are tau-relevant, not particularly amyloid-relevant.
A few qualifications.
This is observational. It cannot prove vitamin D causes lower tau. People developing early pathology may handle vitamin D differently. Reverse causation can't be ruled out.
The clinical cutoff of 30 ng/mL did not reach statistical significance. Only the continuous relationship did. The data doesn't support "below 30 is bad." It supports "more is generally better, smoothly, across the range studied."
The effect size in absolute terms is small. A measurable association, not dramatic protection.
The study did not test supplementation. It measured naturally varying serum levels. Whether pushing your D up with a pill produces the same association is untested.
Practical framing.
Most vitamin D research focused on adults over 65. By 65, tau accumulation has been happening for two decades. The Framingham data suggests the actionable window is earlier.
A reasonable read: test 25-hydroxyvitamin D in your 30s and 40s. The standard reference range labels 30 ng/mL as "sufficient," but in this analysis the association continued above that level. Higher D tracked with less tau across the range.
Reasonable. Not proven causal.
The popular framing of this study is "vitamin D prevents Alzheimer's." The actual finding: a midlife blood marker tracks with a specific brain protein decades later, and the mechanism that fits is narrow and tau-specific.
Mulligan et al., Neurology Open Access, 2026
DOI: 10.1212/WN9.0000000000000057