Vitamin D Shows Mixed Results for Brain Health and COVID Recovery in New Studies

A 16-year study has found an association between higher vitamin D levels in middle age and lower tau protein deposits in the brain later in life, while a major clinical trial shows vitamin D supplementation may not reduce COVID-19 severity but could potentially help with long COVID recovery.

The research from Ireland followed nearly 800 dementia-free participants with an average age of 39. Each had their vitamin D levels tested at the outset, and then received a brain scan roughly 16 years later to measure their levels of tau and amyloid beta proteins. Participants' vitamin D levels were considered high if they were greater than 30 nanograms per milliliter (ng/mL) and low if they fell below that figure. While no correlation was found between vitamin D levels and amyloid beta proteins, those who had more vitamin D in their blood at the start tended to have lower amounts of tau in their brains years later.

Meanwhile, the Vitamin D for COVID-19 (VIVID) trial, led by researchers at Mass General Brigham in Boston, followed 1,747 adults living in the United States and Mongolia who had recently tested positive for COVID, along with 277 of their household members. Patients who were newly diagnosed with COVID were started on a high-dose regimen right away: 9,600 IU of vitamin D3 for the first two days, followed by a daily dose of 3,200 IU for a month. Despite these significant amounts, there was no difference in how much medical care was needed between those taking vitamin D3 and those taking a placebo in the four weeks following their diagnosis. Furthermore, the vitamin did not seem to stop the virus from spreading to other people living in the same home.

However, when researchers looked at the patients' health two months after their initial diagnosis, they found a promising signal for long COVID. In the group that took vitamin D, 21% reported persistent symptoms like brain fog or exhaustion at the eight-week mark. In the placebo group, that number was higher, at 25%. While the difference was only of "borderline" significance statistically, researchers said it may suggest that vitamin D could help the body clear the virus's long-term effects more efficiently.

The Irish study's findings suggest an association between higher vitamin D levels in early middle age and lower tau burden on average 16 years later. Midlife is a time where risk factor modification can have a greater impact, and low vitamin D levels are easily modifiable and treatable. An analysis from 2025 revealed a similar connection between vitamin D levels and Alzheimer's risk, suggesting that low vitamin D can increase a person's likelihood of developing dementia by 49%.

A separate study investigated the effects of an 8-week daily 2000 IU vitamin D₃ supplementation on serum vitamin D, leukocyte count, and performance parameters in healthy recreational runners and non-runners. Supplementation significantly increased serum 25(OH)D₃ in runners (30.45 ± 7.0 to 35.35 ± 8.9 ng/mL) and non-runners (25.0 ± 8.7 to 30.2 ± 9.7 ng/mL), while non-supplemented non-runners showed a marked decline (25.93 ± 6.6 to 17.8 ± 7.3 ng/mL). A significant time-by-supplementation interaction was observed for total leukocytes and neutrophils, indicating more stable immune profiles in supplemented participants.

Researchers hope to expand the COVID study to even larger groups to see if these signals for long COVID recovery remain consistent across different populations. Vitamin D is an essential nutrient that can affect immune and muscle function, skeletal health, cardiovascular health and risk of heart attack and even depression and anxiety. An estimated two-thirds of Americans are considered vitamin D deficient.