High-dose vitamin D didn't clearly cut COVID. The highest tied with low.

People say: “Does a bigger vitamin D dose protect you from COVID?” We read the study.

What they did

Randomised controlled trial (RCT). 923 people. up to 9 months of daily vitamin D3 per participant (some capped at 6 months by funding); enrolment Dec 2020 to May 2023.

A trial. Chance decided who did what.

Who: adults at UChicago Medicine (incl. Ingalls Hospital) and Rush University Medical Center, Chicago, USA; mean age 47, 74.8% women, 63.3% non-Hispanic White, 76% fully COVID-vaccinated at baseline, mean baseline 25(OH)D 35.9 ng/mL (38.8% below 30); people at risk from supplementation excluded

What they found
  • Fewer COVID casesunclear4,000+ vs 400 IU
  • 10,000 vs 400 IUsame12.4% vs 12.4%
  • Blood level vs COVIDnoneno clear link
The catch

More to know, from our read of the paper:

  • Primary result is null: pooled 4,000/10,000 vs 400 IU aHR 0.73 (0.51-1.06), P = 0.10. Say 'no clear drop', never 'vitamin D doesn't work' or 'proven useless'
  • The 4,000 vs 400 IU result (aHR 0.62, P = 0.045) is a prespecified secondary analysis; authors say multiple-comparison adjustment would make it non-significant. Keep it out of findings bars; at most caption with that caveat
  • Comparator is 400 IU/day, not placebo: says nothing about vitamin D vs nothing, or about correcting a deficiency (only 13.6% were below 20 ng/mL)
  • Under-powered: planned 2,000 for 80% power at 3%/month incidence; got 923. COVID-19 outcome = participant-reported or EHR-recorded infection
  • Mostly vaccinated Chicago adults, 2020-2023 variants; may not generalise to other variants, longer use, or other places (authors)
The verdict

Big vitamin D doses: no clear COVID protection.

In short

This double-blind trial randomized 923 adults in Chicago (76% fully vaccinated at the start) to vitamin D every day for up to 9 months: 400 IU versus 4,000 or 10,000 IU. COVID-19 was reported in 10.9% on the higher doses vs 14.7% on 400 IU, not a clear difference (adjusted hazard ratio 0.73, 95% CI 0.51-1.06). 10,000 IU and 400 IU both came in at 12.4%. The 4,000 IU group alone looked better in a planned secondary test, but the authors say that would not hold after correcting for multiple tests. There was no placebo group, and the trial enrolled fewer people than planned.

The video
The paper

Meltzer DO, Moy JN, Schram AW, Tate A, Hyman MJ, Cursio JF, Vokes T, Yeo KJ, Prochaska M, Jacobs JJ, Solway J (2026). Effect of moderate or high versus low dose daily vitamin D on COVID-19 incidence: A double-blind, parallel-group randomized clinical trial. Clinical Nutrition.

Design: Randomised controlled trial (RCT) · double-blind, parallel-group randomized trial; participants chose a branch (4,000 vs 400 IU/day or 10,000 vs 400 IU/day) and were randomized 1:1 within it; active low-dose comparator, no placebo

DOI: 10.1016/j.clnu.2026.106722

Goals:

Healthy old age
How we checked

We read the abstract and the full text of this paper and checked every number on this page against it (17 facts, checked on 2026-10-29). The study design is named above. Cohort and other observational studies are described as linked, not proven. Found an error? Email theis@onestudyaday.com.

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