They took vitamin D for 5 years. Cancer didn't drop.

People say: “Vitamin D pills prevent cancer and heart disease” We read the study.

What they did

Randomised controlled trial (RCT). 25,871 people. 5.3 years median intervention (range 3.8-6.1).

A trial. Chance decided who did what.

Who: US men aged 50+ and women aged 55+ with no cancer (except non-melanoma skin cancer) or cardiovascular disease at entry; 51% women, mean age 67.1, 71% non-Hispanic white, 20% African American (5,106)

What they found
  • Fewer cancersnone
  • Fewer heart eventsnone
  • Started out low1 in 8
The catch

More to know, from our read of the paper:

  • Most participants were not deficient: mean baseline 25(OH)D was 30.8 ng/mL and only 12.7% were below 20 ng/mL (measured in 15,787 people with analyzable blood); authors say a trial in people with extremely low vitamin D might show stronger effects
  • Only one dose tested (2000 IU/day of D3); authors list this as a limitation
  • Median treatment was 5.3 years; authors say cancer has long latency and extended follow-up is necessary (2-year post-intervention follow-up was ongoing)
  • Participants could take up to 800 IU/day of vitamin D from outside supplements; outside use >800 IU/day reached 6.4% (vitamin D) vs 10.8% (placebo) at five years
  • No adjustment for multiple testing: secondary, exploratory and subgroup results (cancer deaths, BMI, African Americans) 'should be interpreted with caution' / 'hypothesis generating'
The verdict

Vitamin D pills: zero proof they prevent cancer.

One thing to try
  1. Don't count on it for cancer2,000 IU a day didn't lower it.
  2. Deficient? Different questionFew people here started low.
  3. Bones weren't tested hereThis paper: cancer and heart disease only.
In short

25,871 US adults (men 50+, women 55+) with no cancer or heart disease were randomly given 2,000 IU of vitamin D3 a day or a placebo, double-blind, for a median of 5.3 years. Invasive cancer (793 vs 824 cases) and major heart events (heart attack, stroke, heart death) didn't drop. In those re-tested, blood vitamin D rose about 40%, so the pills were absorbed. Only about 1 in 8 of those measured started out low, so this wasn't a test in deficient people. Bones and falls weren't part of this paper.

The paper

Manson et al. (2019). Vitamin D Supplements and Prevention of Cancer and Cardiovascular Disease. New England Journal of Medicine.

Design: Randomised controlled trial (RCT) · randomized, double-blind, placebo-controlled trial

DOI: 10.1056/NEJMoa1809944

Goals:

Eat betterHealthy 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 (16 facts, checked on 2026-09-27). 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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