In the VITAL randomized controlled trial, taking 2,000 IU of vitamin D3 every day for a little over five years lowered the chance of being diagnosed with a new autoimmune disease by 22% compared with placebo.
The finding, published in the BMJ in early 2022, is one of the first from a large trial to suggest a routine supplement can shift the odds of these conditions.
The study followed 25,871 US adults. Men aged 50 and older and women aged 55 and older were enrolled, with a mean age of 67 across the group.
Over a median follow-up of 5.3 years, 123 people in the vitamin D group and 155 in the placebo group developed a confirmed autoimmune disease.
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The conditions researchers tracked
The primary outcome pulled together several distinct diagnoses. Confirmed cases included rheumatoid arthritis, polymyalgia rheumatica, autoimmune thyroid disease, psoriasis, and inflammatory bowel disease.
Rheumatoid arthritis is a joint disease driven by the immune system. Polymyalgia rheumatica causes painful stiffness of the shoulders and hips and is more common after 50. Every diagnosis had to be verified through medical record review.
A parallel arm testing 1,000 mg of marine omega-3 fatty acids daily did not produce a statistically significant result on its own. In that arm, 130 people developed a confirmed autoimmune disease versus 148 on placebo.
Why the effect grew after year two
The vitamin D signal became more pronounced the longer people stayed on the supplement. Investigators pointed to a stronger separation between the groups after participants had been supplementing for more than two years, suggesting the effect builds slowly rather than kicks in right away.
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"This is the first direct evidence we have that daily supplementation may reduce [autoimmune disease] incidence," said senior author Karen Costenbader, MD, MPH, of the Brigham's Division of Rheumatology, Inflammation and Immunity.
First author Jill Hahn, ScD, a post-doctoral fellow at Brigham and Women's Hospital, framed the finding in similar terms. "Autoimmune diseases are common in older adults and negatively affect health and life expectancy. Until now, we have had no proven way of preventing them, and now, for the first time, we do."
What the numbers mean in practice
The trial was large and randomized, which is the strongest evidence design for a supplement. Still, its participants were 50 and older, so the results cannot be generalized to younger adults.
A 22% relative reduction is meaningful, but the absolute risk started low. Roughly 12 in 1,000 people on placebo developed a confirmed autoimmune disease across the follow-up.
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Costenbader has publicly recommended 2,000 IU of vitamin D a day and 1,000 mg of marine omega-3 for women 55 and older and men 50 and older, mirroring the doses used in the trial.
Anyone considering a new supplement, especially those already on prescription medication or being treated for a chronic condition, should first check with their doctor.
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This article is made and published by Jesper Bengtson, who may have used AI in the preparation.