Vitamins typically don’t do much for health unless you are deficient to begin with. But many Americans are deficient in Vitamin D, in part because of an error in setting the minimum daily requirements for Vitamin D. One of the consequences of Vitamin D deficiency seems to be autoimmune disease.

The evidence is high-quality evidence from the large-scale vitamin D and omega-3 trial (VITAL):

VITAL is a randomized, double-blind, placebo-controlled research study of 25,871 men (age 50 and older) and women (age 55 and older) across the U.S., conducted to investigate whether taking daily dietary supplements of vitamin D3 (2000 IU) or omega-3 fatty acids (Omacor fish oil, 1 gram) could reduce the risk for developing cancer, heart disease, and stroke in people who do not have a prior history of these illnesses. Participants were randomized to receive either vitamin D with an omega-3 fatty acid supplement; vitamin D with a placebo; omega-3 fatty acid with a placebo; or placebo only. Prior to the launch of VITAL, investigators determined that they would also look at rates of AD among participants, as part of an ancillary study.

The probability the results could be due to chance can be calculated with these additional details:

Among patients who were randomized to receive vitamin D, 123 participants in the treatment group and 155 in the placebo group were diagnosed with confirmed AD (22 percent reduction).

That difference has close to a 5% probability of happening by chance (a p-value near 5%). And of course the chance that one of the set {cancer, heart disease, stroke, autoimmune disease) would show a different incidence this big is considerably higher (multiple hypothesis testing). So the evidence is only suggestive, but with something this serious, a good Bayesian should take the evidence seriously in the absence of other contrary evidence.

Don’t miss these related posts:

• You Probably Need More Vitamin D

• Why You Should Get More Vitamin D: The Recommended Daily Allowance for Vitamin D Was Underestimated Due to Statistical Illiteracy

• Have We Gone Too Far with Sunscreen?

• Vitamin D Seems to Help If You Have Non-Alcoholic Liver Disease