Q: Is it true that vitamin D can ward off colds and flu?

A: There is some evidence that vitamin D is associated with a reduction in the likelihood of developing a respiratory infection—or at least making one less severe.

A recent study led by researchers at Yale University followed 198 healthy adults over the course of a fall and winter and tested their blood every month to measure their levels of vitamin D. Among individuals with vitamin D levels lower than 38 ng/mL, about 45 percent came down with a viral infection.

However, in the group with vitamin D levels of 38 ng/mL or higher, only 16 percent became ill. The findings raise the question of whether taking vitamin D supplements to achieve a blood level of 38 ng/mL or higher could boost the immune system and ward off flu. They may also explain why flu outbreaks don't occur in the summer, when people are more likely to be exposed to sunlight, a natural source of vitamin D. The findings were published in the journal PLoS ONE.

Although these findings don't definitively prove that vitamin D can ward off colds, adequate vitamin D is important, nonetheless, because it helps maintain strong bones. Your doctor can perform a simple blood test to determine if you're vitamin D deficient.

Publication Review By: Peter B. Terry, M.D., M.A.

Published: 19 Sep 2011

Last Modified: 15 Dec 2014