How it helps: According to a 2009 study published in the Archives of Internal Medicine, people with low levels of this vitamin, which helps to regulate the immune system, are more susceptible to catching colds. (The vitamin has also been shown to increase calcium absorption and reduce inflammation.) Multivitamins typically contain 400 international units (IU) of vitamin D, which falls within the current recommended daily allowance (RDA) of 200 to 600 IU. However, experts now suggest taking a 1,000-IU supplement on top of a multivitamin, as strong evidence indicates that the current RDA is too low.
Good to know: If you drink a lot of milk; frequently eat fatty fish, like salmon; or live in a sunny climate, you are probably getting an adequate amount, says Ellert, since these all increase your body’s stores of vitamin D.