The most popular supplement in America isn't Vitamin C or a multivitamin—it's Vitamin D . Vitamin D regulates your calcium levels, vital for healthy bones, and facilitates normal immune system function. (In that regard, it has been promoted by some as a barrier against COVID-19.) Dr. Anthony Fauci takes Vitamin D supplements, because he says he has a lack of it. So how do you know if you need it too? "Lack of vitamin D is not quite as obvious in adults," says the Cleveland Clinic. "Signs and symptoms might include" the following. Read on—and to ensure your health and the health of others, don't miss these Sure Signs You May Have Already Had COVID . 1 You Might Feel Fatigue "Vitamin D deficiency is frequent and has been associated with fatigue and other unspecific symptoms including headache, musculoskeletal pain and weakness, depression, and impaired cognitive performance," says one study published in Medicine . "Vitamin D tr