Two recent studies have shown that millions of American children are not getting enough vitamin D, which helps the body absorb calcium. This puts them at a higher risk for heart and bone problems. Dr. Craig Wittenburg, a family physician at Pella Regional Health Center, says just 15 minutes of unprotected exposure to sunlight can help the body produce vitamin D naturally. Vitamin D is also found in milk and can be taken as a dietary supplement. Connie Leino (LAY-noh), registered dietician at Knoxville Hospital and Clinics says the recommended daily amount of vitamin D for children is 400 units, which is around four cups of milk.