WEDNESDAY, March 4 (HealthDay News) -- Many teens today, especially black teens, aren't getting enough of vitamin D, the so-called sunshine vitamin that is essential for cells to function, say researchers from Weill Cornell Medical College in New York City. Vitamin D deficiency has been linked to many chronic diseases, including heart disease, diabetes, immune system problems and inflammatory diseases. "There is evidence that the levels of vitamin D we have been using in the past may have been too low," said lead researcher Dr. Sandy Saintonge, a fellow in general preventive medicine at Weill Cornell. Vitamin D is measured...