When a child is admitted to a hospital’s intensive care unit (ICU), probably one of the last things on anyone’s mind is, “Are they getting enough vitamin D?”

But this question could be a very important one, according to Kate Madden, MD, and Adrienne Randolph, MD, critical care medicine specialists at Boston Children’s Hospital. In a study of children admitted to Boston Children’s ICU, they found that those with very low vitamin D levels—below what the American Association of Pediatrics (AAP) considers deficient—tended to have more severe illness.

So what’s so important about vitamin D? Turns out, a lot more than most people think. …