Vitamin D plays a key role in calcium absorption in the gut and in skeletal development. Vitamin D deficiency is common, affecting an estimated one billion people worldwide, including developed countries. Vitamin D deficiency causes rickets, the most common non-communicable disease of childhood. Rickets causes poor growth and skeletal deformities and, at its most severe, seizures and death. Recent data suggest that children with rickets have an increased risk for developing diabetes mellitus.