Abstract
Vitamin D insufficiency and deficiency can be diagnosed with measurements of serum 25-hydroxyvitamin D (25OHD). Most vitamin D is derived from sunlight (80%), so serum 25OHD levels are lowest in late winter and early spring. Dietary vitamin D in North America is small, about 100 to 200 IU daily. A recent review of the literature shows many association studies relating vitamin D deficiency and insufficiency to several diseases. Large randomized trials of vitamin D are underway and soon there may be answers as to whether vitamin D is clinically effective and what level of serum 25OHD is necessary.