Vitamin D endocrine physiology.

Quantitative aspects of vitamin D(3) endocrine physiology are briefly reviewed, together with the supporting evidence. Net calcium absorption of at least 200 mg/d is required to offset typical obligatory losses and thereby to protect the skeleton. The ability of the body to regulate intestinal calcium absorption is optimal at serum 25-hydroxyvitamin D(3) concentrations >80 nM. Vitamin D(3) inputs from all sources required to sustain such a level amount to 3600-4200 IU/d. Daily oral intakes as high as 10,000 IU are safe.