Vitamin D
Vitamin D deficiency results in decreased absorption of calcium from the gut, secondary hyperparathyroidism and increased bone turnover and net loss. Cholecalciferol (vitamin D3) is synthesized in skin exposed to UV light. Ergocalciferol (D2) and D3 are absorbed from the gut from food or supplement sources.
D2 and D3 are converted to the active forms, primarily calcitriol (1,25-dihydroxyvitamin D) by hydroxylation in the liver and then the kidney, so liver and kidney disease adversely effects the availability of the active form. Patients with these conditions may need supplementation with active calcitriol and should probably have sub-specialist evaluation.
Vitamin D is a fat soluble vitamin, so toxicity is theoretically possible, though the safe upper limit of intake is probably greater than 5000 IU per day. Sun exposure of hands, face and arms for 10-30 minutes, three times a week produces the equivalent of 400 IU per day. Elderly persons are less efficient at producing vitamin D through sun exposure and persons living in latitudes north of Missouri probably can not depend on sun exposure alone.
In men and women over the age of 60 high doses of vitamin D supplementation (800 IU, but not 400 IU) has been shown to reduce the risk of hip fracture. Milk in the U.S. is fortified with 125 IU per 8 ounces. Serum levels of 25 hydroxyvitamin D can be used when deficiency is suspected, despite supplementation.
Recommended daily vitamin D intake (per NIH consensus and National Osteoporosis Foundation):
Children age 0 to 18 = 200 IU
Women to age 50 = 400 IU
Women above age 50 = 800 to 1000 IU
Men to age 65 = 400 IU
Men over age 65 = 800 to 1000 IU