The Mayo Clinic explained in their Aug. 1, 2006 online article "Drugs and Supplements: Vitamin D," available on their website:

“Vitamin D is found in numerous dietary sources such as fish, eggs, fortified milk, and cod liver oil. The sun is also a significant contributor to our daily production of vitamin D, and as little as 10 minutes of exposure is thought to be enough to prevent deficiencies…

The major biological function of vitamin D is to maintain normal blood levels of calcium and phosphorus. Vitamin D aids in the absorption of calcium, helping to form and maintain strong bones. Recently, research also suggests vitamin D may provide protection from osteoporosis, hypertension, cancer, and several autoimmune diseases…

Adequate Intake (AI) levels have established by the U.S. Institute of Medicine of the National Academy of Sciences to prevent deficiencies in vitamin D. The AI is 5 micrograms (200 IU [International Units]) daily for all individuals (males, female, pregnant/lactating women) under the age of 50 years-old, 10 micrograms daily (400 IU) for all individuals from 50-70 years-old, and 15 micrograms daily (600 IU) for those who are over the age of 70 years-old. The daily upper limit for vitamin D is 25 micrograms (1,000 IU) for infants up to 12 months of age and 50 micrograms (2,000 IU) for children, adults, pregnant and lactating women due to toxicities that can occur when taken in higher doses.”

Aug. 1, 2006