The American Academy of Pediatrics says breast-fed infants should get 400 IUs of supplemental D daily.

WSJ: Vitamin D to Defend Against Disease

Oral doses of up to 2, 000 IUs a day are considered safe and are available over the counter.

CNN: Vitamin D may be essential for heart health

The National Osteoporosis Foundation urges adults over age 50 to get at least 800 to 1, 000 IUs to prevent fractures.

WSJ: Vitamin D to Defend Against Disease

For supplements, the Institute of Medicine set an upper limit of 2, 000 IUs per day, but some experts think that's too low.

WSJ: Vitamin D to Defend Against Disease

But to achieve the daily 1, 000 to 2, 000 international units (IUs) that most people need, a dietary supplement is almost always necessary, according to the report.

CNN: Vitamin D may be essential for heart health

The unofficial RDA for vitamin D as issued by the Institute of Medicine (IOM) in 2010 is 600 ius for those under 70, 800 ius for those over 80.

FORBES: That Vitamin D You're Taking? It May Not Be What It Appears

But many health experts (including the Mayo Clinic) recommend at least 1000 ius for those who are D-deficient, and the fact is you may well be one of them.

FORBES: That Vitamin D You're Taking? It May Not Be What It Appears

Dr. Rosen served on an Institute of Medicine panel that in 2010 recommended a daily vitamin D intake of at least 600 IUs for children and adults up to age 70.

WSJ: Vitamin D and Calcium Supplements May Not Stop Fractures

According to the study, the most variability was found in pills and capsules containing 1000 ius of vitamin D3, so it might make sense to purchase lower dosage pills (and then take more of them).

FORBES: That Vitamin D You're Taking? It May Not Be What It Appears

However, he said, there's no harm in taking a 200-IU dose of vitamin D because it's difficult to get the recommended daily amount of 600 IUs from food alone for children and adults up to age 70.

WSJ: Vitamin D and Calcium Supplements May Not Stop Fractures