An adult should take either the Recommended Dietary Allowance (RDA) of vitamin D, described below, or whatever dose of vitamin D supplement your doctor recommends to get your body's vitamin
D level to an optimal range for your health.
The Recommended Dietary Allowance (RDA) of vitamin D for adults through age 70 is 600 International Units (IU) daily, per the Institute of Medicine's 2010 report on vitamin D. After age 70, 800 IU daily is recommended for the prevention and treatment of osteoporosis.
If vitamin D deficiency is diagnosed, higher doses of vitamin D may be required for weeks or months in order to get up to an optimal level. The exact dose recommended will depend on the person's particular medical situation. Doctors sometimes prescribe a high-dose weekly supplement for four to eight weeks. In other cases, a higher daily dose (1,000-2,000 IU) might be recommended. Regardless of the dose prescribed, if low vitamin D has been diagnosed, it's important to have follow-up blood work after four to six months to make sure the low level has been corrected.
If vitamin D deficiency hasn't been diagnosed, it's probably not a good idea for an adult to take more than 1,000 IU daily without first checking with a doctor. That's because vitamin D affects the body's absorption and processing of calcium. Taking too much vitamin D as a supplement can lead to too-high levels of it in the blood, which in turn can cause problems related to hypercalcemia (high blood calcium). Hypercalcemia can cause a variety of symptoms, including nausea, constipation, depression, and confusion. Elevated blood calcium levels have also been linked to an increased risk of kidney stones, heart attacks, fractures, and other serious health problems.
Note that there remains some controversy among experts as to what blood level of vitamin D is optimal. The Institute of Medicine review found that a vitamin D level of 20 ng/dL (nanograms per deciliter) is adequate for bone health, but the Endocrine Society of America recommends a minimum level of 30 dL/ng. (These numbers refer to measurements of a type of vitamin D called 25-OH, which is considered the best way to check overall vitamin D status.)
Exactly how much vitamin D supplement will get you to an optimal range? It's hard to say, since every individual has a different starting level and we all metabolize vitamin D differently. Taking 600 IUs daily (or 800 IUs after age 70) is expected to keep most people's blood levels above 20 ng/dL and will keep many people's levels above 30 ng/dL.