Vitamin D, well known as essential for bone health, is not one compound but several. In 2010, the U.S. Department of Agriculture revised the guidelines for vitamin D intake. Vitamin D comes in several forms, including D2 and D3 as well as from sunlight. Vitamin D occurs naturally in only a few foods, so many people require dietary supplements. Discuss taking vitamin D supplements with your doctor, who may recommend a specific daily dose based on your blood levels.
Types
Vitamin D2 and D3 come from different sources. Vitamin D2, also called ergosterol, comes from plant sources such as mushrooms and yeast. Very little D2 comes from diet. Vitamin D3, also called cholecalciferol, is synthesized in the skin after sunlight exposure and also comes from fish, egg yolk and fortified foods such as milk and cereal.
Doses
The daily recommended dose of dietary vitamin D is 600 International Units for adults between the ages of 19 and 71 and 800 IU for those over age 71. Dietary supplements contain either D2 or D3; the type of vitamin D is listed on the label. The daily upper limit of vitamin D from supplements is 2,000 IU, unless otherwise prescribed by your medical practitioner.
Sources
A 2006 report published in the "American Journal of Clinical Nutrition" by lead author Lisa Houghton of the University of Toronto compared D2 and D3. The report stated that vitamin D3 is more potent and should replace D2 as the main type of vitamin D supplement. Both types are inactive in their original state and must undergo activation in the liver and kidney. Sunlight exposure of 45 minutes per week provides enough vitamin D3 to meet your needs if you're fair-skinned. Dark-skinned people need as much as three hours a week of exposure to obtain adequate vitamin D3.
Risk
While you can't overdose on vitamin D3 obtained from sunlight, you can overdose on supplements. The Food and Nutrition Board of the Institute of Medicine has set the upper level to avoid toxicity at 4,000 IU per day. Symptoms of overdose include nausea, vomiting, appetite loss, constipation and weight loss. Severe overdose can cause calcium levels in the blood to rise, as vitamin D assists in calcium absorption from the intestines.
References
- "The American Journal of Clinical Nutrition:" The case against ergocalciferol (Vitamin D2) as a Vitamin Supplement; Lisa Houghton; October 2006
- Linus Pauling Institute at Oregon State University: Vitamin D
- The University of Maryland Medical Center: Vitamin D
- Office of Daily Supplements: Vitamin D
- "The Washington Post:" New Vitamin D Recommendations
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