Vitamin D is a fat-soluble vitamin that plays a key role in bone health. Primarily found in fatty fish, eggs, fortified milk and cod liver oil, vitamin D usually comes in the form of cholecalciferol, or D-3. Some mushrooms provide D-2, or ergocalciferol, in variable amounts. Another major source of the vitamin is exposure to ultraviolet radiation from the sun. Adequate levels of vitamin D -- 20 to 50 ng/mL -- are measured using a 25-hydroxyvitamin D blood test. Researchers are also looking at the role of vitamin D in inflammatory process, immune function, brain function and autoimmune diseases.
Reference Intakes
The Institute of Medicine increased its recommended dietary allowance for vitamin D by 200 IU in November 2010. The IOM recommends that all age groups up to age 70 consume 600 IU of vitamin D and those at age 71 years or above consume 800 IU. According to the National Institutes of Health, breastfed infants, older adults, people with dark skin and people with fat malabsorption may be at risk for deficiency and may need supplemental doses.
Natural Sources of Vitamin D
You can get vitamin D from two natural sources: food and sunlight. Among the few foods that naturally contain vitamin D, those that contain the highest amount of vitamin D per serving include cod liver oil, 1,360 IU per tbsp.; trout, 645 IU per 3 oz.; salmon, 447 to 583 IU per 3 oz.; swordfish, 565 IU per 3 oz.; and fortified milk, 120 to 202 IU per cup.
Sun as a Vitamin D Source
Many can meet part of their vitamin D needs through sun exposure. In his book "The Vitamin D Solution," Dr. Michael Holick says that about 15 to 30 minutes of sun exposure between 10 a.m. and 3 p.m. at least twice a week to the face, arms, legs or back without sunscreen can usually produce ample amounts of vitamin D.
Adverse Effects
Chronic intake of more than 4,000 IU daily may increase risks for harm. Blood levels of vitamin D that are consistently more than 200 ng/mL indicate potential toxicity. While you are unlikely to get too much vitamin D from sun exposure or food sources, you could overdose on vitamin D from taking high doses of the dietary supplement. The symptoms of Vitamin D toxicity are nonspecific, including anorexia, weight loss, heart arrhythmias and polyuria. Serious side effects could lead to tissue and blood vessel calcification from rising levels of blood calcium and cause damage to the heart and kidneys. Talk to your doctor before you start taking a vitamin D supplement.
References
- NIH Office of Dietary Supplements: Dietary Supplement Fact Sheet: Vitamin D
- "The FASEB Journal"; Is There Convincing Biological or Behavioral Evidence Linking Vitamin D Deficiency to Brain Dysfunction?; J.C. McCann, et al.; April 2008
- Institute of Medicine: Dietary Reference Intakes for Calcium and Vitamin D
- USDA National Nutrient Database for Standard Reference: Vitamin D Content of Selected Foods
- Wbur.org: Here & Now: Book Excerpt: "The Vitamin D Solution"
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