Vitamin D is actually a collective of five fat-soluble compounds named secosteroids. Of the five forms, vitamins D2 and D3 are most essential. Vitamin D2, or ergocalciferol, is acquired exclusively from the diet or by supplementing. Vitamin D3, or cholecalciferol, is primarily produced within the skin in response to sunshine, but is also in some foods. Vitamin D deficiency in the United States is at an all-time high, with a 2009 study in the journal "Archives of Internal Medicine" suggesting that 75 percent of teens and adults are deficient.
Exposure to Sunshine
The most abundant source of vitamin D, by far, is sunshine. Vitamin D3 is synthesized in the skin when 7-dehydrocholesterol reacts with ultraviolet-B radiation at wavelengths between 270 and 300 nm, as cited in the book "The Vitamins." These wavelengths are present in sunshine when the UV index is greater than three, which occurs daily in the tropics, daily during spring and summer in temperate regions, and rarely in the Arctic and Antarctic Circles.
A unclothed Caucasian adult can produce between 10,000 and 20,000 IU of vitamin D3 in about 30 minutes if exposed to midday summer sunshine. People with darker skin produce up to six times less vitamin D3 when exposed to the sun. Despite these levels, the National Institutes of Health recommends only 200 IU of vitamin D per day for children and adults under the age of 50, which is the absolute minimum required to prevent acute deficiency symptoms.
Animal-based Sources
Few natural foods contain vitamin D, making it difficult to achieve recommended daily levels without the addition of sunshine. According to the National Institutes of Health, fish such as salmon, mackerel, sardines, cod and tuna are excellent food sources of vitamin D. Generally, wild caught fish have more vitamin D content than organically farmed varieties. Cod liver oil contains the greatest amount of vitamin D per gram of any food product. Lesser levels of vitamin D are found in beef liver, shrimp, Swiss cheese and egg yolks.
Fortified Sources
The National Institutes of Health states that fortified foods provide the majority of vitamin D in the average U.S. diet. Most of the U.S. milk supply is fortified with vitamin D, which began in the 1930s in an attempt to reduce deficiency diseases, such as childhood rickets. Other milk-based products are typically not fortified. Breakfast cereals and related flours often contain added vitamin D, as do some brands of yogurt and margarine. Some brands of orange juice and other fruit juices are also vitamin D fortified, but by U.S. law they must also be calcium fortified. Some types of mushrooms are artificially exposed to ultraviolet light to induce excess vitamin D synthesis, but these are not commonly available.
Vitamin D Supplements
The two forms of vitamin D used for supplementation are D2 and D3. Many researchers believe vitamin D3 is the superior choice. Authors of a 2006 article in the "American Journal of Clinical Nutrition" concluded that D2 should no longer be considered suitable for food fortification or as a dietary supplement because D3 has such hormonal advantages. Although government recommendations remain at 200 IU of vitamin D daily, the same authors suggests at least 1,000 IU daily are more appropriate, and up to 10,000 IU taken orally are safe.
References
- "Archives of Internal Medicine"; Demographic Differences and Trends of Vitamin D Insufficiency in the US Population, 1988-2004; Adit A. Ginde, et al; March, 2009
- "The Vitamins, Third Edition"; Gerald F. Combs; 2007
- National Institutes of Health, Office of Dietary Supplements: Vitamin D Intake Levels
- "American Journal of Clinical Nutrition"; The Case Against Ergocalciferol (Vitamin D2) as a Vitamin Supplement; L.A. Houghton and R. Vieth; October, 2006
- "Journal of the American Board of Family Medicine"; Vitamin D: An Evidence-based Review; T. Kulie et al; December, 2009
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