Vitamin D represents a group of five fat-soluble vitamers, or compounds that display similar properties, but have different molecular structures. All five forms act similarly to hormones, although vitamins D2 and D3 are the most biologically active in the body. Both D2 and D3 can be derived from food sources and consumed as supplements, but only D3 is produced within the skin in response to certain frequencies of sunlight. D3 can be synthesized in large amounts within the skin, but is considered nontoxic despite being stored in the body, which may not be the case with supplemental forms.
Vitamins D2 and D3
Vitamin D2, or ergocalciferol, is found within some foods naturally, used to fortify other foods, and can be taken as a supplement. Vitamin D3, or cholecalciferal, is also found in foods, both naturally and fortified, and available as a supplement. In addition, D3 is synthesized within the skin when 7-dehydrocholesterol reacts to UV-B radiation from sunlight at wavelengths between 270 and 300 nm. These wavelengths occur when the UV index is greater than 3, which occurs only seasonally in most regions of the U.S.
Supplemental D2 and D3
Vitamins D2 and D3 are the main types used for supplementing and are often available as liquids. A growing number of nutritionists and health professionals are recommended D3 over D2 as a supplement. A Canadian review article published in a 2006 edition of "The American Journal of Clinical Nutrition" concluded that D2 should no longer be considered suitable for food fortification or as a supplement because D3 has such strong hormonal advantages and is absorbed better. Although D2 is well tolerated in doses up to 10,000 IU per day, D3 seems to have a much higher safety limit, at least 50,000 IU daily and probably higher, as cited in "Human Biochemistry and Disease." The very low toxicity of D3 is likely related to the capacity of the body to produce huge amounts of it under ideal conditions.
D3 Production from Sunlight
The body has the capacity to synthesize many thousands of IU of D3 hourly when exposed to the appropriate sunlight. For example, a fully exposed Caucasian adult exposed to midday, summer sunlight can synthesize at least 10,000 IU of vitamin D3 in 20 minutes, as cited in "Nutritional Sciences." People with darker skin color produce less D3 because of the melanin content, which blocks the UV-B radiation. African-Americans are the most vitamin D-deficient ethnic group in the U.S., with some sources claiming up to 80 percent being affected, as cited in "Nutrition and Public Health."
Vitamin D Recommendations
To prevent deficiency, the National Institutes of Health recommends 400 IU of vitamin D per day for infants and between 600 IU and 800 IU for adults, depending on age. Considering that these levels can be produced in mere minutes with appropriate sun exposure, it would seem that sunbathing is the most economical and efficient way of preventing deficiency. However, indoor lifestyles, fear of wrinkles and skin cancer, use of sunscreens, pollution, cloud cover and seasonal cycles of UV-B radiation all contribute to the vitamin D deficiency epidemic in the U.S. Americans in northern climates should eat vitamin D-rich foods, such as fatty fish, drink vitamin D-fortified milk and consider taking a supplement. Vitamin D3 is considered the best choice for supplementing.
References
- "Nutrition and Diagnosis-Related Care"; Sylvia Escott-Stump; 2008
- "Biochemical, Physiological and Molecular Aspects of Human Nutrition"; Martha Stipanuk; 2006
- "The American Journal of Clinical Nutrition"; The Case Against Ergocalciferol (Vitamin D2) as a Vitamin Supplement; L. Houghton, et al.; October 2006
- "Human Biochemistry and Disease"; Gerald Litwack; 2008
- "Nutritional Sciences"; Michelle McGuire; 2007
- "Nutrition and Public Health"; Sari Edelstein; 2006
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