Vitamin D is not just one substance; rather, it is a collective term representing five different substances. Collectively, vitamin D is essential for healthy bones, strong immunity and improved mood. Some types of vitamin D are more biochemically important than others. Vitamin D-3 is the only type synthesized within people, which occurs when certain frequencies of sunlight strike the skin. Vitamin D is gaining attention because it is estimated that up to 75 percent of American teens and adults are deficient.
Types of Vitamin D
Of the five types of secosteroids known as vitamin D, the D-2 and D-3 types are regarded as the most biologically active and essential, according to "Biochemistry of Human Nutrition." Vitamin D-2, or ergocalciferol, is obtained from dietary sources or from supplements. Vitamin D-3, or cholecalciferol, is also in some foods and supplements, but its primary source originates from the sun. Specifically, vitamin D-3 is synthesized within the skin when 7-dehydrocholesterol reacts to UV-B radiation from the sun at wavelengths between 270 and 300 nm. These wavelengths occur within sunlight when the UV index is greater than three.
Potential for Vitamin D-3 Production
According to "Biochemical, Physiological and Molecular Aspects of Human Nutrition," the fully exposed skin of a Caucasian adult can synthesize between 10,000 and 20,000 IU of vitamin D-3 if exposed to approximately 30 minutes of mid-summer sunlight. People with darker skin color, which contains more melanin, produce much less; up to six-times less in some people. The National Institutes of Health recommends supplementation of 400 IU of vitamin D daily for infants, 600 IU for adults until the age of 70 and 800 IU for adults beyond 70 years.These amounts are much less than what the skin can produce in only a few minutes, although vitamin D is fat-soluble and can be stored within the body and used when needed.
Supplemental D2 vs D3
The two primary types of vitamin D used as supplements are D-2 and D-3. Many health advocates believe that the D-3 type is the much better choice. A 2006 article in "The American Journal of Clinical Nutrition" stated that D-2 should no longer be considered suitable as supplements or to fortify foods because D-3 has such hormonal advantages and is more efficiently utilized by the body.
Diseases that Affect Vitamin D
Even if a person eats foods rich in vitamin D-2 and supplements with it, some diseases may inhibit its absorption. According to "Human Biochemistry and Disease," chronic pancreatitis, celiac disease, Crohn's disease, cystic fibrosis, colitis and biliary obstruction can all lower absorption of dietary or supplemental vitamin D. However, these diseases wouldn't affect the synthesis and use of vitamin D-3 from sunlight. Liver or kidney diseases have the potential to interfere with the formation of calciferol from both vitamins D-2 and D-3. Further, a dysfunctional parathyroid gland leading to reduced parathyroid hormone can interfere with activation of both vitamins D-2 and D-3 in the kidneys.
References
- "Contemporary Nutrition"; Gordon M. Wardlaw; 2010
- "Biochemistry of Human Nutrition"; George Gropper; 2000
- "Biochemical, Physiological and Molecular Aspects of Human Nutrition"; Martha Stipanuk; 2006
- The National Institutes of Health: Office of Dietary Supplements: Vitamin D Fact Sheet
- "The American Journal of Clinical Nutrition"; The Case Against Ergocalciferol (Vitamin D2) as a Vitamin Supplement; L.A. Houghton et al; October 2006
- "Human Biochemistry and Disease"; Gerald Litwack; 2008
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