The vitamin D synthesized in your skin during sun exposure is vitamin D3. Almost daily, news headlines announce another advance in scientific knowledge about the importance of this vitamin in what seems to be an ever expanding list of diseases either caused or made worse by its deficiency. Researchers reporting in the October 2008 issue of "Nutrition Reviews" are alarmed by the current pandemic of vitamin D deficiency and blame the problem on inadequate sun exposure.
Children Need More Sunshine
Dr. Juhi Kumar is a pediatrician working in the Children's Hospital at Montefiore Medical Center in New York. In an article published in the September 2009 issue of "Pediatrics," he calls the prevalence of vitamin D3 deficiency shocking. Dr. Kumar reports that nearly 70 percent of children in the United States have increased risk for bone and heart disease as a result of not spending enough time outdoors. He laments the rise in cases of rickets, a pediatric bone disease thought to have been eliminated when it became mandatory to fortify milk with vitamin D.
How Much Sun is Enough
Researchers reporting in the November 2006 issue of "Photochemistry and Photobiology" calculated the sun exposure needed to raise vitamin D3 blood serum levels to the recommended amount based on time of day. They note that people with darker skin pigment require more sun exposure than those with fair skin. Black skin may require six times the length of exposure to synthesize adequate levels. In conclusion, the researchers recommend that people with average skin tone get 15 minutes exposure between the hours of 10:00 am and 3:00 pm.
Vitamin D Winter
Seasonal and latitudinal changes in sunlight radiation dramatically affects synthesis of vitamin D3 in the skin. Researchers reporting in the August 1998 issue of the "Journal of Clinical Endocrinology and Metabolism" found that in mid-February, under cloudless conditions in Boston, Massachusetts, no synthesis takes place at solar noontime. Boston is 40 degrees latitude north, and the researchers coined the term Vitamin D Winter for the north and south latitudes of 40 degrees and higher -- between November and early March. During the Vitamin D Winter, it is impossible to get adequate sun exposure.
Activating Vitamin D3
It's not enough to get adequate sunshine. Researchers reporting in the December 2004 issue of "The American Journal of Clinical Nutrition" explain that vitamin D3 itself is biologically inert and must be activated. Indeed, genetic defects in children can produce the disease rickets despite adequate sun exposure. After synthesis in the skin from sun exposure, some vitamin D3 is transported to the liver where it is modified into the prohormone named calcidiol. Other vitamin D3 is transported to the kidney where it is modified into the most potent form of prohormone known as calcitriol.
References
- "Nutrition Reviews"; Vitamin D Deficiency: A Global Perspective; Ann Prentice; October 2008
- "Pediatrics"; Prevalence and Associations of 25-Hydroxyvitamin D Deficiency in US Children; Juhi Kumar et al.; September 2009
- "Photochemistry and Photobiology"; Calculated Ultraviolet Exposure Levels for a Healthy Vitamin D Status; Ann Webb et al., November 2006
- "Journal of Clinical Endocrinology and Metabolism"; Influence of Season and Latitude on the Cutaneous Synthesis of Vitamin D3; Ann Webb et al.; August 1988
- "The American Journal of Clinical Nutrition"; Overview of General Physiologic Features and Functions of Vitamin D; H.F. DeLuca; December 2004
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