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Sunlight and Vitamin D
"The current suggested exposure of hands, face and arms for 10-20 minutes, three times a week, provides only 200-400 IU of vitamin D each time or an average of 100-200 IU per day during the summer months. In order to achieve optimal levels of vitamin D, 85 percent of body surface needs exposure to prime midday sun. (About 100-200 IU of vitamin D is produced for each 5 percent of body surface exposed, we want 4,000 iu.) Light skinned people need 10-20 minutes of exposure while dark skinned people need 90-120 minutes.11"
Date: 6/29/2005 3:11:00 AM ( 19 y ) ... viewed 5265 times
Vitamin D From Sunlight
Krispin Sullivan, CN
Pick up any popular book on vitamins and you will read that ten minutes of daily exposure of the arms and legs to sunlight will supply us with all the vitamin D that we need. Humans do indeed manufacture vitamin D from cholesterol by the action of sunlight on the skin but it is actually very difficult to obtain even a minimal amount of vitamin D with a brief foray into the sunlight.4,5
Ultraviolet (UV) light is divided into 3 bands or wavelength ranges, which are referred to as UV-C, UV-B and UV-A.6 UV-C is the most energetic and shortest of the UV bands. It will burn human skin rapidly in extremely small doses. Fortunately, it is completely absorbed by the ozone layer. However, UV-C is present in some lights. For this reason, fluorescent and halogen and other specialty lights may contribute to skin cancer.
UV-A, known as the "tanning ray," is primarily responsible for darkening the pigment in our skin. Most tanning bulbs have a high UV-A output, with a small percentage of UV-B. UV-A is less energetic than UV-B, so exposure to UV-A will not result in a burn, unless the skin is photosensitive or excessive doses are used. UV-A penetrates more deeply into the skin than UV-B, due to its longer wavelength. Until recently, UV-A was not blocked by sunscreens. It is now considered to be a major contributor to the high incidence of non-melanoma skin cancers.7 Seventy-eight percent of UV-A penetrates glass so windows do not offer protection.
The ultraviolet wavelength that stimulates our bodies to produce vitamin D is UV-B. It is sometimes called the "burning ray" because it is the primary cause of sunburn (erythema). However, UV-B initiates beneficial responses, stimulating the production of vitamin D that the body uses in many important processes. Although UV-B causes sunburn, it also causes special skin cells called melanocytes to produce melanin, which is protective. UV-B also stimulates the production of Melanocyte Stimulating Hormone (MSH), an important hormone in weight loss and energy production.8
The reason it is difficult to get adequate vitamin D from sunlight is that while UV-A is present throughout the day, the amount of UV-B present has to do with the angle of the sun's rays. Thus, UV-B is present only during midday hours at higher latitudes, and only with significant intensity in temperate or tropical latitudes. Only 5 percent of the UV-B light range goes through glass and it does not penetrate clouds, smog or fog.
Sun exposure at higher latitudes before 10 am or after 2 pm will cause burning from UV-A before it will supply adequate vitamin D from UV-B. This finding may surprise you, as it did the researchers. It means that sunning must occur between the hours we have been told to avoid. Only sunning between 10 am and 2 pm during summer months (or winter months in southern latitudes) for 20-120 minutes, depending on skin type and color, will form adequate vitamin D before burning occurs.9
It takes about 24 hours for UV-B-stimulated vitamin D to show up as maximum levels of vitamin D in the blood. Cholesterol-containing body oils are critical to this absorption process.10 Because the body needs 30-60 minutes to absorb these vitamin-D-containing oils, it is best to delay showering or bathing for one hour after exposure. The skin oils in which vitamin D is produced can also be removed by chlorine in swimming pools.
The current suggested exposure of hands, face and arms for 10-20 minutes, three times a week, provides only 200-400 IU of vitamin D each time or an average of 100-200 IU per day during the summer months. In order to achieve optimal levels of vitamin D, 85 percent of body surface needs exposure to prime midday sun. (About 100-200 IU of vitamin D is produced for each 5 percent of body surface exposed, we want 4,000 iu.) Light skinned people need 10-20 minutes of exposure while dark skinned people need 90-120 minutes.11
Latitude and altitude determine the intensity of UV light. UV-B is stronger at higher altitudes. Latitudes higher than 30° (both north and south) have insufficient UV-B sunlight two to six months of the year, even at midday.12 Latitudes higher than 40° have insufficient sunlight to achieve optimum levels of D during six to eight months of the year. In much of the US, which is between 30° and 45° latitude, six months or more during each year have insufficient UV-B sunlight to produce optimal D levels. In far northern or southern locations, latitudes 45° and higher, even summer sun is too weak to provide optimum levels of vitamin D.13-15 A simple meter is available to determine UV-B levels where you live.
http://www.westonaprice.org/basicnutrition/vitamindmiracle.html
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