An excerpt from
http://www.westonaprice.org/basicnutrition/vitamindmiracle.html
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.
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
|