Hello ira8711,
An excellent question. Here is the basic answer, from a reputable source:
"Season, geographic latitude, time of day, cloud cover, smog, and
sunscreen affect UV ray exposure and vitamin D synthesis. For
example, sunlight exposure from November through February in Boston is
insufficient to produce significant vitamin D synthesis in the skin.
Complete cloud cover halves the energy of UV rays, and shade reduces
it by 60%. Industrial pollution, which increases shade, also
decreases sun exposure and may contribute to the development of
rickets in individuals with insufficient dietary intake of vitamin D.
Sunscreens with a sun protection factor (SPF) of 8 or greater will
block UV rays that produce vitamin D, but it is still important to
routinely use sunscreen to help prevent skin cancer and other negative
consequences of excessive sun exposure. An initial exposure to
sunlight (10 -15 minutes) allows adequate time for Vitamin D synthesis
and should be followed by application of a sunscreen with an SPF of at
least 15 to protect the skin. Ten to fifteen minutes of sun exposure
at least two times per week to the face, arms, hands, or back without
sunscreen is usually sufficient to provide adequate vitamin D."
"Dietary Supplement Fact Sheet: Vitamin D"
Office of Dietary Supplements -- Warren G. Magnuson Clinical Center --
National Institutes of Health
http://ods.od.nih.gov/factsheets/vitamind.asp
The following passage mentions windows, as well as touching on the
issue of when the exposure to sunlight should occur:
"Sunlight that comes through glass, such as a window, won't cause your
skin to make vitamin D. And you don't have to get 20 minutes of
direct sun exposure all at one time. You can add up the minutes of
exposure you get from things like walking to the end of your driveway
to pick up your newspaper and walking through a parking lot to get to
your car."
"What Should I Eat?"
NASA's Web of Life
http://weboflife.ksc.nasa.gov/exerciseandaging/chapter6.html
As for time of day:
"Skin also manufactures vitamin D under strong sunlight. It has to do
with the sun's angle, though, so go out there between 10 a.m. and 2
p.m. rather than taking a walk in the early morning ...." [Of course,
that's a time for significant sunburn risk too -- but the first
excerpt I've quoted deals with that issue.]
"Brittle Bones Can Affect Older Men As Well" (July 10, 2005)
The National Women's Health Information Center
http://www.4woman.gov/news/english/526224.htm
- justaskscott
Search strategy:
Searched on Google for combinations of the following terms:
"vitamin d"
sunshine
sun
minutes
latitude
site:gov |
