From the NIH:
Sun exposure
Most people meet their vitamin D needs through exposure to sunlight [6,27]. Ultraviolet (UV) B radiation with a wavelength of 290-315 nanometers penetrates uncovered skin and converts cutaneous 7-dehydrocholesterol to previtamin D3, which in turn becomes vitamin D3 [11,27-28]. Season, geographic latitude, time of day, cloud cover, smog, skin melanin content, and sunscreen are among the factors that affect UV radiation exposure and vitamin D synthesis [28]. The UV energy above 42 degrees north latitude (a line approximately between the northern border of California and Boston) is insufficient for cutaneous vitamin D synthesis from November through February [6]; in far northern latitudes, this reduced intensity lasts for up to 6 months. Latitudes below 34 degrees north (a line between Los Angeles and Columbia, South Carolina) allow for cutaneous production of vitamin D throughout the year [14]
Complete cloud cover reduces UV energy by 50%; shade (including that produced by severe pollution) reduces it by 60% [29]. UVB radiation does not penetrate glass, so exposure to sunshine indoors through a window does not produce vitamin D [30]. Sunscreens with a sun protection factor of 8 or more appear to block vitamin D-producing UV rays, although in practice people generally do not apply sufficient amounts, cover all sun-exposed skin, or reapply sunscreen regularly [31]. Skin likely synthesizes some vitamin D even when it is protected by sunscreen as typically applied.
The factors that affect UV radiation exposure and research to date on the amount of sun exposure needed to maintain adequate vitamin D levels make it difficult to provide general guidelines. It has been suggested, for example, that approximately 5-30 minutes of sun exposure between 10 AM and 3 PM at least twice a week to the face, arms, legs, or back without sunscreen usually lead to sufficient vitamin D synthesis and that the moderate use of commercial tanning beds that emit 2-6% UVB radiation is also effective [11,28]. Individuals with limited sun exposure need to include good sources of vitamin D in their diet or take a supplement.
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I had winter depression a few years back and my Vit D was at 27. My friend who is an ND and has done years of bloodwork for Vit D, recommended 40,000 once per week until I was up in the 80's (took 10 weeks). Then she recommended 4,000 as the maintenance but said I could go up to 10,000 (see page 19 of link below). She flat said that if you are low and just taking the recommended doses, you will NOT get your score up quick enough to make a difference. She tested me every 2 weeks during saturation and my bloodwork was perfect. No adverse effects from 40,000 IU's at all.
I used to be a sunblock junkie, but now I get my 20-30 min of sun 3-4x per week in the summer and THEN put on the SPF. I start my Vit D saturation in September and have felt GREAT during the winters ever since. BTW, I live in WA.
I know this is a long post, but Vit D is so important and it changed things for me so much, I thought I'd share my experience.
Here's a link with summaries of studies with graphs starting on page 14. I found page 1 interesting...
cancer.ca/Canada-wide/Prevention/Use%20SunSense/UV%20%20Vitamin%20D%20and%20Health%20Conference/~/media/CCS/Canada%20wide/Files%20List/English%20files%20heading/pdf%20not%20in%20publications%20section/Reinhold%20Vieth%20-%20pdf_899467573.ashx