Some additional references:

Matsuoka LY, Wortsman J, Haddad JG, Hollis BW.
In vivo threshold for cutaneous synthesis of vitamin D3.
J Lab Clin Med. 1989 Sep;114(3):301-5.
PMID: 2549141 [PubMed - indexed for MEDLINE]
<http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&dopt=Abstra
ctPlus&list_uids=2549141>

Webb AR.
Who, what, where and when-influences on cutaneous vitamin D synthesis.
Prog Biophys Mol Biol. 2006 Sep;92(1):17-25. Review.
PMID: 16766240 [PubMed - indexed for MEDLINE]
<http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?itool=abstractplus&db=pubmed&cmd=R
etrieve&dopt=abstractplus&list_uids=16766240>

Lucas RM, Ponsonby AL.
Ultraviolet radiation and health: friend and foe.
Med J Aust. 2002 Dec 2-16;177(11-12):594-8.
PMID: 12463975 [PubMed - indexed for MEDLINE]
<http://www.mja.com.au/public/issues/177_11_021202/luc10478_fm.html>

Matti,

Thanks for that explanation.

I have a couple more questions. It seems like I remember reading years
ago that, for maximum D, one should not bath immediately before or
after exposure. It seemed to have something to do with oil on the skin.
They said this oil needed to be re-absorbed into the skin after
exposure for the body to get the maximum effect. I have not seen this
anywhere since. Is this valid?

Also, what is your opinion regarding tanning beds? At least there is no
variation from day to day and season to season.

Thanks,

John

More information about this in the articles

Webb AR, DeCosta BR, Holick MF.
Sunlight regulates the cutaneous production of vitamin D3 by causing
its photodegradation.
J Clin Endocrinol Metab. 1989 May;68(5):882-7.
PMID: 2541158 [PubMed - indexed for MEDLINE]
<http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&dopt=Abstra
ctPlus&list_uids=2541158>

   "Exposure to sunlight initiates the formation of vitamin D3 in
   skin as the UV B radiation in the solar spectrum causes the
   photoconversion of 7-dehydrocholesterol to previtamin D3. A
   heat-induced isomerization then converts previtamin D3 to
   vitamin D3 over a period of days. A number of irradiation
   products of vitamin D3 are known to form upon irradiation with
   high intensity UV radiation, but the effect of subsequent
   exposures to sunlight on the vitamin D3 formed in skin is not
   known. To investigate this phenomenon, human skin containing
   vitamin D3 was exposed to sunlight in Boston. A model system of
   [3H]vitamin D3 in methanol was also used to study the effects
   of sunlight on vitamin D3 throughout the year. Vitamin D3
   proved to be exquisitely sensitive to sunlight, and once formed
   in the skin, exposure to sunlight resulted in its rapid
   photodegradation to a variety of photoproducts, including 5,6-
   transvitamin D3, suprasterol I, and suprasterol II."

and

Vieth R.
Vitamin D supplementation, 25-hydroxyvitamin D concentrations, and
safety.
Am J Clin Nutr. 1999 May;69(5):842-56. Review.
PMID: 10232622 [PubMed - indexed for MEDLINE]
<http://www.ajcn.org/cgi/content/full/69/5/842>

   "At least 4 studies support the concept that one full-body
   exposure to sunlight can be equivalent to an oral vitamin D
   intake of 250 µg (10000 IU). Stamp (39) compared oral vitamin D
   to the effects of ultraviolet light treatment sessions and
   found that the rise in 25(OH)D was the same in subjects treated
   with ultraviolet light as in those given 250 µg (10000 IU)
   vitamin D/d. In a study of institutionalized elderly, Davie et
   al (28) exposed 600 cm2, {approx}5% of skin surface, to
   ultraviolet light treatments over a 2–3-mo period and compared
   the resulting 25(OH)D concentrations with those achieved with
   oral vitamin D doses. They calculated a production of vitamin D
   in the skin equivalent to 0.045 nmol •d-1•cm-2 exposed skin.
   This is equivalent to 10.9 µg (435 IU) vitamin D/d for 5% of
   skin surface. An almost identical protocol was followed
   recently by Chel et al (30), confirming the relative effects of
   light and supplementation on 25(OH)D concentration (28). If
   these results for the elderly are extrapolated to total body
   surface area, it works out to 218 µg (8700 IU) vitamin D/d that
   can be acquired by the elderly. More recently, Holick (40)
   presented data that compared blood vitamin D concentrations in
   subjects taking vitamin D orally with those given ultraviolet
   light exposure. The ultraviolet treatment produced blood
   vitamin D concentrations comparable with an intake of 250–625
   µg (10000–25000 IU) vitamin D/d orally.

   Ultraviolet exposure beyond the minimal erythemal dose does not
   increase vitamin D production further. The ultraviolet-induced
   production of vitamin D precursors is counterbalanced by
   degradation of vitamin D and its precursors. The concentration
   of previtamin D in the skin reaches an equilibrium in white
   skin within 20 min of ultraviolet exposure (41). Although it
   can take 3–6 times longer for pigmented skin to reach the
   equilibrium concentration of dermal previtamin D, skin
   pigmentation does not affect the amount of vitamin D that can
   be obtained through sunshine exposure (42). However, aging does
   lower the amount of 7-dehydrocholesterol in the skin and lowers
   substantially the capacity for vitamin D production (43, 44)."

I "bothered", Matti, but interpreted your comment differently than you
intended.

Because you answered "Does the adequate supply then last through the
fall, winter and spring?" with "It depends on the latitude" and since
ANY sun "harms the skin", I presumed you meant longer exposure would
overdo the D in the long haul. Thanks for correcting my misinterpretation.

I.P.