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"I discussed in my last post how Dr Vieth has a model of tissue 1,25(OH)2D synthesis and degradation in which the level of active substance is pretty well independent of blood vitamin D level, provided the level is either rising or stable. I think it is also worth pointing out that he is talking, hypothetically, about tissue 1,25(OH)2D, not plasma level... As we know, almost nothing is known about tissue 1,25(OH)2D control.

By Vieth's hypothesis tissue 1,25(OH)2D is OK so long as there is at least SOME vitamin D present in plasma and the level dose not vary too much. Obviously there is a level below which you can have as much of the enzyme for converting vitamin D to the active form as you like, if there is no vitamin D in your blood you can't make any 1,25(OH)2D in your tissues, or in your kidneys for export to your blood to control calcium levels. At the lower extremes we have rickets and osteomalacia. These are clear cut, unarguable markers of vitamin D deficiency, in the absence of confounding factors (there are a few)."

Seasonality of UV-radiation and vitamin D status at 69 degrees north.
Brustad M, Edvardsen K, Wilsgaard T, Engelsen O, Aksnes L, Lund E.
Photochem Photobiol Sci. 2007 Aug;6(8):903-8. Epub 2007 Jun 27.
PMID: 17668121

The generally high dietary intakes of vitamin D, especially in winter, mask largely the effect of seasonal variation in UV-exposure, causing an atypical seasonal variation in vitamin D status. The UV-hour variable significantly predicted 25(OH)D levels in blood when adjusted for intakes and artificial UV-radiation exposure and sun holidays abroad.

"UV-related web tools maintained by Ola Engelsen

* FastRT - Fast simulations of downward UV doses, indices and irradiances at the Earth's surface: http://nadir.nilu.no/~olaeng/fastrt/fastrt.html.
* VitD_quartMED - Calculated Ultraviolet Exposure Levels for a Healthy Vitamin D Status: http://nadir.nilu.no/~olaeng/fastrt/VitD_quartMED.html.
* VitD - Duration of Vitamin D Synthesis in Human Skin: http://nadir.nilu.no/~olaeng/fastrt/VitD.html. "

Simulations of ultraviolet irradiances, doses and indices at the Earth's surface at user specified UV wavelengths. A simpler version for non-experts is available at http://nadir.nilu.no/~olaeng/fastrt/fastrt-ez.html

The solar UV radiation level needed for cutaneous production of vitamin D3 in the face. A study conducted among subjects living at a high latitude (68 degrees N).
Edvardsen K, Brustad M, Engelsen O, Aksnes L.
Photochem Photobiol Sci. 2007 Jan;6(1):57-62. Epub 2006 Nov 10.
PMID: 17200737

Duration of vitamin D synthesis from weather model data for use in prospective epidemiological studies.
Edvardsen K, Engelsen O, Brustad M.
Int J Biometeorol. 2009 Sep;53(5):451-9. Epub 2009 May 15.
PMID: 19444487

"Daily duration of vitamin D synthesis in human skin with relation to latitude, total ozone, altitude, ground cover, aerosols and cloud thickness."

Vitamin D production in human skin occurs only when UV radiation exceeds a threshold. From simulations of UV irradiances, the VitD-ez www page ( http://zardoz.nilu.no/~olaeng/fastrt/VitD-ez.html) computes the daily duration of dermal vitamin D production at midday when UV radiation exceeds the required threshold. The VitD-ez www page is a simplified version of the more complex VitD web page ( http://zardoz.nilu.no/~olaeng/fastrt/VitD.html). The simplified VitD-ez www page is convenient for users unfamiliar to radiative transfer modelling, because the model input is limited and more intuitive.

Daily duration of vitamin D synthesis in human skin with relation to latitude, total ozone, altitude, ground cover, aerosols and cloud thickness.
Engelsen O, Brustad M, Aksnes L, Lund E.
Photochem Photobiol. 2005 Nov-Dec;81(6):1287-90.
PMID: 16354110

Vitamin D production in human skin occurs only when incident UV radiation exceeds a certain threshold. From simulations of UV irradiances worldwide and throughout the year, we have studied the dependency of the extent and duration of cutaneous vitamin D production in terms of latitude, time, total ozone, clouds, aerosols, surface reflectivity and altitude. For clear atmospheric conditions, no cutaneous vitamin D production occurs at 51 degrees latitude and higher during some periods of the year. At 70 degrees latitude, vitamin D synthesis can be absent for 5 months. Clouds, aerosols and thick ozone events reduce the duration of vitamin D synthesis considerably, and can suppress vitamin D synthesis completely even at the equator. A web page allowing the computation of the duration of cutaneous vitamin D production worldwide throughout the year, for various atmospheric and surface conditions, is available on the Internet at http://zardoz.nilu.no/~olaeng/fastrt/VitD.html and http://zardoz.nilu.no/~olaeng/fastrt/VitD-ez.html. The computational methodology is outlined here.

Vitamin D effective ultraviolet wavelengths due to scattering in shade.
Turnbull DJ, Parisi AV, Kimlin MG.
J Steroid Biochem Mol Biol. 2005 Sep;96(5):431-6. Epub 2005 Jul 6.
PMID: 16005208

Decreased bioavailability of vitamin D in obesity.
Wortsman J, Matsuoka LY, Chen TC, Lu Z, Holick MF.
Am J Clin Nutr. 2000 Sep;72(3):690-3. Erratum in: Am J Clin Nutr. 2003 May;77(5):1342.
PMID: 10966885

Conclusions: Obesity-associated vitamin D insufficiency is likely due to the decreased bioavailability of vitamin D3 from cutaneous and dietary sources because of its deposition in body fat compartments.

"Under year round UV exposure conditions (low latitudes, broken line, "High UV") there is no association between 25(OH)D and either prostate or pancreatic cancer. At high latitudes (Solid line, "Low UV") there is a positive association between blood levels of 25(OH)D and these cancers. The average year round levels of 25(OH)D actually tend to be higher in northern latitudes, higher than those where there is year-round solar UVB.

Vieth explains that we know almost nothing about the enzymes controlling tissue 1,25(OH)2D levels and much of his discussion is extrapolated from renal enzyme activity."

Bob has now put this wealth of knowledge to developing arguably the best UVB bulb in the world. Between Reptile UV and Kimbos Reptile World we feel its vital that every reptile and reptile keeper should have access to these products. They have been proven time and time again to represent both excellent value for money, at the same time delivering excellent UVB output to your reptiles and with minimal decay rates. Not only that, every bulb carries Bob's personal gaurantee that your reptile will be happier and healthier with a Mega-Ray lamp in its life

Heliotherapy improves vitamin D balance and atopic dermatitis.
Vähävihu K, Ylianttila L, Salmelin R, Lamberg-Allardt C, Viljakainen H, Tuohimaa P, Reunala T, Snellman E.
Br J Dermatol. 2008 Jun;158(6):1323-8. Epub 2008 Mar 20.
PMID: 18363748
DOI: 10.1111/j.1365-2133.2008.08518.x

CONCLUSIONS: A 2-week course of HT significantly improved vitamin D balance by increasing serum calcidiol concentration, and caused a marked healing of AD. These parallel positive responses should be taken into account when the benefits of HT are considered..

Vitamin D production in human skin occurs only when UV radiation exceeds a threshold. From simulations of UV irradiances, the VitD www page ( http://zardoz.nilu.no/~olaeng/fastrt/VitD.html) computes the daily duration of dermal vitamin D production at midday when UV radiation exceeds the required threshold. The daily duration depends on latitude, time, total ozone, clouds, aerosols, surface reflectivity and altitude all of which can be specified by the user