In the 1990's a test was made of the amount of vitamin D that was generated in a body on a tanning bed.
That test concluded that the human body would not produce vitamin D when the UV intensity is less than 18 milliwatts/square cm.
This inability to produce vitamin D at low UV intensities has been called the UV winter, since in Northern latitudes the UV intensity does not exceed 18 mw/sq cm for many month of the winter. see file gallery Sunshine Hollis 2005
The concept of a "UV winter" ignores the possibility of the body to adapt to the environment over time,
that is, be able to produce vitamin D when there is only a low intensity of UV, such as during the winter.
Serum 25-hydroxyvitamin-D responses to multiple UV exposures from solaria: inferences for exposure to sunlight - 2012
Richard McKenzie , Robert Scragg , Ben Liley , Paul Johnston , John Wishart , Alistair Stewart and Roshani Prematunga
Photochem. Photobiol. Sci., 2012, DOI: 10.1039/C2PP05403E
Received 11 Dec 2011, Accepted 18 Feb 2012; First published on the web 13 Mar 2012
We investigate the relationship between blood serum 25-hydroxyvitamin D (25(OH)D) and UV exposure from two artificial sources. We then use the results to test the validity of the action spectrum for vitamin D production, and to infer the production from summer and winter sunlight.
The results are based on a two-arm randomised clinical trial of biweekly UV exposure for 12 weeks using two different types of dermatological booths:
- one emitting primarily UV-A radiation, and the other emitting
- primarily UV-B radiation (booth A and booth B respectively).
In terms of the vitamin D production per unit erythema, one of the booths mimics summer noon sunlight, while the other mimics winter noon sunlight.
Blood samples were taken before and after the exposures. For all participants, the phototherapy booth treatments arrested the usual wintertime decline in 25(OH)D, and for most the treatments from either booth resulted in significant increases.
The increases were highly non-linear and there was a high degree of variability in 25(OH)D and its response to UV from person to person. By the end of the 12 week period, the mean increase was >30 nmol l?1 from a cumulative exposure of
- 17 SED from the UV-A booth, and
- twice that for the UV-B booth for which the cumulative exposure was 268 SED.
Assuming a logarithmic relationship between UV and vitamin D, the results for the two booths show no obvious inconsistency in the action spectrum for pre-vitamin D production. However, further measurements with similar exposures from each booth are required to confirm its validity.
A model was developed to describe the increases in serum 25(OH)D resulting from the UV exposures, which differed markedly between the two booths.
The deduced initial rate of increase of 25(OH)D was approximately 5 nmol l?1 per SED.
From the large increases in 25(OH)D from each booth, along with knowledge of the spectral distribution of sunlight and assuming the currently-accepted action spectrum for photo-conversion to pre-vitamin D, we infer that the production of 25(OH)D from sunlight should be possible throughout the year, although in winter the exposures necessary to maintain optimal levels of 25(OH)D would be impractically long.
This finding is at variance with the commonly-held view that no vitamin D is produced at mid-latitudes in the winter.
Further work is needed to resolve that inconsistency.
short URL = http://is.gd/UVWinter