Vitamin D and Health

CIE spectrum for photoconversion of 7-DHC to pre-vitamin D
3.16 The validity of the official CIE action spectrum for the photoconversion of 7-DHC to pre-vitamin D (see
Figure 5) has been disputed. However, it continues to be used to calculate the vitamin D efficacy of
solar UVB under different climatic conditions and these calculations have been used for risk/benefit
assessments. For example, one study concluded that the best time to obtain vitamin D is around noon
because the relative efficacy for vitamin D production is greater than that for erythema (Sayre &
Dowdy, 2007). The risk-benefit calculations were based on the pre-vitamin D action spectrum and
assumed that the action spectrum is accurate and that there is no spectral interaction. There is
uncertainty about both these assumptions.
FIGURE 5 - CIE action spectra for erythema and the formation of pre-vitamin D (note
considerable overlap in the solar UVB region).
3.17 One study using ex vivo neonatal foreskin has suggested that UVA degrades vitamin D (Webb et al.,
1989) but this has not been investigated in vivo.
Effect of latitude on skin synthesis of vitamin D
3.18 UVB radiation is sufficient for year round vitamin D synthesis at latitudes below ~37ᵒN. At higher
latitudes, vitamin D is not synthesised during the winter months (Webb et al., 1988). However, the
extent of the effect of latitude on vitamin D synthesis in the UK is not clear. While it probably has
some influence, it could be relatively small compared to other factors. A study which compared
serum 25(OH)D concentrations in postmenopausal women (< 65y) in the North (Aberdeen; 57ᵒN) and
South (Guildford; 51ᵒN) of the UK reported a difference of approximately 10 nmol/L (Macdonald et al.,
2011). However, the difference in serum 25(OH)D concentration might not be due to solar radiation.
Although UVB (as a proportion of UVR) lessens with increasingly northern latitudes, the weather also
gets progressively colder which means that people spend less time outdoors and expose less skin.
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