The above discussion points to an inconsistency between the CIE spectrum for vitamin D (Bouillon et al., 2006) and the statements made in the literature about our ability to photosynthesize vitamin D in summer and winter, namely:
• that a few minutes daily exposure to sunlight in summer is sufficient at mid-latitudes (Holick, 2002; 2007), and
• that no vitamin D is produced in Boston in winter (Webb et al., 1988).
The first statement above is based on the supposition that 1 MED full body exposure corresponds to an oral dose of 10,000 IU to 25,000 IU of vitamin D. For a fair skinned person, 1 MED (i.e., 2.5 SED) is accumulated in approximately 14 minutes when UVI = 12 (UVEry = 0.3 W m ). The vitamin D produced from this is more than 10 times the recommended daily dose of up to ~1,000 IU (Chel et al., 1998; Bischoff-Ferrari et al., 2006; Holick, 2007; Vieth et al., 2007), so that a full body exposure for less than one minute should suffice to meet daily requirements in the summer (see Fig. 2.13). This exposure time is consistent with that recommended in Holick's popular book, "The UV Advantage" (2003), where it is stated (p164) that to maintain adequate vitamin D, one should "expose 25% of your body surface to 25% of 1 MED two to three times per week." Since 1 MED is received in about 15 minutes at noon in summer, that corresponds to less that one minute of full body exposure per day at noon in summer. In the winter, the vitamin D weighted UV incident on a horizontal surface is approximately 1/20th of the summer value at mid-latitudes, but since the sun is lower in the sky in winter, a larger fraction would be incident on vertically oriented cylindrical surfaces that better approximate the exposed surface of our body (Moan et al., 2008). Therefore, sufficient vitamin D should easily be produced in less than 20 minutes of full body exposure. For darker skinned individuals, the exposure time required would be longer, but the amount of vitamin D produced should not be zero. This inconsistency remains, regardless of whether the long wavelength limit of the action spectrum for vitamin D extends to 315 nm or 330 nm.
There is, of course, the question of whether individuals would be prepared to expose a large enough area of their bodies at the temperatures in winter. They probably would not. However, even for more limited exposures, the vitamin D produced would be non-zero. The experiments to determine the action spectrum of pre-vitamin D did not use live subjects. Instead they used samples of skin tissue exposed in a petri dish (Webb et al., 1988). It is surprising that these did not yield any vitamin D since these exposure periods were for three hours over the midday period. This raises the question of whether the action spectrum has been specified sufficiently. For example, it is known that the conversion from pre vitamin D to vitamin D is temperature dependent (Holick, 2007), so it is reasonable to assume some temperature dependence in the overall conversion from sunlight to blood serum vitamin 25(OH)D. Although the temperature-dependent reaction takes place in the skin, the temperature can vary significantly from normal body temperature of 37°C. Some have suggested that there is a threshold below which vitamin D is not produced (Hollis, 2005), but the evidence for this is not strong. More likely, any perceived threshold is actually caused by an inability to detect the smaller amounts produced.
Notwithstanding the above arguments, there is ample evidence that individuals do not receive sufficient UV to maintain optimal vitamin D during the winter (Livesey et al., 2007). This may be in part because of our modern lifestyles, where outdoor exposure is not the norm, even in the summer months. A recent study in New Zealand found that schoolchildren typically received less than 5% of the local ambient daily UV dose, even in the summer (Wright et al., 2007). Another study in Germany found that even outdoor workers receive only 5% - 10% of the ambient daily dose, which is a factor of five more than indoor workers (Knuschke et al., 2007).
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