Vitamin D and Health

increase 25(OH)D concentrations > 50 nmol/L in 97.5 % of the African American women.
Relationship between UVB sunlight exposure and serum 25(OH)D concentration
5.20 The relationship between skin exposure to UVB sunlight and the resulting serum 25(OH)D
concentration is much less well defined because it is complicated by a number of factors (e.g., season,
time of day, amount of skin exposed, skin pigmentation, use of SPF sunscreen) (see also chapter 3).
5.21 It has been suggested that, compared to vitamin D formed in the skin, dietary vitamin D is less
efficient at maintaining serum 25(OH)D concentration (Haddad et al., 1993). This could be because
vitamin D synthesised in the skin is primarily associated with DBP and slowly diffuses into the blood
stream, gradually arriving at the liver (Fraser, 1983). In contrast, dietary vitamin D is associated with
chylomicrons and low density lipoproteins which are readily and rapidly taken up by the liver.
5.22 A systematic review which examined the effect of UVB exposure on serum 25(OH)D concentration
identified 8 randomised trials (Cranney et al., 2007). Four trials evaluated the effect of natural sun
exposure and 4 evaluated the effect of artificial UV exposure on serum 25(OH)D concentration. Study
populations ranged from infants to older adults and interventions were variable, ranging from 1 MED
to specified minutes of exposure to mJ/cm 2 . A quantitative synthesis of the trials of UVB exposure and
serum 25(OH)D concentration was not possible due to the heterogeneous study populations, the
differences in the interventions (length and area of exposure; dose) and lack of complete data
(Cranney et al., 2007).
5.23 Laboratory studies that have investigated the relationship between UVR exposure and vitamin D
synthesis have typically used UVB phototherapy sources which also contain non-solar UVB radiation
(< 295 nm) that is also very effective at vitamin D production. It is, therefore, difficult to make
comparisons with solar UVR. A study that compared doses of natural solar UVR (April-September)
with doses of artificial UVB radiation of hands and face reported a significant increase in serum
25(OH)D concentration with UVB from artificial sources but not with sunlight (Datta et al., 2012). It
was estimated that UVB from a phototherapy source was at least 8 times more effective (in terms of
erythemally equivalent exposure) than solar UVB.
5.24 Laboratory studies (Bogh et al., 2010) report an inverse relationship between baseline serum 25(OH)D
concentration and response to UVB: i.e., the lower the baseline serum 25(OH)D concentration, the
greater the response.
5.25 In a RCT which examined interactions between exposure dose 38 and body surface area (Bogh et al.,
2011), participants (n=92; age, 18-65 y) received 4 UVB exposures (0.75, 1.5 or 3.5 SED) at intervals of
2-3 days. All exposures were for 10 minutes, except in 10 participants who received 5 minutes
exposure (n=5 in each group who received 0.75 & 1.5 SED to 24% body surface area). Increasing the
exposed body surface area from 6% to 24% decreased the effect of increasing UVR dose and
increasing the exposure dose from 0.75 to 3.0 SED decreased the effect of increasing body surface
area. These data indicate higher doses are needed if small areas of the body are exposed and that
lower doses are adequate if larger body surface areas are exposed.
5.26 Chel et al. (1998) reported that exposure of the lower back of older females (mean age, 85 y) residing
in a nursing home in the Netherlands (52 o N) to half the MED (from artificial UVB; individual doses
38 Exposure doses were given in SED units: 1 SED is equivalent to an erythemal effective radiant exposure of 100 Jm -2 ; typically the MED of a fairskinned
individual is about 2-3 SED.
38