Vitamin D and Health

adjusted according to skin sensitivity) 3 times/week for 12 weeks increased serum 25(OH)D
concentration by 42 nmol/L (median of 60 nmol/L at end of trial); a second group who received
10 g/d (400 IU/d) of vitamin D 3 over the same period increased serum 25(OH)D concentration by
37 nmol/L (median, 60 nmol/L at end of trial).
5.27 The amount of sun exposure needed to generate 1 MED (or some fraction of) will depend on external
factors as well as individual factors such as skin type and time spent outdoors. Holick & Jenkins (2003)
have suggested that exposure of approximately 25% of body surface, 2-3 times per week, to 1/4 MED
in spring to autumn is equivalent to an oral dose of 25 g (1000 IU) vitamin D. For the UK, in people
with skin types I to IV, this corresponds to exposure times of around 5-15 minutes in mid-summer and
15-60 minutes in mid-March and mid-September (Webb & Engelsen, 2006). Many solar
recommendations to achieve and maintain serum 25(OH)D at specific concentrations are based on
this guideline; however, it is difficult to extrapolate it to solar UVB exposure since it was derived from
full body exposure to doses of artificial UVR radiation containing non-solar UVB.
5.28 Diffey (2010) developed a mathematical model to estimate changes in serum 25(OH)D concentration
from sun exposure throughout the year using data and calculations for synthesis and decay of serum
25(OH)D concentration following a specific sun exposure and accounting for various factors (including
time outside, month, available UVR in the UK, % skin exposure). The results from this model indicate
that 10-20 minutes of daily sun exposure during summer months in the UK may achieve a maximum
increase of 5-10 nmol/L in serum 25(OH)D concentration.
5.29 A UK group (University of Manchester) has examined and reported the efficacy of a simulated summer
sunlight exposures in raising serum 25(OH)D concentration in UK white-skinned adults (Rhodes et al.,
2010) and in adults of South Asian ethnicity (Farrar et al., 2011; Farrar et al., 2013). Studies were
performed in winter (latitude 53.5°N) to avoid confounding by UVR. Participants wore clothing that
revealed about 1/3 body surface area at commonly sun-exposed skin sites. In white skinned adults
(n=109) low dose, sub-erythemal UVR (1.3 SED, 3 times/week for 6 weeks; total 23.4 SED/week)
produced a mean final serum 25(OH)D concentration of 70 nmol/L (Rhodes et al., 2010). The UVR
dose equates to about 15 minutes (ranging from 13 minutes if lying down to 17 minutes if standing) x6
weekly, exposing about 1/3 skin surface area, in unshaded midday summer sunlight. This estimate
takes account of the fact that in real life, dorsal and ventral body surfaces are not exposed
simultaneously to sunlight, and people adopt postures ranging from horizontal to the vertical
randomly orientated to the sun (Webb et al., 2011).
5.30 Longitudinal studies at mid-UK latitude (Greater Manchester) have examined the relationship
between dose of sunlight UVR exposure received throughout the year and serum 25(OH)D
concentration in adults. In a longitudinal study of white skinned adults (n=109; age, 20-60y), personal
UV dosimeter badges showed that participants were exposed to ~2% of ambient UVR (Webb et al.,
2010), with median exposures of 3.7 SED/week in spring/summer and 0.1 SED in winter. Monthly
serum 25(OH)D measurements revealed a seasonal pattern reaching a peak (mean 71 nmol/L) in
September and a trough (45.8 nmol/L) in February. Sun exposure diaries indicated that relatively
short, frequent solar exposures increased serum 25(OH)D concentration with participants spending a
mean daily time outdoors in spring/summer of 9 (± 13) minutes/day on weekdays and 18 (± 23)
minutes/day at weekends during peak ambient UVB times (11-00 to 13-00). Another longitudinal
study (Kift et al., 2013) examined year-round serum 25(OH)D concentration and sunlight exposure in
adults of South Asian ethnicity (n=125; age, 20-60y). Median serum 25(OH)D concentration was
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