Vitamin D and Health

shorts/skirt and T-shirt) at around noon (12:00-13:00) from March to September, the daily exposure
time to reach the end of summer (September) target serum 25(OH)D concentration (≥ 80.5 nmol/L)
would be 9 minutes for skin types I-IV (white) and 25 minutes for skin type V (South Asian ethnicity).
These exposure durations would not be expected to exceed the sunburn thresholds for skin types I-V.
9.24 If the skin area exposed was modified (35% in June-August and 10% in March-May and September
(equivalent to hands and face being exposed), the target serum 25(OH)D concentrations would be met
across the country in a typical year for weather but the end of summer serum 25(OH)D concentration
would be marginal in Scotland. This means that a summer with particularly poor weather or nonadherence
to the exposure regimen would reduce the end of summer serum 25(OH)D concentration
to below the target concentrations. In addition, if only hands and face (10% surface area) were
exposed throughout the year then the target 25(OH)D concentration would not be met.
9.25 To take account of lower solar radiation at more northerly latitudes, estimated exposure times
required to reach end of summer target serum 25(OH)D concentrations (assuming 35% skin area
exposure from June-August) ranged from 9-14 minutes for skin type I-IV and 25-38 minutes for skin
type V. These estimates would be much higher if exposure was limited to hands and face.
9.26 There are a number of limitations in these estimations since the two main inputs for the model,
weather and human behaviour, have numerous combinations. The calculations represent a typical
outcome for a normal weather year and for an average person following the specified exposure
pattern. The model also assumes exposure in an open environment. Exposure in urban environments
or shade seeking behaviour will reduce the UVB dose received and therefore the likelihood of reaching
the target serum 25(OH)D concentration. It also assumes daily exposure regardless of weather. While
the modelling takes account of cloud, it does not take account of rainy days. When cloud is thick
enough to produce persistent rain, incident UV radiation is significantly reduced. Another assumption
is that skin on all parts of the body synthesises vitamin D at the same rate.
9.27 An important limitation of the modelling is that the mean serum 25(OH)D concentration in winter was
used within the modelling to estimate the required increase in serum 25(OH)D concentration in the
summer. This means that the estimated sunshine exposure required in the summer to maintain
serum 25(OH)D concentration ≥ 25 nmol/L in the winter applies to the average person but would not
cover the requirements of 97.5% of the population.
Modelling the vitamin D intakes required to achieve a serum 25(OH)D concentration ≥ 25 nmol/L
9.28 As outlined in the previous section, making recommendations on sunlight exposure as a means of
achieving and maintaining serum 25(OH)D concentration ≥ 25 nmol/L for 97.5% of the population is
problematic. Taking account of sunlight exposure in setting the RNI (i.e., dietary intake) for vitamin D
is also complicated by the number of factors that impact on cutaneous production of vitamin D.
9.29 The process of translating the target serum 25(OH)D concentration of ≥ 25 nmol/L into an RNI was
therefore based on RCTs that were carried out during winter, in the absence of (or with minimal) UVB
radiation of sufficient strength to allow for production of vitamin D in the skin. The RNI, therefore,
represents the intake needed to achieve a serum 25(OH)D concentration ≥ 25 nmol/L by 97.5% of the
population when UVB exposure is minimal. However, because there are substantial proportions (7-
53%) of some population groups in the UK with a serum 25(OH)D concentration < 25 nmol/L in the
summer, the RNI is required throughout the year to ensure protection of 97.5% of the population.
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