Vitamin D and Health

tight homeostatic control. As a marker of exposure to vitamin D, serum 25(OH)D concentration is
influenced by those factors that affect skin synthesis (including season, latitude, clothing, skin type).
10.14 There are limitations in using serum 25(OH)D concentration as a marker of vitamin D exposure since it
has been observed to decrease in response to acute inflammation. It is therefore possible that low
serum 25(OH)D concentrations (which have been observed in conditions such as cancer) may simply
reflect an underlying inflammatory state. The relationship between exposure and serum 25(OH)D
concentration may also be confounded by BMI and genetic variation.
10.15 Quantification of serum 25(OH)D concentration is influenced by the analytic methodology. The two
main methods in use are antibody or liquid chromatography (LC) based. Most data collected over the
past 20-30 years have been analysed using antibody-based assays. However, LC-based assays which
use a tandem mass spectrometer have high accuracy, specificity, sensitivity and better reproducibility.
10.16 The main limitations associated with the antibody-based methods used for measuring serum 25(OH)D
concentration are accuracy and variability. Measurements can vary considerably (15-20%) depending
on the type of assay used and across different concentration ranges. There is also lack of agreement
between different laboratories using the same methods. This has implications for the interpretation
of epidemiological studies and trials that have examined the relationship between serum 25(OH)D
concentration and health outcomes.
Photobiology
10.17 The sun is the main source of ultraviolet radiation (UVR) which is categorised into three types
according to wavelength: UVA (315-400 nm), UVB (280-315 nm) and UVC (100-280 nm). The UVR
spectrum is modified on its path through the atmosphere by ozone, altitude, ground reflection (e.g.,
by sand or snow), air pollution, cloud cover and shade, time of day and season.
10.18 The amount of vitamin D synthesised in the skin depends on skin exposure to solar UVB radiation and
efficiency of cutaneous synthesis. Factors that affect skin exposure to UVR include the amount of
available sunlight, exposure angle, time spent outdoors, skin coverage and use of sunscreen. The
main determinant of sunlight availability is solar elevation which depends on time of year and time of
day as well as the weather, which affects outdoor activity and skin coverage.
10.19 In the UK, sunlight is not effective for vitamin D synthesis when the sun is low in the sky (sunrise or
sunset). The most effective time for vitamin D synthesis is at midday.
10.20 There is a well observed seasonal cycle in serum 25(OH)D concentrations in the UK. Solar radiation is
greater in the summer because the sun is much higher, the days are longer and the weather less
cloudy. Solar radiation is reduced during winter because of low solar elevation, short days and cloudy
skies. The small amount of UVB radiation in winter sunlight is insufficient to initiate synthesis of any
biologically relevant amounts of vitamin D. Sunlight-induced vitamin D synthesis in white-skinned
populations becomes effective from late March/early April with maximum concentrations observed in
September after a summer of exposure. Serum 25(OH)D concentration decreases from October
onwards throughout the winter months.
10.21 UVB (as a proportion of UVR) lessens with increasingly northern latitudes. The extent to which
increasing latitude reduces vitamin D synthesis is not clear because the weather also gets
progressively colder, people go outdoors less and expose less skin.
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