Vitamin D and Health

a) biochemical indicators of vitamin D status and the validity of the values used to assess
risk of deficiency and excess;
b) association between vitamin D status and health outcomes at all life stages and the
effects of biological modifiers;
c) the contribution of cutaneous vitamin D synthesis to vitamin D status in the UK taking
account of factors that modify skin exposure to sunlight; risks of skin damage and
other adverse health outcomes associated with sunlight exposure;
d) potential adverse effects of high vitamin D intakes; and
e) relative contributions made by dietary vitamin D intake (from natural food sources,
fortified foods and supplements) and cutaneous vitamin D synthesis, to the vitamin D
status of the UK population.
10.7 Data from RCTs, then prospective studies, when available, were preferred for setting DRVs but data
from other study types were also considered (including case-control, cross-sectional studies and case
reports).
Biology & metabolism
10.8 The first step in endogenous vitamin D synthesis is the conversion by solar UVB radiation of 7-DHC to
previtamin D in the skin. The dose response is not linear, which means that longer exposures do not
lead to proportionally greater previtamin D synthesis. Previtamin D 3 is thermodynamically unstable
and is converted at body temperature to vitamin D 3 which enters the circulation and is transported to
the liver bound to vitamin D binding protein.
10.9 Dietary vitamin D is lipid soluble and is incorporated within enterocytes into chylomicrons that are
secreted into lymph and transported through the lymphatic system to the systemic circulation to the
liver.
10.10 Vitamin D is converted to its active metabolite 1,25(OH) 2 D in two hydroxylation steps: firstly to
25(OH)D in the liver and then to 1,25(OH) 2 D in the kidney. 25(OH)D is the major circulating
metabolite of vitamin D. Its concentration in serum reflects vitamin D supply from cutaneous
synthesis and the diet.
10.11 Vitamin D accumulates in both adipose tissue and muscle. Details about accumulation and
mobilisation of vitamin D from adipose tissue and other tissues such as muscle are not clear at this
time.
10.12 Polymorphisms in genes encoding proteins involved in vitamin D metabolism have been identified
which might influence serum 25(OH)D concentrations and functional outcomes but the nutritional
implications of such findings is not clear.
Biomarkers of exposure
10.13 The active metabolite of vitamin D, 1,25(OH) 2 D, is not a suitable indicator of vitamin D exposure
because it is homeostatically regulated and has a short half-life (< 4 hours). Serum 25(OH)D
concentration is widely considered to be the best indicator of total vitamin D exposure (from the diet
and sunlight) because it has a long half-life in the circulation (about 2-3 weeks) and is not subject to
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