Vitamin D, the main forms being vitamin D2 (ergocalciferol) and D3 (cholecalciferol), is a
group of fat-soluble secosteroids which are synthesised from ergosterol in fungi and yeast,
or from 7-dehydrocholesterol in humans, animals including fish and plants including
microalgae, upon exposure to UV radiation. The metabolically inert vitamin D3 is then
converted into 25-hydroxy D3 (25(OH)D3) and subsequently into 1α,25-dihydroxy D3
(1,25(OH)2D3).
A significant proportion of the global population have inadequate vitamin D levels - as
defined by a plasma 25(OH)D concentration of <25 nM, - a level where the risk of poor
musculoskeletal health appears to increase, which is especially apparent in older children,
younger adults, older institutionalised individuals and infants from black and ethnic
minority groups. Data from the NDNS and other studies suggest that between 29 and 54% of
various population groups in the UK have a serum 25(OH)D concentration < 25 nmol/L in the
winter. However, summer synthesis of vitamin D, facilitating maintenance of winter serum
25(OH)D concentration ≥ 25 nmol/L is clearly not occurring for many in the UK population
(Consultation on draft SACN Vitamin D and Health report). If there is inadequate vitamin D3
synthesis within the skin, generally caused by limited exposure to sunlight, then a dietary
supply of vitamin D becomes essential.
Overall however, very few foods contain vitamin D. Among the best dietary sources of vitamin
D are oily fish (including salmon, mackerel, herring and trout) and fish oils, providing up
to 20g of vitamin D per 100 g. Lower amounts of vitamin D are present in red meat, liver and
egg yolks (approximately 1-5g/100 g). Vitamin D in these foods and in fish is primarily in
the form of vitamin D3 and its metabolite 25(OH)D3.
Recently, investigators have found that certain shellfish, especially mussels, contain
significant amounts of 25(OH)D3, ranging from 0.7 to 9.9 µg/100 g wet weight. Thus far, food
composition databases give either very low or no values for levels of vitamin D in shellfish.
The amount of vitamin D3 in mussels is either reported to be below the detection limit, or is
not analysed, in mussels in the food database analysis. The importance of finding 25(OH)D3
rather than vitamin D3 in mussels is illustrated by the fact that this metabolite is
considered 5 times more effective in raising serum 25(OH)D3 levels in humans, and thus
vitamin D status, than vitamin D3 itself. An interesting parallel with meat appears. Meat
contains little native vitamin D but as better measurement techniques detected more
metabolites of vitamin D, which were considered to be more potent, these have been added to
food composition tables. Indeed, the apparent increase in vitamin D intakes in the British
household food data, in 1995 and 1996, is a direct result of including the potency factor for
meat. Taking account of this potency factor, mussels could provide up to 50g of vitamin D per
100g. Whether consumption of mussels improves vitamin D status in humans is however, not
known.
According to exploratory meta-regression analyses of RCT data in a number of European
(51-60°N) winter-based, dose-related RCTs which used supplemental doses of vitamin D between
0-20 μg/d, have reported vitamin D-serum 25(OH)D concentration slope estimates of 1.55-2.43
nmol/L increment per 1 μg vitamin D (Consultation on draft SACN Vitamin D and Health report).
Thus 1, 2 and 3 portions of mussels per week, providing the equivalent of approx. 2.7, 5.4
and 8.0 mg/day (including the potency factor of 5) may increase serum levels of 25(OH)D3 by
4.2-6.6 nmol/L for 1 portion of mussels per week, 8.4-13.1 nmol/L for 2 portions per week or
12.4-19.4 nmol/L for 3 portions of mussels per week
