Research has recently expanded the biochemistry of vitamin D beyond its traditional role in bone metabolism to include diverse functions on cellular regulation. Dietary vitamin D becomes important during winter months at higher latitudes when sunlight is not intense enough to cause the synthesis of enough of the vitamin in the skin. This causes reduction in 25-hydroxyvitamin D [25(OH)D], the accepted biomarker of vitamin D. It is during winter therefore that vitamin insufficiencies become common amongst such populations and the time when dietary intake is of particular importance. Supplements are one solution to prevent reductions in plasma levels of 25(OH)D. It has been shown previously that supplements of cholecalciferol (vitamin D3) are more effective at raising plasma levels of 25(OH)D, when compared to supplements of ergocalciferol (vitamin D2) (here). However, meat, fish and eggs are also important sources of vitamin D because they contain the hydroxylated version of vitamin D3.
The hydroxylated version of vitamin D3 might be nutritionally important because recent research suggests it is more efficient at raising plasma levels of 25(OH)D when compared to the non-hydroxylated version of vitamin D3. For example, researchers1 have investigated the effects of oral supplements of 25(OH)D and vitamin D3 in healthy adults aged over 50 years during 10 weeks of supplementation in winter. Subjects consumed 20 or 7 µg of oral 25(OH)D, 20 µg oral vitamin D3, or a placebo for the duration of the study and had their blood levels of 25(OH)D measured using an enzyme-linked immunoassay. The results showed that over the 10 week study, oral 25(OH)D increased plasma concentrations of 25(OH)D by 4.02 and 4.77 nmol/L per µg of intake for the 7 and 20 µg intakes, respectively. However, vitamin D3 increased plasma 25(OH)D by only 0.93 nmol/L per µg of intake.
For the duration of the study there were no changes in the plasma calcium concentrations, suggesting that vitamin D at these intakes is safe. Using the data from this study, the authors calculated that the 20 µg intake of 25(OH)D was 5 times as effective at raising plasma 25(OH)D when compared to the 20 µg intake of vitamin D3, and that the 7 µg intake of 25(OH)D was 4.2 times as effective at raising the plasma 25(OH)D concentration when compared to the 20 µg intake of vitamin D3. Therefore oral 25(OH)D is significantly more effective at raising plasma levels of 25(OH)D when compared to oral vitamin D3. Like for like intakes of vitamin D from foods such as eggs, fish and meat might therefore be more effective at raising plasma levels of 25(OH)D when compared to vitamin D3 in supplements because the vitamin D in the former is hydroxylated.
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