Ergocalciferol is the form of vitamin D derived from plants. It is designated vitamin D2 to differentiate if from the form of vitamin D synthesised in animals which is designated vitamin D3. Evidence strongly suggests that many individuals living at high latitudes may be deficient in vitamin D. This results from the lack of adequate sources of vitamin D in the diet coupled with the lack of sunlight for the duration of the winter time. Previous estimates of vitamin D requirement were also woefully underestimated, with newer research showing that plasma levels may need to be 40 ng/mL or 100 nmol/L in order to provide adequate vitamin D status. As a result of the large body of research showing inadequate vitamin D levels in large numbers of individuals, supplementation with vitamin D has become more popular amongst the general population. Supplements are available as both vitamin D2 and vitamin D3, and controversy exists as to which is the best form to consume.
Fish oils are a source of vitamin D, but the concentration is quite low for human needs and we simply do not eat enough fish oil for it to be a reliable dietary source of the vitamin.
In particular, vitamin D2 and vitamin D3 may have differing biological activities in humans. This relates to the fact that vitamin D2 and vitamin D3 may not be metabolised in the same way when taken as a supplement, with vitamin D2 being metabolised to non-biologically active metabolites that are not produced from vitamin D3. This renders a larger proportion of the vitamin D2 inactive more quickly when compared to vitamin D3, and thus some have questioned the usefulness of vitamin D2. Studies seem to show that higher doses of vitamin D2 are required to maintain plasma levels of 25-hydroxyvitamin D, the active biological form of vitamin D, to the same levels of vitamin D3. However, if this is taken into account, vitamin D2 can be a useful supplement for maintaining adequate vitamin D status. For example, in a recent study, subjects took 50,000 IU of vitamin D2 and this caused a significant increase in plasma levels of 25-hydroxyvitamin D.
Some foods may contain pre-formed 25-hydroxyvitamin D, the biologically active metabolite of vitamin D in humans. However, it is not clear how much 25-hydroxyvitamin D is in foods, how much is metabolised and what impact this has on human health.
Within 12 weeks the plasma levels of 25-hydroxyvitamin D in the subjects had risen from 18 ng/mL to 43 ng/mL, suggesting that the vitamin D2 supplementation had been effective. However, most of the increase in the plasma levels of 25-hydroxyvitamin D had occurred in the first 4 weeks of supplementation, with subsequent weeks representing a plateau in 25-hydroxyvitamin D levels. The 50,000 IU of vitamin D2 used in this study was large, and the authors admitted that they used vitamin D2 simply because that is the form that is available on prescription at such a high dose. If a lower dose were taken, it would likely require a longer period for the plasma levels of 25-hydroxyvitamin D to reach the equivalent of 43 ng/mL, but there is no reason why this would not be possible as long as adequate amounts of the vitamin were taken. Current recommendations are for 2000 IU of vitamin D3 to be taken daily in the absence of sunlight, but this may need to be in the region of 2500 to 3000 IU of vitamin D2.
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