The primary difference between vitamin D2 and vitamin D3 is that vitamin D2 is not manufactured naturally by humans. It is produced by invertebrate animals, plants and some fungi. Only vitamin D3 is manufactured naturally by vertebrate animals like humans through exposure to the sun. It occurs naturally in a few plants as well.
There are actually five different forms of vitamin D, based upon their chemical structure. The two most common forms are D2, called ergocalciferol, and D3, called cholecalciferol. Vitamin D2 is not produced naturally by humans, but it is used in some over-the counter supplements. Both vitamin D2 and vitamin D3 are available in tablet form at most drugstores, although D3 is more common.
Scientists debate about the effectiveness of vitamin D2 absorption in humans. Some feel that both vitamin D2 and vitamin D3 are equally as effective at raising the vitamin D levels in the blood of humans. Still others suggest that the absorption rate of vitamin D2 might be as little as 40% of that of vitamin D3.
Since vitamin D3 is rarely found in foods, one must either absorb it through direct sunlight or from a tanning bed, or take it in a pill form. The deficiency of vitamin D can result in bone-softening diseases such as rickets or osteomalacia, or it might impede proper hair growth. The recommended dosage of vitamin D by the US Institute of Medicine of The National Academies is 200 international units (IUs) per day up to the age of 50, and higher dosages with increased age. This falls short of most modern science which suggests favorable levels of vitamin D at 1-5,000 IUs per day.
In either form, vitamin D2 and vitamin D3 are helpful to the body in a host of different ways. They facilitate the production of calcium, which is very important in the building and strengthening of bones. In some observational studies, vitamin D is linked to decreases in some cancers, although this remains unproven. In addition, some scientists are now promoting the benefits of vitamin D in fighting depression and tout the link between low vitamin D levels and seasonal affective disorder.
Vitamin D2 and vitamin D3 can be toxic, if ingested in too high doses. Side effects include hypercalcemia, nausea, vomiting and eventually renal failure. Pregnant women should be careful, because too much vitamin D may lead to mental retardation and facial deformities in the fetus.