I have seen some foods fortified with Vitamin D2, and others with Vitamin D3.
Which is better?
-- Tom (last name withheld)
(City withheld), TX
From a fortification standpoint, they are both equal.
Ergocalciferol ("vitamin D2") is a vitamin D precursor that extracted from a yeast sterol, while cholecalciferol ("vitamin D3") is usually synthesized from fish oils or lamb's wool.
Not surprisingly, you will find that vegan alternatives tend to be fortified with the former, while other foods usually rely on the latter.
Both D2 and D3 raise vitamin D levels in the blood equally, although there was always conflicting research on whether or not their half-lives (how long their effects last) are the same.
However, the latest research (a 2008 study out of Boston University) found that doses of both D2 and D3 took the same amount of time to plateau to baseline levels.
Regardless, if you consume Vitamin D on a daily basis (and get some exposure to the sun), half-lives become rather irrelevant.