What it is
Vitamin D3 (cholecalciferol) is the form of vitamin D produced in human skin from sunlight. As a supplement or fortifier it is most often manufactured from lanolin extracted from sheep's wool. A vegan-suitable form is also available, derived from lichen.
Why it matters
Vitamin D3 appears in fortified plant milks, breakfast cereals, multivitamins, and some yoghurts. Without a clear statement on the label, the source is ambiguous.
What to do
Look for 'vegan vitamin D3' or 'D3 from lichen' on the label. If it just says 'vitamin D3', ask the manufacturer.