Vitamin D2 (ergocalciferol) and D3 (cholecalciferol) have long been considered equivalent in human nutrition with the possible preference for Vitamin D3 being a new focus of research only in recent years.  Currently US public health agencies (FDA, CDC) recommend 400IU per day of either Vitamin D2 or D3 for adequate nutrition.  In simple terms Vitamin D2 is found in plants while D3 is from animal sources.  The exception is that D3 is also found in some lichen.  We sourced Vitamin D2 from mushrooms because it is the most widely available form of organic certified Vitamin D.  Currently there are no or very limited sources of organic certified Vitamin D3.