The mystery why fish liver oils only contain vitamin D3

What remains a mystery to scientists and marine experts is why the only form of vitamin D that has been identified in fish oils is vitamin D3. You would expect vitamin D2 would be present especially if vitamin D2 produced in phytoplankton was concentrated in the food chain. The main explanations for this observation include:

• Fish have the ability to discriminate between vitamin D2 and vitamin D3 and therefore, only concentrate vitamin D3 in their visceral oils.
• Alternatively, fish or zooplankton that consume phytoplankton have a similar capacity as Paramecium to convert the side chain of vitamin D2 into the side chain of vitamin D3.
• There is also a possibility the bioavailability of vitamin D2 is lower than for D3 in fish.