- In recent years, controversy has erupted over whether vitamin D2 or vitamin D3 is the predominant form of vitamin D in cod liver oil.
- Research in the 1930s suggested that there were at least four if not six forms of vitamin D in cod liver oil.
- Recent research has shown that fish metabolize vitamin D into at least three other compounds and probably more.
- Although cod liver oil probably does not contain vitamin D2, it probably does contain an array of different compounds derived from vitamin D3.
- The diverse array of vitamin D compounds we would expect to exist in natural cod liver oil likely provides a diverse array of biological activities; many people may experience vitamin D-related benefits from a natural cod liver oil without experiencing as pronounced a rise of 25(OH)D – the blood marker of vitamin D nutritional status—as they would have expected.
- Although it makes sense for someone to increase their sun exposure and vitamin D3 intake if their 25(OH)D is low, low 25(OH)D in and of itself should not be used as evidence that cod liver oil is not providing a vitamin D benefit