For discussion purposes; There are over 3000 metabolites of Vitamin D known. Many of the metabolites are slightly understood, others very little is known about their action/pathway and few are routinely discussed (D2/D3). Many of the Metabolites will have different pathway's and actions in our health. For all you chemists out there, a new publication, "The Vitamin D Handbook' is now available that identifies and describes 900+ of these metabolites. Todays standard testing can only measure a handful of the different metabolites (maybe 4 to 10 of the species and at a very high cost).

I have found no indication that we have any base of knowledge of the vitamin D pathway in fish. In fact, I have been told that very little is known/studied in this area of science.

By the way, Vitamin D2 is a very useful nutrient to our bodies. All the information I have found on the subject is contradictory to the story told in many blogs. Maybe there is a difference between naturally occuring Vitamin D2 vs synthetic D2.

Side note; There are Vitamin A metabolites other than what is typically measured and reported in foods/supplements. If memory serves me, there are at least 15 metabolites identifed in nature.

So much we know, so much we don't know. I think the turth lies in the latter. This is why it is exhilarating to get up each morning!