I first started taking fermented COD beacuse I wanted a natural source of vitamin D (I live in Sweden and don't get much sun.
I regulary take a vitamin D test and this fermented COD from Green Pastures really works.
The good thing about this COD is that it also contains butter oil witch makes it a great source of important nutrients like, vitamin D, A, K, Omega 3, 6, 7, 9