Our bodies do not use Vitamin D3 or D2 but rather these Hormones follow a enzyme pathway converting to a common usable form 1-25 dihydroxy vitamin D. As we age,. or if a person has issues that interrupt/slow down the natural conversion pathway, ones body might not convert as efficiently the vitamin D to 1-25 dihydroxy. I have seen specific studies that show the aging process does negatively effect the pathway efficiency.

couple other bullets of interest

*Vitamin D Helpers include: Vitamins A, Bp, C, F, T, calcium, phosphorus activator x, and i found a reference that vitamin C is a detoxifier of vitamin D

* I found a old reference to improving the effectiveness of cod liver oil through exposing it to the sun for a 'short time'. Short time was not defined but my understanding of the refernce is if exposed too long teh sun would have an opposite effect. My guess is the fermented clo would be the better product to test this theory/observation.