He says that a study on people with repeated respiratory infections found that cod liver oil also contains vitamin A, and that vitamin D and A compete with each other metabolically, so if you have too much vitamin A in your vitamin D supplement, you are not getting as much use out of the D. "Vitamin A and vitamin D compete for each other’s function. For example, even the vitamin A in a single serving of liver can impair vitamin D’s rapid intestinal calcium response." Mercola calls the amounts of vitamin A in the supplements "detrimental".
A separate study from the New York City Department of Health and Mental Hygiene suggests that one form of vitamin D (calcitriol, aka 1,25-dihydroxyvitamin D) may be one of your body's main protections against damage from low levels of radiation. Calcitriol is the active form of vitamin D might be an appropriate protective agent before or after a low-level nuclear incident!! Calcitriol is involved in cell cycle regulation, hence affects proliferation, differentiation, encourages apoptosis and autophagy, and inhibits angiogenesis.