A landmark paper from the Vitamin D Council asserts that a form of vitamin A, retinoic acid, can block the activity of vitamin D by weakly activating the vitamin D response element on genes. Since vitamin D levels are crucial for human health, that means it is essential to have the proper ratio of vitamin D to vitamin A in your body.
This means that vitamin A supplementation is potentially dangerous. Vitamin A production is tightly controlled in your body, the source (substrate) being carotenoids from vegetables in your intestine. Your body uses these carotenoid substrates to make exactly the right amount of retinol. But when you take vitamin A as retinol directly, such as in cod liver oil, you intervene in this closed system and bypass the controls.
The goal is to provide all the vitamin A and vitamin D substrate your body would have obtained in a natural state, so your body can regulate both systems naturally. This is best done by eating colorful vegetables and by exposing your skin to sun every day.
Sources: Vitamin D Newsletter December 2008
Read Dr. Mercola's comments at: http://articles.mercola.com/sites/articles/archive/2009/01/03/why-vitamin-a-may-not-be-as-useful-or-harmless-as-you-thought.aspx