WARNING!
This is an older article that may not reflect Dr. Mercola’s current view on this topic. Use our search engine to find Dr. Mercola’s latest position on any health topic.
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.