Mercola I believe was referring to the balance of Vitamin A & D as they work in tandem or unison.
There is evidence he states that without vitamin D, vitamin A can be ineffective or even toxic. But if you’re deficient in vitamin A, vitamin D cannot function properly either.
Mercola stated that modern cod liver oil contains far less vitamin D than it used to, due to the deodorization process used today which removes much of this essential nutrient.
It is certainly a very complex issue, but I personally believe that to receive the maximum benefit from Vitamin A, it should be accompanied by an appropriate dosage of Vitamin D, which would also prevent any dangers of Vitamin A toxicity
It has generally been known for sometime that Vitamin A taken to excess, and in isolation from other synergistic nutrients, can result in toxicity and potential Liver damage. This is the view of Dr Cannell MD of the Vitamin D Council. In their view the amount of Vitamin D3 should exceed that of Vitamin A or retinol, as opposed to its precursor, beta-carotene from plant sources.
Mercola was just warning us of the potential risks, as there are huge variations in the ratio of both A & D in Cod-liver oil (in favor of Vitamin A) from many manufacturers, except in Carlsons own-brand where the D outweighs the A.