Vitamin A and D Work as Partners
"At least 2,000 genes, or nearly 10 percent of your genes, have been identified that are directly influenced by vitamin D, which in turn impact a wide variety of health issues, from preventing the common cold and flu to inhibiting at least sixteen different types of cancer. There’s even evidence linking vitamin D to the process of brain detoxification of heavy metals such as mercury. Widespread vitamin D deficiency has also been strongly linked to the childhood epidemics of autism, asthma, and diabetes, both type 1 and 2. Vitamin A, which is essential for your immune system just like vitamin D, is also a precursor to active hormones that regulate the expression of your genes, and they work in tandem. For example, there is evidence 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." (source)
Risks of Vitamin A and D Toxicity
"Some forms of synthetic vitamin A found in supplements can be toxic at only moderately high doses, fat-soluble vitamin A naturally found in foods like cod liver oil, liver, and butterfat is safe at up to ten times the doses of water-soluble, solidified and emulsified vitamin A found in some supplements that produce toxicity. Additionally, the vitamin D found in cod liver oil and butterfat from pasture-raised animals protects against vitamin A toxicity, and allows one to consume a much higher amount of vitamin A before it becomes toxic." (source)
Importance of Balanced Ratio of Vitamin A to D (source)
"The Weston Price Foundation’s strong belief is that vitamin A is not at all toxic but is necessary for optimal vitamin D function. However they believe there is sufficient vitamin A in the diet of most Americans, especially if they are taking a multivitamin.
Consuming such high amounts of vitamin A as contained in cod liver oil and most multi-vitamins, while not getting nearly enough vitamin D, combined with the fact that most people are deficient in vitamin D to begin with, could potentially cause vitamin A to become toxic. We agree with this statement and have consistently warned people not to use multivitamins and not to take brands of cod liver oil that are low in vitamin D."
|An example of a very|
low vitamin D to A ratio in cod liver oil.
A ratio of vitamin A to D of 5:1 is ideal (source)
For further reading about vitamin A and D
Vitamin A Saga
Am I Getting Too Much Vitamin D
From Seafood to Sunshine: A New Understanding of Vitamin D Safety
Cod Liver Oil Basics and Recommendations