Important Cod Liver Oil Update

For years, I have recommended cod liver oil as a dietary supplement to support healthy vitamin D levels. However, based on more recent findings, I am updating my recommendations regarding cod liver oil, as it may not serve you as well as previously believed.

My previous recommendation was based on the fact that cod liver oil contains vitamins D and A in addition to healthy omega-3 fats. These vitamins are essential for most everyone who cannot get regular sun exposure year-round.

But more recent research has discovered that the ratios of these two vitamins may be of paramount importance in order to extract optimal health benefits, and unfortunately, modern cod liver oil does not supply these vitamins in healthy ratios to each other.

What You Need to Know About Vitamins A and D in Cod Liver Oil

More than 2,000 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 16 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.

There are many problems with modern cod liver oil but one of the primary ones is that there is no standard definition of what constitutes cod liver oil. Manufacturers are free to add or subtract as much vitamin A or D as they see fit. In fact cod liver oil was discovered in the sewers of England several hundred years ago by starving children who drank it and they noticed they did not get rickets. Cod liver oil is in fact a highly processed food that was never consumed by humans prior to this.

Primary Justification Why You Should AVOID Cod Liver Oil

There have been two recent meta-analyses done. The first one showed that people who took vitamin A supplements in cod liver oil, or in supplements, had an 18 percent increase in death rates. The other study showed that unlike third world countries where vitamin A supplementation appears to decrease infections, vitamin A supplementation in developed countries like the US actually increases infections.

The researchers believe this is due to massive nutritional deficiencies in the third world because most of their calories are from grains and they simply don’t have an opportunity to consume as many fresh fruits, vegetables, butter, eggs and other vitamin A containing foods that those in the developed world do.

In fact current research could not find any vitamin A deficiency at all, but approximately 5% had vitamin A toxicity. The converse is true in the third world where vitamin A toxicity is virtually unheard of yet vitamin A deficiency is pervasive.

Additionally new research has shown that vitamin D protects against cancer. But a paradox was found as those with higher vitamin D levels did not seem to have this benefit. A bright Harvard researcher carefully analyzed the data in the study that showed this and found that when he removed the people with high vitamin A and vitamin D levels those with normal vitamin A levels and high vitamin D levels continued to have reduced risk of colon cancer. So those that did not take vitamin A had the protective effect from higher levels of vitamin D.

Other research is now showing a connection between high levels of vitamin A and osteoporosis. In fact many Scandinavian countries that regularly supplement with cod liver oil have rampant osteoporosis even though they are getting adequate amounts of oral vitamin D.

Dr. John Cannell, head of the Vitamin D Council, along with 15 other researchers, recently released an article “Cod Liver Oil, Vitamin A Toxicity, Frequent Respiratory Infections, and the Vitamin D Deficiency Epidemic" in the November issue of Annals of Otology, Rhinology and Laryngology. In this paper Dr. Cannell raised questions about the efficacy of cod liver oil due to its highly variable and frequently excessive amount of vitamin A. Typically 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.

Dr. Cannell and other prominent researchers believe the vitamin A contained in most cod liver oil is excessive, and can reduce the effectiveness of vitamin D by inhibiting the binding of its active form to your DNA, effectively preventing its ability to regulate the expression of your vitamin D-responsive genes.

The Weston Price Foundation, of which I am an advisory member, holds a contradictory view. They believe vitamin D can only effectively target genes when its “partner receptor” is activated by vitamin A. If vitamin A is absent, certain molecules called co-repressors bind to the receptors and prevent vitamin D from functioning. It is their position that cod liver oil is still a highly recommended supplement.

However even they acknowledge that there are dangerous versions out there, even from some highly reputable companies, like Nordic Naturals, that produces a version of cod liver oil that is clearly excessive in vitamin A as it only has 3 to 60 units of vitamin D per tablespoon but between 150 and 12,000 times as much vitamin A.

So it’s a delicate balance.

Both vitamins are essential to obtain optimal health benefits, however, the ratios of the can become dangerously unbalanced -- much like the omega-3/omega-6 balance, which has become inversed in our modern diet.

Nearly all brands of cod liver oil provide a token amount of vitamin D, typically a mere 400 to 1,200 IU of vitamin D per tablespoon but anywhere between 4,000 to 30,000 IU of vitamin A. This is clearly inappropriate. About the lowest ratio I have seen is ten times as much vitamin A as vitamin D but, as I stated above, it can be as high as 12,000 times as much vitamin A.

First of all, this is clearly an insufficient amount of vitamin D for even the smallest child. This is in part due to the government recommendations, which are FAR too low to offer any health benefits; the recommended daily dosage being no more than 200 to 600 IU, depending on age. Meanwhile, researchers have since established that the therapeutic dosage is anywhere between 2,000 to 10,000 IU per day, depending on your weight and other factors, such as skin color and level of regular sun exposure. (Some people may require, and can safely take, as much as 20,000 IU daily.)

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.

The concern Dr. Cannell and the other researchers have is that vitamin A in cod liver oil is excessive and actually antagonizes vitamin D by inhibiting the binding of its active form to DNA and thus preventing its ability to regulate the expression of vitamin D-responsive genes.

Their 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. In the third world this is not the case and they would likely benefit from vitamin A supplementation.

The Weston Price Foundation does not agree with Dr. Cannell’s conclusion that cod liver oil itself may cause vitamin A toxicity, however they also do not recommend taking any cod liver oil that is low in vitamin D. Yet even their recommendations, in my opinion have far too low amounts of vitamin D to be clinically useful. But more importantly it appears that the high amounts of vitamin A may limit the effectiveness of vitamin D even if more is taken in addition to that received in the cod liver oil.

Although it’s still unclear exactly what the balance should be, Dr. Cannell and most of the prominent expert researchers in this area believe that the ratios of these two essential nutrients likely should be inversed, as you need far greater amounts of vitamin D as opposed to vitamin A.

After carefully reviewing the arguments on both sides of the issue I am convinced that Dr. Cannell’s approach is far more likely to be consistent with producing high levels of health and decreased illness.

My Revised Cod Liver Oil Recommendations

As the prevalence of vitamin A deficiency (which would benefit from cod liver oil) in the U. S. is much lower than the prevalence of subclinical vitamin A toxicity, while most everyone suffers from vitamin D deficiency, I no longer recommend taking cod liver oil for either adults or children. You’re likely getting the vitamin A you need if you regularly consume fresh vegetables high in this nutrient, such as sweet potatoes, carrots, cantaloupe, and other colorful fruits and vegetables, and butter especially, if obtained from grass fed cows.

Although you can obtain Vitamin D, from your diet it is very difficult, and I believe it is very unnatural. It is my strong belief that we were designed to obtain virtually all of our vitamin D from exposing appropriate areas of our skin to sunshine. If this is not possible, the next best choice would be UVB from safe tanning beds, and if that is not possible then one should resort to a high quality vitamin D3 supplement.

As it stands, it is my strong belief that you’re simply not getting the appropriate balance of vitamin A to vitamin D from cod liver oil, which is why I believe it is best to avoid it.

Please note that this new recommendation does NOT apply to either fish oil or krill oil, as neither of them contain the vitamins A or D, but rather are excellent sources of essential omega-3 fats. EVERYONE still needs a regular high quality source of these absolutely essential and vital nutrients.

Another potential point of confusion is that beta carotene is not a concern, as that is PRE vitamin A. Your body will simply not over convert beta carotene to excessive levels of vitamin A. So taking beta carotene supplements is not going to interfere with vitamin D. Instead, focus on getting your vitamin A from your diet, and ensuring your vitamin D levels are optimized, either by appropriate sun bathing or taking an oral vitamin D3 (cholecalciferol) supplement.

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About the author

author-pictureDr. Mercola is the founder of the world’s most visited natural health web site, You can learn the hazardous side effects of OTC Remedies by getting a FREE copy of his latest special report The Dangers of Over the Counter Remedies by going to his Report Page.


Anonymous's picture


Thanks for this update Dr. Mecola!

delorespayne's picture

Ms Delores Payne

I have interstitial cystitis. I've found that if I take Vit D (pill or liquid) that it increases urination & causes pain in my bladder & sometimes even my kidneys.
I always take my vitamins with my meals.
I also take Vit E, Milk Thistle, Bilberry, FlaxSeed Oil (2capsules) & B12 with my Vit D pill.
Is there something I can do to alleviate this pain.
About 1 1/2 years ago my physician said I was deficient in Vit D. I've tried to take the Vitamin with little success at reducing the pain attributed to my intake.
The drops are 2000 IU per drop. The pills are 600 per pill.
Could my pain be from mixing the vitamins together?

Anonymous's picture


Well are these findings based on NATURALLY OCCURRING vitamin A and D levels or synthetic vitamins A and D that are added back to cod liver oil???

The vitamin A and D work synergically in cod liver if not tainted with and should not have a toxic effect.

Anonymous's picture


What about Krill Supp. are they any better or worse for you?

Anonymous's picture


your honesty and courage made me add you to my favorite links.

Anonymous's picture


Great info Dr. Mercola as usual but i did want to mention that Carlsson's makes a low vitamin A cod liver oil and i must say i feel great since taking it and getting more sun bathing in for the vitamin D...thanks, jules

Anonymous's picture

Hillel Fridman

Vitamin A has had a bad press lately. Conversion of beta carotene not only occurs in the liver, but the intestines as well. This conversion therefore depends on intestinal health and also decreases in winter months. So observations that apply to low vitamin D status in winter may also apply to Vitamin A, though less so, given it has a half-life twice that of Vitamin D. Considering the fall in liver consumption and increasing issues with digestion we may see a rise in Vitamin A population deficiencies. Of course the correct hypothesis must be less vitamin A and more vitamin D, but to rely solely on caretanoid sources of vitamin A may be wrong.

Anonymous's picture

Sean Cherian

You ROCK!!!!!

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