Why taking cod liver oil is not a good idea
I recently wrote an article on the importance of vitamin D in the fight against influenza (1). The discussion focused mainly on the role of vitamin D in supporting innate immunity, but adaptive immunity obviously plays a role in the battle against infection as well. The innate branch of the immune system produces so-called anti-microbial peptides (AMPs), compounds with broad-spectrum effectiveness against bacteria, viruses, and fungi. Vitamin D is necessary for the expression of some of these AMPs.
The cells of the lining (endothelium) of the upper airways produce AMPs constitutively, i.e. regardless of whether or not pathogens are present. These anti-microbial peptides are released into a thin layer of fluid above the endothelium; this layer in turn is covered with mucus. When a pathogen penetrates the mucous layer and encounters anti-microbial peptides, the AMP molecules bind to its glycoprotein coat, effectively destroying the invader.
If a pathogen makes it past this protective coating and binds to the endothelium, this initiates the synthesis of high concentrations of specific inducible AMPs, thus providing a backup anti-microbial shield. The expression of the genes coding for these inducible AMPs requires vitamin D.
Unfortunately, it is virtually impossible to get adequate amounts of vitamin D from the diet. In the winter it is also impossible to make enough vitamin D by exposing the skin to the sun. In fact, vitamin D deficiency is now even a problem during the summer months, because of widespread sun avoidance. Vitamin D supplementation is therefore essential.
How much vitamin D do we need? It is now believed that serum 25(OH)D (25-hydroxyvitamin D) levels of 50 ng/mL (nanograms per milliliter) or more are required to ensure adequate vitamin D stores. To get to this level, adults would need to take 2,000 IU/day or more. These levels are considered quite safe — the tolerable upper intake level is now believed to be at least 10,000 IU/day (1).
What kind of vitamin D supplement should you take? In my article I recommended getting vitamin D from cod liver oil. No sooner had I written this than I came across a paper warning against cod liver oil (2). The problem? Cod liver oil contains excessive amounts of vitamin A. For every 1 IU of vitamin D, cod liver oil contains at least 10 IU of vitamin A. If you were to take enough to get 2,000 IU of vitamin D, you′d get 20,000 IU or more of vitamin A as retinol. This amount is well above the tolerable upper intake level (UL) of 3,000 μg/day (10,000 IU/day) set by the Food and Nutrition Board of the U.S. Institute of Medicine (3). 0.3 μg equals 1 IU of vitamin A (4).
In contrast to vitamin D, vitamin A is readily available in the diet in the form of provitamin A — carotenoids in coloured vegetables. Consequently, excess vitamin A intake is much more widespread in developed countries than vitamin A deficiency. Taking cod liver oil is therefore a recipe for subclinical vitamin A toxicity. Not only that, but its vitamin A content actually masks the effect of its vitamin D. In other words, the vitamin A in cod liver oil defeats the very purpose of taking it for its vitamin D content.
Of course, the warning against cod liver oil does not apply to fish or krill oil. Fish oil is extracted from the body of the fish and contains neither vitamin A nor D in any meaningful amount. It is, however, an important source of the polyunsaturated omega-3 fatty acids EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid). In fact, fish (or krill) oil is one of the most important supplements you can take!
Forget about cod liver oil and take vitamin D3 (cholecalciferol) supplements instead!
Sources:
The cells of the lining (endothelium) of the upper airways produce AMPs constitutively, i.e. regardless of whether or not pathogens are present. These anti-microbial peptides are released into a thin layer of fluid above the endothelium; this layer in turn is covered with mucus. When a pathogen penetrates the mucous layer and encounters anti-microbial peptides, the AMP molecules bind to its glycoprotein coat, effectively destroying the invader.
If a pathogen makes it past this protective coating and binds to the endothelium, this initiates the synthesis of high concentrations of specific inducible AMPs, thus providing a backup anti-microbial shield. The expression of the genes coding for these inducible AMPs requires vitamin D.
Unfortunately, it is virtually impossible to get adequate amounts of vitamin D from the diet. In the winter it is also impossible to make enough vitamin D by exposing the skin to the sun. In fact, vitamin D deficiency is now even a problem during the summer months, because of widespread sun avoidance. Vitamin D supplementation is therefore essential.
How much vitamin D do we need? It is now believed that serum 25(OH)D (25-hydroxyvitamin D) levels of 50 ng/mL (nanograms per milliliter) or more are required to ensure adequate vitamin D stores. To get to this level, adults would need to take 2,000 IU/day or more. These levels are considered quite safe — the tolerable upper intake level is now believed to be at least 10,000 IU/day (1).
What kind of vitamin D supplement should you take? In my article I recommended getting vitamin D from cod liver oil. No sooner had I written this than I came across a paper warning against cod liver oil (2). The problem? Cod liver oil contains excessive amounts of vitamin A. For every 1 IU of vitamin D, cod liver oil contains at least 10 IU of vitamin A. If you were to take enough to get 2,000 IU of vitamin D, you′d get 20,000 IU or more of vitamin A as retinol. This amount is well above the tolerable upper intake level (UL) of 3,000 μg/day (10,000 IU/day) set by the Food and Nutrition Board of the U.S. Institute of Medicine (3). 0.3 μg equals 1 IU of vitamin A (4).
In contrast to vitamin D, vitamin A is readily available in the diet in the form of provitamin A — carotenoids in coloured vegetables. Consequently, excess vitamin A intake is much more widespread in developed countries than vitamin A deficiency. Taking cod liver oil is therefore a recipe for subclinical vitamin A toxicity. Not only that, but its vitamin A content actually masks the effect of its vitamin D. In other words, the vitamin A in cod liver oil defeats the very purpose of taking it for its vitamin D content.
Of course, the warning against cod liver oil does not apply to fish or krill oil. Fish oil is extracted from the body of the fish and contains neither vitamin A nor D in any meaningful amount. It is, however, an important source of the polyunsaturated omega-3 fatty acids EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid). In fact, fish (or krill) oil is one of the most important supplements you can take!
Forget about cod liver oil and take vitamin D3 (cholecalciferol) supplements instead!
Sources:
- Helmut Beierbeck. Why you need extra vitamin D during the flu season. Natural News Dec. 2, 2008, and references therein.
[Full Text] - Cannell JJ et al. Cod liver oil, vitamin A toxicity, frequent respiratory infections, and the viamin D deficiency epidemic. Ann Otol Rhinol Laryngol 2008;117:864-870.
- Dietary Reference Intakes – vitamins. [Table}
- Vitamin A. Micronutrient Information Center. Linus Pauling Institute. http://lpi.oregonstate.edu/infocenter/vitamins/vitaminA/ (accessed Jan. 15, 2009)
I can say that a high intake of cod liver oil by pregnant women is associated with a nearly five-fold increased risk of gestational hypertension.
Therefore 'Cholecalciferol' can be used as a dietary supplement as vitamin D has causion of containing elevated levels of toxins such as 'mercury' and 'PCBs' found in fish.
Thanks for the information
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thanks for this post. Its a great post.
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According to Adelle Davis in her book “Let’s Get Well” vitamin A deficiencies is more widespread than is generally admitted by the medical establishment. Vitamin A can be toxic but only when taken in very high amounts, such as 200,000 units per day, for several months. Taking up to 50,000 units per day is safe and is often useful when having to cope with infections like measles, scarlet fever, pneumonia, sinusitis, etc. Children should be limited to 20,000 units per day and infants to 10,000 units per day.
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Cannell′s article on the connection between serum vitamin D (25(OH)D) levels and upper respiratory tract infections (URTIs) (1) persuaded another group of investigators to go through the 1988 - 1994 NHANES (National Health And Nutrition Examination Survey) data in search of corroborative evidence. Here is what they found.
People with 25(OH)D levels below 10 ng/mL were 36% more likely to come down with URTIs than those with 25(OH)D > 30 ng/mL (OR = 1.36). For those with levels between 10 and 30 ng/mL the risk was 24% higher (OR = 1.24). The effect was even more pronounced when only patients with asthma or chronic obstructive pulmonary disease (COPD) were considered. Here the odds ratios (ORs) were 5.67 and 2.26, respectively. The authors concluded:
"Serum 25(OH)D levels are inversely associated with recent URTI. This association may be stronger in those with respiratory tract diseases. Randomized controlled trials are warranted to explore the effects of vitamin D supplementation on RTI."
Sources:
Cannell JJ et al. Cod liver oil, vitamin A toxicity, frequent respiratory infections, and the vitamin D deficiency epidemic. Ann Otol Rhinol Laryngol 2008;117:864-870.
Ginde AA, Mansbach JM, Camargo CA. Association between serum 25-hydroxyvitamin D level and upper respiratory tract infection in the Third National Health and Nutrition Examination Survey. Arch Intern Med 2009;169(4):384-390.
[Abstract]
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Just wondering why there was such a bad idea about cod liver oil? As far as I know, cod liver oil is good for the body because of its essential nutrients. But like any other supplements, correct dosage should be followed because anything in excess is harmful.
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Why is it such a bad idea to take cod liver oil? Because of its high vitamin A/vitamin D ratio. In the developed world vitamin A deficiency is rare, but vitamin D deficiency is common. Cod liver oil is therefore taken for its vitamin D content.
If you take enough cod liver oil for the "correct dosage" of vitamin D, your vitamin A intake will exceed safe upper limits.
In addition, vitamin A actually interferes with the absorption of vitamin D, i.e. it defeats the very purpose of taking cod liver oil in the first place.
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Thank you for this info. It gives updated data on this commodity. For many years, people had been thinking of cod liver oil as superfood. Now, it's clear it is not exactly what they thought it was.
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Thanks for the post. I think tha additional benefits of cod liver oil are being overlooked though.
Sofus
www.nammi.is
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Can cod liver oil cause hypothyroidism? Because both my child and I developed acute symptoms of hypothyroidism (enlarged thyroid, decreased heartrate, increase TSH blood levels) upon ingestion for a few days of cod liver oil. I am wondering if this is the causative factor, as I can't think of anything else "new" that could've caused it.
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Michelle, you'll have to consult your doctor to confirm that you are really suffering from hypothyroidism, and what might be causing it. I can make one suggestion though. If you are taking cod liver oil for vitamin D, you could switch to vitamin D3 (not vitamin D2) tablets.
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