Cod liver oil is a nutritional supplement derived from liver of cod fish. As with most fish oils, it has high levels of the omega-3 fatty acids, EPA and DHA. Cod liver oil also contains vitamin A and vitamin D. It has historically been taken because of its vitamin A and vitamin D content. It was once commonly given to children, because vitamin D has been shown to prevent rickets and other symptoms of vitamin D deficiency.[1]
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Cod liver oil was traditionally manufactured by filling a wooden barrel with fresh cod livers and seawater and allowing the mixture to ferment for up to a year before removing the oil.[2][3] Modern cod liver oil is made by cooking the whole cod body tissues of fatty fish during the manufacture of fish meal.
Cod liver oil is widely taken to ease the pain and joint stiffness associated with arthritis,[4]. It may have a positive effect on heart,[5][6] bone,[7] as well as helping to repair wounded skin,[8] hair, and nails.
Cod liver oil and fish oil are similar, but cod liver oil has higher levels of vitamins A and D. People consuming cod liver oil as a source of omega-3 fatty acids should pay attention to how much vitamin A and vitamin D this adds to their diet.
Cod liver oil may be an effective complementary measure for long-term treatment of multiple sclerosis.[9]
Use of cod liver oil during pregnancy is associated with lower risk of Type I diabetes in the offspring.[10] This effect was found only in mothers taking cod liver oil, not in mothers taking multivitamin supplements. Cod liver oil taken by nursing mothers improves the breast milk by increasing the amount of fatty acids, which promotes brain development, and the amount of vitamin A, which helps prevent infections, but the level of vitamin D is unchanged.
A Norwegian study of more than 68,000 women reported that female cancer patients who took daily cod-liver oil supplements had significantly reduced mortality (25% for all cancers, 45% for lung cancer) compared to women who did not take such supplements.[11][12]
Because cod liver oil has a very high level of vitamin A, it is possible to exceed the Recommended Dietary Allowance (RDA) of vitamin A. Vitamin A accumulates in body fat, and can reach harmful levels sufficient to cause hypervitaminosis A.[13] Pregnant women may want to consider consulting a doctor when taking cod liver oil because of the high amount of natural forms of vitamin A such as retinol. High doses of synthetic vitamin A (retinoids) have been shown to cause birth defects.[14]
The risks of hypervitaminosis and of exposure to environmental toxins such as mercury, polychlorinated biphenyls (PCBs), dioxins , and other contaminants, are reduced when purification processes are applied to produce refined fish-oil products, which consequently contain raised levels of omega-3 fatty acids, such as EPA and DHA.[15]
A high intake of cod liver oil by pregnant women is associated with a nearly fivefold increased risk of gestational hypertension,[16] although this study did not control for mercury, which can be present in harmful amounts in fish[15] and which is another cause of hypertension.[17]
Some urge caution when taking cod liver oil and other fish-based supplements since they may contain elevated levels of toxins such as mercury and PCBs found in fish.[15] Some supplement companies regularly test cod liver oil for purity. Consumerlab.com tested 44 fish and cod liver oil supplements and found that all were free of detectable levels of mercury.[18]
In Newfoundland, cod liver oil was sometimes used as the liquid base for traditional red ochre paint, the coating of choice for use on outbuildings and work buildings associated with the cod fishery.
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