Cod liver oil
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, eicosapentaenoic acid (EPA) and docosahexaenoic acid (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.
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. 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 may have a positive effect on the heart after a first heart attack.
Cod liver oil and fish oil are similar, but cod liver oil has higher levels of vitamins A and D. According to the USDA, a tablespoon (4 drams or 15 ml) of cod liver oil (13.6 g) contains 4080 μg of retinol (vitamin A) and 34 μg of vitamin D. The Recommended Dietary Allowance of vitamin A is 900 μg per day for adult men and 700 for women, while that for vitamin D is 15 μg per day. The "tolerable upper intake levels" are 1000 000 μg/day and 100 μg/day respectively, so 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. A 300 mg soft gelatin capsule like those made by brands like Seven Seas and seacod by sanofi contain approximately 88 μg vitamin A per dose.
Per tablespoon (13.6 g), cod liver oil contains 136% of the established daily Tolerable Upper Intake Level (UL) for Preformed Vitamin A (Retinol). Vitamin A accumulates in the liver, and can reach harmful levels sufficient to cause hypervitaminosis A. 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. A toxic dose of retinol (vitamin A) is around 25 000 IU/kg (see Retinol#Retinoid overdose (toxicity)), or the equivalent of about 1.25 kg of cod liver oil for a 50 kg person.
There is an increased risk of prostate cancer in men who had a higher blood level of omega-3 fatty acids, however this specific study did not assess whether or not a person was taking supplements or what their dietary intake of omega-3s were.
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.
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