Cod liver oil
Cod liver oil is a nutritional supplement derived from liver of cod fish (Gadidae).  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 during the manufacture of fish meal.
Though similar in composition to fish oil, cod liver oil has higher concentrations of vitamins A and D. According to the United States Department of Agriculture, a tablespoon (4 drams or 15 ml) of cod liver oil contains 4080 μg of retinol (vitamin A) and 34 μg of vitamin D. The Dietary Reference Intake of vitamin A is 900 μg per day for adult men and 700 μg per day for women, while that for vitamin D is 15 μg per day. The "tolerable upper intake levels" are 3000 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.
Rich in omega-3 fatty acids, cod liver oil may be beneficial in secondary prophylaxis after a heart attack. Diets supplemented with cod liver oil have also been demonstrated to have beneficial effects on psoriasis.
Potential adverse effects
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. 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.
The risks of fatty acid oxidation, hypervitaminosis, and exposure to environmental toxins are reduced when purification processes are applied to produce refined fish oil products.
A high intake of cod liver oil by pregnant women is associated with a nearly fivefold increased risk of gestational hypertension. The study noted that "possibly, the amount of n-3 LCPUFA may have positive effects up to a certain level, while becoming detrimental in high doses."
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