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Cod liver oil is derived from cod livers. It is a nutritional supplement, in the past commonly given to children. Cod liver oil is one of the most effective providers of omega-3 fatty acids, and an excellent source of vitamin A and vitamin D. It is widely taken to ease the pain and joint stiffness associated with arthritis but has also been clinically proven to have a positive effect on heart, bone and brain health, as well as helping to nourish skin, hair and nails. Empirically however, Rheumatologists attribute this effectiveness to a placebo effect, and have called into question the validity of these studies.

Depending on the quality of the oil, the flavor and aroma range from a mild sardine-like flavor, to an intense and obnoxious odor of rotten fish and rancid oil. High quality cod liver oil is "a pale-yellow, thin, oily liquid, having a peculiar, slightly fishy, but not rancid odor, and a bland, slightly fishishly taste." It has recently become popular to flavor cod liver oil with citrus or mint essence to make it more palatable.

Cod liver oil is made by cooking cod livers with steam and then pressing/decanting the cooked livers to extract the oil. By contrast, fish oils are extracted from the cooked whole body tissues of fatty fish during the manufacture of fish meal. Cod liver oil and fish oil are similar but have a somewhat different composition: fish oil has a much lower content of vitamins A and D compared to liver oils.

This may pose a problem in that one may need to exceed the Recommended Dietary Allowance (RDA) of vitamins A and D in order to obtain therapeutic amounts of EPA and DHA from cod liver oil. These vitamins are fat soluble, so it's possible for dietary excess (well above the RDA) to accumulate and become harmful.

Because the body naturally produces vitamin D when exposed to sunlight, a common way to benefit from both oils while avoiding a vitamin D overdose is to take cod liver oil during late fall through winter, and fish oil during spring through summer. The ideal dosage and timing of consumption depends on your seasonal sun exposure (and therefore natural vitamin D production). The only way to be certain of any possible vitamin D deficiencies or overdosing (from supplements) is to have your levels checked.

On the other hand, the RDA of vitamin D is considered by many to be strongly understated. Many adults don't meet the RDA.^{[citation needed]}

In 2005 researchers at the University of California reported that Vitamin D may lower the risk of developing different types of cancers, cutting in half the chances of getting breast, ovarian, or colon cancer. [1].

Pregnant women using cod liver oil have infants with a lower risk for juvenile type 1 diabetes. Use of cod liver oil during pregnancy associated with lower risk of Type I diabetes in the offspring. (Stene LC, Ulriksen J, Magnus P, Joner G. Diabetologia 2000;43:1093-8). 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 fatty acid profile in breast milk to promote optimal brain development and also increases levels of vitamin A to prevent infections. Interestingly, cod liver oil does not provide increased vitamin D in breast milk. Nevertheless, pregnant women should be wary and not consume large amounts, since researchers in Iceland report that a high intake of cod liver oil is associated with a nearly five-fold increased risk of gestational hypertension (BJOG 2006;113:301-309)

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.

Some urge caution when taking cod liver oil and other fish-based supplements since they may contain elevated levels of toxins increasingly found in fish such as mercury and PCBs. Some supplement companies regularly test cod liver oil for purity, while others do not.