Cod Liver Oil

What Is Cod Liver Oil?

Cod liver oil is an oil extracted from the liver of cod fish. Like other marine oils, it contains the long-chain omega-3 fatty acids EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid), but it also naturally contains significant amounts of vitamins A and D. This combination makes cod liver oil nutritionally different from ordinary fish oil, which is generally obtained from the tissues of oily fish and does not naturally contain the same concentrations of these vitamins.

The exact amounts of EPA, DHA, vitamin A and vitamin D vary considerably among cod liver oil products. Modern supplements are available as liquid oil and softgel capsules, and some liquid products are flavored to make them easier to take. When comparing products, it is more useful to read the supplement facts label than to judge quality by color, taste or the total amount of oil alone.

History of Cod Liver Oil

Cod liver oil has a long history in northern Europe, particularly in coastal and fishing communities where cod was an important food resource. Traditional uses of fish liver oils existed long before researchers understood the roles of omega-3 fatty acids or vitamins A and D.

Cod liver oil began attracting greater medical attention in Europe during the late eighteenth and nineteenth centuries. One of its best-known historical uses was in the treatment and prevention of rickets, a childhood bone disease that was once widespread in industrialized cities.

During the early twentieth century, researchers established the connection between vitamin D, sunlight and bone health. By the 1920s and 1930s, the vitamin D content and anti-rickets properties of cod liver oil were much better understood. Cod liver oil consequently became a common nutritional supplement for children in many countries before vitamin D fortification of foods became widespread.

Cod Liver Oil Benefits

Many of the potential benefits of cod liver oil come from its three principal nutritional components: EPA and DHA omega-3 fatty acids, vitamin A and vitamin D. It is important, however, to distinguish established nutritional roles from claims that cod liver oil can prevent or treat specific diseases.

Omega-3 Fatty Acids

EPA and DHA are involved in cardiovascular function, cell membranes and inflammatory signaling. Omega-3 fatty acids can reduce blood triglyceride levels, although therapeutic treatment of very high triglycerides generally uses standardized prescription omega-3 products rather than ordinary cod liver oil supplements.

EPA and DHA have also been studied in inflammatory conditions such as rheumatoid arthritis. Some research suggests that long-chain omega-3 fatty acids can provide modest additional benefit for joint symptoms, but cod liver oil should not be considered a replacement for established treatment.

Vitamin A

Vitamin A is essential for normal vision, immune function, reproduction and the maintenance of skin and other tissues. Cod liver oil contains preformed vitamin A, or retinol, which the body can use directly.

This can be nutritionally useful, but it is also one of the main reasons to pay attention to dosage. Unlike beta-carotene from plant foods, preformed vitamin A can accumulate in the body when consumed in excessive amounts.

Vitamin D and Bone Health

Fish liver oils are among the naturally rich dietary sources of vitamin D. Vitamin D helps the body absorb calcium and is important for maintaining healthy bones and normal muscle function.

Historically, this was one of the most important benefits of cod liver oil. Its vitamin D content helped prevent and treat nutritional rickets long before vitamin D-fortified foods and modern supplements became widely available.

Brain and Eye Health

DHA is an important structural component of the brain and retina. Adequate omega-3 intake is therefore relevant to normal neurological and visual development. However, this does not mean that cod liver oil has been proven to treat depression, improve intelligence or prevent neurological diseases. Research into omega-3 fatty acids and mental health continues, but results from clinical studies remain mixed.

Cod Liver Oil vs. Regular Fish Oil

Cod liver oil and regular fish oil both provide EPA and DHA, but they are not interchangeable products.

Regular fish oil is usually obtained from the tissues of fatty fish and is taken primarily as a source of EPA and DHA. Cod liver oil is obtained from fish liver and additionally provides substantial amounts of vitamins A and D.

This means that taking more cod liver oil simply to obtain a higher omega-3 dose can also substantially increase vitamin A and vitamin D intake. Anyone primarily interested in EPA and DHA may prefer a product that provides clearly stated amounts of these omega-3 fatty acids without large additional quantities of fat-soluble vitamins.

Cod Liver Oil Side Effects and Safety

At ordinary supplemental amounts, cod liver oil is generally well tolerated by many adults. Possible side effects include a fishy aftertaste, belching, heartburn, nausea or other digestive discomfort.

The more important concern is excessive intake of the fat-soluble vitamins A and D. Because these vitamins can accumulate in the body, taking large amounts of cod liver oil together with multivitamins or other vitamin A or vitamin D supplements can unintentionally result in a much higher total intake than expected.

Vitamin A

Long-term excessive intake of preformed vitamin A can cause toxicity. Depending on the amount and duration of exposure, excessive vitamin A may affect the liver, bones, skin and nervous system. This is why the vitamin A content on a cod liver oil label deserves particular attention.

Cod liver oil is especially important to consider during pregnancy. Health authorities generally advise pregnant women to avoid cod liver oil and other supplements containing retinol because excessive preformed vitamin A can harm fetal development. Anyone who is pregnant or planning pregnancy should check the supplement label and discuss vitamin A-containing products with a healthcare professional.

Vitamin D

Vitamin D is essential, but very high supplemental intake over time can also be harmful. Excessive vitamin D can increase calcium levels in the blood and may affect the kidneys and other organs. Again, total intake matters: vitamin D from cod liver oil should be considered together with any other vitamin D supplements being used.

Omega-3s and Medications

High doses of EPA and DHA can affect platelet activity and bleeding time. People taking anticoagulant or antiplatelet medications, or anyone considering high-dose omega-3 supplementation, should discuss the amount with a healthcare professional.

For more information about omega-3 amounts and supplement dosing, see Dosage of Fish Oil.

How to Choose Cod Liver Oil

When comparing cod liver oil products, look at the actual amounts of EPA, DHA, vitamin A and vitamin D per serving. Two products containing the same amount of total oil can have very different nutrient profiles.

It is also useful to choose products from manufacturers that provide clear information about purification, testing, oxidation and contaminant control. The vague marketing term “high quality” is less informative than a complete label showing precisely what the product contains.

Because cod liver oil provides several nutrients at once, the best amount depends on the rest of the diet and on any other supplements being taken. More is not automatically better.

Is Cod Liver Oil Worth Taking?

Cod liver oil can be a useful source of EPA, DHA, vitamin A and vitamin D, but those same characteristics mean that it requires a little more attention than ordinary fish oil. Someone who needs additional vitamin D as well as omega-3s may view its combined nutrient profile as an advantage, while someone already receiving plenty of vitamins A and D may prefer a standard fish oil or another omega-3 source.

The most useful way to evaluate a cod liver oil supplement is therefore not by treating it as a universal remedy, but by looking at what it actually provides and whether those nutrients fit the rest of the diet and supplement routine.

For detailed information about omega-3 fatty acids and supplements, see the NIH Office of Dietary Supplements omega-3 fact sheet. Information about vitamin A intake and fish liver oils is also available from the NHS vitamin A guidance.

This page provides general nutritional information and is not intended to diagnose, treat or prevent any medical condition.