cod liver oil

Cod liver oil is an oil pressed from the livers of cod, sold as a dietary supplement. The NIH Office of Dietary Supplements counts it among omega-3 supplements (alongside fish oil, krill oil, and algal oil); it provides the long-chain omega-3 fatty acids EPA and DHA, and because it is derived from fish liver it also delivers meaningful amounts of preformed vitamin A and vitamin D.

Evidence is mixed and mostly attaches to the individual nutrients it contains rather than to cod liver oil as a product. NIH notes that people who eat fish tend to have a lower risk of several chronic diseases, but says it is not clear whether that benefit comes from the food itself or from its omega-3s; for the heart, the American Heart Association does not recommend omega-3 supplements for people who are not at high cardiovascular risk. For vitamin D, clinical trials have not shown that supplements reduce the risk of developing or dying from heart disease, and results for cancer, mood, and other conditions are inconsistent. For vitamin A, NIH states that supplements have not been shown to prevent cancer in people who are already well nourished.

There is no established recommended dose for cod liver oil itself, and because it concentrates vitamins A and D, how much is in a serving matters. For context, NIH sets the recommended intake of vitamin D at 15 mcg (600 IU) per day for most adults and 20 mcg (800 IU) from age 71, each with a safe upper limit; for preformed vitamin A it is 900 mcg RAE per day for men and 700 mcg RAE for women, with an adult upper limit of 3,000 mcg from all sources. Separately, the FDA advises taking no more than 5 g per day of combined EPA and DHA from supplements. Because product strengths vary widely and vitamins A and D can accumulate to harmful levels, check with a clinician before starting cod liver oil, especially if you are pregnant, already take other vitamin A or D supplements, or have a medical condition.

The concern most specific to cod liver oil is its preformed vitamin A, which can be harmful in excess. NIH cautions that too much preformed vitamin A can bring on severe headache, vision changes, nausea, dizziness, muscle aches, and trouble with coordination, and in serious cases coma or death; in pregnancy it can cause birth defects, so anyone who is or may become pregnant should avoid high-dose preformed vitamin A. Excess vitamin D can likewise reach toxic levels. Anyone taking a multivitamin or a separate A or D product should add up these overlapping sources.

For the omega-3 portion, side effects are usually mild, and per NIH may include a fishy aftertaste, bad breath, heartburn, nausea, stomach upset, diarrhea, headache, and body odor. Taking in more vitamin A or vitamin D than the body can handle can add the further symptoms described under safety.

NIH notes that high doses of omega-3s can raise the risk of bleeding when combined with warfarin (Coumadin) or other anticoagulants. The vitamin A component can interact with the weight-loss drug orlistat (Alli, Xenical), which lowers vitamin A absorption, and with vitamin A-derived medicines such as acitretin (Soriatane) and bexarotene (Targretin), where the added vitamin A could push blood levels dangerously high. Tell your doctor and pharmacist about every supplement and medicine you take.