Cod Liver Oil vs Fish Oil: Vitamin A, Vitamin D and Who Should Take It

Read the label for vitamin A per serving, count any vitamin A in your multivitamin, and talk to your doctor first if you are pregnant or may become pregnant, take a blood thinner, or take a retinoid medication.

What is cod liver oil, and what is it used for?

Cod liver oil (in Spanish, aceite de hígado de bacalao) is a fish liver oil. It carries three nutrients at once: the long-chain omega-3s EPA and DHA, vitamin D, and preformed vitamin A (retinol). NCCIH, the NIH's center for complementary health, notes that the amounts of A and D differ from brand to brand, so the label tells you more than the name does.

So what is it for ("para qué sirve")? Mostly three things:

• Omega-3 intake. EPA and DHA are the fats in salmon, sardines and other oily fish. Our omega-3s explainer covers what research does and does not show about them.
• Vitamin D. One tablespoon supplies about 34 mcg (1,360 IU), or 170% of the Daily Value (NIH ODS).
• Tradition. In several northern countries a daily spoonful through fall and winter has long been the family habit for keeping vitamin D up when sunlight is short (Forsmo et al., American Journal of Epidemiology, 2008).

Cod liver oil and rickets

Rickets is a childhood bone disease caused by too little vitamin D. Around 1900 it was common among poor children in polluted industrial cities of the northern US and in England. Folk practice already favored cod liver oil, and by the 1930s researchers had shown why: the oil was rich in the newly identified vitamin D (Rajakumar, Pediatrics, 2003; Wilton, CMAJ, 1995). Vitamin D added to milk, plus cod liver oil, then made rickets rare in the US (NIH ODS).

Cod liver oil vs fish oil: what's the difference?

Both give you EPA and DHA. The difference is the vitamins. NCCIH lists fish oil as a source of EPA and DHA, and fish liver oils such as cod liver oil as a source of those plus vitamins A and D. Don't count on ordinary fish oil for either vitamin unless the label says the maker added it.

If you want omega-3s only, or already get plenty of vitamin A, plain fish oil lets you take more omega-3 without stacking vitamin A. If you want a moderate all-in-one source and have room for the vitamin A, cod liver oil fits.

How much vitamin A and D is too much?

The NIH ODS sets these upper limits for adults 19 and older:

• Preformed vitamin A: 3,000 mcg RAE a day, including during pregnancy and breastfeeding. For retinol, 1 IU equals 0.3 mcg RAE, so that is 10,000 IU.
• Vitamin D: 100 mcg (4,000 IU) a day for everyone 9 and older.

Children's limits are much lower: 600 to 1,700 mcg of preformed vitamin A depending on age, and 63 to 100 mcg of vitamin D for ages 1 to 13 (NIH ODS). Give children only a product with directions for their age, at the labeled serving.

Strength varies by brand and has changed over time. Norway's cod liver oil held 1,000 mcg of vitamin A per 5 mL until a 1999 reformulation (Mai et al., Thorax, 2013). Read the Supplement Facts panel on your own bottle.

With a multivitamin: many multis also contain vitamin A and vitamin D, so add up every product. Beta-carotene is counted separately; the ODS says it has no upper limit and is not known to cause birth defects.

Who should be careful with cod liver oil?

• Pregnancy, or trying to conceive. Preformed vitamin A above the upper limit can cause birth defects of the eyes, skull, lungs and heart. The ODS advises people who are or might be pregnant, or are breastfeeding, to avoid vitamin A supplements above 3,000 mcg RAE (10,000 IU) a day.
• Blood thinners. Fish oil may lengthen clotting time with warfarin. Studies found 3 to 6 g a day did not significantly change anticoagulation, but the ODS still advises periodic INR checks.
• Retinoid drugs. Acitretin and bexarotene can raise the risk of vitamin A toxicity when combined with vitamin A supplements (NIH ODS).
• Stacking products. Long-term vitamin A excess can cause dry skin, aching joints, fatigue and abnormal liver tests; too much vitamin D raises blood calcium, with nausea, weakness and kidney stones (NIH ODS).

Everyday side effects of omega-3s are usually mild: unpleasant taste, bad breath, heartburn, nausea, loose stools, headache or smelly sweat (NCCIH).

Find it at William's

• Now cod liver oil 1,000 mg softgels, 90 or 180 count. Now calls this its extra-strength version, with vitamins A and D3, and says the oil is screened for pollutants including mercury, other heavy metals, PCBs and dioxins.
• Now cod liver oil 650 mg softgels, 250 count, a smaller softgel from the same line, also with vitamins A and D3.
• Carlson cod liver oil, green apple flavor (liquid), with 1,100 mg of omega-3s per teaspoon plus vitamins A and D3, from Norwegian Arctic cod.

Compare each label's vitamin A per serving with the limits above. To leave vitamin A out entirely, pair fish oil with vitamin D3; our vitamin D IUs guide explains those doses.

Frequently Asked Questions