Quick answer

Omega-3s are a family of fats, and fish oil is one source of two of them, EPA and DHA. A typical 1,000 mg fish oil softgel holds about 300 mg of EPA plus DHA. Krill oil, cod liver oil and algal oil also supply EPA or DHA, while flaxseed oil gives the plant omega-3 ALA.^1,2

Key facts

Every common chemical form of omega-3 in supplements raises EPA and DHA in the blood, though ethyl esters are absorbed somewhat less well.¹
Krill oil's omega-3s are mostly phospholipids, and studies disagree on whether they are absorbed better than fish oil's.¹
Prescription omega-3 is a different product: one holds about 840 mg of EPA plus DHA in each 1 g capsule.³
Fish oil supplements have not been found to contain methyl mercury.¹

Omega-3 is the family, fish oil is a source

Omega-3 is the name for a family of fats that share one chemical feature, a double bond three carbons from the end of the chain. Three matter most in food and supplements: ALA from plants, and EPA and DHA from seafood. Fish oil is oil from fish, and its omega-3s are EPA and DHA, carried alongside other fats.^1,4

So the word omega-3 on a label can mean EPA and DHA from fish, DHA from algae, or ALA from flax. What counts is which omega-3s a product contains and how much of each.¹

Supplement sources compared

Common omega-3 supplements and what they provide^1,2
Supplement	Omega-3s	Notes
Fish oil	EPA and DHA	Typically 180 mg EPA and 120 mg DHA per 1,000 mg of oil
Krill oil	EPA and DHA, mainly as phospholipids	Whether it is absorbed better than fish oil is unsettled
Cod liver oil	EPA and DHA	Also vitamins A and D, in amounts that vary
Algal oil	DHA, sometimes EPA	Vegetarian; usually about 100 to 300 mg of DHA
Flaxseed oil	ALA	Turned into EPA and DHA only in small amounts

Fish oil supplements contain EPA and DHA. A typical one provides about 1,000 mg of fish oil with 180 mg of EPA and 120 mg of DHA, but amounts vary widely between products.^1,2

Krill oil carries its omega-3s mainly as phospholipids. Some studies suggest these are absorbed somewhat better than the omega-3s in fish oil, and others do not.^1,2

Cod liver oil supplies EPA and DHA plus vitamins A and D, in amounts that vary from product to product. Vitamins A and D can be harmful in excessive amounts.^1,2

Algal oil is a vegetarian source of DHA, usually providing about 100 to 300 mg, and some products also contain EPA. In a small study, DHA from algal oil was as available to the body as DHA from cooked salmon.^1,2

Flaxseed oil contains the plant omega-3 ALA, not EPA or DHA.²

Triglyceride, ethyl ester and phospholipid forms

Omega-3s in supplements come in the natural triglyceride form, as re-esterified triglyceride, as free fatty acids, as ethyl esters or as phospholipids. The triglyceride and free fatty acid forms are absorbed somewhat better than ethyl esters, but every form raises EPA and DHA in the blood.¹

The natural triglyceride form is the one found in fish. Ethyl esters are made from it by swapping the glycerol backbone for ethanol, and the re-esterified triglyceride form is ethyl ester turned back into a triglyceride.¹

The gap between forms is largest when there is little fat in the meal. In one crossover trial, a free fatty acid product was absorbed about four times better than an ethyl ester product during low-fat eating, but only about 1.3 times better with high-fat meals.⁵

Prescription omega-3 is a different product

Prescription omega-3 medicines differ in composition from typical supplements and are tested and regulated as drugs, so their effects may not match those of supplements. One holds about 465 mg of EPA and 375 mg of DHA, as ethyl esters, in each 1 g capsule.^2,3

Prescription products are taken at 4 g a day under a doctor's care. Their trial results may not apply to supplements taken at a few hundred milligrams a day.^2,3

Reading a label

Find the lines that give EPA and DHA in milligrams, and add them. The total oil, 1,000 mg in a typical softgel, is not the omega-3 dose.¹

NIH's Dietary Supplement Label Database holds the labels of many omega-3 products, which makes it easier to compare EPA and DHA per serving across brands.¹

What's in ours

What’s in ours

Omega-3: Fish oil concentrate, 1,000 mg per serving, from the label

Cautions by form

Cod liver oil adds vitamins A and D, and both can be harmful in excessive amounts. Count them with any multivitamin or vitamin D supplement you take.²

Fish, krill and cod liver oils come from seafood. If you are allergic to fish or shellfish, ask your doctor first. Algal oil comes from algae rather than fish.^3,4

Whatever the form, high doses of omega-3s can affect clotting, so ask first if you take a blood thinner.⁴

Questions people ask

Is krill oil better than fish oil?

Not clearly. Krill oil's omega-3s are mostly phospholipids, and some studies suggest they are absorbed somewhat better than those in fish oil while others do not. Compare the EPA and DHA per serving on the labels.¹

Is omega-3 from algae as good as fish oil?

For DHA, a small study found that DHA from algal oil was as available to the body as DHA from cooked salmon. Some algal oils contain EPA as well and others only DHA, so check the label.¹

Is cod liver oil the same as fish oil?

Both supply EPA and DHA, but cod liver oil also contains vitamins A and D, in amounts that vary from product to product.²

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Sources

This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.