Fish oil comes from the tissue of oily, cold-water fish like anchovies, mackerel, herring, and sardines. You can get it in two ways: by eating these fish directly or by taking a concentrated supplement. Most commercial fish oil is extracted through an industrial cooking-and-pressing process, then purified and sold as softgels, liquids, or capsules.
Where Commercial Fish Oil Comes From
The fish species used for mass oil production are chosen for their high fat content. Atlantic mackerel, herring, anchovies, sardines, menhaden, and capelin are the primary sources. These small, fast-reproducing species sit low on the food chain, which means they accumulate fewer contaminants than larger predatory fish like tuna or swordfish. A 100-gram portion of Atlantic mackerel contains about 2.5 grams of the two omega-3 fatty acids that matter most (EPA and DHA), making it one of the richest natural sources available.
Salmon, while popular as a food fish, is less commonly used for oil extraction at industrial scale. Most salmon oil sold as supplements is a byproduct of the salmon farming and canning industries rather than a primary product.
How Fish Oil Is Extracted
Nearly all commercial fish oil is made using a method called wet pressing. The process has three core steps: cooking, pressing, and separating.
First, whole fish or fish byproducts are heated to coagulate the protein. This breaks open the fat deposits inside the tissue and releases the oil along with water. The cooked mass then moves through a strainer and into a twin-screw press, which squeezes out a liquid mixture of oil, water, dissolved proteins, and minerals. What’s left behind is a solid “presscake” that becomes fish meal for animal feed.
The liquid from the press still contains solids, so it goes through a series of centrifuges. The first spin removes the remaining sludge. The second separates the oil from the water. What comes out is crude fish oil, a golden-orange liquid that still needs refining before it ends up in a supplement bottle. Some newer facilities skip the press entirely and use high-capacity centrifuges for the entire separation, but the cooking step remains the same.
How Supplements Are Refined and Purified
Crude fish oil goes through several rounds of processing to remove contaminants and concentrate the omega-3 content. Refining typically involves degumming (removing phospholipids), neutralizing free fatty acids, bleaching to remove pigments, and deodorizing with steam to eliminate the strong fishy smell.
To concentrate EPA and DHA beyond what raw fish tissue provides, manufacturers use a process called molecular distillation. This separates fatty acids based on their molecular weight under a vacuum, allowing producers to create supplements with 50%, 60%, or even 90% omega-3 content compared to the roughly 20-30% found in crude oil.
Finished products must meet strict purity limits. The GOED voluntary monograph, which most reputable brands follow, sets maximum contaminant levels at 0.05 mg/kg for lead, 0.1 mg/kg for mercury, and 0.1 mg/kg for cadmium. These thresholds are far below what you’d encounter eating whole fish.
Triglyceride vs. Ethyl Ester Forms
Not all fish oil supplements are chemically identical. The two main forms you’ll find on shelves are triglycerides and ethyl esters, and the difference affects how well your body absorbs them.
In whole fish, omega-3 fats naturally exist as triglycerides: three fatty acid chains attached to a glycerol backbone. During concentration, manufacturers often strip the fatty acids off that backbone and attach them to ethanol molecules instead, creating ethyl esters. This form is cheaper to produce but doesn’t occur naturally in food.
Your body has to work harder to use ethyl esters. The pancreatic enzyme that breaks down dietary fat digests ethyl esters 10 to 50 times more slowly than triglycerides. In human absorption studies, EPA from triglyceride-form oil was absorbed at 68% compared to just 20% from ethyl esters. DHA showed a similar gap: 57% versus 21%. Taking ethyl ester supplements with a high-fat meal narrows this difference somewhat, because the meal provides the glycerol molecules your body needs to reassemble the fats.
Some brands take the extra step of converting concentrated ethyl esters back into triglyceride form (called re-esterified triglycerides). These cost more but combine high omega-3 concentration with better absorption. If you’re comparing products, look for “triglyceride form” or “rTG” on the label.
How to Tell If Your Fish Oil Is Fresh
Fish oil oxidizes when exposed to heat, light, and air, producing off flavors and potentially harmful breakdown products. Freshness is measured by three values you’ll sometimes see on a certificate of analysis from the manufacturer.
Peroxide value measures early-stage oxidation and should be 5 or below. Anisidine value captures later-stage breakdown products and should stay under 20. The combined measure, called TOTOX (total oxidation), should be 26 or lower. If a brand won’t share these numbers, that’s a red flag. In practice, fishy-tasting burps or a strong smell when you open a bottle usually signals oxidation has already gone too far.
Store liquid fish oil in the refrigerator after opening. Keep softgels in a cool, dark place and use them before the expiration date. Some brands add a small amount of vitamin E as a natural preservative to slow oxidation.
Getting Omega-3s From Whole Fish
Eating fatty fish two to three times per week is the simplest way to get meaningful amounts of EPA and DHA without supplements. A single 3-ounce serving of the richest species delivers more omega-3 than most supplement capsules:
- Atlantic herring: 1.71 g combined EPA and DHA
- Wild Atlantic salmon: 1.57 g
- Farmed Atlantic salmon: 1.83 g
- Sardines (canned): 1.19 g
- Atlantic mackerel: 1.02 g
- Rainbow trout (wild): 0.84 g
Compare that to a standard fish oil softgel, which typically contains 300 mg (0.3 g) of combined EPA and DHA per capsule, though concentrated versions may offer 500 to 1,000 mg. Two servings of salmon per week gives you roughly the same weekly omega-3 intake as taking a standard capsule every day.
At the lower end of the spectrum, popular lean fish deliver far less. A 3-ounce serving of cod provides just 0.14 g, tilapia gives you 0.15 g, and canned light tuna only 0.19 g. These are fine sources of protein but won’t move the needle much on omega-3 intake.
Plant-Based and Algal Alternatives
Fish don’t actually produce EPA and DHA themselves. They accumulate these fats by eating microalgae (or eating smaller creatures that ate the algae). Algal oil supplements cut out the middlefish entirely by growing omega-3-rich microalgae in controlled fermentation tanks, then extracting the oil using solvents or mechanical methods.
Algal oil is the only plant-based source of preformed DHA and EPA. Flaxseed, chia seeds, and walnuts contain a different omega-3 called ALA, which your body converts to EPA and DHA at very low rates, typically under 10%. If you’re vegetarian or vegan and want the same fatty acids found in fish oil, algal oil supplements are the practical option.
Most algal oil products emphasize DHA over EPA, so check the label if you want a specific ratio. Potency has improved significantly in recent years, with many algal supplements now offering 500 mg or more of combined omega-3s per serving.