Omega-3 (EPA and DHA)
EPA and DHA are long-chain omega-3 fatty acids built into the phospholipids of cell membranes, where they compete with arachidonic acid as substrates for eicosanoids and give rise to pro-resolving lipid mediators. Humans convert plant ALA to EPA poorly and to DHA barely at all, so tissue levels track fish intake or supplements. The number that matters on a label is milligrams of EPA plus DHA; "1,000 mg fish oil" often means 300 mg of the fatty acids you are paying for.
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What It Does
DHA is concentrated in the retina and in neuronal membranes. EPA displaces arachidonic acid in the cyclooxygenase and lipoxygenase pathways, shifting eicosanoid output toward less inflammatory products. The best-established effect is on triglycerides: 2-4 g a day of EPA plus DHA lowers them by roughly 20-30%, mainly by reducing hepatic VLDL secretion.
Cardiovascular outcomes are contested. REDUCE-IT gave 4 g a day of icosapent ethyl, a purified EPA ethyl ester, to statin-treated patients with elevated triglycerides and reported a 25% reduction in major cardiovascular events (Bhatt 2019). STRENGTH tested 4 g a day of an EPA and DHA combination in a similar population and stopped for futility (Nicholls 2020). The leading explanation for the difference is the mineral oil placebo in REDUCE-IT, which raised LDL and inflammatory markers in the control arm; whether EPA alone has a distinct benefit is still argued. In a general population taking 1 g a day, VITAL found no reduction in its primary cardiovascular or cancer endpoints (Manson 2019).
The same trials surfaced a harm. A meta-analysis of outcome trials found a higher risk of atrial fibrillation with omega-3 supplementation, rising with dose and clearest above 1 g a day (Gencer 2021).
The omega-3 index, EPA plus DHA as a percentage of red cell fatty acids, was proposed as a risk marker with 8% or more as the target and 4% or less as high risk (Harris 2004). It is a reasonable measure of status; the targets rest on observational data.
People who eat little or no fish, vegans, and pregnant women (DHA accumulates in the fetal brain in the third trimester) are the groups most likely to have low status.
Forms and Bioavailability
Absorption ratings are broad tiers. Head-to-head human trials rarely support a finer ranking among well-absorbed forms.
| Form | Absorption | Typical dose | Cost | Labels |
|---|---|---|---|---|
| Natural Triglyceride Fish OilTG fish oil, natural TG | HighEPA and DHA on a glycerol backbone, the form found in fish. Pancreatic lipase releases free fatty acids and monoglycerides for micellar absorption. | 250-1,000 mg EPA+DHA daily for general use | Low | 31 |
| Ethyl Ester Fish OilEE fish oil, omega-3-acid ethyl esters | ModerateFatty acids esterified to ethanol to allow concentration by distillation. The ethyl ester bond is hydrolyzed more slowly by pancreatic lipase, so absorption depends heavily on taking it with a fat-containing meal. | 1-4 g EPA+DHA daily | Low | 83 |
| Re-esterified Triglyceride Fish OilrTG, re-esterified triglyceride | HighConcentrated as ethyl esters, then enzymatically converted back to triglycerides. Produced higher plasma EPA+DHA than ethyl esters in a comparative trial (Dyerberg 2010). | 500-2,000 mg EPA+DHA daily | Moderate | |
| Krill Oil (Phospholipid)krill oil, phospholipid omega-3 | HighA large share of EPA and DHA is bound to phospholipids, which are emulsifiers and absorb without heavy dependence on dietary fat. | 1-3 g krill oil daily (typically 150-400 mg EPA+DHA) | High | 5 |
| Algal Oilalgae omega-3, vegan DHA | HighDHA (and in some strains EPA) from microalgae, usually in triglyceride form. Absorbed like fish-derived triglyceride oil. | 250-1,000 mg EPA+DHA daily | High |
Labels column: number of labels in the Fish Oil and Omega-3 category that list this form. A product can list more than one form, and some labels do not state a form at all.
Fish oil starts as natural triglyceride, about 30% EPA plus DHA. Concentrating it requires converting the fatty acids to ethyl esters, which some products sell as is and others re-esterify back into triglycerides (rTG). In a single-dose comparison, rTG produced the highest blood levels and ethyl esters the lowest (Dyerberg 2010). Over six months of daily use, triglyceride oil raised the omega-3 index more than ethyl esters did, though both worked (Neubronner 2011). Ethyl ester absorption depends heavily on a fatty meal; taken with one, the gap narrows.
Krill oil carries much of its EPA and DHA in phospholipids. It is well absorbed, but a 1 g capsule typically holds 100-250 mg EPA plus DHA, so reaching a meaningful dose takes many capsules. Algal oil is how fish get their omega-3s in the first place; many products are DHA-dominant with little EPA.
Oxidation is a real quality problem. A New Zealand survey of 32 retail fish oils found most exceeded recommended oxidation limits and few contained the labeled amount of EPA and DHA (Albert 2015).
Typical Dosing
| Group | Adequate Intake (AI) |
|---|---|
| Men (ALA) | 1.6 g |
| Women (ALA) | 1.1 g |
| EPA and DHA | No established intake recommendation |
| Upper limit (UL) | None established. FDA advises that supplemental EPA plus DHA not exceed 5 g per day |
General supplementation commonly provides 250-1,000 mg EPA plus DHA. Triglyceride-lowering regimens use 2-4 g per day under medical supervision.
Food sources: salmon, herring, sardines, mackerel, trout, anchovies, oysters.
What to Look For
Find the EPA and DHA lines on the Supplement Facts panel and add them. Then check the serving size, which is often two or three softgels. A product that lists only "fish oil" or "omega-3 fatty acids" without separate EPA and DHA values is hiding a low dose.
For freshness, look for a stated TOTOX or peroxide value, or certification from IFOS, which publishes oxidation and contaminant results by lot. A strong fishy smell when you cut a capsule open means the oil has oxidized. Store opened bottles away from heat. Molecularly distilled oils from reputable manufacturers test low for mercury and PCBs; the fish species matters less than the purification.
Red flags: "omega 3-6-9" blends (most diets already supply plenty of omega-6 and the body makes omega-9), krill marketed as more potent per capsule, and front labels that advertise total oil weight. For triglyceride lowering at 2-4 g a day, prescription products have verified content and are often covered by insurance.
Re-esterified Triglyceride Fish Oil
Concentrated and well absorbed. The practical choice when you need 1-2 g of EPA plus DHA in two or three softgels.
What to look for: At least 600 mg EPA plus DHA per softgel, a published TOTOX or IFOS certificate, and an expiration date.
Find on iHerb Affiliate linkEthyl Ester Fish Oil
The form of both prescription products and most budget concentrates. Fine with a fatty meal, poorly absorbed without one.
What to look for: EPA plus DHA per serving rather than per softgel. Take with the largest meal of the day.
Find on iHerb Affiliate linkAlgal Oil
The vegan source, and the direct one.
What to look for: Both EPA and DHA stated; many algal oils are DHA-only, which suits pregnancy better than triglyceride lowering.
Find on iHerb Affiliate linkKrill Oil (Phospholipid)
Krill oil absorbs well, but the dose per capsule is small and the cost per milligram of EPA plus DHA is high.
What to look for: Total EPA plus DHA in mg. Ignore krill oil weight and astaxanthin content when comparing.
Find on iHerb Affiliate linkProducts on Titrate
The Fish Oil and Omega-3 category in Titrate's index holds 7,169 labels from the NIH Dietary Supplement Label Database, 3,579 of them currently on market, out of 214,745 labels indexed in total. The search page ships the 6,000 most detailed of these.
Top on-market labels, ranked by label completeness and simplicity:
- High Concentrate EPA plus CoQ10 with KD-Pur EPA and BioPerine
- Children's DHA IQ Fish Formula Natural Strawberry Flavor
- Wild Red Sockeye Salmon Oil (Astaxanthin Added)
- Wild Red Sockeye Salmon Oil (Astaxanthin Added)
- Wild Red Sockeye Salmon Oil (Astaxanthin Added)
Search fish oil and omega-3 supplements
Supplement Search loads one category at a time. Choose the Fish Oil and Omega-3 tab, then type a form such as "triglyceride" under Must contain, or a form to avoid under Must not contain.
Related Tools
- Omega-3 Dosing
- Inflammation Score
- Algal Oil vs Ethyl Ester Fish Oil
- Algal Oil vs Krill Oil
- Algal Oil vs rTG Fish Oil
- Ethyl Ester Fish Oil vs Krill Oil
- Ethyl Ester Fish Oil vs rTG Fish Oil
- Krill Oil vs rTG Fish Oil
- Algal Oil vs Triglyceride Fish Oil
- Ethyl Ester Fish Oil vs Triglyceride Fish Oil
- All omega-3 (EPA and DHA) form comparisons
- Best Fish Oil Without Soy
- Best Fish Oil Without Gelatin
- Best Fish Oil Without Lemon Flavor
- All supplement guides
References
NIH Office of Dietary Supplements. Omega-3 Fatty Acids: Fact Sheet for Health Professionals.
Harris WS, von Schacky C. The Omega-3 Index: a new risk factor for death from coronary heart disease? Prev Med. 2004;39(1):212-220.
Dyerberg J, et al. Bioavailability of marine n-3 fatty acid formulations. Prostaglandins Leukot Essent Fatty Acids. 2010;83(3):137-141.
Neubronner J, et al. Enhanced increase of omega-3 index in response to long-term n-3 fatty acid supplementation from triacylglycerides versus ethyl esters. Eur J Clin Nutr. 2011;65(2):247-254.
Albert BB, et al. Fish oil supplements in New Zealand are highly oxidised and do not meet label content of n-3 PUFA. Sci Rep. 2015;5:7928.
Bhatt DL, et al. Cardiovascular Risk Reduction with Icosapent Ethyl for Hypertriglyceridemia. N Engl J Med. 2019;380(1):11-22.
Manson JE, et al. Marine n-3 Fatty Acids and Prevention of Cardiovascular Disease and Cancer. N Engl J Med. 2019;380(1):23-32.
Nicholls SJ, et al. Effect of High-Dose Omega-3 Fatty Acids vs Corn Oil on Major Adverse Cardiovascular Events in Patients at High Cardiovascular Risk: The STRENGTH Randomized Clinical Trial. JAMA. 2020;324(22):2268-2280.
Gencer B, et al. Effect of Long-Term Marine omega-3 Fatty Acids Supplementation on the Risk of Atrial Fibrillation in Randomized Controlled Trials of Cardiovascular Outcomes: A Systematic Review and Meta-Analysis. Circulation. 2021;144(25):1981-1990.
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