Most people who take fish oil supplements buy a generic capsule without considering whether the omega-3 content is primarily EPA, DHA, or a combination — and at what dose. This matters because EPA and DHA have fundamentally different biological mechanisms. They're both omega-3 fatty acids, but they operate on different targets and have different strongest evidence bases. Taking a 500mg "omega-3 supplement" that's mostly DHA for cardiovascular protection is like taking the wrong medication.
n=8,179, statin-treated patients with elevated triglycerides (135–499 mg/dL) and established cardiovascular disease or diabetes + risk factors. Mean 4.9 years.
Vascepa (icosapentaenoic acid — pure EPA, no DHA) at 4g/day reduced 3-point MACE by 25% and cardiovascular death by 20% vs. mineral oil placebo. The triglyceride reduction alone (~20%) does not explain the cardiovascular benefit magnitude, suggesting direct plaque-stabilizing and anti-inflammatory effects of EPA. This trial produced the only prescription omega-3 product (Vascepa) approved by the FDA for cardiovascular risk reduction.
The STRENGTH trial counterpoint: STRENGTH used a high-dose EPA+DHA combination (omega-3 carboxylic acids) and showed no cardiovascular benefit vs. corn oil. This suggests the EPA-specific mechanism — not omega-3s in general — drives the REDUCE-IT result. The EPA:DHA ratio and purity appear to matter for cardiovascular outcomes.
| Goal | Recommended Form | Target Daily Dose | Notes |
|---|---|---|---|
| Cardiovascular protection (high TG) | High-EPA fish oil or prescription Vascepa | 2–4g EPA/day | Rx Vascepa required for 4g; OTC high-EPA oils at 2g may provide partial benefit |
| General cardiovascular + anti-inflammatory | Balanced EPA+DHA fish oil (2:1 EPA:DHA) | 1–2g combined EPA+DHA | Standard recommendation; minimum effective dose |
| Depression / mood support | EPA-dominant (≥60% EPA) | 1–2g EPA/day | Meta-analyses: EPA more effective than DHA for depression; minimum 60% EPA ratio |
| Brain health / cognitive aging | DHA-dominant or balanced | 500mg–1g DHA/day | DHA is the structural brain fatty acid; EPA also contributes |
| Pregnancy / infant | DHA-dominant prenatal omega-3 | 200–300mg DHA/day (WHO minimum) | Critical for fetal brain development; most prenatal vitamins contain inadequate DHA |
| Vegan / vegetarian | Algae oil (DHA-dominant) | 250–500mg DHA + some EPA | Algae is the original source — fish get DHA from eating algae |
Omega-3 fatty acids are highly unsaturated and oxidize rapidly — rancid fish oil produces aldehydes and lipid peroxides that may negate cardiovascular benefits and cause harm. Key quality markers:
TOTOX (total oxidation) score: Should be below 26; below 10 is excellent. Few brands publish this; request a certificate of analysis or choose brands that voluntarily disclose.
IFOS certification: International Fish Oil Standards certification verifies purity, oxidation levels, and heavy metal testing. A reliable quality proxy when TOTOX data isn't available.
Smell test: Fresh fish oil should not smell strongly fishy. Strong, offensive fish odor indicates significant oxidation. If your capsules smell bad after cutting them open, throw them out.
Storage: Refrigerate after opening. Keep away from light and heat. Replace every 90 days once opened.
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