Fish oil is one of the most purchased supplements globally — and one of the most quality-variable. The active compounds, EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid), have robust evidence for reducing triglycerides, evidence for cardiovascular benefit at high doses, emerging evidence in depression and cognitive function, and clear roles in resolving inflammatory signaling. The supplement aisle problem is that the dose needed for these effects (2–4g EPA+DHA per day) is rarely achieved by standard "1000mg fish oil" capsules, which typically contain 300mg EPA+DHA per capsule — requiring 7–14 capsules for therapeutic dosing. Understanding the distinction between total fish oil dose and EPA+DHA content is the first correction most people need to make.
The second correction is quality. Fish oil is a highly oxidizable lipid, and a significant proportion of commercial fish oil products are rancid at the time of sale — with TOTOX scores (total oxidation, measured as peroxide value × 2 + anisidine value) exceeding the 26-point threshold recommended by the Global Organization for EPA and DHA Omega-3s (GOED). A 2015 study (Albert 2015, Journal of Nutritional Science) tested 171 New Zealand fish oil products and found 83% exceeded international oxidation standards at some point during the product's shelf life. Consuming oxidized fish oil is not just ineffective — oxidized lipids may be pro-inflammatory.
| Property | EPA (20:5n-3) | DHA (22:6n-3) |
|---|---|---|
| Primary location | Plasma phospholipids, platelets | Brain, retina, sperm (30–40% of brain PUFA) |
| Anti-inflammatory mechanism | Resolvin E series; competes with AA for COX-2; reduces LTB4 | Resolvin D series; neuroprotectins; reduces neuroinflammation |
| Cardiovascular | Primary — REDUCE-IT evidence; reduces TG, fibrinogen, platelet aggregation | Secondary; also reduces TG but less CV trial evidence for EPA dose |
| Depression/mood | Primary antidepressant signal — Liao 2019 meta-analysis: EPA >60% of dose | Weak antidepressant signal alone; synergistic with EPA |
| Brain development | Minor role | Critical — DHA is required for synaptic membrane fluidity, neuronal migration |
| Pregnancy | Standard supplementation | 200–300mg DHA/day recommended during pregnancy for fetal brain/retina |
General anti-inflammatory / maintenance: 1–2g combined EPA+DHA per day; read the label — a "1000mg fish oil" capsule typically contains 180mg EPA + 120mg DHA = 300mg active; need 3–7 standard capsules OR 1–2 concentrated-form capsules; take with the largest meal of the day (fat increases absorption of ethyl ester form by 35%).
Triglyceride reduction: 3–4g EPA+DHA per day; requires prescription (Vascepa, Lovaza) or high-potency concentrated supplement; at this dose: expect 20–30% TG reduction in 6–8 weeks; recheck lipid panel at 8 weeks; may mildly increase LDL-C in some patients (DHA effect more than EPA).
Depression (adjunctive): 1–2g EPA per day, with EPA comprising >60% of total omega-3 dose; pure EPA formulas (Vascepa, or supplements listing EPA-dominant content) are preferred over DHA-dominant or balanced formulas; 4–8 weeks to observe mood effect; most evidence as adjunct to antidepressants, not monotherapy.
Quality selection checklist: Choose triglyceride form (natural oil) or re-esterified triglyceride form over ethyl ester for best absorption without food requirement; verify TOTOX <26 on COA; IFOS or NSF certified preferred; store refrigerated after opening; discard if fishiness develops; molecular distillation removes heavy metals — verify on COA.
Safety notes: At 4g/day: mild increase in bleeding time (clinically significant primarily if on anticoagulants like warfarin or direct oral anticoagulants — discuss with prescriber); may lower blood pressure slightly (beneficial in most contexts); safe in pregnancy at standard doses (DHA is beneficial); very rare risk of AF at very high doses per REDUCE-IT secondary analysis.
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