DHA is a structural component of neuronal membranes — about 40% of the brain's polyunsaturated fat. It's what you want for age-related cognitive support. EPA is more anti-inflammatory and shows stronger data in depression and mood disorders. Most quality fish oils have both; the ratio should shift depending on your goal.
The structural vs signaling divide
Docosahexaenoic acid (DHA) is the omega-3 the brain uses as building material. It's incorporated into neuronal membranes, particularly at synapses. Roughly 40% of the brain's polyunsaturated fatty acid content is DHA. The retina is even more DHA-rich.
Eicosapentaenoic acid (EPA) is barely detectable in brain tissue at any age — it's rapidly beta-oxidized. But EPA is a precursor to resolvins and specialized pro-resolving mediators (SPMs) that modulate inflammation. Its brain effects are indirect, mediated by systemic inflammatory tone.
Same molecule family. Very different jobs.
What the human trials actually show
| Endpoint | DHA | EPA |
|---|---|---|
| Brain membrane content | ~40% of brain PUFA | Trace |
| Cognitive aging (MIDAS trial) | Improved episodic memory | Not tested alone |
| Depression meta-analyses | Weak effect alone | Stronger effect (EPA-dominant formulas) |
| Anti-inflammatory potency | Moderate | Higher (resolvin precursor) |
| Cardiovascular (REDUCE-IT) | Not tested (icosapent ethyl = pure EPA) | Meaningful reduction in MACE |
| Ratio for cognitive aging | DHA-dominant preferred | - |
| Ratio for mood/depression | - | EPA-dominant preferred (2:1+ EPA:DHA) |
The MIDAS trial (Yurko-Mauro 2010) enrolled 485 older adults with age-related cognitive decline and gave them 900 mg algal DHA or placebo daily for 24 weeks. The DHA group showed significant improvement on the CANTAB Paired Associate Learning test — a validated episodic memory measure.[1]
The depression story tilts the other way. Sublette 2011 meta-analyzed 15 depression RCTs and found EPA-dominant formulas (defined as ≥60% EPA of total EPA+DHA) showed a significant benefit for depressive symptoms, while DHA-dominant formulas showed no effect.[2] A more recent 2019 network meta-analysis reached broadly similar conclusions.[3]
REDUCE-IT (Bhatt 2019) tested pure EPA (as icosapent ethyl, 4 g/day) in patients with elevated triglycerides on statin therapy. Major adverse cardiovascular events dropped 25% vs placebo over 4.9 years.[4] That's a cardiovascular trial, not a cognitive one, but it validates EPA's biological potency at high doses.
The ratio question
Standard fish oil is usually 18% EPA + 12% DHA by weight — a ratio driven by what's in cold-water fish naturally, not by any therapeutic logic.
For cognitive aging: prioritize higher DHA content. Algal DHA products (500-1000 mg DHA per serving) or fish oil labeled as DHA-dominant are the fit. Aim for at least 500 mg of DHA daily.
For mood support: prioritize EPA-dominant formulas. A 2:1 or higher EPA:DHA ratio is what the depression meta-analyses point to. Total EPA in the 1000-2000 mg range daily.
For general health with no specific goal: standard fish oil at 1-2 g combined EPA+DHA daily is fine. The question of ratio matters less when you're just meeting baseline needs.
Algal oil (from marine microalgae like Schizochytrium) delivers DHA directly, no fish required. The MIDAS trial used algal DHA specifically. EPA from algal sources is less common but exists — look for products with both.
ALA (from flax, chia, walnuts) is a poor substitute. Conversion of ALA to EPA is 5-10% in most people; conversion to DHA is under 1%. If you want the brain-relevant omega-3s from a plant source, take the DHA directly.
Testing your omega-3 status
The Omega-3 Index measures EPA+DHA as a percentage of total fatty acids in red blood cell membranes. Under 4% is high-risk; 8%+ is protective in most cardiovascular studies. Most Americans test between 4-6% without supplementation.
You can test through Omega Quant or Grassroots Health for around $50. Not required — but if you're supplementing seriously, one test at baseline and one after 3-4 months of consistent intake tells you if your dose is actually moving your levels.
What to buy
Three things matter more than brand:
- Third-party purity testing for heavy metals, PCBs, and oxidation (IFOS, USP, or NSF).
- Actual EPA and DHA per serving — not "1000 mg fish oil" (that's mostly other fatty acids).
- Cold storage or antioxidant protection — oxidized fish oil is worse than none. Rancid taste on burp is a red flag.
What to actually buy
Nordic Naturals Algae DHA
Vegan algal DHA at 500 mg per serving — the format used in the MIDAS trial. IFOS 5-star tested, no fish burp.
Check Amazon price →Nordic Naturals ProOmega 2000
High-EPA fish oil at 1125 mg EPA + 875 mg DHA per two-softgel serving. IFOS tested, TG form for absorption.
Check Amazon price →Carlson Elite Omega-3
Reasonable balance of EPA and DHA at ~1600 mg total per serving. IFOS 5-star, freshness-guaranteed.
Check Amazon price →Citations
- Yurko-Mauro K, et al. Beneficial effects of docosahexaenoic acid on cognition in age-related cognitive decline. Alzheimers Dement. 2010;6(6):456-464.
- Sublette ME, et al. Meta-analysis of the effects of eicosapentaenoic acid (EPA) in clinical trials in depression. J Clin Psychiatry. 2011;72(12):1577-1584.
- Liao Y, et al. Efficacy of omega-3 PUFAs in depression: A meta-analysis. Transl Psychiatry. 2019;9(1):190.
- Bhatt DL, et al. Cardiovascular Risk Reduction with Icosapent Ethyl for Hypertriglyceridemia. N Engl J Med. 2019;380(1):11-22.
- Dyall SC. Long-chain omega-3 fatty acids and the brain: a review of the independent and shared effects of EPA, DPA and DHA. Front Aging Neurosci. 2015;7:52.
- Harris WS, et al. The Omega-3 Index: a new risk factor for death from coronary heart disease? Prev Med. 2004;39(1):212-220.