Omega-3 fatty acids are a type of polyunsaturated fat. "Essential" means the body can't manufacture them - they must come from food. Unlike most fats, omega-3s have distinct biological functions beyond just providing energy.
They're structural components of cell membranes throughout the body, particularly concentrated in the brain, eyes, and heart. They also serve as precursors to anti-inflammatory signalling molecules (prostaglandins, resolvins, and protectins) that regulate the immune response.
The Three Types: EPA, DHA, and ALA
EPA (Eicosapentaenoic Acid)
EPA is a 20-carbon omega-3 found primarily in fatty fish and seafood. It's the main anti-inflammatory omega-3 - it competes with omega-6 fatty acids in the production of inflammatory signalling molecules, producing less inflammatory versions.
The strongest research on EPA is in cardiovascular health. A 2018 NEJM trial (REDUCE-IT) found high-dose EPA supplementation (4g/day icosapentaenoic acid) reduced cardiovascular events by 25% in people with elevated triglycerides and existing cardiovascular risk factors. This is one of the most significant cardiovascular nutrition trials of the past decade.
EPA also has evidence for reducing triglycerides, supporting mood regulation (research on depression), and reducing inflammation markers including CRP.
DHA (Docosahexaenoic Acid)
DHA is a 22-carbon omega-3, also found in fatty fish and algae. It's the primary structural omega-3 in the brain (particularly in synaptic membranes and the cortex) and in the retina of the eye.
DHA is particularly important during pregnancy and infancy for brain and eye development. Adults need ongoing DHA to maintain neurological function. Low DHA status is associated with cognitive decline and has been studied in relation to depression and ADHD.
ALA (Alpha-Linolenic Acid)
ALA is an 18-carbon omega-3 found in plant foods: flaxseeds, chia seeds, walnuts, and hemp seeds. The human body can convert ALA to EPA and DHA - but inefficiently. Conversion rates are typically 5-15% to EPA and under 1% to DHA.
This has practical significance: eating walnuts and flaxseeds is not a reliable substitute for eating fatty fish or taking fish/algae oil if your goal is to maintain adequate EPA and DHA levels. ALA provides the building blocks but the body doesn't use them very effectively.
Why Omega-3 to Omega-6 Ratio Matters
Omega-6 fatty acids (primarily linoleic acid from vegetable oils) and omega-3 fatty acids compete for the same enzymes in the body. They produce different downstream signalling molecules - omega-6 tend toward more pro-inflammatory, omega-3 toward anti-inflammatory.
The estimated omega-6 to omega-3 ratio in ancestral human diets was roughly 1:1 to 4:1. The modern Western diet typically sits at 15:1 to 20:1 - dominated by vegetable oil-heavy ultra-processed foods with low fatty fish intake.
This imbalance is a plausible mechanistic driver of the low-grade chronic inflammation seen across modern Western populations. Increasing EPA and DHA intake while moderating refined vegetable oil consumption shifts this ratio toward less inflammatory territory.
Dietary Sources
Richest in EPA and DHA:
- Salmon (farmed, Atlantic): ~2.2g per 100g
- Mackerel: ~2.7g per 100g
- Sardines (tinned): ~1.5g per 100g
- Herring: ~1.7g per 100g
- Anchovies: ~2.1g per 100g
- Oysters: ~0.4g per 100g
Rich in ALA (plant sources):
- Flaxseeds (ground): ~2.4g per tablespoon
- Chia seeds: ~5g per 28g
- Walnuts: ~2.6g per 28g
- Hemp seeds: ~1g per tablespoon
For comparison between fish oil and algae-based omega-3 supplements, see the fish oil vs algae omega-3 article.
How Much Do You Need?
No universal RDA for EPA and DHA has been established in all countries, but common guidance:
- General health maintenance: 250-500mg combined EPA+DHA daily
- Cardiovascular risk reduction: 1,000-4,000mg EPA+DHA daily (prescription-level doses require medical supervision)
- Pregnancy and breastfeeding: 200-300mg DHA daily minimum
Two portions of fatty fish per week provides roughly 500-700mg EPA+DHA per day averaged out. For people who don't eat fish, supplementation is the practical alternative. As noted in good fats vs bad fats, the quality and type of fat in the diet has significant metabolic consequences.