Omega-3 Fatty Acids: EPA, DHA, ALA — What They Do and Where to Get Them

The three primary dietary omega-3 fatty acids differ in chain length, number of double bonds, and — critically — their biological roles and sources. Alpha-linolenic acid (ALA) is an 18-carbon fatty acid with three double bonds, found predominantly in plant sources including flaxseed, chia seeds, walnuts, hemp seeds, and rapeseed oil. It serves as a precursor to the longer-chain omega-3s and has its own modest anti-inflammatory effects, but its direct biological activity is limited compared with EPA and DHA. The body can elongate and desaturate ALA to produce EPA (20 carbons, 5 double bonds) and subsequently DHA (22 carbons, 6 double bonds), but the efficiency of this conversion is poor. Studies suggest that only 5–10% of dietary ALA is converted to EPA, and less than 1% reaches DHA — rates that are further reduced in males compared with females, in people consuming high amounts of linoleic acid (the competing omega-6 fat), and in those with certain genetic variants of the delta-5 and delta-6 desaturase enzymes. EPA is the primary precursor to anti-inflammatory eicosanoids — prostaglandins, leukotrienes, and thromboxanes of the 3-series — which oppose the pro-inflammatory eicosanoids derived from arachidonic acid. EPA also gives rise to resolvins and protectins that actively resolve inflammation rather than merely suppressing it. DHA is a structural component of brain grey matter, the retina, and sperm, where it is present at high concentrations; it also plays a key role in membrane fluidity, neuronal signalling, and synaptic function. Both EPA and DHA are found preformed in fatty fish, shellfish, and algae.

The cardiovascular benefits of omega-3 fatty acids have been the subject of research for decades, beginning with observations in the 1970s that Greenlandic Inuit populations — who consumed large quantities of marine fats — had remarkably low rates of heart disease despite high total fat intakes. Subsequent mechanistic research identified multiple cardioprotective pathways: EPA and DHA reduce serum triglycerides by 20–50% at doses of 2–4g daily, lower resting heart rate, reduce platelet aggregation (thereby decreasing clot risk), improve arterial compliance, and have modest anti-arrhythmic effects — possibly explaining reduced rates of sudden cardiac death in high fish-consuming populations. High-dose EPA (4g daily as the pharmaceutical drug icosapentaenoic acid ethyl ester, brand name Vascepa) demonstrated a 25% reduction in major cardiovascular events in the REDUCE-IT trial in people with elevated triglycerides on statin therapy. However, the broader cardiovascular evidence picture is nuanced: large trials testing standard fish oil doses (1g/day) in primary prevention have shown more modest effects, and some meta-analyses have found that the benefits are concentrated in populations with high background cardiovascular risk or low baseline omega-3 status. Current evidence most strongly supports omega-3 supplementation for triglyceride reduction, secondary prevention after heart attack, and in those with established cardiovascular disease. The protective mechanism may extend to omega-3 fatty acids' anti-inflammatory effects in atherosclerotic plaque stabilisation, reducing the risk of plaque rupture and acute coronary events.

DHA is the dominant structural fat in the human brain, constituting approximately 40% of the polyunsaturated fatty acids in brain grey matter, where it is particularly concentrated in synaptic membranes. Its presence is critical for membrane fluidity, which governs the speed and efficiency of neurotransmitter receptor function, ion channel activity, and cell signalling cascades. During pregnancy and the first two years of life, DHA accretion in the developing brain is rapid, and inadequate maternal intake is associated with poorer cognitive, language, and visual outcomes in children. For this reason, DHA supplementation is widely recommended during pregnancy and breastfeeding, with evidence supporting benefits in foetal brain development and visual acuity. In adult brain health, higher omega-3 status is associated with greater brain volume in regions vulnerable to age-related atrophy, better performance on memory and cognitive tests, and reduced risk of cognitive decline and Alzheimer's disease in prospective studies — though randomised trials of supplementation in already-cognitively-impaired individuals have yielded mixed results, suggesting that prevention rather than treatment may be the key application. In mental health, EPA has attracted particular research attention for depression: several meta-analyses have found that EPA supplementation (at least 1g/day) significantly reduces depressive symptoms, with effect sizes comparable to mild antidepressant medication. The combination of EPA and DHA appears more effective than either alone in some studies. Anxiety, ADHD, and bipolar disorder have also been studied in relation to omega-3 status, with generally supportive findings for EPA in particular, though evidence quality varies.

Marine foods are the primary source of preformed EPA and DHA for most people. Among fish, oily cold-water species are the richest sources: a 100g serving of farmed Atlantic salmon provides approximately 2.2g of combined EPA and DHA, wild-caught salmon around 1.8g, mackerel 2.5g, herring 2.2g, and sardines around 1.5g. Anchovies are particularly concentrated, providing around 2.1g per 100g. Tuna is a reasonable source — particularly bluefin at around 1.5g per 100g — though canned tuna (typically skipjack) provides much less, around 0.2–0.3g per 100g. Shellfish including mussels, oysters, and scallops contain meaningful EPA and DHA at 0.4–1.0g per 100g. Seaweed and microalgae are important plant-based sources: while most sea vegetables contain only small amounts, certain microalgae — particularly Schizochytrium and Nannochloropsis species — are rich in DHA and are now commercially grown specifically for supplement production. These algae-derived omega-3 supplements are the most practical option for vegans and vegetarians seeking preformed EPA and DHA. Among plant foods, flaxseed (linseed) is the richest ALA source at approximately 2.4g per tablespoon of ground flaxseed; chia seeds provide around 5g ALA per two tablespoons; walnuts offer 2.6g per 28g serving; hemp seeds contain about 1g per tablespoon. Rapeseed (canola) oil is a reasonable cooking oil choice with around 1.3g ALA per tablespoon. Consuming these ALA-rich foods regularly is worthwhile, but they should not be relied upon as sole omega-3 sources for those without marine food intake.

Understanding omega-3 fatty acids' anti-inflammatory action requires understanding their relationship with omega-6 fatty acids, specifically arachidonic acid (AA). When cells are damaged or stimulated, phospholipases release fatty acids from cell membrane phospholipids as signalling precursors. If AA is released, it is metabolised by cyclooxygenase and lipoxygenase enzymes to produce pro-inflammatory prostaglandins, leukotrienes, and thromboxanes (2-series and 4-series eicosanoids). If EPA is released instead, the same enzymes produce 3-series and 5-series eicosanoids that are significantly less potent inflammatory mediators. The balance between AA and EPA in cell membranes — which directly reflects dietary intake patterns over preceding weeks and months — therefore determines the inflammatory tone at the cellular level. Additionally, EPA and DHA give rise to a family of recently characterised pro-resolving mediators — resolvins, protectins, and maresins — that actively terminate inflammatory responses, promote tissue clearance of cellular debris, and facilitate return to homeostasis. This resolution phase of inflammation is increasingly understood as critically important: chronic inflammation in many modern diseases may represent not excessive inflammatory initiation but failure of resolution, and EPA/DHA-derived mediators are central to this resolution biology. The practical implication is that reducing omega-6 intake (primarily by reducing consumption of refined vegetable oils such as sunflower, corn, and soybean oil) while increasing omega-3 intake from marine sources creates a more anti-inflammatory cellular environment — a dual dietary strategy more powerful than increasing omega-3 intake alone.

Fish oil is the most widely used omega-3 supplement globally, but quality varies enormously. The key parameters to look for are total EPA and DHA content per dose, oxidation status, purity from heavy metals and PCBs, and form of the omega-3 (triglyceride vs ethyl ester). Natural triglyceride-form fish oils are generally better absorbed than ethyl ester forms — particularly when taken without food — though high-quality ethyl ester products taken with fat can match triglyceride bioavailability. Oxidation is a significant quality concern: rancid fish oil generates toxic oxidation products, smells unpleasant, and may paradoxically increase cardiovascular risk. Always look for products that have been independently tested for oxidation markers (TOTOX score) and contaminants. Third-party certifications from IFOS (International Fish Oil Standards), GOED, or NSF International provide meaningful quality assurance. For general health maintenance, 1–2g combined EPA and DHA daily is a typical evidence-based dose. For specific conditions — triglyceride reduction, depression management, or pregnancy — doses of 2–4g are commonly used. Krill oil is an alternative to fish oil; it contains phospholipid-bound EPA and DHA which may be slightly better absorbed at lower doses, plus the antioxidant astaxanthin. Algae oil is the recommended choice for vegans and those who prefer to avoid fishy taste or sustainability concerns — it is often DHA-dominant and some formulations include EPA. Cod liver oil provides EPA and DHA alongside significant quantities of vitamins A and D, which can be an advantage but also requires attention to upper limits for fat-soluble vitamins. Store fish oil in the refrigerator after opening to slow oxidation.