Omega-3 fatty acids EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid) are long-chain polyunsaturated fats essential for cardiovascular, neurological, and inflammatory health. Predominantly found in oily fish and microalgae, EPA and DHA cannot be efficiently synthesised by the body from plant-based omega-3 sources, making direct dietary intake or supplementation important for many individuals. This article examines the biochemistry, health benefits, recommended dosages, food sources, and safety considerations of EPA and DHA, with reference to UK clinical guidance from NICE, the NHS, and the MHRA. Understanding the evidence base helps inform appropriate use in both general health maintenance and specific therapeutic contexts.

What Are EPA and DHA Omega-3 Fatty Acids?

Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are long-chain omega-3 polyunsaturated fatty acids that play important roles in human health. Unlike alpha-linolenic acid (ALA), which is an essential omega-3 fatty acid found in plant sources, EPA and DHA are considered conditionally essential because the body can synthesise them from ALA, but in quantities that may be insufficient for optimal health, making dietary intake beneficial.

From a biochemical perspective, EPA contains 20 carbon atoms with five double bonds, whilst DHA contains 22 carbon atoms with six double bonds. These structural differences influence their distinct physiological functions. EPA serves as a precursor for eicosanoids—signalling molecules that help regulate inflammation, blood clotting, and immune responses. DHA is a major structural component of cell membranes, particularly abundant in the brain, retina, and nervous system.

The body can convert alpha-linolenic acid (ALA), a plant-based omega-3 found in flaxseeds and walnuts, into EPA and DHA. However, this conversion is relatively inefficient in humans, with estimates suggesting approximately 5-10% of ALA is converted to EPA and less than 0.5-5% to DHA, with considerable individual variation. Women generally have higher conversion rates than men. This limited conversion capacity highlights the potential value of obtaining EPA and DHA directly from dietary sources or supplements.

Key characteristics of EPA and DHA:

• EPA (C20:5 n-3): Primarily anti-inflammatory and cardiovascular protective

• DHA (C22:6 n-3): Important for brain structure and neurological function

• Marine origin: Predominantly sourced from cold-water fish and microalgae

• Limited endogenous synthesis: Direct dietary intake is more efficient than relying on ALA conversion

Health Benefits of EPA and DHA Supplements

Clinical evidence supports several health benefits of EPA and DHA supplementation. The GB Nutrition and Health Claims Register recognises that omega-3 fatty acids contribute to the maintenance of normal heart function (at 250mg EPA+DHA daily) and that EPA and DHA contribute to the maintenance of normal blood triglyceride levels (at 2g daily). Systematic reviews indicate that omega-3 supplementation may reduce triglyceride concentrations by 15–30%, which is particularly relevant for individuals with hypertriglyceridaemia. There is also evidence suggesting modest effects on blood pressure and endothelial function in some individuals, though results vary between studies.

It's important to note that NICE guidance (NG238) advises against offering omega-3 fatty acid supplements for primary or secondary prevention of cardiovascular disease. However, NICE technology appraisal guidance (TA805) recommends icosapent ethyl, a highly purified EPA ethyl ester, for reducing cardiovascular risk in specific adults with raised triglycerides who are taking statins.

Neurological and cognitive health represents another area of interest. DHA is important for foetal brain development, and maternal intake during pregnancy may support neurodevelopmental outcomes in children. The NHS recommends that pregnant women consume fish within pregnancy-specific limits, whilst avoiding high-mercury species and fish liver oils. In adults, observational studies suggest associations between higher omega-3 intake and cognitive health, though intervention trials have produced inconsistent findings.

Anti-inflammatory properties of EPA may benefit individuals with inflammatory conditions. Cochrane reviews indicate modest improvements in rheumatoid arthritis symptoms, including reduced joint pain and morning stiffness. Additionally, omega-3 fatty acids are being investigated for potential roles in mental health, including depression and anxiety disorders, though NICE does not currently recommend omega-3 supplements for treating these conditions.

Other areas of research include:

• Potential support for eye health, though large trials such as AREDS2 have not shown omega-3 supplements prevent progression of age-related macular degeneration

• Possible improvements in dry eye symptoms, though evidence remains mixed

• Potential benefits for inflammatory bowel conditions, though evidence remains preliminary

Recommended Dosages for Omega-3 EPA and DHA

Dosage recommendations for EPA and DHA vary depending on the intended health outcome and individual circumstances. The European Food Safety Authority (EFSA) has established reference values that inform UK guidance. For general health maintenance in adults, a combined EPA and DHA intake of 250mg daily is typically suggested as an adequate intake. This amount can usually be achieved through consuming at least two portions of fish per week, including one portion of oily fish, as advised by the NHS.

For specific therapeutic purposes, higher doses may be appropriate under medical supervision. For triglyceride reduction, 2–4g of combined EPA and DHA daily may be used. In the UK, Omacor (omega-3-acid ethyl esters 90) is licensed for hypertriglyceridaemia at doses of 2–4g daily. Vazkepa (icosapent ethyl) is licensed at 4g daily (2g twice daily) for cardiovascular risk reduction in specific high-risk adults with raised triglycerides despite statin therapy. These pharmaceutical-grade products differ from over-the-counter supplements in their purity, concentration, and regulatory status.

Pregnancy and lactation require particular attention to omega-3 intake. The NHS recommends that pregnant women eat at least two portions of fish a week, including one of oily fish (but no more than two portions of oily fish weekly). Women should avoid shark, swordfish and marlin completely, and limit tuna consumption due to mercury concerns. The NHS does not recommend routine omega-3 supplementation during pregnancy but advises avoiding supplements derived from fish liver oils (such as cod liver oil) due to excessive vitamin A content, which poses teratogenic risks.

For children, EFSA suggests an adequate intake of 100mg DHA for infants 7–24 months, and 250mg EPA+DHA for older children. Paediatric formulations are available, though whole food sources should be prioritised where possible.

Important dosing considerations:

• Supplement labels: Check the actual EPA and DHA content, not just total fish oil weight

• Divided doses: Higher doses (>2g daily) may be better tolerated when split across meals

• Medical supervision: Doses exceeding 3g daily should be taken under healthcare professional guidance, as EFSA considers intakes up to 5g daily from all sources to be generally safe

• Consistency: Regular daily intake is more important than occasional high doses

Food Sources vs Supplements: Getting Enough EPA and DHA

Obtaining EPA and DHA through whole food sources offers nutritional advantages beyond isolated omega-3 content. Oily fish remains the most significant dietary source, with the NHS recommending at least two portions of fish weekly, including one portion of oily fish (approximately 140g). Species particularly rich in EPA and DHA include salmon, mackerel, sardines, herring, and trout. A typical 100g serving of Atlantic salmon provides approximately 1.5-2.5g of combined EPA and DHA, though content varies by species, farming method, and cooking.

There are specific UK recommendations for certain groups: women of childbearing age and girls should eat no more than two portions of oily fish weekly, while men and boys can eat up to four portions weekly. Pregnant women should eat no more than two portions of oily fish weekly, avoid shark, swordfish and marlin completely, and limit tuna to no more than two tuna steaks or four medium-sized cans weekly.

Practical barriers often limit fish consumption. These include cost, availability, taste preferences, dietary restrictions (vegetarian or vegan diets), and concerns about environmental contaminants such as mercury, polychlorinated biphenyls (PCBs), and microplastics.

Supplementation becomes particularly relevant for individuals who cannot or choose not to consume fish. Fish oil supplements are the most common form, typically providing 180–300mg EPA and 120–200mg DHA per standard 1000mg capsule, though concentrations vary considerably between products. Pharmaceutical-grade omega-3 preparations offer higher concentrations and undergo more rigorous purification processes to remove contaminants.

Algal oil supplements provide a sustainable, vegetarian alternative, as microalgae are the original source of EPA and DHA in the marine food chain. These supplements typically contain higher DHA relative to EPA, though some formulations now include both fatty acids in balanced ratios. Algal oil has comparable bioavailability to fish oil and avoids concerns about marine contamination and overfishing.

Practical considerations for choosing between food and supplements:

• Additional nutrients: Fish provides high-quality protein, vitamin D, selenium, and iodine

• Sustainability: Consider Marine Stewardship Council (MSC) certification for fish and supplements

• Cost-effectiveness: Tinned sardines and mackerel offer economical omega-3 sources

• Convenience: Supplements provide consistent dosing for those with irregular dietary patterns

Potential Side Effects and Safety Considerations

EPA and DHA supplements are generally well-tolerated at recommended doses, though several side effects and safety considerations warrant attention. The most commonly reported adverse effects are gastrointestinal symptoms, including fishy aftertaste, belching, nausea, and loose stools. These effects are typically mild and dose-dependent, often improving when supplements are taken with meals or by switching to enteric-coated formulations. Refrigerating capsules may also reduce fishy reflux.

Bleeding risk represents a theoretical concern due to omega-3 fatty acids' antiplatelet effects, which may prolong bleeding time. However, clinical evidence suggests that even at doses up to 3–4g daily, the risk of significant bleeding complications is low. Nonetheless, caution is advised for individuals taking anticoagulants (warfarin, DOACs) or antiplatelet agents (aspirin, clopidogrel). If taking warfarin, your INR should be monitored when starting, stopping or changing the dose of omega-3 preparations. Patients should inform their GP or anticoagulation clinic before starting omega-3 supplements, particularly before surgical procedures.

Drug interactions may occur with certain medications. There is no established interaction with statins; indeed, omega-3 fatty acids may complement lipid-lowering therapy. Patients taking multiple cardiovascular medications should discuss supplementation with their healthcare provider to ensure coordinated management.

Quality and contamination concerns vary between products. Fish oil supplements may contain trace amounts of environmental pollutants, including mercury, PCBs, and dioxins. Reputable manufacturers employ molecular distillation and other purification processes to minimise contaminants. Consumers should look for third-party testing certifications and avoid products with excessive vitamin A or D, which can accumulate to toxic levels.

When to seek medical advice:

• Persistent gastrointestinal symptoms affecting quality of life

• Unusual bruising or bleeding, particularly if taking anticoagulants

• Planning surgery or invasive procedures whilst taking high-dose omega-3

• Allergic reactions (rare but possible, especially with fish or shellfish allergies)

• Uncertainty about appropriate dosing for specific health conditions

Special populations require particular consideration. Individuals with fish or shellfish allergies should exercise caution, though refined fish oil typically contains minimal allergenic protein. Algal oil offers a safer alternative. Those with diabetes should be aware that omega-3 supplements generally have neutral effects on blood glucose control, though monitoring is advisable with high doses. Patients with liver disease should consult their hepatologist before high-dose supplementation.

If you experience any side effects from omega-3 supplements, report them to the MHRA Yellow Card Scheme.

Frequently Asked Questions