Fish oil EPA and DHA are essential omega-3 fatty acids that support cardiovascular, neurological, and visual health. Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are long-chain polyunsaturated fatty acids found predominantly in oily fish such as mackerel, sardines, and salmon. The human body cannot efficiently synthesise these fatty acids from plant-based omega-3 sources, making dietary intake or supplementation particularly important. EPA primarily influences inflammation and cardiovascular function, whilst DHA is crucial for brain structure and retinal health. Fish oil supplements provide a concentrated source of both fatty acids, though formulations vary considerably in potency, purity, and EPA-to-DHA ratios.

What Are EPA and DHA in Fish Oil?

Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are long-chain omega-3 polyunsaturated fatty acids found predominantly in oily fish and marine sources. These fatty acids are not efficiently synthesised by the human body from alpha-linolenic acid (ALA, the essential omega-3), making dietary intake particularly important. Fish oil supplements represent a concentrated source of both EPA and DHA, typically derived from species such as anchovies, sardines, mackerel, and salmon.

EPA is a 20-carbon fatty acid that serves as a precursor to various bioactive compounds called eicosanoids, which play crucial roles in regulating inflammation, immune function, and cardiovascular health. DHA, containing 22 carbons, is the most abundant omega-3 fatty acid in the brain and retina, where it contributes to structural integrity of cell membranes and supports neurological function throughout life.

The body can convert alpha-linolenic acid (ALA), a plant-based omega-3 found in flaxseed and walnuts, into EPA and DHA, but this conversion is notably inefficient—typically less than 8% for EPA and less than 1% for DHA. This limited conversion capacity explains why direct consumption of EPA and DHA from marine sources is considered more effective for achieving optimal omega-3 status.

Fish oil supplements are available in various formulations, including natural triglyceride form, ethyl ester form, and re-esterified triglycerides. The total omega-3 content varies considerably between products, with some concentrated formulations providing 60–90% EPA and DHA combined, whilst standard fish oil may contain only 30% omega-3 fatty acids by weight.

Health Benefits of EPA and DHA Omega-3 Fatty Acids

Cardiovascular health represents one of the most extensively researched areas for EPA and DHA. The GB Nutrition and Health Claims Register recognises that a daily intake of 250mg EPA and DHA contributes to normal heart function. Evidence suggests omega-3 fatty acids may help reduce triglyceride levels and modestly lower blood pressure in some individuals. However, it's important to note that NICE guidance (CG172) specifically advises not to offer omega-3 fatty acid capsules for secondary prevention after myocardial infarction, and the MHRA has withdrawn this indication for licensed omega-3 medicines.

EPA demonstrates particular anti-inflammatory properties through its influence on eicosanoid metabolism, competing with arachidonic acid pathways to produce less inflammatory mediators. This mechanism may explain observed benefits in conditions characterised by chronic inflammation, including rheumatoid arthritis, where some patients report reduced joint stiffness and pain with omega-3 supplementation. However, this is not a NICE-recommended or licensed treatment, and evidence is limited to modest symptomatic relief.

Neurological and mental health applications centre primarily on DHA's structural role in brain tissue. Adequate DHA status during pregnancy and early childhood supports foetal brain development and visual function. In adults, observational studies suggest associations between higher omega-3 intake and reduced risk of cognitive decline, though intervention trials have produced mixed results. Regarding depression, it's important to note that NICE guidance (NG222) does not recommend offering omega-3 supplements as a treatment for depression.

DHA is essential for retinal health, comprising approximately 60% of photoreceptor outer segment fatty acids. Adequate intake throughout life may support visual function, though there is no established evidence that omega-3 supplements prevent age-related macular degeneration. The NHS recognises the importance of omega-3 fatty acids as part of a balanced diet for overall health maintenance.

Recommended Dosages for EPA and DHA Supplements

General health maintenance recommendations from UK health authorities suggest consuming at least one portion (approximately 140g) of oily fish weekly. The EPA and DHA content varies considerably by fish species: mackerel, herring and sardines typically provide higher amounts than tuna. For individuals who do not consume fish regularly, the GB Nutrition and Health Claims Register indicates that 250mg daily of combined EPA and DHA contributes to normal heart function. However, it's important to note that NHS and NICE do not recommend omega-3 supplements specifically for preventing cardiovascular disease.

For specific therapeutic applications, higher doses may be appropriate under medical supervision:

• Elevated triglycerides: 2–4 grams of combined EPA and DHA daily, often as prescription omega-3 preparations (the only licensed indication for omega-3 medicines in the UK)

• Pregnancy: The NHS advises eating 1-2 portions of oily fish weekly during pregnancy, while limiting oily fish to 2 portions weekly due to potential pollutant exposure. Pregnant women should avoid shark, swordfish and marlin due to mercury content. Supplements specifically formulated for pregnancy may be an alternative source of DHA.

The MHRA regulates omega-3 products differently depending on their intended use. Prescription-only omega-3 medicines contain highly concentrated EPA and DHA (typically 80–90% omega-3 content) and are licensed specifically for treating hypertriglyceridaemia. Over-the-counter supplements vary considerably in potency and are regulated as food supplements by the Food Standards Agency and local authorities, not as medicines.

Dosing considerations should account for the actual EPA and DHA content rather than total fish oil weight. A 1000mg fish oil capsule may contain only 300mg of combined EPA and DHA, necessitating multiple capsules to achieve higher doses. Patients should carefully read supplement labels to determine the precise EPA and DHA quantities per serving. Those taking anticoagulant medications (particularly warfarin, which requires INR monitoring when starting omega-3s) or with bleeding disorders should consult their GP before commencing supplementation, particularly at higher doses exceeding 3 grams daily.

Choosing Quality Fish Oil: EPA and DHA Ratios Explained

EPA to DHA ratios vary considerably across fish oil products, and understanding these differences can help individuals select appropriate supplements for their needs. Standard fish oil typically provides EPA and DHA in roughly equal proportions or with slightly more EPA (ratios around 1.5:1 to 2:1). However, concentrated formulations may be specifically designed to favour one fatty acid over the other.

EPA-predominant supplements (ratios of 2:1 to 6:1 EPA:DHA) have different fatty acid profiles than DHA-rich formulations. DHA-rich products are often included in pregnancy supplements to support foetal development, with some products providing DHA with minimal EPA content. While some research suggests different effects of EPA versus DHA, it's important to note that specific ratio selection for conditions like mood or inflammation is based on limited evidence and is not endorsed by NICE as a treatment approach.

When evaluating fish oil quality, several factors warrant consideration:

• Purity and contaminants: Reputable manufacturers test for heavy metals (mercury, lead), polychlorinated biphenyls (PCBs), and dioxins. The Food Standards Agency sets maximum levels for contaminants in fish and fish products. Look for products displaying third-party testing certification or adherence to international purity standards.

• Oxidation status: Omega-3 fatty acids are susceptible to oxidation, which reduces efficacy and may produce unpleasant taste. Quality indicators include peroxide value, anisidine value, and TOTOX (total oxidation) measurements, though these are rarely displayed on consumer packaging.

• Form and bioavailability: Different forms of omega-3 supplements (triglyceride, ethyl ester) may have different absorption characteristics, particularly when taken without food. Taking supplements with meals generally improves absorption regardless of form.

Sustainability credentials are increasingly important, with certifications from the Marine Stewardship Council (MSC) or Friend of the Sea indicating responsibly sourced fish. Patients concerned about fish-derived products may consider algal oil supplements, which provide EPA and DHA from microalgae and represent a vegetarian alternative, though typically with lower EPA content.

Potential Side Effects and Safety Considerations

Fish oil supplements are generally well-tolerated at recommended doses, but patients should be aware of common adverse effects that, whilst typically mild, can affect adherence:

• Gastrointestinal symptoms: Fishy aftertaste, belching (eructation), indigestion (dyspepsia), nausea, and loose stools are common side effects. Taking supplements with meals or using enteric-coated formulations may reduce these effects.

• Fishy body odour or breath: Occasionally reported, particularly at higher doses

• Mild bleeding tendency: At doses exceeding 3 grams daily, omega-3 fatty acids may prolong bleeding time, though clinically significant bleeding is rare

Drug interactions require careful consideration. Patients taking anticoagulants (warfarin, DOACs), antiplatelet agents (aspirin, clopidogrel), or non-steroidal anti-inflammatory drugs should consult their GP before commencing fish oil supplementation, particularly at higher doses above 3 grams daily. If you take warfarin, your doctor may recommend additional INR monitoring when starting omega-3 supplements. Whilst the risk of clinically significant bleeding appears low, caution is appropriate in those with bleeding disorders.

Allergic reactions to fish oil supplements are uncommon but possible in individuals with fish or shellfish allergies. Symptoms may include rash, itching, or respiratory difficulties. Call 999 immediately if you experience signs of severe allergic reaction (anaphylaxis) such as difficulty breathing, swelling of the face/throat, or feeling faint. Patients with confirmed fish allergies should consider algal-derived omega-3 alternatives and seek medical advice before supplementation.

When to contact your GP: Patients should seek medical attention if they experience unusual bruising, prolonged bleeding from minor cuts, black or bloody stools, severe abdominal pain, or signs of allergic reaction. Those with diabetes should be aware that omega-3 supplements generally have neutral effects on blood glucose, though individual responses may vary.

Pregnant and breastfeeding women should choose supplements specifically formulated for pregnancy, avoiding cod liver oil or any supplements containing vitamin A (retinol) which may be harmful at high doses. The NHS advises limiting oily fish consumption to 2 portions weekly during pregnancy due to potential pollutant exposure.

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

Frequently Asked Questions