Fish oil and omega-3 are often confused, but they are not the same. Omega-3 refers to a family of polyunsaturated fatty acids—including alpha-linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA)—that support cardiovascular health, brain function, and inflammatory regulation. Fish oil is a specific dietary source rich in EPA and DHA, extracted from oily fish such as mackerel, salmon, and sardines. Understanding this distinction is important when considering supplementation or dietary choices, as different sources provide varying types and amounts of omega-3 fatty acids with distinct biological effects.

What Is the Difference Between Fish Oil and Omega-3?

The terms 'fish oil' and 'omega-3' are often used interchangeably, but they are not synonymous. Omega-3 refers to a family of polyunsaturated fatty acids that play crucial roles in cardiovascular health, brain function, and inflammatory regulation. Of these, only alpha-linolenic acid (ALA) is considered essential, meaning the body cannot synthesise it and must obtain it through diet.

Fish oil is a specific dietary source rich in certain types of omega-3 fatty acids, primarily eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Fish oil is extracted from the tissues of oily fish such as mackerel, salmon, sardines, and anchovies. While fish oil is an excellent source of EPA and DHA, it is not the only one—omega-3 fatty acids are also found in plant-based sources, though in different forms.

Understanding this distinction is important when considering supplementation or dietary choices. When healthcare professionals recommend 'omega-3 supplementation', they may be referring to fish oil capsules, algal oil (a vegetarian alternative that provides DHA and sometimes EPA), or plant-based sources such as flaxseed (which provide ALA). Each source provides different types and amounts of omega-3 fatty acids, which may influence their biological effects.

For patients seeking to increase their omega-3 intake, it is essential to recognise that whilst fish oil is a concentrated source of EPA and DHA, alternative sources exist for those with dietary restrictions, allergies, or ethical preferences. The Scientific Advisory Committee on Nutrition (SACN) recommends a daily intake of approximately 450mg of EPA and DHA combined, which can be achieved through diet or supplementation.

Types of Omega-3 Fatty Acids: EPA, DHA and ALA

There are three main types of omega-3 fatty acids relevant to human health: alpha-linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA). Each has distinct sources, metabolic pathways, and physiological functions.

ALA is an 18-carbon omega-3 fatty acid found predominantly in plant-based sources such as flaxseeds, chia seeds, walnuts, and rapeseed oil. ALA is considered an essential fatty acid because the body cannot produce it. However, ALA must be converted into EPA and DHA to exert many of the health benefits associated with omega-3s. This conversion process is inefficient in humans—typically less than 10% of ALA is converted to EPA, and less than 1% to DHA. Consequently, relying solely on ALA may not provide adequate levels of EPA and DHA for optimal health.

EPA is a 20-carbon omega-3 fatty acid with anti-inflammatory properties. It is primarily found in marine sources, particularly oily fish and fish oil supplements. EPA plays a role in modulating inflammatory pathways and has been studied for its potential benefits in cardiovascular disease and inflammatory conditions. While some studies suggest EPA may have benefits for depression, the evidence is mixed, and NICE does not currently recommend omega-3 supplements for the treatment of depression.

DHA is a 22-carbon omega-3 fatty acid that is a major structural component of the brain, retina, and cell membranes. DHA is important for neurological development in infants and cognitive function throughout life. Like EPA, DHA is predominantly obtained from marine sources, though algal oil supplements provide a vegetarian alternative.

For clinical purposes, EPA and DHA are considered the most biologically active forms of omega-3 fatty acids. Patients who do not consume fish regularly may benefit from direct supplementation with EPA and DHA rather than relying on ALA conversion alone.

Health Benefits: Fish Oil vs Plant-Based Omega-3 Sources

The health benefits of omega-3 fatty acids vary depending on the source—whether fish oil or plant-based—and the specific types of omega-3 fatty acids they contain.

Cardiovascular health: NICE does not recommend standard fish oil supplements for the prevention of cardiovascular disease. However, NICE Technology Appraisal 805 recommends icosapent ethyl (a highly purified EPA-only preparation) for reducing cardiovascular risk in specific high-risk patients already taking statins. For hypertriglyceridaemia, the licensed medicine Omacor (omega-3-acid ethyl esters) may be prescribed at doses of 2–4 grams daily under specialist supervision. Plant-based ALA has shown some modest cardiovascular benefits, but the evidence is less robust compared to marine-derived omega-3s.

Blood pressure effects from omega-3 supplementation are generally modest and not included in NICE hypertension management guidelines.

Brain and mental health: DHA is important for foetal brain development, and adequate maternal intake during pregnancy is advised through dietary sources. Some studies suggest EPA may have a role in mood disorders, but the evidence is mixed, and NICE does not currently recommend omega-3 supplements for treating depression. Plant-based omega-3s do not provide direct DHA unless converted from ALA, which happens inefficiently.

Anti-inflammatory effects are associated with EPA. Fish oil supplementation has shown modest benefits in reducing joint pain and stiffness in some patients with rheumatoid arthritis, though it is not a primary treatment. For inflammatory bowel disease, the evidence for omega-3 supplementation is insufficient, and it is not recommended in UK clinical guidelines. Plant-based ALA has some anti-inflammatory properties, but these are generally weaker than those of EPA.

Eye health: While DHA is a component of retinal tissue, current evidence does not clearly support omega-3 supplementation for preventing or slowing age-related macular degeneration.

In summary, whilst both fish oil and plant-based omega-3 sources offer potential health benefits, fish oil (or algal oil) provides direct EPA and DHA, which have different physiological effects than plant-derived ALA.

Which Should You Take: Fish Oil or Omega-3 Supplements?

The choice between fish oil and other omega-3 supplements depends on individual health goals, dietary preferences, and specific clinical indications.

For cardiovascular health, standard fish oil supplements are not recommended by NICE for routine cardiovascular disease prevention. For patients with hypertriglyceridaemia, prescription omega-3-acid ethyl esters (Omacor) may be considered at doses of 2–4 grams daily, typically under specialist supervision. Icosapent ethyl (a purified EPA preparation) is recommended by NICE for specific high-risk patients already taking statins.

For vegetarians and vegans, algal oil supplements are an alternative. Derived from marine algae, these supplements provide DHA and sometimes EPA without animal products. Plant-based ALA sources (flaxseed, chia, walnuts) can contribute to overall omega-3 intake but should not be relied upon exclusively if higher EPA/DHA levels are needed.

For pregnant and breastfeeding women, the NHS recommends consuming no more than two portions of oily fish per week. This provides omega-3 fatty acids while limiting exposure to pollutants. The NHS does not routinely recommend omega-3 supplements during pregnancy, but women who don't eat fish might consider them after discussing with their healthcare provider. Importantly, fish liver oils (such as cod liver oil) should be avoided during pregnancy due to their vitamin A content, which can be harmful to the developing baby.

For general health maintenance, dietary sources are preferable to supplements. The NHS recommends eating at least one portion (140 grams) of oily fish per week. The Scientific Advisory Committee on Nutrition (SACN) advises an intake of approximately 450mg of EPA and DHA combined daily, which can be achieved through diet.

Quality and purity are important considerations. Fish oil supplements should be tested for contaminants such as mercury, polychlorinated biphenyls (PCBs), and dioxins. Patients should discuss supplementation with their GP or pharmacist, particularly if taking anticoagulant medications, as omega-3s may affect blood clotting.

Safety, Side Effects and NHS Guidance on Omega-3 Intake

Omega-3 supplements, including fish oil, are generally well-tolerated, but patients should be aware of potential side effects and safety considerations.

Common side effects of fish oil supplementation include:

• Gastrointestinal symptoms such as nausea, diarrhoea, and indigestion

• A fishy aftertaste or 'fish burps', which can be minimised by taking capsules with meals or choosing enteric-coated formulations

• Mild increases in bleeding time, particularly at higher doses

Drug interactions are an important consideration. Omega-3 supplements may enhance the effects of anticoagulant and antiplatelet medications (such as warfarin, aspirin, or clopidogrel), potentially increasing bleeding risk. According to the Omacor SmPC (Summary of Product Characteristics), patients taking anticoagulants should be monitored, particularly when starting or stopping omega-3 treatment. Patients taking these medications should consult their GP before starting omega-3 supplementation and may require additional monitoring.

Allergies to fish or shellfish are a contraindication for fish oil supplements. While highly purified fish oils contain minimal protein, patients with fish allergies should generally avoid fish oil products and consider algal oil as a safer alternative after discussing with their healthcare provider.

NHS guidance emphasises dietary sources over supplements for general health. The NHS recommends:

• At least one portion of oily fish per week for adults

• No more than two portions weekly for women of childbearing age, pregnant women, and breastfeeding mothers (to limit pollutant exposure)

• Up to four portions weekly for men and postmenopausal women

Regulatory status: In the UK, most fish oil supplements are regulated as food supplements under food law (overseen by the Food Standards Agency and local authorities). Prescription omega-3 preparations (such as Omacor and icosapent ethyl) are licensed medicines regulated by the MHRA.

Patients should contact their GP if they experience persistent gastrointestinal symptoms, unusual bleeding or bruising, or allergic reactions after starting omega-3 supplementation. Suspected side effects from prescription omega-3 products should be reported through the MHRA Yellow Card scheme. As with all supplements, omega-3 products should be stored safely out of reach of children, and patients should not exceed recommended doses without medical supervision.

Frequently Asked Questions