Cod liver oil and fish oil are popular marine-derived supplements rich in omega-3 fatty acids, offering potential health benefits for cardiovascular function, inflammation, and brain health. Whilst both contain eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), cod liver oil uniquely provides vitamins A and D. Understanding the differences between these supplements, their evidence-based benefits, and appropriate usage is essential for safe and effective supplementation. This article examines the clinical evidence, dosing guidance, and safety considerations to help you make informed decisions about incorporating these supplements into your health regimen, aligned with current UK medical guidance.

What Are Cod Liver Oil and Fish Oil?

Cod liver oil and fish oil are both marine-derived supplements rich in omega-3 fatty acids, but they differ in source and nutritional composition. Fish oil is extracted from the tissues of oily fish such as mackerel, herring, sardines, and anchovies, whilst cod liver oil comes specifically from the liver of cod fish. Both contain long-chain omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which the body can synthesise in limited amounts from alpha-linolenic acid (ALA), the essential omega-3 fatty acid found in plant sources.

The key distinction lies in their vitamin content. Cod liver oil naturally contains significant amounts of vitamins A and D, which are fat-soluble vitamins stored in the liver of the fish. Cod liver oil products typically provide vitamin A (commonly 750-1500 µg/5ml [2500-5000 IU]) and vitamin D (commonly 5-10 µg/5ml [200-400 IU]), alongside omega-3 fatty acids. The exact amounts vary between products, so it's important to check labels. In contrast, standard fish oil supplements contain higher concentrations of EPA and DHA but negligible amounts of vitamins A and D unless these are added during manufacturing.

Both supplements are available in liquid and capsule forms, with varying concentrations of active ingredients. The omega-3 content is typically expressed as the combined total of EPA and DHA per serving. It's important to distinguish between over-the-counter supplements (regulated as foods by the Food Standards Agency) and prescription omega-3 medicines available on the NHS for specific indications. These prescription products, such as omega-3-acid ethyl esters for hypertriglyceridaemia or icosapent ethyl for cardiovascular risk reduction in certain high-risk patients, contain highly purified forms of omega-3 fatty acids at standardised doses.

Key Health Benefits of Cod Liver Oil and Fish Oil

The health benefits of omega-3 fatty acids vary across different conditions, with evidence strength differing considerably. For cardiovascular health, while omega-3 fatty acids may influence blood pressure and triglyceride levels (particularly at higher doses of 2-4g daily), NICE does not recommend routine omega-3 fish oil supplementation for primary or secondary prevention of cardiovascular disease. However, prescription icosapent ethyl (a highly purified EPA ethyl ester) is recommended by NICE (TA805) for reducing cardiovascular risk in specific high-risk patients with elevated triglycerides who are already on statin therapy.

Anti-inflammatory properties constitute a key mechanism of action. Omega-3 fatty acids are incorporated into cell membranes and serve as precursors for specialised pro-resolving mediators (SPMs) that help resolve inflammation. This mechanism may benefit individuals with inflammatory conditions such as rheumatoid arthritis, where some studies suggest modest improvements in joint pain and stiffness, though omega-3 supplements should complement rather than replace conventional disease-modifying treatments.

Brain and cognitive function may be supported by DHA, which is a major structural component of neuronal membranes. Observational evidence suggests associations between higher omega-3 intake and reduced cognitive decline, though intervention trials have produced inconsistent results. The evidence is stronger for maternal and infant health, with adequate DHA intake during pregnancy supporting foetal brain and eye development. The NHS recommends that pregnant women consume oily fish but advises specific limits: no more than 2 portions of oily fish weekly, and avoiding fish with higher mercury levels.

The vitamin D content in cod liver oil provides additional benefits, particularly for bone health and immune function. Given the high prevalence of vitamin D insufficiency in the UK, especially during winter months, cod liver oil may help maintain adequate vitamin D status. However, pregnant women should avoid cod liver oil supplements due to their high vitamin A content, which may harm the developing foetus. Benefits of omega-3 supplementation are most evident in those with deficiencies or specific medical conditions rather than in routine use by healthy individuals.

How to Choose and Take These Supplements Safely

Selecting high-quality supplements requires attention to several key factors. Purity and quality standards are paramount, as fish-derived products may contain environmental contaminants such as mercury, polychlorinated biphenyls (PCBs), and dioxins. Look for products that display third-party testing certification or compliance with international standards. In the UK, fish oil supplements are regulated as foods by the Food Standards Agency, and quality may be indicated by voluntary certification from programmes such as the International Fish Oil Standards (IFOS).

Dosage recommendations vary depending on the intended use. For general health, the NHS recommends adults eat at least two portions of fish per week, including one portion (approximately 140g) of oily fish. The omega-3 content varies significantly between fish species, with oily fish providing the highest amounts. When supplementing, typical doses range from 250mg to 1,000mg of combined EPA and DHA daily for general health support. Higher doses (2–4g daily) may be prescribed under medical supervision for specific conditions such as hypertriglyceridaemia. For cod liver oil, it is crucial to check the vitamin A content, as excessive intake can be harmful. The UK safe upper limit for vitamin A is 1,500 micrograms (approximately 5,000 IU) daily for adults. Pregnant women should avoid cod liver oil supplements due to the risk of vitamin A-related birth defects.

Practical considerations include taking supplements with meals to enhance absorption and reduce gastrointestinal side effects. Capsules should be stored in a cool, dark place to prevent oxidation, which can reduce efficacy and cause a rancid odour. Liquid formulations should be refrigerated after opening and used within the recommended timeframe. Patients taking anticoagulant medications (such as warfarin) or antiplatelet drugs should consult their GP before starting omega-3 supplements, as there is a theoretical risk of enhanced bleeding effects, though evidence for clinically significant interactions at standard doses is limited. If scheduled for surgery, seek advice from your surgical team about whether to continue or temporarily stop supplements. Always inform healthcare professionals about all supplements being taken.

Potential Side Effects and Who Should Avoid Them

Whilst cod liver oil and fish oil are generally well-tolerated, several common adverse effects may occur, particularly at higher doses. Gastrointestinal symptoms are most frequent and include:

• Nausea and indigestion

• Diarrhoea or loose stools

• Fishy aftertaste or belching

• Abdominal discomfort

These effects can often be minimised by taking supplements with food, choosing enteric-coated capsules, or dividing the daily dose. Some individuals may experience a fishy body odour with high-dose supplementation.

Bleeding risk is a theoretical concern due to the mild antiplatelet effects of omega-3 fatty acids. At standard supplement doses, this effect is rarely clinically significant, but patients taking anticoagulants (warfarin, DOACs), antiplatelet agents (aspirin, clopidogrel), or NSAIDs should discuss supplementation with their healthcare provider. If you're scheduled for surgery, consult your surgical team about whether to continue or temporarily stop supplements.

Specific populations who should avoid or use with caution include:

• Pregnant women: Should avoid cod liver oil due to high vitamin A content (teratogenic risk), though standard fish oil may be appropriate under medical guidance

• Individuals with fish or seafood allergies: May experience allergic reactions, though highly purified products may be tolerated

• Those with bleeding disorders: Require medical supervision due to theoretical anticoagulant effects

• Patients with diabetes: May need monitoring if taking high doses, though effects on glycaemic control are generally minimal at standard doses

High-dose prescription omega-3 preparations have been associated with a small increased risk of atrial fibrillation; seek medical advice if you experience palpitations.

Vitamin A toxicity is a particular concern with cod liver oil. Chronic excessive intake can cause hepatotoxicity, bone abnormalities, and central nervous system effects. Symptoms of hypervitaminosis A include headache, nausea, dizziness, and skin changes. The UK upper safe limit for vitamin A is 1,500 micrograms (approximately 5,000 IU) daily for adults. Contact your GP if you experience persistent side effects, unusual bleeding or bruising, or signs of allergic reaction such as rash, swelling, or breathing difficulties. Suspected adverse reactions can be reported through the MHRA Yellow Card Scheme (yellowcard.mhra.gov.uk).

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