Cod liver oil and omega-3 supplements are often confused, yet they are not the same product. Whilst cod liver oil does contain omega-3 fatty acids—specifically EPA and DHA—it also provides significant amounts of vitamins A and D, which standard fish oil supplements do not. This distinction affects dosing, safety, and suitability for different individuals. Understanding the differences between these products is essential for making informed choices about supplementation, particularly given NHS guidance on cardiovascular health and the potential risks of excessive vitamin A intake. This article clarifies what sets cod liver oil apart from omega-3 supplements and helps you determine which option may be appropriate for your needs.

What Are Omega-3 Fatty Acids?

Omega-3 fatty acids are a family of essential polyunsaturated fatty acids that play crucial roles in human health. The term 'essential' indicates that the body cannot synthesise these compounds independently, necessitating dietary intake. The three principal omega-3 fatty acids of clinical significance are alpha-linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA).

ALA is predominantly found in plant sources such as flaxseeds, chia seeds, walnuts, and rapeseed oil. Whilst the body can convert ALA to EPA and DHA, this conversion process is notably inefficient, with conversion rates typically in the low single digits for EPA and even lower for DHA. Conversion efficiency varies by sex, physiological status, and dietary composition. Consequently, direct dietary sources of EPA and DHA are considered more effective for meeting physiological requirements.

EPA and DHA are primarily obtained from marine sources, particularly oily fish such as salmon, mackerel, sardines, and herring. These long-chain omega-3 fatty acids are integral to numerous physiological processes, including cardiovascular function, inflammatory modulation, neurological development, and cellular membrane integrity. EPA is particularly associated with anti-inflammatory effects, whilst DHA is essential for brain structure and function, forming a substantial proportion of brain polyunsaturated fatty acids.

For individuals who do not consume fish, algal oil supplements provide a vegetarian and vegan source of EPA and DHA, offering a suitable alternative for those avoiding marine products or with fish allergies.

The NHS recommends consuming at least two portions of fish weekly (approximately 140g each), including one portion of oily fish, to ensure adequate omega-3 intake. For individuals who do not consume fish regularly, supplementation may be considered, though dietary sources remain the preferred option for obtaining these essential nutrients alongside other beneficial compounds found naturally in whole foods.

What Is Cod Liver Oil and What Does It Contain?

Cod liver oil is a nutritional supplement derived from the livers of Atlantic cod (Gadus morhua) and related species. Unlike standard fish oil supplements, which are extracted from the flesh of oily fish, cod liver oil is specifically sourced from hepatic tissue, resulting in a distinct nutritional profile that extends beyond omega-3 fatty acids alone.

The primary constituents of cod liver oil include EPA and DHA (the marine omega-3 fatty acids), alongside significant quantities of fat-soluble vitamins A and D. The omega-3, vitamin A, and vitamin D content varies considerably between manufacturers and formulations. It is essential to check the product label for specific amounts of EPA, DHA, vitamin A (retinol), and vitamin D (cholecalciferol) to ensure safe intake levels. The omega-3 content in cod liver oil is generally lower per unit volume compared to concentrated fish oil supplements.

The distinguishing feature of cod liver oil is its vitamin content. Vitamin A supports immune function, vision, and skin health, whilst vitamin D is essential for calcium absorption, bone health, and immune regulation. This combination made cod liver oil historically valuable in preventing rickets and vitamin deficiencies, particularly in northern latitudes with limited sunlight exposure. However, the presence of vitamin A also limits the amount of cod liver oil that can be safely consumed daily, as excessive vitamin A intake carries health risks.

Modern cod liver oil supplements are typically processed to remove contaminants such as heavy metals and persistent organic pollutants, and many formulations are standardised to specific vitamin and omega-3 concentrations. Some products are also flavoured to improve palatability, as traditional cod liver oil has a distinctive taste that some individuals find unpleasant.

Key Differences Between Cod Liver Oil and Omega-3 Supplements

Whilst cod liver oil contains omega-3 fatty acids, it is not synonymous with omega-3 supplements, and several clinically relevant differences exist between these products. Understanding these distinctions is essential for making informed supplementation decisions.

Omega-3 concentration represents the most significant difference. Standard fish oil supplements typically contain higher concentrations of EPA and DHA per capsule or serving compared to cod liver oil. Concentrated omega-3 preparations may provide 500–1,000mg of combined EPA and DHA per capsule, whereas equivalent volumes of cod liver oil generally contain lower amounts. For individuals requiring higher intakes of omega-3 fatty acids, fish oil supplements may be more practical, as cod liver oil's vitamin A content limits the safe daily dose.

Vitamin content distinguishes cod liver oil from pure omega-3 products. The presence of vitamins A and D can be advantageous for individuals with deficiencies or increased requirements, such as during winter months when vitamin D synthesis is limited. However, this characteristic also introduces potential safety considerations, particularly regarding vitamin A toxicity with excessive consumption. Standard fish oil supplements contain negligible amounts of these vitamins, eliminating this concern.

Source and processing differ between products. Cod liver oil is hepatic-derived, whilst fish oil supplements are extracted from fish body tissues. This distinction affects the lipid profile and potential contaminant exposure, though modern purification processes generally ensure safety across both product categories.

Clinical guidance is important to note. NICE does not recommend over-the-counter fish oil or cod liver oil supplements for cardiovascular disease prevention. Only prescription icosapent ethyl (a highly purified EPA product) is recommended by NICE for specific high-risk patients on statin therapy with persistently raised triglycerides. Cod liver oil may be preferable when combined vitamin and omega-3 supplementation is desired, provided vitamin intake remains within safe limits.

Which Should You Choose: Cod Liver Oil or Omega-3?

The choice between cod liver oil and omega-3 supplements depends on individual health status, nutritional requirements, existing dietary intake, and specific therapeutic goals. Neither product is universally superior; rather, selection should be personalised based on clinical context.

Cod liver oil may be appropriate for individuals seeking combined omega-3 and vitamin supplementation, particularly those with:

• Vitamin D deficiency or limited sunlight exposure, especially during autumn and winter months, who have low vitamin A intake and can keep total vitamin A within safe limits (≤1.5mg daily from all sources)

• Historical or cultural preferences for traditional cod liver oil use

• No contraindications to vitamin A supplementation (see safety section)

The NHS recommends that adults and children over one year consider taking 10 micrograms of vitamin D daily during autumn and winter, making cod liver oil a potential option for meeting this recommendation whilst simultaneously providing omega-3 fatty acids, provided vitamin A intake remains safe.

Concentrated omega-3 fish oil supplements may be preferable for:

• Individuals requiring higher intakes of EPA and DHA, where cod liver oil's vitamin A content would limit safe dosing

• Those already meeting vitamin A and D requirements through diet or separate supplementation

• Pregnant women or those planning pregnancy (due to vitamin A teratogenicity concerns)

• Individuals taking medications that interact with vitamin A (such as oral retinoids)

• People seeking maximum omega-3 concentration per capsule

Algal oil supplements (providing DHA and EPA from algae) are suitable alternatives for:

• Vegetarians and vegans

• Individuals with fish or shellfish allergies

• Those preferring non-marine omega-3 sources

For cardiovascular health, it is important to note that NICE does not recommend over-the-counter omega-3 supplements or cod liver oil for primary or secondary prevention of cardiovascular disease. Only prescription icosapent ethyl is recommended for selected high-risk patients on statin therapy with raised triglycerides. Individuals with established cardiovascular disease should discuss supplementation with their GP or cardiologist.

Dietary intake should always be the primary consideration. The NHS emphasises consuming oily fish twice weekly as the preferred method for obtaining omega-3 fatty acids, alongside a balanced diet rich in fruits, vegetables, and whole grains. Supplementation should complement, not replace, healthy dietary patterns.

Safety Considerations and NHS Guidance

Both cod liver oil and omega-3 supplements are generally well-tolerated, but several safety considerations warrant attention, particularly regarding cod liver oil's vitamin content and potential drug interactions.

Vitamin A toxicity represents the primary concern with cod liver oil supplementation. Excessive vitamin A intake (hypervitaminosis A) can cause hepatotoxicity, bone abnormalities, and teratogenic effects. The NHS specifically advises that individuals consuming cod liver oil should avoid exceeding 1.5mg of vitamin A daily from all sources combined, including diet, multivitamins, and other supplements. Pregnant women and those planning pregnancy should avoid cod liver oil and any vitamin A–containing supplements entirely due to the risk of foetal abnormalities, particularly during the first trimester. Check product labels carefully, as vitamin A content varies.

Interactions with oral retinoids (such as isotretinoin for acne or acitretin for psoriasis) are important: do not combine cod liver oil with these medications, as this may increase the risk of vitamin A toxicity. Always check whether multivitamins or other supplements contain vitamin A before taking cod liver oil.

Vitamin D excess, whilst less common than vitamin A toxicity, can occur with excessive supplementation, leading to hypercalcaemia. The adult upper safe limit is 100 micrograms daily. Individuals taking separate vitamin D supplements should account for the vitamin D content in cod liver oil to avoid exceeding this limit.

Common adverse effects of both cod liver oil and fish oil supplements include:

• Gastrointestinal disturbance (nausea, diarrhoea, fishy aftertaste or burping)

• Mild anticoagulant effects at high doses

• Allergic reactions in individuals with fish or shellfish allergies

Individuals taking anticoagulant medications (warfarin, DOACs) or antiplatelet agents should consult their GP before commencing omega-3 supplementation. Whilst standard supplementation doses rarely cause clinically important bleeding, caution is appropriate, and clinician discussion is advised. High-dose prescription EPA (icosapent ethyl) has been associated with a small increased risk of atrial fibrillation and bleeding in clinical trials; such dosing requires clinician supervision.

Fish and shellfish allergy: individuals with fish or shellfish allergies should avoid marine-derived omega-3 supplements (including cod liver oil and fish oil) and consider algal oil alternatives instead.

Quality and regulation are important considerations. In the UK, supplements are regulated as foods rather than medicines, meaning they do not undergo the same rigorous testing as pharmaceutical products. Consumers should select products from reputable manufacturers that provide third-party testing certification and clear labelling of omega-3, vitamin A, and vitamin D content.

Reporting adverse effects: if you experience a suspected side effect from any supplement, you can report it via the MHRA Yellow Card Scheme (website: yellowcard.mhra.gov.uk or via the Yellow Card app).

The NHS advises that most people can obtain adequate omega-3 fatty acids through dietary sources alone. Consult your GP before starting supplementation if you have existing medical conditions, take regular medications, are pregnant or breastfeeding, or have concerns about nutritional deficiencies. Blood tests can assess vitamin D status if deficiency is suspected, enabling targeted supplementation recommendations.

Frequently Asked Questions