Cod liver oil is a traditional dietary supplement derived from Atlantic cod livers, uniquely rich in omega-3 fatty acids (EPA and DHA) alongside fat-soluble vitamins A and D. Understanding the cod liver oil fatty acid profile is essential for healthcare professionals and patients considering its use, as composition varies between products and influences both potential benefits and safety considerations. Whilst omega-3s support cardiovascular and neurological health, the high vitamin A content requires caution in pregnancy and older adults. This article examines the detailed nutritional composition, fatty acid ratios, EPA and DHA content, and important safety guidance aligned with NHS and NICE recommendations.

What Is Cod Liver Oil and Its Nutritional Composition?

Cod liver oil is a dietary supplement derived from the livers of Atlantic cod (Gadus morhua) and related species. Historically used to prevent rickets and support general health, it remains a popular nutritional supplement in the UK and worldwide. Unlike standard fish oils extracted from the flesh of oily fish, cod liver oil is uniquely rich in both omega-3 fatty acids and fat-soluble vitamins A and D, making it a distinctive source of multiple essential nutrients.

The nutritional composition of cod liver oil varies depending on the source, processing methods, and whether it has been fortified. A typical serving (approximately 5 ml or one teaspoon) provides around 40–45 kcal, derived almost entirely from fat. The total fat content is approximately 4.5–5 grams per serving, comprising a mixture of polyunsaturated fatty acids (PUFAs), monounsaturated fatty acids (MUFAs), and saturated fatty acids. The precise ratios depend on the manufacturing process and whether the oil has undergone molecular distillation or other purification techniques to remove contaminants such as heavy metals, dioxins, and persistent organic pollutants. Reputable products comply with UK and retained EU maximum levels for these contaminants.

Beyond its fatty acid content, cod liver oil is particularly valued for its naturally occurring vitamins. A single teaspoon typically contains 400–1,000 micrograms of retinol equivalents (vitamin A) and 10–20 micrograms (400–800 IU) of vitamin D3 (cholecalciferol). These concentrations can vary significantly between products, and some manufacturers adjust levels to align with Nutrient Reference Values (NRVs). Most cod liver oil products sold in the UK are marketed as food supplements and regulated under UK food law overseen by the Food Standards Agency (FSA). Only products making specific medicinal claims are regulated as medicines by the Medicines and Healthcare products Regulatory Agency (MHRA). Consumers should check product labels for exact composition and choose reputable brands.

Cod liver oil is derived from fish and may contain fish proteins. Individuals with fish allergy should avoid it. Some capsule formulations may also contain gelatin or soya; check labels if you have relevant allergies or dietary restrictions.

Key nutritional components per 5 ml serving (product-dependent):

• Total fat: 4.5–5 g

• Omega-3 fatty acids: 0.9–1.2 g (varies by brand)

• Vitamin A: 400–1,000 µg retinol equivalents

• Vitamin D: 10–20 µg (400–800 IU)

Fatty Acid Profile: Omega-3, Omega-6, and Saturated Fats

The fatty acid profile of cod liver oil is characterised by a predominance of long-chain omega-3 polyunsaturated fatty acids, alongside smaller amounts of omega-6 PUFAs, monounsaturated fats, and saturated fats. Understanding this profile is relevant for healthcare professionals advising patients on cardiovascular health, inflammatory conditions, and overall nutritional adequacy. However, composition varies between products, and patients should check individual product labels.

Omega-3 fatty acids typically constitute approximately 20–25% of the total fatty acid content in cod liver oil. The principal omega-3s are eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which are marine-derived long-chain PUFAs with established roles in cardiovascular and neurological health. A standard 5 ml serving may provide 400–500 mg of EPA and 500–600 mg of DHA, though concentrations vary between products. These omega-3 fatty acids exert anti-inflammatory effects through multiple mechanisms, including modulation of eicosanoid synthesis and resolution of inflammatory pathways.

Omega-6 fatty acids are present in much smaller quantities, typically comprising less than 2% of total fatty acids. The primary omega-6 in cod liver oil is linoleic acid, with trace amounts of arachidonic acid. The omega-6 to omega-3 ratio in cod liver oil is generally favourable, often around 1:10 or lower, which contrasts sharply with typical Western diets where ratios may exceed 15:1.

Saturated fatty acids account for approximately 20–25% of the total fat content, predominantly palmitic acid (C16:0) and myristic acid (C14:0). Whilst saturated fats have historically been associated with cardiovascular risk, current evidence suggests that the overall dietary pattern and specific fatty acid chain length influence health outcomes more than total saturated fat intake alone.

Monounsaturated fatty acids, primarily oleic acid (C18:1), comprise roughly 45–50% of cod liver oil's fatty acid profile, contributing to its overall lipid stability. Any health effects depend on overall diet, dosing, and individual patient factors; cod liver oil composition alone does not determine clinical outcomes.

EPA and DHA Content in Cod Liver Oil

Eicosapentaenoic acid (EPA, C20:5n-3) and docosahexaenoic acid (DHA, C22:6n-3) are the two principal omega-3 fatty acids in cod liver oil, each with distinct but complementary physiological roles. The combined EPA and DHA content varies by brand and formulation; typical non-concentrated cod liver oil products may provide 600–1,200 mg per 5 ml serving, whilst concentrated formulations may provide higher amounts. Patients should check product labels for exact content.

EPA is a 20-carbon omega-3 fatty acid that serves as a precursor for series-3 prostaglandins and series-5 leukotrienes, which possess anti-inflammatory and vasodilatory properties. EPA competes with arachidonic acid (an omega-6 fatty acid) for incorporation into cell membranes and for metabolism by cyclooxygenase and lipoxygenase enzymes. This competitive inhibition reduces the production of pro-inflammatory eicosanoids such as prostaglandin E2 and leukotriene B4.

DHA is a 22-carbon omega-3 fatty acid that is highly concentrated in neural tissue, particularly the brain and retina. It constitutes approximately 40% of the polyunsaturated fatty acids in neuronal cell membranes and 60% in photoreceptor outer segments. DHA is essential for normal brain development during pregnancy and infancy, and maintains structural and functional integrity of neural membranes throughout life. It also serves as a precursor for specialised pro-resolving mediators (SPMs) such as protectins and resolvins, which actively resolve inflammation rather than simply suppressing it.

The ratio of EPA to DHA in cod liver oil typically ranges from 1:1.2 to 1:1.5, favouring DHA slightly. This differs from some concentrated fish oil supplements that may be enriched in EPA.

Bioavailability of EPA and DHA from cod liver oil is generally enhanced when consumed with meals containing fat. Absorption and incorporation into plasma lipids occur within hours of ingestion, though this varies by formulation (triglyceride versus ethyl ester forms) and individual factors.

Important distinction regarding cardiovascular disease prevention: NICE guidance (CG181: Cardiovascular disease: risk assessment and reduction, including lipid modification) does not recommend omega-3 fatty acid supplements (including fish oils or cod liver oil) for the primary or secondary prevention of cardiovascular disease in the general population. NICE technology appraisal TA805 applies specifically to prescription icosapent ethyl (a purified EPA ethyl ester medicine) in selected high-risk patients with elevated triglycerides despite statin therapy; this is not equivalent to cod liver oil supplements. Patients requiring cardiovascular risk management should follow their clinician's guidance and not substitute prescribed treatments with supplements.

Safety considerations: At higher intakes, omega-3 fatty acids may increase bleeding tendency, though the risk is small at typical supplement doses. Patients taking anticoagulants (such as warfarin, apixaban, rivaroxaban) or antiplatelet agents (such as aspirin, clopidogrel), or those scheduled for surgery, should consult their GP or pharmacist before starting cod liver oil. If you experience unusual bruising or bleeding, seek medical advice promptly.

Vitamins A and D: Additional Nutritional Benefits and Safety Considerations

Beyond its omega-3 content, cod liver oil is distinguished by naturally high concentrations of vitamins A and D, both fat-soluble vitamins with critical roles in immune function, bone health, and cellular regulation. This dual provision of essential vitamins alongside omega-3 fatty acids makes cod liver oil unique among marine oil supplements. However, the high vitamin A content requires careful consideration in certain groups.

Vitamin A in cod liver oil exists primarily as retinol and retinyl esters, the preformed active forms of the vitamin. A typical 5 ml serving provides 400–1,000 micrograms of retinol equivalents, which represents 50–125% of the UK Reference Nutrient Intake (RNI) for adults. Vitamin A is essential for vision (particularly night vision), immune function, epithelial cell differentiation, and reproduction. It supports the integrity of mucosal barriers in the respiratory and gastrointestinal tracts, contributing to first-line immune defences.

Important safety advice for pregnancy: The NHS advises that pregnant women and women trying to conceive should avoid taking supplements containing vitamin A (retinol), including cod liver oil, because high intakes of vitamin A can harm the developing baby. Pregnant women should also avoid eating liver and liver products (such as pâté) for the same reason. If you are pregnant, planning pregnancy, or of childbearing age and not using reliable contraception, do not take cod liver oil; consider a vitamin D-only supplement instead.

Bone health and older adults: Long-term intakes of vitamin A exceeding 1.5 mg (1,500 micrograms) per day from diet and supplements combined may increase the risk of bone fractures, particularly in older adults. If you are taking cod liver oil regularly, check the total vitamin A content from all sources (including fortified foods and multivitamins) and consider switching to a vitamin D-only supplement if your intake approaches or exceeds this level.

Vitamin D content in cod liver oil typically ranges from 10–20 micrograms (400–800 IU) per 5 ml serving. Vitamin D3 (cholecalciferol) is the form naturally present in cod liver oil and is more effective at raising serum 25-hydroxyvitamin D concentrations than vitamin D2 (ergocalciferol). Vitamin D is crucial for calcium homeostasis, bone mineralisation, immune modulation, and potentially cardiovascular and metabolic health. Given the high prevalence of vitamin D insufficiency in the UK population, particularly during autumn and winter months, cod liver oil can contribute meaningfully to vitamin D status. The NHS recommends that all adults consider taking a daily supplement containing 10 micrograms of vitamin D during autumn and winter. Some groups (including people with dark skin, those who cover their skin for cultural reasons, and those who are housebound or rarely go outdoors) should consider taking a daily supplement all year round.

Monitoring total intake: Patients taking cod liver oil alongside multivitamins or other supplements should check total vitamin A and D intake from all sources to avoid exceeding safe upper limits. The tolerable upper intake level for vitamin A is 3,000 micrograms daily for adults, whilst for vitamin D it is 100 micrograms daily. However, as noted above, long-term vitamin A intakes above 1.5 mg/day may affect bone health.

Drug interactions and special populations:

• Retinoid medicines (such as isotretinoin for acne or acitretin for psoriasis): Do not take cod liver oil or vitamin A supplements whilst taking these medicines, as this may lead to vitamin A toxicity.

• Orlistat or cholestyramine: These medicines reduce absorption of fat-soluble vitamins; if you are taking them, discuss vitamin supplementation with your GP or pharmacist.

• Thiazide diuretics (such as bendroflumethiazide, indapamide): Taking vitamin D alongside thiazides may increase the risk of high blood calcium levels (hypercalcaemia); your doctor may monitor your calcium levels if you take both.

• Liver or kidney disease: If you have liver or kidney disease, seek medical advice before taking cod liver oil, as metabolism and excretion of fat-soluble vitamins may be affected.

Symptoms requiring medical attention: If you experience symptoms that may indicate vitamin A excess (such as persistent headache, nausea, dry or peeling skin, bone or joint pain) or vitamin D toxicity leading to high blood calcium (such as excessive thirst, passing urine frequently, confusion, vomiting, or constipation), stop taking cod liver oil and consult your GP promptly.

Reporting side effects: If you suspect you have experienced an adverse effect from cod liver oil, you can report it via the MHRA Yellow Card Scheme at https://yellowcard.mhra.gov.uk or by searching for 'Yellow Card' in the Google Play or Apple App Store.

Frequently Asked Questions