Cod liver oil has been used for generations as a traditional source of vitamin D, particularly valued in the UK where sunlight exposure is limited during autumn and winter months. Derived from the liver of Atlantic cod, this nutritional supplement provides vitamin D3 alongside vitamin A and omega-3 fatty acids. With UK Government guidance recommending that everyone consider taking 10 micrograms (400 IU) of vitamin D daily during darker months, cod liver oil offers a convenient option. However, its vitamin A content means it is not suitable for everyone, particularly pregnant women. Understanding the benefits, appropriate dosage, and potential risks is essential for safe and effective use.

What Is Cod Liver Oil and How Much Vitamin D Does It Contain?

Cod liver oil is a nutritional supplement derived from the liver of Atlantic cod (Gadus morhua) and related species. It has been used for centuries as a traditional remedy, particularly in Northern European countries, and remains a popular source of fat-soluble vitamins and omega-3 fatty acids. The oil is extracted through various processing methods, with modern techniques designed to preserve nutritional content whilst removing potential contaminants such as heavy metals and environmental pollutants.

The vitamin D content of cod liver oil varies widely depending on the source, processing method, and manufacturer. Many UK products provide approximately 5–10 micrograms (200–400 IU) of vitamin D per 5 ml teaspoon, though this can differ significantly between products. It is essential to check the product label for precise vitamin D content, as some manufacturers fortify their cod liver oil to standardise vitamin levels. The oil also naturally contains substantial amounts of vitamin A (retinol), typically 700–1,300 micrograms per teaspoon, alongside omega-3 fatty acids including eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA).

Vitamin D exists in two main forms: vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Cod liver oil contains vitamin D3, which is generally considered more effective at raising and maintaining vitamin D levels in the body, though both forms can increase serum 25-hydroxyvitamin D concentrations. This makes cod liver oil a bioavailable source of this essential nutrient, particularly relevant in the UK where sunlight exposure is limited during autumn and winter months. Cod liver oil sold as a food supplement is regulated under UK food law, with oversight from the Food Standards Agency (FSA) and enforcement by local authorities, ensuring products meet safety, quality and labelling standards.

Health Benefits of Vitamin D from Cod Liver Oil

Vitamin D plays crucial roles in multiple physiological processes, and obtaining adequate amounts through cod liver oil can support several aspects of health. The primary function of vitamin D is to regulate calcium and phosphate metabolism, which is essential for maintaining healthy bones, teeth, and muscle function. Vitamin D promotes intestinal absorption of calcium and helps maintain appropriate serum calcium concentrations, working in concert with parathyroid hormone to prevent hypocalcaemia. Deficiency can lead to rickets in children and osteomalacia in adults, conditions characterised by bone softening and deformity.

Beyond skeletal health, vitamin D has important immunomodulatory properties. It influences both innate and adaptive immune responses, and adequate vitamin D status may support normal immune function. The vitamin D receptor is expressed in various immune cells, including T lymphocytes, B lymphocytes, and antigen-presenting cells. Evidence suggests associations between vitamin D status and respiratory infections, with modest benefits mainly observed in people with deficiency. The UK Government and NHS recommend that everyone consider taking a daily supplement containing 10 micrograms (400 IU) of vitamin D during autumn and winter, with some groups requiring year-round supplementation.

Cod liver oil provides additional nutritional benefits beyond vitamin D. The omega-3 fatty acids EPA and DHA contribute to normal heart function at a daily intake of 250 mg, though NICE does not recommend fish oil supplements for primary prevention of cardiovascular disease. Vitamin A supports vision, immune function, and cellular differentiation. However, it is important to note that whilst cod liver oil can be a convenient source of multiple nutrients, the evidence base for specific health claims varies considerably. NICE guidance emphasises the importance of vitamin D for bone health but notes that evidence for other purported benefits requires further robust clinical investigation. Individuals should maintain realistic expectations and view cod liver oil as part of a balanced approach to nutrition rather than a panacea.

Recommended Dosage and How to Take Cod Liver Oil Safely

The appropriate dosage of cod liver oil depends on individual vitamin D requirements, existing dietary intake, and baseline vitamin D status. The UK Government and NHS recommend that adults and children over one year of age should have a daily intake of 10 micrograms (400 IU) of vitamin D, particularly during autumn and winter months (October to early March in the UK). For individuals at higher risk of deficiency—including those with limited sun exposure, darker skin tones, or who are housebound—year-round supplementation is advised.

Pregnant women should avoid cod liver oil supplements due to their vitamin A (retinol) content, which may harm the unborn baby. Instead, they should use pregnancy-specific vitamin D supplements that do not contain vitamin A. Breastfeeding women should ensure adequate vitamin D intake to support infant health.

When selecting a cod liver oil supplement, carefully review the product label to determine the vitamin D content per serving. A standard teaspoon (5 ml) typically provides 5–10 micrograms (200–400 IU) of vitamin D in many UK products. It is crucial to consider total vitamin D intake from all sources, including other supplements, fortified foods, and any prescribed medications. Adults and children over 11 years should not exceed 100 micrograms (4,000 IU) daily for prolonged periods. For children aged 1-10 years, the upper limit is 50 micrograms (2,000 IU) daily, and for infants under 1 year, 25 micrograms (1,000 IU) daily, unless prescribed by a healthcare professional.

Cod liver oil can be taken with or without food, though consuming it with a meal containing fat may enhance absorption of fat-soluble vitamins. Liquid formulations should be measured accurately using the provided dosing device. Capsule forms offer convenience and avoid the characteristic taste of liquid cod liver oil. Store cod liver oil in a cool, dark place and refrigerate after opening to prevent oxidation and rancidity. Check expiration dates and discard products that develop an unpleasant odour or appearance.

Individuals taking anticoagulant medications (such as warfarin) or antiplatelet drugs should consult their GP before starting cod liver oil, as omega-3 fatty acids may have mild antiplatelet effects, though these are typically more significant at higher doses than found in standard cod liver oil supplements. Those with fish allergies should avoid cod liver oil, and individuals with conditions that predispose to hypercalcaemia (such as kidney disease or sarcoidosis) should seek medical advice before supplementing.

Potential Side Effects and Risks of Cod Liver Oil

Cod liver oil is generally well tolerated when taken at recommended doses, but several potential adverse effects and safety considerations warrant attention. The most commonly reported side effects are gastrointestinal in nature, including:

• Nausea and indigestion

• Fishy aftertaste or belching

• Diarrhoea or loose stools

• Abdominal discomfort

These effects are typically mild and may be minimised by taking cod liver oil with meals, using enteric-coated capsules, or refrigerating liquid formulations. Some individuals find that starting with a lower dose and gradually increasing helps improve tolerance.

A significant concern with cod liver oil supplementation is the risk of vitamin A toxicity (hypervitaminosis A) with excessive or prolonged use. Cod liver oil contains preformed vitamin A (retinol), and the liver has limited capacity to metabolise excess amounts. Adults should not exceed 1.5 mg (1,500 micrograms) of retinol per day from supplements. Chronic vitamin A toxicity can cause hepatotoxicity, bone abnormalities, and central nervous system effects. Pregnant women must avoid cod liver oil supplements due to the risk of birth defects associated with excessive vitamin A intake during pregnancy. Instead, they should take pregnancy-specific vitamin D supplements that do not contain vitamin A.

Vitamin D toxicity, whilst rare, can occur with excessive supplementation over extended periods. Hypervitaminosis D leads to hypercalcaemia, which may manifest as nausea, vomiting, weakness, and frequent urination. Severe cases can result in kidney damage and cardiac arrhythmias. Individuals should not exceed recommended doses without medical supervision. Seek urgent medical attention if experiencing severe vomiting, confusion, dehydration, severe constipation, or cardiac symptoms that could indicate hypercalcaemia.

Cod liver oil may interact with certain medications. The omega-3 content can potentially enhance the effects of anticoagulants and antiplatelet drugs, though this risk is generally greater at higher omega-3 doses than typically found in cod liver oil. Patients taking warfarin, aspirin, or other blood-thinning medications should consult their GP before commencing cod liver oil. Additionally, medications that reduce fat absorption (such as orlistat) may decrease the absorption of fat-soluble vitamins from cod liver oil.

Patients should contact their GP if they experience persistent gastrointestinal symptoms, signs of vitamin toxicity (such as severe headache, vision changes, or bone pain), or any unexpected adverse effects. Those with liver disease, bleeding disorders, or scheduled for surgery should seek medical advice before using cod liver oil supplements. Quality assurance is important—choose products from reputable manufacturers that test for contaminants and provide clear labelling of vitamin content. If you suspect side effects from cod liver oil, you can report them through the MHRA Yellow Card scheme.

Frequently Asked Questions