What's Inside Cod Liver Oil
The three main active components all come together in a form the body absorbs readily:
Vitamin A (retinol) is the preformed, ready-to-use version — not the beta-carotene found in carrots, which the body must convert. Retinol is directly used for: maintaining the mucosal lining of the respiratory and digestive tracts, supporting night vision, regulating immune cell development (particularly T cells and natural killer cells), and keeping skin and reproductive tissue healthy. A single teaspoon of high-quality cod liver oil typically provides 1,000–2,000 IU (300–600 mcg RAE) of vitamin A.
Vitamin D3 in cod liver oil is cholecalciferol — the same form your skin produces in sunlight. Most people in northern latitudes are deficient by winter's end. Vitamin D regulates calcium absorption, influences hundreds of genes involved in immunity and cell growth, and supports mood. A teaspoon of cod liver oil provides roughly 400–1,000 IU of D3 depending on the product.
EPA and DHA are the long-chain omega-3 fatty acids behind cod liver oil's anti-inflammatory effects. EPA (eicosapentaenoic acid) competes with arachidonic acid in the inflammatory pathway, reducing prostaglandin synthesis. DHA (docosahexaenoic acid) is a structural component of brain and retinal cell membranes. Together, they reduce serum triglycerides, support vascular function, and may modulate mood via their role in neuronal membrane fluidity [3][4].
Synergy Between the Three Nutrients
Vitamins A and D are interdependent in ways that explain why cod liver oil performs differently from isolated supplements of either nutrient. They share nuclear receptors (RAR and RXR for vitamin A; VDR for vitamin D), and both receptors heterodimerize with RXR — meaning adequate vitamin A is necessary for vitamin D signaling to work properly. Taking large doses of vitamin D without corresponding vitamin A can actually impair immune function rather than support it. Cod liver oil provides them together in ratios that mirror traditional diets.
Anti-inflammatory and Joint Benefits
The n-3 fatty acids in cod liver oil reduce the production of inflammatory cytokines (IL-1, IL-6, TNF-alpha) and eicosanoids derived from arachidonic acid. In a 9-month randomized double-blind trial of 97 patients with rheumatoid arthritis, those taking 10g of cod liver oil daily (providing 2.2g of n-3 fatty acids) were significantly more likely to be able to reduce or eliminate their daily NSAID use compared to placebo — 39% vs. 10% achieved a clinically significant reduction [3]. This NSAID-sparing effect has real implications: long-term NSAID use carries GI and cardiovascular risks, and reducing dependence on them is clinically meaningful.
In a prospective study of 1,002 recreational cyclists who completed a 91 km race, habitual cod liver oil users showed 27% lower exercise-induced C-reactive protein (CRP) response compared to non-users, as well as lower resting CRP levels, suggesting reduced baseline inflammation [4].
Mood and Depression
The Hordaland Health Study, a population-based cross-sectional study of 21,835 adults in Norway, found that daily cod liver oil use was associated with a 30% lower prevalence of depressive symptoms (2.5% vs. 3.8%) after controlling for confounders including fish consumption and lifestyle factors [2]. The mechanism is thought to involve EPA's role in reducing neuroinflammation and DHA's structural role in neuronal membranes, with vitamin D's influence on serotonin synthesis likely contributing.
Immune and Respiratory Infection
A large Norwegian randomized controlled trial enrolled 34,601 adults during the COVID-19 pandemic (November 2020 to June 2021) to test whether daily cod liver oil supplementation reduced rates of acute respiratory infection [1]. Participants received 5 ml/day of cod liver oil (providing roughly 400 IU vitamin D3). While the trial did not show a statistically significant reduction in COVID-19 specifically, it found a meaningful reduction in other respiratory tract infections and was notable for its scale and rigorous blinding.
Dosing and Safety
Standard dosing is 1 teaspoon (5 ml) per day, providing approximately:
- 1,000–2,000 IU vitamin A
- 400–1,000 IU vitamin D3
- 1,000–1,500 mg EPA+DHA combined
This range is safe for most adults. The concern with cod liver oil is chronic over-supplementation of vitamin A. Preformed retinol accumulates in the liver, and intakes above 10,000 IU/day over time are associated with liver damage, bone density loss, and teratogenicity in pregnancy. At typical 1-teaspoon doses, cod liver oil is safe, but taking multiple supplements containing retinol simultaneously warrants attention.
Pregnancy: The vitamin A content is the reason cod liver oil is sometimes flagged during pregnancy. At 1 teaspoon/day, the vitamin A dose is well within safe limits, but as with all supplements, it's worth discussing with a healthcare provider.
Quality matters: Cod livers accumulate fat-soluble toxins from the ocean (PCBs, dioxins) along with the vitamins. Choose products from brands that test for and publish contaminant levels. Norwegian and Icelandic brands with third-party testing are generally highest quality.
See our Omega-3 page for the broader evidence on EPA and DHA. For vitamin D specifically, see Vitamin D.
Evidence Review
Brunvoll et al. (2022) conducted a quadruple-blinded randomized placebo-controlled trial with 34,601 Norwegian adults aged 18–75 during the COVID-19 pandemic. Participants received 5 ml/day of cod liver oil or placebo from November 2020 to June 2021. The primary outcome was SARS-CoV-2 infection confirmed by PCR. While the trial found no statistically significant reduction in COVID-19 specifically (OR 0.82, 95% CI 0.66–1.02), it did find a reduction in symptomatic respiratory infections broadly. The trial is notable for its exceptional scale — among the largest supplement RCTs ever conducted — and its rigorous quadruple-blinding methodology. The absence of effect on COVID-19 may reflect the relatively low vitamin D dose (approximately 400 IU/day from the cod liver oil) and the fact that Norway had high baseline vitamin D intake [1].
Raeder et al. (2007) analyzed data from the Hordaland Health Study, a population-based cross-sectional study of 21,835 adults in Norway. After adjusting for potential confounders including age, sex, smoking, physical activity, income, marital status, and dietary fish intake, daily cod liver oil users had significantly lower prevalence of high depressive symptoms than non-users (2.5% vs. 3.8%, OR 0.66, 95% CI 0.50–0.88). The effect was independent of dietary fish intake, suggesting a contribution from the concentrated EPA/DHA dose or the fat-soluble vitamins in cod liver oil beyond what comes from food. This study design cannot establish causality — people who take cod liver oil may differ in unmeasured ways from those who do not [2].
Galarraga et al. (2008) conducted a 9-month dual-centre, double-blind placebo-controlled RCT in 97 patients with established rheumatoid arthritis. Patients were randomized to 10g/day of cod liver oil (2.2g n-3 EPA+DHA) or placebo capsules, with all patients maintaining their background DMARD therapy. The primary outcome was the ability to reduce NSAID dose by 30% or more while maintaining disease control. Thirty-nine percent of cod liver oil patients achieved this threshold versus 10% of placebo patients (p=0.003). Secondary outcomes including pain VAS score, morning stiffness duration, and Ritchie articular index all trended favorably in the cod liver oil group, though not all reached significance individually. This is one of the most robust clinical demonstrations of cod liver oil's anti-inflammatory efficacy [3].
Bjørnsen et al. (2021) conducted a prospective observational study of 1,002 recreational cyclists participating in a 91 km mountain bike race. CRP was measured before and 24 hours after the race. Of the cohort, 173 subjects reported habitual cod liver oil use. After controlling for age, sex, BMI, and training volume, cod liver oil users had significantly lower basal CRP (geometric mean 0.93 mg/L vs. 1.28 mg/L) and a 27% attenuated CRP response to exercise compared to non-users (p<0.05). The observational design limits causal inference, but the finding is consistent with the known mechanism of EPA/DHA inhibiting arachidonic acid-derived eicosanoid production [4].
Evidence summary: The anti-inflammatory and NSAID-sparing effects in rheumatoid arthritis are supported by well-designed RCT evidence. Depression associations are from large observational studies — plausible mechanistically but not proven causally. The COVID-19 trial provides reassurance about safety at typical doses and suggests modest benefit for respiratory infections broadly. The primary evidence gap is a large RCT powered to detect effects on depression or long-term cardiovascular outcomes.