Cod Liver

Cod liver is one of the few foods that bridges the worlds of everyday eating and old-fashioned medicine. Eaten as a soft, spreadable canned delicacy across Scandinavia, Iceland, and Russia, or taken by the spoonful as cod liver oil — the supplement that helped wipe out childhood rickets — it delivers a nutrient combination no land-animal liver can match: vitamins A and D side by side with the marine omega-3 fats EPA and DHA. That same richness is its one real catch. Because cod liver oil concentrates two fat-soluble vitamins that the body stores rather than excretes, a normal teaspoon is a gift while megadosing — or stacking it with other vitamin A or D supplements — risks toxicity, and its high preformed vitamin A makes it a supplement pregnant women should specifically avoid. Used sensibly and in modest amounts, though, cod liver remains a time-tested, genuinely nourishing food.

Table of Contents

  1. What Cod Liver Is
  2. Historical Medical Use (1926)
  3. Nutritional Profile (Unlike Any Land Liver)
  4. Health Benefits
  5. How to Use It
  6. How Much — and Don't Megadose
  7. Cautions — Vitamin A AND D
  8. Research Papers
  9. Connections
  10. Featured Videos

What Cod Liver Is

Cod liver is exactly what it sounds like: the liver of the cod fish. Unlike the white, flaky fillet most people picture when they think of cod, the liver is a soft, fatty organ — and that fattiness is the whole point, because it is where the fish concentrates its richest nutrients. People consume cod liver in two very different forms, and it helps to keep them straight.

Canned (tinned) cod liver is the actual organ, gently cooked and packed in its own oil. It is a traditional delicacy across Nordic, Icelandic, and Russian cuisines, where it is prized as an affordable, nutrient-dense food. The texture is soft and spreadable, almost like a rich pâté, and it is typically eaten on dark rye bread or crackers, sometimes mashed with onion, egg, or a squeeze of lemon. In Russian it is known as pechen treski; in Norway and Iceland it appears on the table much as canned sardines or liver pâté might elsewhere.

Cod liver oil is the oil rendered (pressed) from those same fresh livers. It is one of the oldest and most famous dietary supplements in the world — given by the spoonful to generations of children — and it is sold today as a plain or flavored liquid and as softgel capsules. The food and the supplement come from the same place, but they are used differently: canned cod liver is eaten as part of a meal, while cod liver oil is taken in small measured doses.

Historical Medical Use (1926 U.S. Dispensatory)

Cod liver oil was not always a quiet supplement on a pharmacy shelf — a century ago it was a mainstream medicine. The United States Dispensatory of 1926 (Wood & LaWall) was the authoritative American drug reference of its day, written when roughly three-quarters of all remedies were still natural substances, and it took cod liver oil seriously as a therapeutic agent. The uses physicians put it to back then make a fascinating window into pre-antibiotic, pre-vitamin medicine. The honest way to read this history is to separate what doctors did (a matter of record) from why it sometimes worked (which we understand far better now than they did). The short version: cod liver oil's value in 1926 was almost entirely nourishment — concentrated calories plus the fat-soluble vitamins A and D and marine omega-3 fats — not any direct drug-like action against disease.

Tuberculosis and the sanatorium diet

In the early 1900s, before streptomycin and other antibiotics existed, tuberculosis was a leading cause of death, and the standard treatment was the sanatorium: rest, fresh air, sunlight, and a deliberately rich, fattening diet. Cod liver oil was a staple of that regimen, and period sources describe it as helping to "inhibit" the disease and aid recovery. It is crucial to frame this correctly. Cod liver oil never cured tuberculosis and was not an antibiotic; the bacterium that causes TB is killed by modern drugs, not by fish oil. What cod liver oil genuinely provided was nutritional support — dense calories to fight the severe wasting ("consumption") that TB caused, plus vitamins A and D in an era of widespread deficiency. There is a real and interesting modern footnote here: vitamin D activates parts of the immune system that help contain the TB bacterium, which is partly why sunlight (a natural source of vitamin D) seemed to help sanatorium patients. Modern randomized trials of adding vitamin D to standard antibiotic treatment have shown, at best, modest and selective benefits — useful biological insight, but a confirmation that the antibiotics do the curing and the vitamin plays only a supporting role.

Rheumatoid arthritis and "rheumatism"

The 1926-era materia medica also reached for cod liver oil in rheumatoid arthritis and the painful joint conditions then lumped together as "rheumatism." This is the claim that has aged best. We now know that the long-chain omega-3 fats in cod liver oil — especially EPA — are genuinely anti-inflammatory: the body uses them to make signalling molecules that calm inflammation, partly displacing the more inflammatory products it makes from omega-6 fats. Modern controlled trials and meta-analyses back this up in a measured way: omega-3 supplements modestly reduce joint tenderness, morning stiffness, and the need for anti-inflammatory painkillers in rheumatoid arthritis. They are an adjunct that helps symptoms — not a replacement for the disease-modifying drugs that actually slow joint damage. So the old physicians were onto something real, even though they had no idea of the underlying chemistry.

Calcium, tetany and nerve pain

Finally, the 1926 sources paired cod liver oil with calcium salts to treat tetany — the muscle twitching, cramps, and spasms caused by low blood calcium — and used it for various forms of nerve pain. This, too, has a sound modern explanation. The body cannot properly absorb and regulate calcium without vitamin D, and cod liver oil is one of the richest natural sources of it. In a population riddled with vitamin D deficiency and rickets, low vitamin D meant poor calcium handling, which can produce exactly the neuromuscular irritability — twitching and spasm — that "tetany" describes. Supplying vitamin D (via cod liver oil) alongside calcium corrected the deficiency at its root. The nerve-pain uses are harder to substantiate and are best read as a product of their time rather than established practice.

The safety lesson the old usage hides

Reading these historical uses with modern eyes points to one essential caution that the enthusiasm of a century ago glossed over. The very thing that made cod liver oil therapeutic — its concentration of fat-soluble vitamins A and D — is also what makes it possible to overdo. Both vitamins are stored in the body rather than excreted, so both can build up to toxic levels if megadosed or stacked on top of other A- or D-containing supplements (a far easier mistake to make today, with high-potency products everywhere, than it was in 1926). Cod liver oil's omega-3 content can also mildly thin the blood, which matters for anyone on anticoagulant or antiplatelet medication. The historical record is a tribute to cod liver oil as nourishment; it is not a license to take it in large doses. The dedicated Cautions section below covers the vitamin A and D ceilings, the specific pregnancy warning, and the blood-thinning interaction in detail.

Nutritional Profile (Unlike Any Land Liver)

Liver is famously nutritious no matter which animal it comes from — beef, chicken, and pork liver are all packed with iron, vitamin B12, and vitamin A. But fish liver, and cod liver in particular, has a nutritional signature that land-animal livers simply do not share.

The difference comes down to two things: vitamin D and long-chain omega-3 fatty acids (EPA and DHA). Beef and chicken liver contain essentially none of either. Cod liver is one of the richest natural food sources of both. On top of that, it carries a large amount of vitamin A in its most usable form, retinol. So cod liver pairs the two fat-soluble vitamins A and D together with marine omega-3 fats — a combination you will not find in any liver from a land animal.

The approximate figures are striking. Canned cod liver is roughly half fat by weight and runs around 500 calories per 100 grams, most of it from that beneficial oil. A single teaspoon of cod liver oil (about 4–5 mL) typically supplies, very approximately, 4,000–4,500 IU of vitamin A, 400–1,000 IU of vitamin D, and 800–1,000 mg of combined EPA and DHA. These numbers vary considerably from brand to brand and batch to batch, so always read the label on the specific product you buy rather than assuming a fixed amount.

Health Benefits

Cod liver's benefits flow directly from its three standout nutrients: vitamin D, omega-3 fats, and vitamin A.

Vitamin D — and the conquest of rickets

Vitamin D helps the body absorb calcium and is essential for strong bones, and it plays a role in normal immune function. The most important chapter in cod liver oil's history is genuine medical fact, not folklore: in the 19th and early 20th centuries, rickets — a disease that softens and bends children's growing bones — was rampant among poor children in the crowded, smoky industrial cities of northern Europe and the United States. Cod liver oil was found to prevent and treat it, and once scientists worked out in the 1920s that the active ingredient was vitamin D, cod liver oil became a frontline tool that helped largely eradicate rickets. Spoon-feeding children cod liver oil remained common practice well into the 1950s. This is one of the clearest examples in history of a single food correcting a widespread nutritional disease.

Omega-3 fatty acids (EPA and DHA)

The long-chain omega-3 fats in cod liver are the same ones found in oily fish like salmon and sardines. They are best known for heart health: at high (prescription-strength) doses, EPA and DHA reliably lower elevated blood triglycerides, a type of blood fat tied to cardiovascular risk. Omega-3s also have anti-inflammatory effects, and DHA is a major structural building block of the brain and the retina of the eye. It is worth being honest about the limits of the evidence here: while omega-3s clearly lower triglycerides, large trials of fish-oil supplements have produced mixed results on actually preventing heart attacks and strokes in the general population, so they are not a guaranteed shield against heart disease.

Vitamin A (retinol)

Vitamin A supports normal vision (especially seeing in dim light), immune defense, and healthy skin. Cod liver delivers it as preformed retinol, the form the body uses directly. This is a real benefit for people who are short on vitamin A — but, as the cautions section explains, that same potency is exactly why cod liver oil must be treated with respect.

A note on observational links

Beyond these established roles, regular cod liver oil users in observational studies tend to show some favorable health patterns — for example, in people with rheumatoid arthritis, cod liver oil has been shown in a controlled trial to let many patients cut back on their anti-inflammatory painkillers. It is important to read associations like these honestly: people who take cod liver oil for years may also differ in other ways (diet, income, general health habits), so an observed link is not proof that the oil itself caused the outcome.

How to Use It

As a food (canned cod liver): Drain it lightly and spread or mash it onto dark rye bread, crispbread, or plain crackers — the classic Scandinavian and Russian open-faced snack. It pairs well with thinly sliced raw onion, chopped hard-boiled egg, fresh dill, black pepper, and a squeeze of lemon to cut the richness. It also works flaked into a green salad or a potato salad. Because it is so rich, a little goes a long way; a tablespoon or two is a typical portion.

As a supplement (cod liver oil): The traditional approach is a single daily spoonful of the liquid oil. Modern lemon- or mint-flavored versions exist precisely because plain cod liver oil's fishy taste was the bane of generations of children — the flavoring makes it genuinely pleasant. If you dislike any liquid oil, softgel capsules deliver the same nutrients with no taste at all. Many people find it sits more comfortably when taken with a meal that contains some fat, which also helps absorption of the fat-soluble vitamins.

How Much — and Don't Megadose

For cod liver oil, a sensible everyday amount is about one teaspoon (or the equivalent in capsules — check the label, as concentrations differ). At that level, cod liver oil is a reasonable way to top up omega-3s, vitamin D, and vitamin A together.

The single most important rule is this: more is not better. The temptation to take a big spoonful, or several capsules, or to keep cod liver oil going on top of every other supplement in the cabinet, is exactly where cod liver oil gets people into trouble. The reason is the fat-soluble vitamin load it carries — which is the subject of the next section, and the part of this page worth reading most carefully.

Cautions — Vitamin A AND D

Here is the headline that sets cod liver oil apart from ordinary fish oil: it is rich in both vitamin A and vitamin D. These are fat-soluble vitamins, which means the body stores excess amounts in the liver and fatty tissue rather than flushing them out in urine the way it does with vitamin C or the B vitamins. Because they accumulate, taking too much over time can build up to toxic levels.

Don't stack it. The practical danger is doubling up. If you take a generous dose of cod liver oil and a separate high-dose vitamin D supplement and a multivitamin (which usually contains vitamin A), the totals can quietly climb into the danger zone. Excess preformed vitamin A can cause vitamin A toxicity (hypervitaminosis A) — headaches, nausea, bone and joint pain, dry skin, hair loss, and, with chronic overload, liver damage. Excess vitamin D can cause vitamin D toxicity, which raises blood calcium and can harm the kidneys. Both are avoidable simply by keeping to a normal serving and tallying up the vitamins A and D coming from all your supplements together, not just from the cod liver oil.

Pregnancy — a specific and important warning. This is the one caution that genuinely surprises people. Pregnant women are usually told that fish oil is fine and even beneficial — and that is true of plain fish oil, which contains little or no vitamin A. Cod liver oil is different, because it is high in preformed vitamin A (retinol), and high intakes of preformed vitamin A in early pregnancy are teratogenic, meaning they can cause birth defects. For this reason, women who are pregnant or planning a pregnancy are generally advised to avoid cod liver oil specifically (and other liver-derived vitamin A supplements), even though plain fish oil and dietary omega-3s remain encouraged. If you are pregnant and want the omega-3 benefits, a plain fish-oil or algae-based product avoids the vitamin A problem entirely.

Blood thinning and surgery. High doses of omega-3 fats can have a mild blood-thinning (anti-clotting) effect. At a normal cod liver oil serving this is rarely a concern, but if you take anticoagulant or antiplatelet medication, or you have surgery scheduled, mention your cod liver oil use to your doctor.

Buy a reputable, purified brand. Because cod liver oil comes from an organ that filters and processes substances, choose products from established manufacturers that test and purify for environmental contaminants (such as heavy metals and other pollutants). Reputable brands publish purity testing; this is one area where the cheapest option is not worth it.

Research Papers

  1. Rajakumar K. Vitamin D, cod-liver oil, sunlight, and rickets: a historical perspective. Pediatrics. 2003;112(2):e132–e135. doi:10.1542/peds.112.2.e132 — Traces how cod liver oil and, later, the discovery of vitamin D within it conquered childhood rickets.
  2. Siscovick DS, Barringer TA, Fretts AM, et al. Omega-3 polyunsaturated fatty acid (fish oil) supplementation and the prevention of clinical cardiovascular disease: a science advisory from the American Heart Association. Circulation. 2017;135(15):e867–e884. doi:10.1161/CIR.0000000000000482 — A balanced expert review showing omega-3 supplements have real but limited, population-dependent cardiovascular benefits.
  3. Skulas-Ray AC, Wilson PWF, Harris WS, et al. Omega-3 fatty acids for the management of hypertriglyceridemia: a science advisory from the American Heart Association. Circulation. 2019;140(12):e673–e691. doi:10.1161/CIR.0000000000000709 — Confirms that EPA and DHA reliably lower elevated blood triglycerides.
  4. Galarraga B, Ho M, Youssef HM, et al. Cod liver oil (n-3 fatty acids) as a non-steroidal anti-inflammatory drug sparing agent in rheumatoid arthritis. Rheumatology (Oxford). 2008;47(5):665–669. doi:10.1093/rheumatology/ken024 — In a controlled trial, daily cod liver oil let many rheumatoid arthritis patients cut their painkiller use.
  5. Senftleber NK, Nielsen SM, Andersen JR, et al. Marine oil supplements for arthritis pain: a systematic review and meta-analysis of randomized trials. Nutrients. 2017;9(1):42. doi:10.3390/nu9010042 — Pooled randomized trials find marine omega-3s produce a modest but real reduction in arthritis pain — the modern evidence behind the 1926 rheumatism use.
  6. Martineau AR, Timms PM, Bothamley GH, et al. High-dose vitamin D₃ during intensive-phase antimicrobial treatment of pulmonary tuberculosis: a double-blind randomised controlled trial. Lancet. 2011;377(9761):242–250. doi:10.1016/S0140-6736(10)61889-2 — A modern trial of adding vitamin D to standard TB antibiotics, showing only a modest, selective benefit — confirming vitamin D is supportive, not curative, in tuberculosis.
  7. Martineau AR, Jolliffe DA, Hooper RL, et al. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ. 2017;356:i6583. doi:10.1136/bmj.i6583 — Large meta-analysis showing vitamin D modestly supports respiratory immune defense, especially in those who are deficient — context for cod liver oil's role in the pre-antibiotic sanatorium diet.
  8. Rothman KJ, Moore LL, Singer MR, et al. Teratogenicity of high vitamin A intake. N Engl J Med. 1995;333(21):1369–1373. doi:10.1056/NEJM199511233332101 — The landmark study linking high preformed-vitamin-A intake in early pregnancy to birth defects — the basis for the cod-liver-oil pregnancy warning.
  9. Olson JM, Ameer MA, Goyal A. Vitamin A toxicity. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2023. NCBI Bookshelf NBK532916 — Explains how fat-soluble vitamin A accumulates in the liver and the acute and chronic signs of overdose.

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Connections

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