Comprehensive Overview of Fish Liver Oils

Fish liver oils are among the oldest nutritional remedies known to humanity. Long before modern medicine identified vitamins, ancient civilizations from Egypt to Greece were using the livers of fish to treat conditions like night blindness and bone deformities. Today, we understand that fish liver oils owe their therapeutic power to their rich concentrations of vitamins A and D, along with beneficial omega-3 fatty acids. This post covers the key fish species used, the various extraction methods employed, and the wide-ranging health benefits and applications of fish liver oils.

What are fish liver oils? 🔗

Fish liver oils are oils derived specifically from the livers of certain fish species, as opposed to fish body oils which come from the flesh and tissues. What sets liver oils apart is their exceptionally high content of fat-soluble vitamins A and D. While fish body oils are valued primarily for their omega-3 fatty acid content, liver oils serve a dual nutritional purpose – they supply both essential vitamins and beneficial fatty acids like eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA).

Fish liver oils also contain unique compounds not typically found in body oils. For instance, shark liver oils are notable for their high levels of diacylglyceryl ethers and squalene, a hydrocarbon compound with applications in cosmetics, pharmaceuticals, and even vaccine adjuvants. The DHA concentration in liver oils also tends to be higher than in common body oils from species like herring or menhaden.

A brief history of fish liver oils in medicine 🔗

The medicinal use of fish liver oils goes back thousands of years. Ancient Egyptian medical texts, including the Papyrus Ebers (circa 1500 B.C.), documented the use of animal liver – including fish liver – to treat night blindness. Greek physicians from Hippocrates to Galen also recommended liver consumption for vision disorders. In the Arab world, the 9th-century physician Hunayn Ibn Ishaq prescribed rubbing liver oil directly into the eyes and eating cooked liver to cure night blindness.

In northern Europe, cod liver oil had a long tradition in folk medicine. Scandinavian Vikings are believed to have produced it by suspending birch branches over kettles of boiling water, laying fresh livers on the branches, and collecting the oil that dripped out. By the 1800s, cod liver oil was being bottled and sold as a popular oral medicine. European doctors in the 1820s observed that children given cod liver oil did not develop rickets, and the practice of administering it to prevent bone disease persisted well into the 1950s.

The scientific breakthrough came in the 1920s when vitamin D deficiency was identified as the cause of rickets. Cod liver oil, already proven effective in practice, was then understood to work because of its high vitamin D content. Meanwhile, vitamin A – the compound responsible for curing night blindness – was isolated and named in the late 1920s by Swiss scientist Paul Karrer. The commercial fish liver oil industry thrived until the late 1940s, when the development of inexpensive synthetic vitamins A and D reduced demand for natural sources.

Key fish species used for liver oil production 🔗

Not all fish livers are equal in oil content or vitamin potency. The species chosen for liver oil extraction are selected based on their liver size, oil yield, and vitamin concentration. Fish liver oils are broadly categorized into two types based on these characteristics.

High oil, low vitamin A potency species 🔗

Species like cod (Gadus morhua), coalfish, and haddock fall into this category. Cod is the most widely used species for liver oil production globally. Lean fish like cod store most of their fat in their relatively large livers rather than in their muscle tissue. Cod liver oil typically contains vitamin A at a potency of around 1,000-3,000 I.U. per gram, which is relatively modest compared to some other species. However, the high oil yield from cod livers makes them commercially efficient to process.

Atlantic cod (Gadus morhua) remains the primary species, but Pacific cod (Gadus macrocephalus) and Greenland cod (Gadus ogac) are also used. Norway is one of the leading producers, with fish often processed on the same day they are caught to ensure freshness and quality.

Low oil, high vitamin A potency species 🔗

Species such as halibut (Hippoglossus vulgaris), tuna (Thunnus sp.), and certain sharks produce liver oils with much lower fat content but extraordinarily high vitamin A concentrations. Halibut liver oil can be 50 to 100 times richer in vitamin A than cod liver oil. Tuna liver oil can contain up to 400,000 I.U. per gram of vitamin A, making it one of the most potent natural sources available.

Shark species used in liver oil production include the spiny dogfish (Squalus acanthias), Greenland shark, and basking shark. Shark liver oil is distinct because it contains high levels of alkylglycerols (AKGs) and squalene, compounds with potential immune-boosting and antioxidant properties. A shark’s liver can constitute up to 20% of its total body weight – it functions as a buoyancy organ since sharks lack swim bladders.

Methods of extracting fish liver oil 🔗

The extraction method used depends largely on the characteristics of the liver being processed – specifically, its oil content and vitamin A potency. Fresh, well-preserved livers are essential for producing high-quality oil with minimal free fatty acids and good colour and flavour. The main methods are outlined below.

Direct steaming 🔗

This is the simplest and most traditional method. Fish livers are heated with low-pressure steam at 85-89°C, causing the oil to separate from the liver protein. The released oil is skimmed off, placed in settling tanks, and then centrifuged to remove suspended solids and moisture. Direct steaming works well for species with high oil content like cod, where the oil separates relatively easily from the tissue. The resulting product is generally classified as medicinal-grade liver oil.

However, direct steaming is not suitable for all species. Halibut liver oil, for example, cannot be obtained by simple steaming and requires more advanced methods. The heat involved can also lead to some vitamin degradation – research on dogfish liver oil showed a 30% loss of vitamin A in stored steamed oil over just three weeks.

Alkali digestion 🔗

In this method, minced liver is mixed with a dilute alkali solution – typically 1-2% sodium hydroxide (NaOH) or 2-5% sodium carbonate (Na₂CO₃). The mixture is then heated with live steam at 82-88°C with constant stirring. The alkali breaks down the protein matrix, releasing the tightly bound oil. After digestion, the pulp is centrifuged and the oil collected from the surface. This method achieves a higher recovery of vitamin A compared to direct steaming, making it preferred for livers where the oil is bound more tightly to protein.

Acid digestion 🔗

Acid digestion involves adjusting the pH of the ground liver to approximately 1.5 using hydrochloric acid. The acidified mixture is then cooked under constant stirring, which breaks down cellular structures and releases the oil. Like alkali digestion, this method is used for livers where simple steaming fails to achieve adequate oil recovery.

Enzyme digestion 🔗

This is a more refined approach that uses commercial proteolytic enzymes like papain, alcalase, or pepsin to break down the liver tissue. In a typical process, commercial pepsin (about 0.5% by weight) is mixed with minced liver and digested at 43-49°C. The pH is then raised to around 9.0 using sodium bicarbonate, and the temperature is increased to 80°C. After complete digestion, the oil that has gathered at the top is collected.

Enzyme-assisted extraction offers several advantages over purely chemical methods. It operates under milder conditions, results in better protein-lipid separation, and can produce oil with less damage to sensitive compounds like omega-3 fatty acids. Studies on cobia liver extraction showed that enzyme-assisted methods doubled the oil yield compared to aqueous extraction alone.

Solvent extraction 🔗

This is the most advanced and expensive method, but it yields the highest amount of oil. The minced liver is first autoclaved to rupture cell walls and reduce moisture, then mixed with organic solvents such as ethylene dichloride, diethyl ether, or light petroleum. After extraction, the solvent is recovered through vacuum distillation, leaving behind the purified oil. Solvent extraction is particularly important for livers with low oil content but high vitamin potency, like those of halibut and tuna, where other methods fail to fully release the oil.

Nutritional profile and health benefits 🔗

The primary nutritional value of fish liver oils comes from their vitamins A and D, though they also provide omega-3 fatty acids and other bioactive compounds.

Vitamin A 🔗

Vitamin A in fish liver oil exists mainly as retinyl palmitate – a preformed, readily absorbable form. Unlike plant-based beta-carotene (provitamin A), the retinol in fish liver oil does not need to be converted by the body, making it highly bioavailable. Vitamin A is essential for maintaining normal vision, supporting immune function, ensuring healthy skin and mucous membranes, and aiding in cellular communication. Deficiency of vitamin A leads to night blindness, and in severe cases, xerophthalmia – a condition that can cause irreversible corneal damage.

Vitamin D 🔗

Fish liver oil naturally contains vitamin D3 (cholecalciferol), the form most efficiently used by the human body. Vitamin D plays a critical role in calcium absorption, bone mineralisation, and immune regulation. Cod liver oil gained its historical reputation largely because it prevented and treated rickets – a childhood bone disease caused by vitamin D deficiency. Fish liver oils also contain small amounts of vitamin K, which works synergistically with vitamin D to regulate mineral uptake in bones.

Omega-3 fatty acids 🔗

Both EPA and DHA are present in fish liver oils. These omega-3 fatty acids support cardiovascular health, reduce inflammation, and contribute to brain and retinal development. The DHA concentration in liver oils tends to be higher than in many common fish body oils, adding to their nutritional value.

Squalene and alkylglycerols 🔗

Shark liver oil is uniquely rich in squalene – a triterpene hydrocarbon that was first isolated from shark liver in 1916. Squalene serves as a precursor for cholesterol and steroid biosynthesis in the body and has applications as a vaccine adjuvant. Deep-sea sharks can have livers composed of 40-70% squalene. Commercially, squalene derived from shark liver is used in cosmetics, pharmaceuticals, and dietary supplements. The cosmetics industry alone accounts for the majority of global shark liver oil usage. However, sustainability concerns around deep-sea shark fishing have driven research into plant-based and microbial alternatives for squalene production.

Grades of fish liver oil 🔗

Not all fish liver oils are the same quality. The grade depends on the freshness of the liver and the extraction method used. There are broadly four grades:

Medicinal grade – Produced from fresh, properly preserved livers using gentle methods like direct steaming. This oil is pale, straw-coloured, with a mild flavour and odour, and contains minimal free fatty acids. It is the standard for human consumption.

Veterinary grade – Obtained from residues after initial extraction, often through caustic treatment to destroy remaining protein and release additional oil. This grade may actually have a higher vitamin content than medicinal grade but is less refined in taste and appearance.

Industrial grade – Produced from diseased or discoloured livers, or from livers that were not promptly processed. These oils are darker, more pungent, and higher in free fatty acids. They are used in technical applications rather than for consumption.

Sun-dried or putrefied grade – In traditional methods, livers are placed in earthen pots and exposed to sunlight for several days to undergo decomposition, allowing the oil to separate naturally. This method produces low-quality oil suitable only for non-dietary uses.

Modern applications and market relevance 🔗

Although synthetic vitamins A and D largely replaced fish liver oils in the mid-20th century, there has been a renewed interest in natural liver oil supplements. This resurgence is driven by the recognition that naturally occurring vitamins in their whole-food matrix may offer better absorption and synergistic benefits than isolated synthetic forms.

Cod liver oil remains the most popular supplement globally, valued for its balanced ratio of vitamins A and D along with omega-3s. Halibut liver oil is used where a higher concentration of vitamin A is desired. Shark liver oil has carved out a distinct market niche due to its squalene and alkylglycerol content, with applications in immune support and skin care.

In India, fish liver oils are gaining traction as dietary supplements, especially for addressing vitamin deficiencies in children and adults in underserved communities. The country’s extensive coastline and active fishing industry in states like Kerala and Tamil Nadu support domestic liver oil production.

From a sustainability standpoint, the fishing of deep-sea sharks for liver oil raises conservation concerns. Many targeted species are slow-growing, late-maturing, and vulnerable to overexploitation. Organisations like Oceana have advocated for plant-based squalene alternatives, and companies are increasingly turning to olive oil, sugarcane, and engineered microorganisms as substitute sources.

Precautions and potential risks 🔗

Despite their benefits, fish liver oils must be used with care. Because vitamins A and D are fat-soluble, they accumulate in the body and can reach toxic levels if consumed in excess. Vitamin A toxicity (hypervitaminosis A) can cause symptoms ranging from nausea and headaches to more serious liver damage. A single tablespoon of cod liver oil can contain well over the recommended daily intake of vitamin A for adults, and even a teaspoon may exceed safe limits for young children.

Pregnant women should be particularly cautious, as excessive vitamin A intake has been linked to birth defects. The key is to follow recommended dosages and consult a healthcare professional before starting supplementation, especially for children, pregnant women, or individuals on other vitamin supplements.

What do you think? Given the growing interest in natural supplements, do you see fish liver oils making a significant comeback in everyday nutrition? And with sustainability concerns around shark-derived products, how should the industry balance nutritional demand with marine conservation?

References

1. https://pmc.ncbi.nlm.nih.gov/articles/PMC3340802/
2. https://en.wikipedia.org/wiki/Squalene
3. https://pmc.ncbi.nlm.nih.gov/articles/PMC4348990/
4. https://en.wikipedia.org/wiki/Cod_liver_oil
5. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/fish-liver-oils
6. https://consensus.app/questions/vitamin-a-in-fish-oil/
7. https://www.healthline.com/nutrition/shark-liver-oil
8. https://www.nature.com/articles/129726a0
9. https://www.e-fas.org/archive/view_article?pid=fas-25-2-76
10. https://www.mdpi.com/2073-4344/10/11/1323
11. https://usa.oceana.org/shark-squalene/