The species and tissue source — cod liver versus cod body
Per FAO 2023 (State of World Fisheries) and standard ichthyology references, Atlantic cod (Gadus morhua) is a demersal predatory fish distributed through the North Atlantic, primarily in the Barents Sea, Iceland, Norwegian, North Sea, Celtic Sea, and Northwest Atlantic. Cod is a major commercial whitefish species, harvested primarily for human food. The lean white muscle tissue is the primary product; the liver is a substantial co-product. Cod liver represents 5–10 percent of total body weight in mature fish and is exceptionally lipid-rich (up to 50–70 percent oil by fresh weight), supporting cod liver oil as an efficient industrial extraction.
The historical context for cod liver oil predates modern aquaculture nutrition. Per ATA (American Thyroid Association) and broader medical history, cod liver oil was used as a folk and pharmaceutical remedy for "weakness," rickets, and various pediatric conditions in 19th and early 20th century Europe and North America, before the discovery of vitamins A and D in the 1910s-1930s. The therapeutic effect — correction of vitamin A and D deficiency — was established once these vitamins were chemically identified. Cod liver oil thus enters modern pet-food formulation with both a marine omega-3 role (a 20th century recognition) and a fat-soluble vitamin role (the original 19th century recognition).
The composition — EPA + DHA plus concentrated vitamin A and D
Per Bauer 2007/2008/2011 (JAVMA) canine omega-3 reviews and standard fish-oil references, cod liver oil’s EPA + DHA composition is typically 10–15 percent of total fatty acids, with EPA usually slightly higher than DHA. The remaining fatty acid composition includes substantial monounsaturated fatty acids (oleic acid, palmitoleic acid, cetoleic acid), saturated fatty acids (palmitic acid, myristic acid), and minor amounts of other long-chain omega-3 species including DPA (docosapentaenoic acid). The EPA + DHA density is slightly lower than body-oil fish oils because liver tissue accumulates a broader fatty acid profile than the muscle and adipose tissue that supplies body-oil fish oils.
The distinguishing feature is fat-soluble vitamin content. Per FDA 21 CFR 184 (vitamin A GRAS status) and standard United States Pharmacopeia (USP) cod liver oil monograph specifications, cod liver oil contains approximately 50,000–100,000 IU vitamin A per 100g (predominantly as retinol and retinyl esters) and 1,000–2,500 IU vitamin D per 100g (predominantly as cholecalciferol, vitamin D3). These concentrations are 10-100 times higher per gram than typical body-oil fish oils and 100-1000 times higher per gram than most other pet-food ingredients. The biochemical reason: fish liver concentrates fat-soluble vitamins as part of normal hepatic storage physiology, in a manner analogous to mammalian liver vitamin A storage.
AAFCO-complete formulation and the Safe Upper Limit
Per AAFCO 2024 Official Publication Dog Food Nutrient Profiles, the minimum vitamin A requirement for dog food is 5,000 IU/kg dry matter (growth and maintenance) and the Safe Upper Limit (SUL) is 250,000 IU/kg DM. The minimum vitamin D requirement is 500 IU/kg DM and the SUL is 3,000 IU/kg DM. Per the AAFCO formulation framework, cumulative vitamin A and D intake from all ingredient sources plus the vitamin premix must fall within the minimum-to-SUL window. Cod liver oil at typical pet-food inclusion (1–3 percent) contributes 500–3,000 IU vitamin A/kg diet and 10–75 IU vitamin D/kg diet on top of the premix contribution. AAFCO-compliant formulators account for this by reducing the synthetic vitamin A and D premix addition when cod liver oil is included as an ingredient.
Per Hall 1996 (Vet Med) hypervitaminosis A review and English 1971 (Vet Rec) feline hypervitaminosis A case series, the concern with vitamin A excess is cumulative liver storage with potential bone, hepatic, and skin pathology. The concern is more acute in cats than dogs because cats are particularly sensitive to vitamin A excess (they have limited capacity to convert excess vitamin A to inactive metabolites for excretion). Hypervitaminosis A in dogs has been documented mainly in scenarios of long-term over-supplementation rather than AAFCO-formulated complete-and-balanced diet consumption. The practical pet-owner consideration: cod liver oil added as a supplement on top of a complete-and-balanced commercial diet can in principle exceed the AAFCO SUL if dosing is high and prolonged. Veterinary guidance for additional fish-oil supplementation typically defaults to body-oil fish oils (salmon oil, krill oil) rather than cod liver oil for this reason.
Sustainability — Atlantic cod stock status
Per ICES 2024 (International Council for the Exploration of the Sea) annual cod stock assessments and FAO 2023 (State of World Fisheries) sustainability reviews, Atlantic cod stocks have mixed status. The Barents Sea cod stock (Norway-Russia joint management) is one of the largest cod stocks globally and is at sustainable biomass levels per recent assessments. The Iceland cod stock is also at sustainable levels and is MSC-certified. The North Sea cod stock has a history of overfishing through the late 20th century with subsequent recovery efforts; current biomass is below historic peaks but within management plan reference points. The Northwest Atlantic cod stocks (Newfoundland Grand Banks) collapsed in the 1990s and have not fully recovered; harvest is restricted.
Cod liver oil sustainability therefore depends on the source fishery. Cod liver oil sourced from MSC-certified Barents Sea or Iceland fisheries has a defensible sustainability story; cod liver oil from unspecified Atlantic sources should be evaluated case-by-case. The KibbleIQ rubric does not currently differentiate scoring based on fishery certification because supply-chain documentation is rarely transparent to consumers; this is a trust-signal consideration rather than a rubric-defined penalty.
How KibbleIQ scores cod liver oil
The KibbleIQ Dry Kibble Rubric awards marine omega-3 credit for cod liver oil at the same tier as salmon oil, herring oil, sardine oil, anchovy oil, and krill oil. The credit applies when the fish-oil ingredient appears with sufficient inclusion to deliver meaningful EPA + DHA. The vitamin A and D contributions are not double-credited because AAFCO-complete formulation already accounts for them; in an AAFCO-complete diet, the cod liver oil’s vitamin A and D contribution is offset by a reduction in the synthetic premix vitamin A and D, leaving the final diet within the AAFCO minimum-to-SUL window. The rubric does not penalize cod liver oil because AAFCO-formulated complete diets account for cumulative vitamin A safety.
To check whether your dog’s food carries marine omega-3 ingredients, paste the ingredient list into the KibbleIQ analyzer. For peer marine oil context, see our omega-3 fatty acids explainer, salmon oil explainer, herring oil explainer, sardine oil explainer, anchovy oil explainer, krill oil explainer, and algae oil explainer. For broader fat-soluble vitamin context, see our vitamin E forms explainer, vitamin K menaquinones explainer, and our KibbleIQ methodology page.