Cod Liver Oil: Omega-3 + Vitamins A and D in One Spoonful

Cod liver oil has been given to children since the 19th century, originally for rickets. It fell from favour as standalone vitamin D and concentrated fish-oil products became available, then had a minor revival in the COVID-19 era when interest in vitamin D status surged. It has real advantages over separate supplements but also non-trivial risks — particularly around pre-formed vitamin A.

What's in a spoonful

A 5 mL teaspoon of standard cod liver oil typically provides 400–500 mg of EPA+DHA combined, 400–1,000 IU vitamin A as retinol, and 400–1,200 IU vitamin D — depending on brand and concentration. The density is useful: one supplement covers three evidence-based deficiencies common in northern-latitude populations. Brand labels vary widely; read the per-serving values, not the front-of-pack claims.

Vitamin A is the underrated risk

The pre-formed retinol in cod liver oil adds up quickly. A landmark cohort study reported that women consuming >10,000 IU/day of pre-formed vitamin A in early pregnancy had ~5-fold higher risk of cranial-neural-crest birth defects compared with those consuming ≤5,000 IU/day (Rothman 1995, NEJM; PMID 7477116; DOI 10.1056/NEJM199511233332101). The Institute of Medicine subsequently set a Tolerable Upper Intake Level for adults of 3,000 µg RAE/day (~10,000 IU) for pre-formed vitamin A. A teaspoon of standard cod liver oil daily fits within that limit; a tablespoon every day will, for many products, breach it. Pregnant women should pick a vitamin-A-free fish oil plus a separate vitamin D supplement, not cod liver oil.

Omega-3 content vs concentrated fish oil

Per dollar of EPA+DHA, high-quality cod liver oil is competitive with generic fish oil but is less concentrated. For high-dose EPA+DHA goals (e.g. 2–4 g/day for cardiovascular indications), dedicated fish-oil capsules or icosapent ethyl (prescription) deliver more per pill. For general supplementation in northern-latitude winters — modest omega-3, fat-soluble vitamins, traditional palatability — cod liver oil has a reasonable case.

Quality and source

Arctic cod from Norwegian or Icelandic waters is the traditional source and tends to have the best sustainability and contaminant profile. Third-party testing for mercury, PCBs, dioxins, and oxidation (peroxide value, anisidine value, TOTOX) matters — rancid fish oil is pro-inflammatory and unpleasant. Heavily flavoured products (lemon, orange) can mask early rancidity, so prefer transparent brands.

Historical evidence and modern context

The 1930s demonstration that cod liver oil prevented rickets is one of the oldest proven nutritional interventions. Population-level observational data during respiratory-virus seasons (including pandemic influenza work cited by Cannell 2006 in Epidemiology and Infection) suggest plausibly modest infection-rate effects mediated by vitamin D, but randomised trials of cod liver oil specifically for respiratory infection have not produced conclusive benefit. Bottom line: a sensible supplement in vitamin-D-poor latitudes if dosed within the vitamin A UL, but not a magic preventive.