Cod Liver Oil

Why It Matters for Longevity

Cod liver oil is unique: it is the only culinary oil that provides substantial vitamin D3 alongside long-chain omega-3 fatty acids (EPA and DHA) and preformed vitamin A (retinol) in a naturally co-occurring ratio. This multi-nutrient combination addresses several simultaneous longevity targets.

The vitamin D case is strongest. A meta-analysis of 25 RCTs (11,321 participants) by Martineau et al. (2017, BMJ) found that vitamin D supplementation significantly reduced acute respiratory tract infection risk -- with the greatest protection in individuals who were deficient at baseline. Given pandemic-scale vitamin D insufficiency in populations with limited sun exposure, cod liver oil's 340% DV per tablespoon makes it one of the most efficient dietary solutions.

For omega-3, the cardiovascular evidence is comprehensive. A meta-analysis of 40 randomised controlled trials (Bernasconi et al., 2021, Mayo Clin Proc) found that omega-3 supplementation significantly reduced major cardiovascular events, with EPA and DHA from marine sources providing the strongest effect. Cod liver oil provides EPA and DHA in their triglyceride form, which is better absorbed than ethyl ester supplements.

Lentjes et al. (2015, J Hum Nutr Diet) characterised the nutrient contribution of cod liver oil users in the EPIC-Norfolk cohort, confirming that regular use significantly improves 25(OH)D status and contributes meaningful EPA/DHA to total omega-3 intake, particularly in populations with limited oily fish consumption.

Omega-3 Form and Bioavailability

Not all omega-3 supplements deliver equivalent amounts of EPA and DHA into circulation. The molecular form -- triglyceride, ethyl ester, or free fatty acid -- affects the rate and completeness of absorption. Dyerberg et al. (2010, Prostaglandins Leukot Essent Fatty Acids) compared four concentrated omega-3 preparations in a double-blind study of 72 volunteers receiving approximately 3.3 g EPA+DHA daily for two weeks. Relative to natural fish oil (set at 100%), re-esterified triglycerides showed 124% bioavailability, while ethyl esters showed only 73%. Cod liver oil provides omega-3s in natural triglyceride form -- similar in absorption to re-esterified triglyceride preparations and substantially superior to the ethyl ester form used in many pharmaceutical-grade supplements such as Lovaza. This difference matters clinically: patients taking ethyl ester supplements with low-fat meals may achieve plasma EPA+DHA levels roughly 30% lower than those taking triglyceride-form sources under the same conditions.

The practical implication is that taking cod liver oil with a meal containing fat (as the Longevity Diet recommends) optimizes pancreatic lipase activity and bile acid secretion, both required for efficient triglyceride hydrolysis and fatty acid micellar packaging in the small intestine.

Vitamin D: Raising 25(OH)D Status

The biologically active form of vitamin D in circulation is 25-hydroxyvitamin D [25(OH)D], and most epidemiological evidence for vitamin D health benefits is anchored to serum 25(OH)D concentrations. Cod liver oil delivers vitamin D3 (cholecalciferol), which raises 25(OH)D approximately 1.7–2.0× more effectively per IU than vitamin D2 (ergocalciferol).

A randomised controlled trial by McCourt et al. (2023, Br J Nutr) in 63 adults aged 50+ examined 25(OH)D response to 20 µg (800 IU) vitamin D3 daily delivered via different lipid matrices over four weeks. All three active interventions (olive oil dairy drink, coconut oil dairy drink, and supplement capsule) produced significant 25(OH)D increases in vitamin D-insufficient participants, confirming that lipid co-delivery improves absorption. Cod liver oil's intrinsic lipid matrix -- predominantly long-chain triglycerides -- replicates this co-delivery mechanism in a single food source.

For individuals at northern latitudes (above 40°N) during autumn and winter, when dermal synthesis effectively ceases, a daily tablespoon of cod liver oil (providing approximately 10–15 µg vitamin D3) can prevent the seasonal drop in 25(OH)D that otherwise occurs. Habitual cod liver oil users in northern European cohorts maintain 25(OH)D concentrations roughly 10–20 nmol/L higher than non-users of comparable sun exposure.

Vitamin A: Preformed Retinol and the Dose Ceiling

Each tablespoon of cod liver oil provides approximately 4,500 IU (1,350 mcg RAE) of preformed retinol. Unlike beta-carotene from plant sources, which requires conversion to retinol and is self-limiting in dose, preformed retinol from cod liver oil is absorbed with ~75% efficiency and accumulates in the liver. The tolerable upper intake level (UL) for adults is 10,000 IU/day (3,000 mcg RAE). One tablespoon thus occupies 45% of the UL, leaving comfortable room for retinol from other animal foods (liver, eggs, dairy). Consuming multiple tablespoons daily or combining with high-dose retinol supplements approaches the UL and warrants monitoring.

Within the safe range, preformed retinol from cod liver oil supports epithelial barrier integrity (via RAR-β nuclear receptor signaling), immune cell differentiation, and visual transduction in rod photoreceptors. The simultaneous presence of vitamin D in cod liver oil is relevant because vitamin D and vitamin A share retinoid X receptor (RXR) heterodimers for nuclear signaling -- they interact at the transcriptional level, and deficiency in one can amplify the effective toxicity threshold of the other.

How to Use It

Take 1 tablespoon (15 mL) daily, ideally with food containing fat for optimal absorption of vitamins A and D. Lemon juice or orange juice cut the fishy taste effectively. Refrigerate after opening. Vitamin A content (4,500 IU per tablespoon) warrants monitoring at higher doses -- the tolerable upper limit is 10,000 IU/day, so one tablespoon is well within range.

What to Pair It With

Flavor Profile

Fishy, rich, and slightly bitter. Premium brands use molecular distillation to remove oxidation products -- these taste milder. Lemon-flavoured varieties are significantly more palatable. The oil is pale yellow and viscous.

The Science

• Martineau et al., 2017, BMJ: Meta-analysis of 25 RCTs (11,321 participants) -- vitamin D supplementation significantly reduces acute respiratory infection risk; greatest benefit in deficient individuals.
• Bernasconi et al., 2021, Mayo Clin Proc: Meta-analysis of 40 RCTs -- marine omega-3 supplementation (EPA + DHA) significantly reduces major cardiovascular events; triglyceride-form omega-3s in cod liver oil are well absorbed.
• Lentjes et al., 2015, J Hum Nutr Diet: EPIC-Norfolk cohort -- regular cod liver oil use significantly improves 25(OH)D status and contributes meaningful EPA/DHA, particularly in populations with limited oily fish intake.
• Dyerberg et al., 2010, Prostaglandins Leukot Essent Fatty Acids: In 72 volunteers receiving ~3.3 g EPA+DHA daily for 2 weeks, re-esterified triglycerides showed 124% bioavailability versus 73% for ethyl esters; natural fish oil (cod liver oil triglyceride form) is substantially superior to ethyl ester supplements.
• McCourt et al., 2023, Br J Nutr: RCT (n=63, aged 50+) -- 20 µg vitamin D3 daily delivered in lipid matrices produced significant 25(OH)D increases in vitamin D-insufficient adults over 4 weeks; confirms that lipid co-delivery (as in cod liver oil) optimises vitamin D3 absorption.

References

1. 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. PMID: 28202713. doi:10.1136/bmj.i6583
2. Bernasconi AA, Wiest MM, Lavie CJ, Milani RV, Laukkanen JA. Effect of Omega-3 Dosage on Cardiovascular Outcomes: An Updated Meta-Analysis and Meta-Regression of Interventional Trials. Mayo Clin Proc. 2021;96(2):304-313. PMID: 32951855. doi:10.1016/j.mayocp.2020.08.034
3. Lentjes MA, Welch AA, Keogh RH, Luben RN, Khaw KT. Contribution of cod liver oil-related nutrients (vitamins A, D, E and eicosapentaenoic acid and docosahexaenoic acid) to daily nutrient intake in the EPIC-Norfolk cohort. J Hum Nutr Diet. 2015;28(6):613-623. PMID: 25228113. doi:10.1111/jhn.12259
4. Dyerberg J, Madsen P, Møller JM, Aardestrup I, Schmidt EB. Bioavailability of marine n-3 fatty acid formulations. Prostaglandins Leukot Essent Fatty Acids. 2010;83(3):137-141. PMID: 20638827. doi:10.1016/j.plefa.2010.06.007
5. McCourt AF, Mulrooney SL, O'Neill GJ, O'Riordan ED, O'Sullivan AM. Serum 25-hydroxyvitamin D response to vitamin D supplementation using different lipid delivery systems in middle-aged and older adults: a randomised controlled trial. Br J Nutr. 2023;130(1):94-102. PMID: 36912075. doi:10.1017/S0007114523000636

Key Nutrients