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Cod Liver Oil
Cod liver oil is the original fish oil, the bottle that cured rickets a century ago. What almost nobody tells you: it has a ceiling. A teaspoon gives you omega-3, vitamin D, and vitamin A together; two teaspoons pushes the vitamin A toward levels tied to hip fractures and, in pregnancy, to birth defects. So it earns its place as a daily floor-lift for people short on fish and winter sun, not as the heart shield the label implies. Take one teaspoon of a fresh product; don't double it.
Do ยท Daily Evidence Emerging Chapter Supplements

Three things ride in one teaspoon. EPA and DHA are the long-chain omega-3s; they slot into your cell membranes and nudge inflammatory signalling down a notch. Vitamin D is stored in the cod's liver because cod live where there's no sun; in you it runs calcium absorption, muscle, and immune tone. Preformed vitamin A as retinol rides alongside, both the point and the problem: it accumulates in your liver, and that ceiling is the single most important fact about CLO as a daily habit.

The active ingredients work, but modestly, and mostly where a real gap existed. At the dose a teaspoon supplies, roughly a gram of EPA plus DHA, the big cardiovascular trials came back null: VITAL in 25,000 adults 1, ASCEND in 15,000 diabetics 2, and a Cochrane pooling of 86 trials showing little effect on death rates 3. The one clear win used four grams of pure prescription EPA, a drug that isn't what's in the bottle 4.

What it does deliver is real. The vitamin D fraction, around 800 IU, cuts falls in older adults by about a fifth 5, and in coastal Norway it's one of the strongest predictors of adequate winter vitamin D 6. Infants given it in their first year had about a quarter lower risk of type 1 diabetes 7. It gives you sufficiency in three nutrients a lot of people lack, without being a shortcut to a different life.

One teaspoon a day, and the whole trick is not going bigger. The dose is capped by the vitamin A, not by the omega-3.

Three months in, if you started short. Blood vitamin D climbs from the low twenties into the thirties, across the line the falls-prevention data uses 5, and your omega-3 tissue level drifts into the healthier range. You won't feel either directly; these are floor changes that show up in averages over years. What you might notice: one less winter cold, or less joint stiffness on cold mornings if you had inflamed joints to begin with. The fish you skipped that week, made up for.

The fine print โ€” when to skip it, and what people get wrong

Pregnant or trying: switch to a vitamin-A-free fish or algal oil plus separate D. Excess preformed vitamin A around conception is teratogenic, risking heart, face, and brain defects 8. On a blood thinner or with hemochromatosis or fish allergy, ask your clinician first.

Not the same as fish oil: CLO keeps the liver's vitamins, which is why it has a ceiling that refined fish-body oil doesn't. "Fermented" or "raw" CLO isn't better; some traditional-craft bottles arrive heavily oxidised 9.

Vitamin A creeps up when you stack CLO with a retinol multivitamin and liver pรขtรฉ; over years that lifts hip-fracture risk about 50% 10. Rancid oil: most fish-oil products fail freshness testing 9; a sharp paint-like burp means throw the bottle out.

References
  1. 1Manson JE, Cook NR, Lee IM, Christen W, Bassuk SS, Mora S, et al. (2019). Marine n-3 Fatty Acids and Prevention of Cardiovascular Disease and Cancer. New England Journal of Medicine. link
  2. 2ASCEND Study Collaborative Group (Bowman L, Mafham M, Wallendszus K, et al.) (2018). Effects of n-3 Fatty Acid Supplements in Diabetes Mellitus. New England Journal of Medicine. link
  3. 3Abdelhamid AS, Brown TJ, Brainard JS, Biswas P, Thorpe GC, Moore HJ, et al. (2020). Omega-3 fatty acids for the primary and secondary prevention of cardiovascular disease. Cochrane Database of Systematic Reviews. link
  4. 4Bhatt DL, Steg PG, Miller M, Brinton EA, Jacobson TA, Ketchum SB, et al. (2019). Cardiovascular Risk Reduction with Icosapent Ethyl for Hypertriglyceridemia. New England Journal of Medicine. link
  5. 5Bischoff-Ferrari HA, Dawson-Hughes B, Staehelin HB, Orav JE, Stuck AE, Theiler R, et al. (2009). Fall prevention with supplemental and active forms of vitamin D: a meta-analysis of randomised controlled trials. BMJ. link
  6. 6Brustad M, Sandanger T, Aksnes L, Lund E (2004). Vitamin D status in a rural population of northern Norway with high fish liver consumption. Public Health Nutrition. link
  7. 7Stene LC, Joner G (2003). Use of cod liver oil during the first year of life is associated with lower risk of childhood-onset type 1 diabetes: a large, population-based, case-control study. American Journal of Clinical Nutrition. link
  8. 8Penniston KL, Tanumihardjo SA (2006). The acute and chronic toxic effects of vitamin A. American Journal of Clinical Nutrition. link
  9. 9Albert BB, Derraik JGB, Cameron-Smith D, Hofman PL, Tumanov S, Villas-Boas SG, Garg ML, Cutfield WS (2015). Fish oil supplements in New Zealand are highly oxidised and do not meet label content of n-3 PUFA. Scientific Reports. link
  10. 10Feskanich D, Singh V, Willett WC, Colditz GA (2002). Vitamin A intake and hip fractures among postmenopausal women. JAMA. link
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