Cod Liver Oil: Omega-3 with Vitamins A and D, and Caution

Long before anyone knew what vitamin D was, grandmothers across Europe forced children to swallow a spoonful of a yellowish, foul-smelling liquid every morning. That liquid was cod liver oil, and it saved entire generations from the disease rickets (rickets is a bone disease caused by vitamin D deficiency). This is one of the oldest and most documented natural remedies in history, and as early as the 19th century, it was considered a home miracle cure for strengthening children.

But cod liver oil is not just another Omega-3 supplement. It is unique because it delivers three components at once: Omega-3 fatty acids (EPA and DHA), vitamin D, and a high dose of vitamin A. This combination is both its greatest advantage and its greatest risk. In this article, we will explain exactly what is in it, what the research actually shows, why vitamin A makes it a supplement that requires caution, and why we rated it yellow and not green like fish oil and algae oil.

What is Cod Liver Oil?

Cod liver oil is extracted, as its name suggests, from the liver of cod fish and related fish. Unlike regular fish oil, which is extracted from the fish's flesh, liver oil concentrates the fat-soluble vitamins that the fish stores in its liver. Here is what is important to understand about it:

• It is a source of active Omega-3. Like fish oil, it provides the fatty acids EPA and DHA in their ready-to-use form, which are essential for heart, brain, and eye health.
• It naturally contains vitamin D. This is one of the richest dietary sources of vitamin D, and this is the historical reason why it successfully fought rickets.
• It contains a very high dose of vitamin A. One tablespoon of cod liver oil can provide more than four times the recommended daily intake of vitamin A. This is the component that distinguishes it from any other Omega-3 supplement, for better and for worse.
• It also contains vitamin E. In smaller amounts, usually as an antioxidant that preserves the oil.

In other words, cod liver oil is not just Omega-3, but a combined package of Omega-3 plus two fat-soluble vitamins (A and D). And anyone considering taking it must understand that they are also taking these vitamins, not just the fatty acids.

The Connection to Health: A Triple Mechanism

To understand why cod liver oil is interesting, one must understand what each of its three components does in the body.

Omega-3 (EPA and DHA) acts on the heart, brain, and inflammation. DHA is a structural building block of nerve cell and retinal membranes, essential for brain and vision function. EPA is a precursor to inflammation-balancing molecules and is linked to cardiovascular health and triglyceride levels. Together, they help regulate the chronic inflammation associated with aging, sometimes called inflammaging.

Vitamin D is a hormone in every sense. It is essential for calcium absorption and bone health (hence the historical victory over rickets), but it also affects the immune system, muscles, and many other processes. Vitamin D deficiency is very common, especially in populations with little sun exposure, and cod liver oil historically provided this deficiency through diet.

Vitamin A is essential for vision, skin, and immunity, but it is also the problematic component. Vitamin A is necessary for night vision, skin health, and immune system function. But unlike vitamin D and Omega-3, vitamin A is fat-soluble and accumulates in the liver. The body does not get rid of it easily, so a high amount over time builds a store that can reach toxic levels. This is precisely the fundamental difference between cod liver oil and regular fish oil, and the core of our caution regarding it.

Current Evidence

Study 1: Cod Liver Oil in Adolescence and Risk of Multiple Sclerosis, Cortese et al. 2015

One of the most interesting and high-quality studies on cod liver oil examined its connection to an autoimmune disease. In 2015, Cortese and colleagues published in the Multiple Sclerosis Journal an analysis from the Norwegian EnvIMS study, a case-control study that included 953 multiple sclerosis patients and 1717 controls, who reported cod liver oil use from childhood to adulthood.

The result was clear: Taking cod liver oil between the ages of 13 and 18 was associated with a reduction of about one-third in the risk of multiple sclerosis (odds ratio 0.67). Interestingly, the association was significant mainly for winter use and not in early childhood. The researchers interpreted this as support for the hypothesis that vitamin D deficiency is a risk factor for multiple sclerosis, and that adolescence is a particularly sensitive period. This is an example of how the vitamin component in liver oil, not just the Omega-3, can carry real health benefits.

Study 2: Cod Liver Oil Consumption and Dietary Contribution, EPIC-Norfolk Cohort

Cod liver oil is the most common supplement in the UK, and a large cohort study examined what it actually contributes to the diet. Researchers from the EPIC-Norfolk cohort calculated the intake of vitamins A, D, and E, and EPA and DHA, among cod liver oil users, and examined the association with blood levels.

The finding: The accompanying components of cod liver oil provided about 15 to 33 percent of the total daily intake, nearly doubling their median intake. In other words, those who take cod liver oil receive a significant contribution of vitamin A and vitamin D from it, not just Omega-3. This illustrates exactly why it is important to think of this supplement as a combined source, and why careless combination with additional vitamin A sources can accumulate to a dangerous amount.

Study 3: The Historical Victory Over Rickets

The oldest evidence for the benefit of cod liver oil is also the most convincing. In the 18th and 19th centuries, cod liver oil was used to treat and prevent rickets, a bone disease caused by vitamin D deficiency, long before science understood why it worked. After it was proven in the 1920s that vitamin D deficiency is the cause of rickets, health authorities in many countries recommended a daily teaspoon of cod liver oil during the autumn and winter months, as a means of protection against rickets and vitamin D deficiency.

This is powerful historical evidence that the vitamin D in liver oil really works and prevents a real disease. But it is important to remember that today we have cleaner and more targeted ways to get vitamin D, and this is one of the reasons cod liver oil is no longer necessarily the first choice for someone who only wants vitamin D or only Omega-3.

What About Regular Fish Oil and Algae Oil?

This is the central question: if cod liver oil provides three components in one dose, why not always choose it? The answer is that precisely the "bonus" of vitamins makes it more complex. Regular fish oil and algae oil provide clean Omega-3, without the load of vitamin A, and are therefore easier and safer for long-term use at high doses.

The distinction is simple. If your goal is only Omega-3, or you are already taking a multivitamin or a separate vitamin A or D supplement, regular fish oil or algae oil are the cleaner and safer choice. In such a situation, cod liver oil would simply add vitamin A and D that you may already be getting, and could accumulate to an excessive amount. Conversely, if you specifically want the combined package, and you can ensure you are not taking an additional vitamin A source, cod liver oil can be an effective and convenient solution, especially in winter and for those with little sun exposure. Algae oil, by the way, is the only one of the three suitable for vegans, as it is derived from algae and not fish.

Should You Start Taking Cod Liver Oil?

We rated cod liver oil Yellow, and not green like fish oil and algae oil. The reason is not that it doesn't work—it certainly does—but that it requires real caution due to vitamin A. Here are the most important considerations:

• Vitamin A accumulates, and this is the main risk. Unlike vitamin D and Omega-3, vitamin A is fat-soluble and stored in the liver. A high dose over time can cause vitamin A excess (hypervitaminosis A), a condition that manifests as headaches, bone pain, skin changes, and in severe cases, liver damage.
• During pregnancy, this is critical caution. Excess vitamin A during pregnancy is linked to birth defects (teratogenic effect). For this reason, pregnant women or those planning pregnancy should avoid cod liver oil and vitamin A supplements, unless a doctor instructs otherwise. This is one of the most prominent situations where regular fish oil or dedicated pregnancy DHA, without vitamin A, are clearly preferable.
• It must not be combined with a multivitamin or an additional vitamin A supplement. This is the point most easily missed. Someone already taking a multivitamin (most of which contain vitamin A) or a separate vitamin A supplement, and adding cod liver oil, may unknowingly cross the safe threshold of vitamin A. Always check labels and calculate the total amount.
• Mild blood-thinning effect. Like any Omega-3 source, cod liver oil slightly slows clotting. Those taking anticoagulant medications like warfarin or aspirin, or about to undergo surgery, should consult a doctor.

The bottom line: Cod liver oil is an effective supplement but not for everyone. It is mainly suitable for those who intentionally want the combination of Omega-3 with vitamin D and A, who are not taking another vitamin A source, and who are not pregnant. For anyone who only wants Omega-3, regular fish oil or algae oil are simply cleaner and safer.

What to Take Away from the Research?

1. If you only want Omega-3, choose regular fish oil or algae oil. They provide the same EPA and DHA without the load of vitamin A, and are therefore safer for the long term.
2. If you do take cod liver oil, count your vitamin A. Check if you are already getting vitamin A from a multivitamin or another supplement, and do not combine without calculating the total amount.
3. During pregnancy or when planning pregnancy, avoid it. Excess vitamin A is dangerous for the fetus. Prefer dedicated pregnancy DHA without vitamin A, and in any case consult a doctor.
4. Do not assume more is better. Precisely because of the accumulation of vitamin A, a moderate and consistent dose is far preferable to high doses. A single teaspoon per day in winter is the historical recommendation, not several servings.
5. On anticoagulant medications or before surgery, consult a doctor. The effect on clotting is mild but present.

For those who want to consider the option, you can purchase cod liver oil on iHerb in a variety of dosages, but remember to check the amount of vitamin A on the label. To check which supplements are suitable for your health goals, including heart health, according to your age and condition, and to see our honest rating for each, you can use our personal supplement checker. And for those interested only in clean Omega-3, we also recommend reading our guides on fish oil and algae oil.

The Broader Perspective

Cod liver oil is an excellent reminder of the principle we hold: A supplement can be both effective and dangerous at the same time, and everything depends on the dose, the context, and what you are already taking. The same combined package that made it a miracle cure against rickets a hundred years ago is precisely what makes it a supplement that needs to be taken with open eyes today, when many of us already take a multivitamin that contains vitamin A.

The practical lesson is twofold. First, Omega-3 and vitamin D are important components for heart, brain, and bone health, and cod liver oil does provide both, but not for free—together with vitamin A that accumulates. Second, the right choice is not "the most components in one dose," but the clean component that exactly fits your need. For those who only want Omega-3, fish oil or algae oil will do the job safely. For those who intentionally want the full package and know how to calculate vitamin A, cod liver oil is a legitimate choice. And that is precisely our angle: not to promise that a supplement is magic, but to honestly explain when it works, when it is dangerous, and for whom it is truly suitable.

References:
Cortese M. et al., Timing of use of cod liver oil, a vitamin D source, and multiple sclerosis risk: The EnvIMS study, Multiple Sclerosis Journal, 2015;21(14):1856-1864 (DOI: 10.1177/1352458515578770)
Lentjes MAH et al., Contribution of cod liver oil-related nutrients (vitamins A, D, E and EPA and DHA) to daily nutrient intake and their associations with plasma concentrations in the EPIC-Norfolk cohort, Journal of Human Nutrition and Dietetics, 2014;28(6):568-582 (DOI: 10.1111/jhn.12271)