Cod Liver Oil: What This Traditional Remedy Can Really Do

Many people remember taking cod liver oil as children. Today, the supplement is making a comeback. But which of its health benefits are actually backed by science, and where do the claims go beyond the evidence?

For many people, cod liver oil brings back childhood memories: the spoonful with that distinct fishy taste, given by parents or grandparents in winter to ward off a possible vitamin D deficiency. For a long time, it was regarded as a somewhat old-fashioned home remedy. Today it is making a comeback, largely thanks to its content of omega-3 fatty acids as well as vitamins A and D.

Unlike conventional fish oil, cod liver oil delivers not only omega-3 fatty acids but also significant amounts of fat-soluble vitamins. That is precisely what makes it interesting — but it also introduces potential risks. So which of its health effects are actually supported by science?

Cod Liver Oil vs. Fish Oil: What Is the Difference?

Cod liver oil and fish oil are often used interchangeably, yet they differ in their composition. Both supply the omega-3 fatty acids EPA and DHA, which are important for the heart and brain, among other things. Cod liver oil, however, is extracted from the liver of fish — usually cod — and also contains notable amounts of vitamin A and vitamin D.

Fish oil, by contrast, is derived mainly from the fatty tissue of fish and generally provides little or none of these vitamins. It is the high vitamin A content in particular that sets cod liver oil apart as a dietary supplement: it can offer health benefits, but at persistently high doses it also raises the risk of an oversupply.

Heart Health: The Best Data Is for Triglycerides

When it comes to heart health, research offers its most compelling results in the area of blood lipid levels. Elevated triglycerides in particular appear to be positively influenced by cod liver oil.

A placebo-controlled study, published in the journal Cureus, examined 600 people with elevated cholesterol levels. Participants received 415 milligrams of cod liver oil daily alongside the cholesterol-lowering drug rosuvastatin. Compared with the control group, their blood lipid levels improved to a greater degree.

A second study by the same research group, involving 870 people with elevated cardiovascular risk, observed numerically fewer heart attacks among those taking additional cod liver oil. However, the difference did not reach statistical significance, meaning that no confirmed protective effect can be drawn from this finding.

The available data therefore suggest that cod liver oil may support an existing cholesterol-lowering therapy. As a standalone measure for reducing cholesterol, however, the evidence is considerably more limited.

Inflammation: Good Indications of Lower CRP Levels

There is also relatively solid data on anti-inflammatory effects. The Norwegian NEEDED study (North Sea Race Endurance Exercise Study) is cited particularly often in this context.

In this study, CRP levels — an important marker of inflammation — were measured in more than 1,000 recreational athletes before and after a 91-kilometer cycling race. People who regularly took cod liver oil had lower CRP levels both at rest and after exertion compared with non-users.

The study points to an association between regular cod liver oil consumption and lower inflammation markers. Whether cod liver oil alone is responsible for this effect, however, cannot be established with certainty from an observational study.

Bone Health: The Studies Contradict Each Other

When it comes to bone health, advertising claims and the research evidence are particularly far apart.

A Norwegian study involving more than 3,000 women between the ages of 50 and 70 produced a surprising finding: women who had not received cod liver oil in childhood actually had slightly higher bone density. As a possible explanation, the researchers pointed to the high retinol content of cod liver oil. High amounts of vitamin A have long been suspected of adversely affecting bone metabolism.

A later Icelandic study with nearly 4,800 participants, however, could not confirm this association. No evidence of reduced hip bone density was found — regardless of whether cod liver oil had been taken during childhood or adulthood.

Overall, the current evidence is insufficient either to recommend cod liver oil for the protection of bones or to clearly establish a harmful effect.

Eye Health: Small Studies Raise Hopes, Large Studies Temper Them

Cod liver oil has been debated in relation to eye health for years. While small studies raised hopes, larger investigations reached considerably more sobering conclusions.

Smaller pilot studies suggested that high-dose omega-3 fatty acids might improve visual acuity in dry AMD. However, those studies examined omega-3 supplements — not cod liver oil specifically.

The considerably larger AREDS2 study conducted by the US National Eye Institute, on the other hand, found no benefit of omega-3 fatty acids for slowing the progression of the disease. A Cochrane review involving more than 2,300 participants reached a similar conclusion.

That said, vitamin A is essential for retinal function. However, this does not mean that taking additional amounts will improve eye health in people who already have an adequate supply.

What to Keep in Mind When Taking Cod Liver Oil

Commercially available products typically recommend around five milliliters per day. The appropriate amount and duration of use should be discussed with a doctor on an individual basis.

Since cod liver oil contains not only omega-3 fatty acids but also vitamins A and D, more is not automatically better. Persistently excessive intake of vitamin A can lead to health problems. Anyone already taking other supplements that contain vitamin A or vitamin D should be mindful of the risk of a double dose.

People who take blood-thinning medications should also speak with their doctor before starting cod liver oil, as omega-3 fatty acids can affect blood clotting.

Conclusion

Cod liver oil has earned its good reputation — at least in part. There are meaningful indications of benefit for elevated triglycerides and inflammation markers in particular. However, many other claims — for example regarding bones or eyes — are not well supported by the current body of research. Those who take cod liver oil should also bear in mind that, in addition to omega-3 fatty acids, the supplement contains significant amounts of vitamin A, and it should therefore not be taken indefinitely.