Is cod liver oil healthy even though it contains environmental toxins?

Cod liver oil comes from fish liver and therefore contains environmental toxins. Is cod liver oil or capsules with fish oil still a healthy way to get enough vitamin D?

Experts disagree.

Recommends highly purified vitamin D supplements

Senior physician Anne-Lise Bjørke-Monsen at Førde Central Hospital does not recommend cod liver oil, even though it also contains omega-3 fatty acids, which are often linked to positive health effects.

“What you need in the winter months is vitamin D. I would recommend taking it as a purified vitamin supplement rather than taking cod liver oil,” says Bjørke-Monsen.

She points out that cod liver oil contains environmental pollutants, and she is sceptical about whether we need the extra omega-3 fatty acids.

In the summer, the body produces vitamin D when our skin is exposed to the sun. But from October to April, many in Norway need vitamin D supplements, according to Helsenorge.no, the Norwegian national health portal.

Few sources of vitamin D are found in food. Helsenorge.no states that the most important food sources are fatty fish, cod liver oil, and vitami- D-enriched milk and margarine.

The Norwegian health authorities emphasise taking cod liver oil.

Environmental pollutants exceed tolerance limit by up to 332 per cent

Bjørke-Monsen has previously co-authored an article in the Journal of the Norwegian Medical Association about environmental pollutants in cod liver oil.

The researchers point out that the recommended amount of cod liver oil for children and adults is five millilitres a day. That's a large teaspoon or a child's spoon. Children therefore ingest more environmental pollutants relative to body weight than adults.

The researchers calculated that a one-year-old child would ingest between 43 and 332 per cent of the tolerable weekly intake of these toxins, depending on how much contamination the cod liver oil contains.

“For young children, cod liver oil is a significant source of environmental pollutants,” says Bjørke-Monsen.

We ingest twice the recommended amount of environmental pollutants

The particular environmental pollutants in question are dioxins and dioxin-like PCBs, called dl-PCBs.

Food is the main source, especially fish and shellfish, followed by dairy products and meat.

“Norwegians’ intake is above the tolerable weekly intake,” says Helle Katrine Knutsen, a toxicologist at the Norwegian Institute of Public Health.

Norwegians are therefore consuming too much – actually twice as much as they should.

Cod liver oil accounts for around one quarter

On average, marine oils are the source of 5 per cent of the weekly intake of this type of environmental toxin, Knutsen explains. In 4-year-olds, it is 7.6 per cent and in 2-year-olds, 11 per cent.

The amount of cod liver oil taken by the average Norwegian does not make much of a difference, since their intake is already over the recommended limit.

But not everyone takes cod liver oil. How much does cod liver oil add to the contaminant intake level for those who do?

If you are an adult and follow the dosage on the bottle, you will get about a quarter of the recommended maximum weekly intake of dioxins and dl-PCBs from cod liver oil, says Knutsen – assuming average contamination levels.

For a two-year-old, cod liver oil contributes as much as the rest of the child’s diet.

Cod liver oil therefore exposes us to environmental toxins even though it is purified, Knutsen summarises.

Cod liver oil is a Norwegian tradition

Community medicine expert Haakon Eduard Meyer is sceptical about discouraging the use of cod liver oil.

Vitamin D is produced when UVB rays from the sun hit the skin. But not in winter.

“In the middle of winter there’s no production, even on a bright sunny day, because the UVB rays are filtered out in the atmosphere,” he says. “So we can eat food that contains vitamin D. But only a few foods are rich sources of vitamin D. Fatty fish is the traditional good source, followed by products that are fortified with vitamin D.”

It’s a good idea to take supplements if you tend to eat too little of these foods, according to Meyer.

Meyer is a professor at the University of Oslo's Department of Community Medicine and Global Health and a senior medical officer at the Norwegian Institute of Public Health.

“When it comes to vitamin D, a multivitamin tablet and cod liver oil accomplish the same thing. You don't have to take cod liver oil, but it’s a tradition in the Norwegian diet. And the fact that people are used to taking it has value,” he says.

If people stop taking cod liver oil, the question is whether they would replace it with another source of vitamin D, says the professor.

Sceptical about being too strict

"I’m sceptical about being too strict when it comes to cod liver oil," says Meyer. “We know that a good number of people get too little vitamin D, and I think it’s wise to get some of the long-chain omega-3 fatty acids as well. Then we have a product that provides both.”

But you should not take more cod liver oil than what is stated on the bottle, he says.

Quite a lot of Norwegians take cod liver oil.

“We need to have good arguments if we say that people shouldn’t take it. I don't think we have that. There's a lot of focus on environmental pollutants, and this is being monitored,” says Meyer.

So is cod liver oil really that dangerous? What do these environmental toxins actually do to us?

“It’s the next generation that's affected”

Dioxins and dl-PCBs are persistent, fat-soluble organic toxins that accumulate in the food chain.

You don't notice the symptoms yourself if you ingest too much of these substances, says toxicologist Knutsen.

The main problem lies in what the mother has ingested throughout her life, according to Knutsen.

“It’s the next generation that's affected,” she says.

Foetuses and baby boys who are exposed to dioxins and dl-PCBs may have reduced sperm quality, explains Knutsen.

Tooth enamel in children may be affected, as can the level of thyroid hormones in newborns, according to VKM (link in Norwegian).

Health authorities nevertheless recommend fish oil to pregnant women. If pregnant women eat fatty fish less than two or three times a week, they should be advised to take vitamin D supplements in the form of cod liver oil or another supplement, the Norwegian Directorate of Health states on its website (link in Norwegian).

“Yes, cod liver oil can be used as a source of vitamin D and omega-3 fatty acids for pregnant women who do not eat enough fatty fish,” Thomas Berg at the Norwegian Directorate of Health tells Science Norway.

You can read the directorate's entire response about environmental toxins in cod liver oil in the fact box further down.

Recommends avoiding certain types

Fish oils that are not purified can have very high levels of fat-soluble organic environmental toxins, says Bente Nilsen at the Institute of Marine Research.

Sometimes researchers discover fish oils with illegally high levels of pollutants, she says. In those cases, the Norwegian Food Safety Authority will have the manufacturer withdraw the product from the market.

According to Nilsen, well-known brands are generally safe. But she recommends avoiding certain types of oils in which the highest levels of toxins have repeatedly been found – especially oils made from the liver of the rabbit fish, also called rat fish.

“We have also seen that many of the marine oils with the highest levels of pollutants are cold-pressed oils, which may be labelled 'extra virgin' or 'handcrafted'," she says.

Möller’s analyses show low and stable levels

Tone Hagerup-Lyngvær works with quality assurance at Orkla Health, which produces Möller's Tran, a brand of fish oil.

“In Möller's Tran, these substances are purified in several steps to ensure very low levels. The purification process is continuously monitored and verified through regular tests at independent, accredited labs,” she says.

The levels are well below both the EU's limit values and the GOED industry standard, according to Hagerup-Lyngvær.

“The figures in the Journal of the Norwegian Medical Association article show variations in cod liver oil in general based on historical analyses, including products with much higher values than what is found in Möller’s Tran,” she writes in an email to Science Norway.

“Möller’s own analyses show low and stable levels, and the exposure from our products at the recommended dose is well within the tolerable limits defined by EFSA,” she adds.

Hagerup-Lyngvær points out that the product Min Første Tran (my first cod liver oil) has had among the lowest measured levels in its category in independent studies.

“Analyses conducted for Möller’s over time show that we are in the lower range of the tolerable limit for children for all cod liver oil types. Norwegian marine products generally have lower levels than the European average, and Möller’s Tran does not contribute to consumers exceeding the TWI (tolerable weekly intake),” Hagerup-Lyngvær writes.

She points out that vitamin D supplements and cod liver oil cover different needs, since cod liver oil also contains the omega-3 fatty acids EPA and DHA.

Stock photo at the top of the article: Shutterstock / NTB.

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Translated by Ingrid P. Nuse

Read the Norwegian version of this article on forskning.no

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