It’s well-known that herring is a rich source of the healthy marine omega-3 fatty acids. Scientists at Nofima have now discovered that another fat component present in herring also promotes health.

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Herring has a naturally high content of cetoleic acid, which is a fatty acid whose importance we haven’t previously been aware of. We now know, however, that it has properties that promote health. Senior scientist Bente Ruyter and her colleagues at the food research institute Nofima have carried out experiments showing that cetoleic acid stimulates cells to convert short omega-3 fatty acids into the healthy, longer marine omega-3 fatty acids.

The experiments used human liver cells and liver cells from salmon. Both of these showed that pure cetoleic acid stimulates increased formation of the healthy, longer marine omega-3 fatty acids. The results have been confirmed in salmon that have been given feed with herring oil that had a high content of cetoleic acid. Fish oil is an extremely important ingredient in the feed given to farmed salmon, but it is not the dominating one.

Cetoleic acid added to feed causes the salmon to store 10% more marine omega fatty acids (EPA and DHA) in the body than they do otherwise.

“It’s too early to say how large an effect cetoleic acid has in humans, but the experiments in cells in culture suggest that it influences how much healthy, marine omega fatty acids we store after eating herring,” says Bente Ruyter.

Other rich sources of cetoleic acid are sand eels and capelin, in contrast to oily fish from South America, such as sardines. The latter have very low levels of cetoleic acid, while having much higher levels of EPA and DHA.

The results of the feeding experiments show that salmon fed with herring oil, which is rich in cetoleic acid, acquire higher deposits of EPA and DHA in the body than fish fed with South American sardine oil.

Further, the scientists saw lower levels of the condition known as “fatty liver” in farmed fish that had been given herring oil in the feed.

“Salmon that had been given herring oil had less fat in the liver, which suggests that they had a higher level of fat metabolism. This is good for the salmon,” says Ruyter.

The research has been financed by the Norwegian Seafood Research Fund (FHF).

Fat from fish

  • The content of omega- 3 in salmon fillet is 1.5%, while it is 4.3% in cured herring. (The level differs somewhat between different types of herring.)
  • This means that 130 grams of salmon or around 50 grams of cured herring are sufficient to provide the recommended weekly dose of omega-3. (This calculation is based on the recommendation that we need 250 mg/day, EFSA.)

Food composition database
European Food Safety Authority

Experimental diets

In the cell-based experiments, pure cetoleic acid and the EPA/DHA precursor 18:3 n-3 were added to liver cells in culture.

In the feed experiments carried out on salmon, two feeds were supplemented with two different levels of sardine oil with relatively low levels of cetoleic acid, approximately 1% of the total fatty acid content. Two other feeds were supplemented with two different levels of herring oil with high levels of cetoleic acid, approximately 11% and 15% of the total fatty acid content, respectively.  A fortified oil was used to balance the feeds such that the feeds with different levels of sardine oil and herring oil contained approximately the same levels of EPA and DHA.

The two feeds with sardine oil contained 7.5% and 9.8%, respectively, of the total fatty acids in the form of EPA + DHA, while those with herring oil were slightly lower at 7.2 and 9.1% EPA+DHA.

The salmon were given the various diets for 67 days.

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