Effect of heat treatment on the n-3/n-6 ratio and content of polyunsaturated fatty acids in fish tissues

Abstract: The aim of this study was to compare the effect of different heat treatments (pan-frying, oven-baking, and grilling) on the contents of polyunsaturated fatty acids (PUFAs) in fish tissue. Four fish species were examined: pike, carp, cod, and herring. High performance liquid chromatography, coupled with electrospray ionization and mass spectrometric detection (HPLC/ESI/MS), was employed for determination of intact lipid molecules containing n-3 and n-6 PUFAs. Although mostly non-polar lipids (triacylglycerols, TGs) were present in the fish tissue, the PUFAs were present preferentially in the phospholipid fraction. Omnivorous fish species (carp, herring) contained more TGs than did predatory ones (pike, cod). Higher amounts of PUFAs were detected in the marine species than in the freshwater ones. The impact of heat treatments on the lipid composition in the fish tissue seems to be species-specific, as indicated by multivariate data analysis. Herring tissue is most heat-stable, and the mildest heat treatment for PUFA preservation was oven-baking.


Alex’s Notes: Our good friend Adel Moussa from Suppversity has written several excellent articles on the effects of cooking various forms of fat. Of these, two should be mandatory reading for all Super Humans. The first looks at the effects of cooking on the often claimed “superior” animal-based fats rich in saturated fat and cholesterol, while the second looks at the effects of cooking on unsaturated vegetable-based fats. What these articles make blatantly clear is that there are consequences to cooking all forms of fats and fatty foods, but that some methods are superior to others when it comes to minimizing the detrimental effects of cooking.

With that in mind, many of us eat fish on a consistent basis for the long-chained omega-3 fatty acids, EPA & DHA. However, with the exception of an occasional sushi binge, I would bet that most of us cook the fish before eating it. At least, that’s what people living in the Czech Republic do, and thus the researchers of the study at hand wanted to examine how cooking affected the polyunsaturated fatty acid (PUFA) composition of various fish species. As a bonus, they also wanted to examine how the habitat and feeding status of the fish affected the baseline fatty acid profile.




Pure Carnivore






As we can see in the table above, four fish from the North Atlantic region with different ecological demands were bought from a local fish market where the fish had been freshly killed and placed on ice. The carnivorous fish stand on a higher level of the food chain and feed on other fish while the omnivorous fish feed primarily on primary sources of PUFA such as algae but also small crustaceans.

Raw Differences

It is important to acknowledge that the fatty acid profiles are strongly influenced by environmental factors such as season, locality, life phase, and sex, so what follows is only a snapshot of one possible set-up. That said, the marine fish contained significantly more omega-3s than the freshwater fish (as a percent of total fat composition), with all species except Carp having the majority be DHA. The freshwater fish also contained significantly more omega-6 fatty acids (linoleic acid) than the marine species, which contained virtually none.

But percentages can be misleading when we are more interested in the absolute amount we would receive from eating the fish. While the current study doesn’t tell us what the amount of fat in the fish is, a quick search on the USDA nutrient database tells us that Atlantic cod and pike are very lean (1g fat per 100g fish) compared to herring (9g/100g) and carp (6g/100g). Thus, the carnivorous fish are not going to be a great source of omega-3s regardless of this discussion. With that in mind, it is important to point out that herring has an n-3:n-6 ratio of 34.5:1 while carp’s ratio is 5.3:1.

As would be expected from the fattiness of the fish, the herring and carp showed significantly greater amounts of triglyceride-bound PUFAs, whereas the lean fish had mainly phospholipid-bound PUFAs. Nevertheless, all the fish except carp had pretty much their entire EPA/DHA content stored in muscle-bound phospholipids. The difference in triglycerides comes from the storage of other fats, primarily myristic acid (14:0), palmitic acid (16:0), palmitoleic acid (16:1), and linoleic acid (18:1).

Effects of cooking

The researchers used the three most common cooking methods for fish:

  1. Frying in extra virgin olive oil in a non-stick (Teflon) pan for 5 min,
  2. Baking at 200 °C for 10–15 min in an electric oven without addition of fat, and
  3. Grilling on a contact electric grill for 2–5 min without addition of fat

Overall, baking had the lowest impact on PUFA levels while pan-frying had the greatest. The effects were most pronounced in the cod, but given that this fish has so little fat to begin with it likely isn’t a concern. Conversely, the carp demonstrated the greatest heat-stability with all cooking treatments, while the herring was very heat-stable during baking and had all things go to hell when pan-fried and grilled. Similar results have been obtained for sardines as well, where baking and grilling were less harmful than frying.

Since the pike and cod have too little fat to make the changes from cooking relevant, I’m going to focus on the herring and carp. Baking had virtually no impact on the n-3 content of herring compared to raw, whereas the total percentage of PUFA in the carp actually increased. Grilling and frying, on the other hand, reduced the overall PUFA content of both fish. The changes can be accounted for by three potential mechanisms: water loss in the food, leaching of triglycerides from the food, and/or oxidation.

Bottom line

If you are going to eat a fatty fish, it appears that baking at 200°C (392°F) is the best choice when compared to pan-frying or grilling. Additionally, we can take comfort in the fact that most of the n-3 content is phospholipid bound and thus more thermally stable.


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