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Providing perspectives on recent research into vitamins and nutritionals


Is Oxidized Fish Oil a Cause For Concern?

By Julia Bird

Ever eaten flaxseed and noticed a fishy taste? One of the compounds that gives fish (and some unexpected foods) their characteristic taste  is the omega-3 polyunsaturated fatty acids in them. The unsaturated double bonds in the molecule are prone to oxidation, and it is this oxidation that can cause an unpleasant smell.

The amount of fishy smell depends on how well the fish oil is purified: the process that takes crude fish oil to the final product used in many applications involves the removal of gums and acids, taking out any compounds that are already oxidized, and removing pesticides and heavy metals, the oil is “winterized” to make sure that it stays clear at room temperature, and the oil is deodorized with steam. Antioxidants such as alpha-tocopherol are added to prevent further oxidation (information from colleague Jaroslav Kralovec).

Dietary supplement and food manufacturers are usually interested in preventing oxidation in their products containing fish oil. The fishy aftertaste is unpleasant for consumers, and a lack of oxidation is a key part of third party testing programs for fish oils, such as the International Fish Oil Standards (IFOS) program.

So how prevalent is oxidation in fish oil supplements? Jackowski looked at supplements in the US and found around 40% of 171 brands did not meet voluntary standards for oxidation. Likewise, of 32 fish oil supplements in New Zealand, around were unacceptable oxidized, according to Albert and colleagues. This same group of researchers has also speculated on the role of oxidized fish oil in human health. On the other hand, the third party testing company Labdoor found that the “majority” of fish oil supplements passed the freshness test and 74% had peroxide levels under the maximum threshold.

Like other dietary supplements, there are some programs available to guide consumers to products that meet quality and safety standards. To be certain that supplements do not contain unacceptable levels of oxidation, consumers can view the test reports on the Labdoor website, or the IFOS program also publishes consumer reports containing the results of analytical tests on their website for 74 brands of fish oil on the North American market.

Even so, the effect of oxidized fish oil on human health has not been well characterized. A clinical study by Ottestad and associates assigned 54 participants to received 8 g per day of non-oxidized or oxidized fish oil, or a sunflower oil placebo, for seven weeks. Oxidation was induced by flushing oxygen through the oil twice a day for 20 minutes for 21 days. The oxygen treatment increased the total oxidation value of the oxidized oil, the amount of volatile oxidative products of omega-3 fatty acids was around 10-20 fold higher than the oil that had not been oxidized. There were no changes in markers of inflammation or oxidative stress in the subjects over 3 or 7 weeks of the study. This study provides some evidence that oxidized fish oil does not affect health over a few weeks of supplementation.

It seems that while there are a considerable number of fish oil supplements that are oxidized available on the market, some third party companies provide publicly available test results to allow consumers to find supplements that are not oxidized. Even so, oxidized fish oil was not found to have a negative effect on health or markers of inflammation in a 7-week clinical trial. I would like to see a longer term study on the safety of oxidized fish oils, however from this shorter study, it seems like there is no cause for concern.



Albert BB, Derraik JG, Cameron-Smith D, Hofman PL, Tumanov S, Villas-Boas SG, Garg ML, Cutfield WS. Fish oil supplements in New Zealand are highly oxidised and do not meet label content of n-3 PUFA. Sci Rep. 2015 Jan 21;5:7928. doi: 10.1038/srep07928.

Albert BB, Cameron-Smith D, Hofman PL, Cutfield WS. Oxidation of marine omega-3 supplements and human health. Biomed Res Int. 2013;2013:464921. doi: 10.1155/2013/464921. Epub 2013 Apr 30.

Jackowski SA, Alvi AZ, Mirajkar A, Imani Z, Gamalevych Y, Shaikh NA, Jackowski G. Oxidation levels of North American over-the-counter n-3 (omega-3) supplements and the influence of supplement formulation and delivery form on evaluating oxidative safety. J Nutr Sci. 2015 Nov 4;4:e30. doi: 10.1017/jns.2015.21. eCollection 2015.

Ottestad I, Vogt G, Retterstøl K, Myhrstad MC, Haugen JE, Nilsson A, Ravn-Haren G, Nordvi B, Brønner KW, Andersen LF, Holven KB, Ulven SM. Oxidised fish oil does not influence established markers of oxidative stress in healthy human subjects: a randomised controlled trial. Br J Nutr. 2012 Jul;108(2):315-26. doi: 10.1017/S0007114511005484. Epub 2011 Dec 5.