Fish Oil Oxidation and Rancidity: TOTOX, Peroxide Value and How to Tell

The terms

• Peroxide value (PV) reflects early oxidation products; anisidine value (p-AV) reflects later ones; TOTOX is a total calculated from the two. [1][2] Labs measure these with standard assays. [1]
• Voluntary limits. The trade association GOED publishes a voluntary monograph covering oxidative quality, environmental contaminants and EPA and DHA measurement, and requires its members to follow it. [3] Studies cite its limits as PV 5 meq/kg, p-AV 20 and TOTOX 26. [2]
• Other limits. Pharmacopeial and Australian limits cited in one study are less strict: p-AV 30, PV 10 and TOTOX 50. [4] Results depend on which limit is used.

What independent analyses found

Evidence label: mixed. Samples, years, labs and limits differ, and two New Zealand studies disagreed sharply; the authors of one are linked to a trade association and the other had an erratum. We cannot resolve this here.

What predicts a fresher product?

In Canada, capsules without flavor additives had lower secondary and TOTOX levels than bulk oils and flavored products, and children's products were higher on all three markers. [7] In the US sample, flavored products were more often over the limit. [9] In the New Zealand study, best-before date, cost, country of origin and exclusivity were poor markers of quality. [5] In China, price was independent of quality. [10]

Can you tell by smell?

A Chinese study noted a distinct fishy odor in four capsule products and a pronounced rancid odor in one. [10] Another found aroma and taste generally changed in a negative way during storage. [11] How well smell predicts lab results: Proper evidence not found in the sources we read. Flavorings can also mask odor and interfere with the p-AV test. [6]

Does oxidized fish oil harm health?

A 2013 review said oxidized oils may be less effective or harmful, that animal studies show oxidized lipid products can cause harm, and that human trials had not reported the oxidation state of their oil. [1] Evidence of harm in people from typical retail products: Proper evidence not found in the sources we read. That is not proof of safety. If a product smells or tastes strongly off, ask the seller or a pharmacist.

Practical points

Follow the storage directions on the label; see Storing omega-3 supplements. Certification programs are described on Quality testing and certifications. For the rest of the label, see How to read a fish oil label and Fish oil. Fish oil can affect bleeding and interact with medicines; see Omega-3 safety and interactions.