Omega-3 fatty acids are valuable precisely because of the chemistry that makes them fragile. EPA and DHA carry five and six double bonds respectively, and every one of those bonds is a site where oxygen can attack. The result of that attack is rancidity — first invisible, then smellable — and an oil that has gone rancid has not merely lost its benefit; it now delivers lipid peroxides and reactive aldehydes instead. The certificate of analysis for a marine oil carries three numbers that track this process. Learning to read them is the single most useful skill a buyer of omega-3 can acquire.
The chemistry of rancidity in three steps
Lipid oxidation proceeds through a chain reaction. Initiation: heat, light or trace metals abstract a hydrogen atom from a carbon next to a double bond, creating a lipid radical. Propagation: the radical reacts with oxygen to form a peroxyl radical, which steals a hydrogen from a neighbouring fatty acid — creating a lipid hydroperoxide and a new radical, so the process feeds itself. Termination and decomposition: hydroperoxides are unstable and break down into secondary products — aldehydes (hexanal, malondialdehyde, 4-hydroxynonenal), ketones and short-chain acids — which are what you smell as “fishy” or “paint-like.” Primary oxidation (hydroperoxides) comes first and is odourless. Secondary oxidation (aldehydes) comes later and is not. An oil can therefore be significantly oxidised before the nose detects anything — which is why a number is needed.
The three numbers
Peroxide value (PV) measures primary oxidation: the concentration of hydroperoxides, expressed in milliequivalents of active oxygen per kilogram of oil (meq/kg). Fresh, well-refined oil is typically below 1–2 meq/kg. Anisidine value (AV) measures secondary oxidation — principally the aldehydes — and is dimensionless. Because hydroperoxides decompose into aldehydes, an oil can have a low PV and a high AV if it was oxidised some time ago: the peroxides have already broken down. TOTOX (total oxidation) combines the two as 2 × PV + AV, giving a single figure that captures both recent and historical oxidation. The internationally used limits, set by the Global Organization for EPA and DHA Omega-3s voluntary monograph and mirrored in the Codex standard for fish oils (CXS 329-2017) and the European Pharmacopoeia fish-oil monographs, are PV ≤ 5 meq/kg, AV ≤ 20, TOTOX ≤ 26. These are maximums for a product at release; a high-quality oil will typically show PV around 1–3 and TOTOX well under 15.
What the shelf surveys found
A New Zealand study of 32 retail fish-oil products reported that 83% exceeded the recommended PV limit, 25% exceeded AV and 50% exceeded TOTOX; on average products contained only 68% of the labelled EPA+DHA (Albert et al. 2015). A later, independently funded re-analysis of the New Zealand market using a different analytical approach found much lower oxidation and better label compliance, and argued that the earlier method — applied to flavoured and emulsified products — had overestimated oxidation (Bannenberg et al. 2017). The disagreement is itself instructive: the test method must be validated for the product matrix, and flavourings and antioxidants can interfere. Across the Atlantic, a North American survey of 171 products found that 50% exceeded at least one of the voluntary limits for PV, AV or TOTOX, with flavoured oils and some capsule types performing worst (Jackowski et al. 2015). A review of the health implications concluded that while oxidised fish oil clearly loses efficacy, the direct human harms of consuming it remain insufficiently studied — an uncomfortable gap given how much oxidised oil is apparently on sale (Albert et al. 2013).
Why it matters beyond “it doesn’t work”
Oxidised omega-3s are not inert. Lipid hydroperoxides and aldehydes are pro-inflammatory and cytotoxic in laboratory systems; malondialdehyde and 4-hydroxynonenal are the same molecules measured as biomarkers of oxidative stress in patients. Animal studies report adverse effects of oxidised oil on lipid profiles and inflammation that fresh oil does not produce. Clinically, the large omega-3 outcome trials that failed to show benefit have prompted the question of whether the oxidative state of the intervention oil was ever controlled — a question most of them cannot answer. For the individual taking a daily dose for cardiovascular or cognitive reasons, the conclusion is simple: a rancid oil is a different substance from the one the evidence base studied.
Reading the certificate — and protecting the oil after purchase
On a certificate of analysis (how to read one), look for PV, AV and TOTOX as three separate lines with a method (usually AOCS Cd 8b-90 or Ph. Eur. 2.5.5 for PV; AOCS Cd 18-90 for AV), a specification limit, and a result for your lot. If only PV is reported, ask for AV — low PV alone can hide an older oxidation event. Check that the certificate date is recent relative to your purchase and that the lot number matches your bottle (batch traceability). Then hold up your end: oils protected by added tocopherols or astaxanthin, bottled in amber glass under nitrogen, oxidise far more slowly (glass vs plastic) — but every opening admits oxygen, so refrigerate after opening, keep the cap tight, and use within the period stated. Heat is the enemy at every stage (how heat destroys potency). Finally, trust your nose as a last line: a strong fishy or paint smell means secondary oxidation is advanced, whatever the label says.
The clinical bottom line
Peroxide value measures fresh oxidation, anisidine value measures older oxidation, and TOTOX combines them; the recognised limits are PV ≤ 5, AV ≤ 20, TOTOX ≤ 26, and a good oil sits well under all three. Shelf surveys find between a quarter and four-fifths of retail omega-3 products outside at least one limit, depending on market and method. An oil that has oxidised is no longer the substance the clinical trials studied — so the three numbers on the certificate, for your lot, are the difference between taking omega-3 and taking its breakdown products.
Primary sources
- Albert BB, Derraik JGB, Cameron-Smith D, et al. Fish oil supplements in New Zealand are highly oxidised and do not meet label content of n-3 PUFA. Sci Rep. 2015;5:7928. doi:10.1038/srep07928
- Bannenberg G, Mallon C, Edwards H, et al. Omega-3 long-chain polyunsaturated fatty acid content and oxidation state of fish oil supplements in New Zealand. Sci Rep. 2017;7:1488. doi:10.1038/s41598-017-01470-4
- Jackowski SA, Alvi AZ, Mirajkar A, et al. 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;4:e30. doi:10.1017/jns.2015.21
- 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
- Codex Alimentarius Commission. Standard for fish oils. CXS 329-2017. fao.org
- Global Organization for EPA and DHA Omega-3s. GOED Voluntary Monograph (oxidation limits: PV, AV, TOTOX). goedomega3.com
Educational information on supplement quality and consumer protection, not medical advice.
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