Fish Oil Quality: How to Choose a Product That Isn't Rancid
Fish oil is one of the most widely consumed supplements globally and one of the most frequently mislabeled and oxidized. Independent surveys of retail fish oil products in the US, UK, New Zealand, and Australia have repeatedly found that a substantial share of tested products exceed at least one voluntary oxidation threshold — sometimes before the consumer ever opens them. Rancid fish oil doesn't just smell bad; oxidized lipids may attenuate the cardiovascular benefits and could potentially cause harm.
Understanding Oxidation
Omega-3 fatty acids are highly unsaturated, which makes them biologically active and also chemically unstable. Exposure to heat, light, and oxygen triggers lipid peroxidation, measured via peroxide value (PV), anisidine value (AnV), and the combined TOTOX score (2×PV + AnV). The Global Organization for EPA and DHA Omega-3s (GOED) sets voluntary thresholds (PV ≤ 5, AnV ≤ 20, TOTOX ≤ 26). The Albert et al. New Zealand survey published in Scientific Reports (2015) tested 32 retail fish oil products and reported that the majority exceeded at least one of these voluntary oxidation limits.
How to Choose a High-Quality Product
Look for third-party certification. IFOS (International Fish Oil Standards) is the most rigorous fish oil-specific certification, testing for oxidation, potency, contaminants, and heavy metals. IFOS 5-star certified products are the benchmark.
Check the form. Triglyceride (TG) form fish oil is absorbed 70% better than ethyl ester (EE) form, which is the cheap industrial form used in many products. Re-esterified triglyceride (rTG) form is the best absorbed.
Dose reality check. Many products advertise "1,000 mg fish oil" but contain only 300 mg of EPA+DHA. Aim for at least 1,000–2,000 mg of combined EPA+DHA daily from the Supplement Facts panel. Refrigerate after opening and use within 90 days.
Sources
- Albert BB, Derraik JG, Cameron-Smith D, et al. "Fish oil supplements in New Zealand are highly oxidised and do not meet label content of n-3 PUFA." Scientific Reports, 2015;5:7928. DOI: 10.1038/srep07928.
- 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." Journal of Nutritional Science, 2015;4:e30. DOI: 10.1017/jns.2015.21.
- Ghasemifard S, Turchini GM, Sinclair AJ. "Omega-3 long chain fatty acid 'bioavailability': a review of evidence and methodological considerations." Progress in Lipid Research, 2014;56:92–108. DOI: 10.1016/j.plipres.2014.09.001.
- Global Organization for EPA and DHA Omega-3s (GOED). "GOED Voluntary Monograph and oxidation standards for EPA and DHA omega-3 supplements." goedomega3.com, 2024.
Reviewed against 4 peer-reviewed and standards sources.