Fish Oil Quality: Oxidation, Contamination, and How to Choose

Fish oil is one of the most widely purchased supplements globally, driven by substantial evidence for omega-3 fatty acids (EPA and DHA) in cardiovascular health, inflammation reduction, and brain function. What is less widely understood is that fish oil is chemically unstable — it oxidises readily from exposure to light, heat, and air — and that a significant proportion of fish oil products on retail shelves are measurably oxidised. An oxidised fish oil may not just be less effective; some research suggests it may be actively pro-inflammatory.

The short answer

Fish oil goes rancid. The chemical markers of oxidation — peroxide value (PV), anisidine value (AV), and the combined TOTOX score — should be below specific limits to indicate fresh, high-quality oil. A 2015 Norwegian study found that 83% of retail fish oil products exceeded at least one of these limits. Oxidised fish oil smells and tastes distinctly fishy or rancid — but opaque capsules conceal this. Buy from brands with published IFOS certification or third-party oxidation testing. Store in the fridge. Use within 90 days of opening. Mercury and PCB contamination is effectively addressed by molecular distillation in quality products — this is less of a concern than oxidation for well-sourced brands.

The oxidation problem

Omega-3 fatty acids (EPA and DHA) have multiple double bonds in their chemical structure. These double bonds are why they are biologically active and anti-inflammatory — but they are also why they react readily with oxygen. This oxidation process:

• Degrades EPA and DHA into rancid byproducts (aldehydes, ketones, alcohols)
• Reduces the effective omega-3 content below what the label states
• May produce compounds that are themselves pro-inflammatory or cytotoxic in cell studies
• Produces the characteristic fishy, rancid smell that indicates poor quality

Oxidation begins when fish are caught and accelerates during processing, storage, and after opening. Heat, light, and oxygen exposure all speed the process.

The oxidation markers to know

• Peroxide value (PV): Measures primary oxidation products. GOED (Global Organisation for EPA and DHA Omega-3s) recommends ≤5 meq/kg. IFOS recommends ≤5 meq/kg.
• Anisidine value (AV): Measures secondary oxidation products (the ones that form after peroxides break down — often more toxic). GOED recommends ≤20.
• TOTOX score: Combined measure (2×PV + AV). GOED recommends ≤26. This is the most comprehensive single oxidation indicator.

The 2015 Norwegian study that tested 171 retail fish oil products found 83% exceeded at least one of these limits. A more recent (2021) US study found similar rates of oxidation in retail products.

Contamination: mercury, PCBs, and dioxins

Marine fish bioaccumulate persistent organic pollutants (PCBs, dioxins) and mercury from their environment. This is a real concern for dietary fish consumption and for fish oil derived from large, long-lived fish. However, quality fish oil manufacturing addresses this effectively:

• Molecular distillation: A purification process that uses high temperature under vacuum to separate omega-3 fatty acids from contaminants. Molecular distillation effectively removes mercury, PCBs, and dioxins to very low levels. Most reputable fish oil brands use this process.
• Small fish sourcing: Fish oil derived from small, short-lived, low-trophic fish (anchovies, sardines, mackerel) starts with much lower contamination levels than oil from tuna or shark, because these species do not bioaccumulate contaminants to the same degree.

If a fish oil product has IFOS 5-star certification or equivalent independent testing, its contamination levels are verified. Contamination is a much lower concern than oxidation for brands with documented purification processes.

Triglyceride vs ethyl ester form

Fish oil comes in two main forms:

• Triglyceride (TG) form: The natural form of omega-3s as they occur in fish. Better absorbed (approximately 70% higher bioavailability in some studies compared to ethyl ester). More stable against oxidation.
• Ethyl ester (EE) form: The most common form in supplements — cheaper to produce because it is an intermediate step in omega-3 concentration. Lower bioavailability than triglyceride form. Used in most standard fish oil capsules.
• Re-esterified triglyceride (rTG) form: Ethyl ester that has been converted back to triglyceride form — high concentration, high bioavailability. Used in premium products.

For the best bang for your money on omega-3s, look for "triglyceride form" or "rTG" on the label.

IFOS certification

The International Fish Oil Standards (IFOS) program, run by Nutrasource Diagnostics, independently tests fish oil for oxidation markers (PV, AV, TOTOX), contaminants (mercury, PCBs, dioxins), and omega-3 content accuracy. Products that pass earn an IFOS 5-star rating — the most meaningful quality certification specific to fish oil. Batch-level results are published on the IFOS website, so you can verify the specific product lot you are buying.

How to buy and store fish oil correctly

1. Buy IFOS 5-star certified or equivalent. Nordic Naturals, Carlson, Thorne, and a few others consistently earn high IFOS ratings.
2. Smell it before buying if possible. Fresh fish oil in triglyceride form has a mild, clean oceanic smell. Rancid fish oil smells strongly fishy or like paint/cardboard. If capsules smell rancid when you cut one open, discard the bottle.
3. Check the expiry date. Buy with maximum shelf life remaining.
4. Store in the fridge immediately on opening. Cold temperature significantly slows oxidation. Dark glass or opaque bottles are better than clear plastic.
5. Use within 90 days of opening. Even well-stored fish oil degrades over time once opened.
6. Choose small fish sourcing. Anchovy, sardine, or mackerel sourced products start cleaner.

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