Supplement Guide | Fish Oil 04 | The Quality Problem: Oxidation, Label Gaps, and What Testing Reveals

This is part 4 of SuppCo’s four-part series on fish oil, covering its sources, health effects, quality considerations, and other key topics consumers should understand.

To read part 1 in the series, click here.

To read part 2 in the series, click here.

To read part 3 in the series, click here.

Fish oil is one of the most popular supplements on the market. It is also one of the most fragile. The same omega-3 fats that make fish oil beneficial are highly unstable. Over time, they can break down, changing the quality of the oil inside the capsule.

This creates a basic problem for consumers. 

The nutrients people want from fish oil, EPA and DHA, are also the nutrients most likely to degrade before the product is ever used.

Why oxidation is such a problem for fish oil

EPA and DHA are long-chain polyunsaturated fatty acids. This structure makes them biologically active, but it also makes them sensitive to damage. Heat, light, oxygen, and even trace metals can trigger oxidation.

Oxidation happens in stages. First, unstable compounds called lipid peroxides form. Over time, these break down into secondary compounds like aldehydes and ketones. These later compounds are what give rancid oils their unpleasant smell and taste.

More importantly, oxidation changes the oil itself. Once this happens, the product no longer matches the fish oil used in most clinical studies. Even if the label still lists EPA and DHA, the oil may not behave the same way in the body.

Oxidation is not rare or unusual. It is a predictable risk in a supply chain that includes fishing, oil extraction, global shipping, heat-based refining, encapsulation, and long storage times.

What testing studies have found

Multiple independent studies have tested fish oil supplements sold in stores around the world. The results are consistent and concerning.

• In Canada, researchers tested 171 fish oil supplements and found that half exceeded voluntary limits for at least one oxidation marker.

• In the United States, about one quarter of tested products had peroxide levels more than twice recommended limits.

• In New Zealand, more than 80 percent of tested products exceeded recommended oxidation thresholds.

Oxidation is typically assessed using two standardized laboratory measures. Peroxide value reflects early, primary oxidation products, while anisidine value reflects secondary aldehydic compounds. 

Together, they are combined into a total oxidation, or TOTOX, value. Trade organizations such as GOED recommend maximum limits of PV ≤ 5 mEq/kg, AV ≤ 20, and TOTOX ≤ 26. Many retail products exceed one or more of these thresholds.

Why labels do not tell the full story

One of the biggest challenges for consumers is that oxidation data rarely appears on supplement labels. Most products do not disclose peroxide value, anisidine value, or TOTOX score. They also do not indicate how old the oil is or how it was processed.

Two fish oil products can list the same EPA and DHA amounts, yet differ greatly in freshness and quality. From the label alone, there is no reliable way to tell the difference.

This is partly due to how fish oil is produced. Most fish oil is treated as a global commodity. It is extracted in one location, stored in large tanks, shipped across countries, and refined through multiple heating steps. Only a small portion of global fish oil production is intended for supplements, and early processing steps are not designed with consumer products in mind.

By the time the oil reaches a finished capsule, much of its history is invisible to the person buying it.

What quality testing actually helps answer

Independent testing does not guarantee a perfect product, but it answers questions that labels cannot.

In the context of fish oil, testing can help determine:

• Whether oxidation levels stay below accepted safety limits

• Whether the EPA and DHA amounts still match what the label claims

• Whether added antioxidants are actually protecting the oil over time

Without this information, consumers are left guessing. Brand reputation and marketing language do not reliably predict oxidation status.

Do oxidized fish oils affect health?

This is where the science becomes less clear. While oxidation in fish oil supplements is well documented, the long-term health effects of consuming oxidized omega-3s are not fully understood.

The long-term health effects of consuming oxidized omega-3s are not fully understood.

Regulatory bodies such as the European Food Safety Authority have stated that data on oxidation-related health effects in humans is limited. An additional concern raised by researchers is that some past omega-3 clinical trials may have unknowingly used oxidized oils, which could influence study results.

Product quality may not just affect consumers, it may also affect the evidence base itself.

Why fish oil is different from most supplements

All supplements benefit from quality testing, but fish oil stands apart. Small differences in handling and storage can lead to large differences in chemical quality, without any visible signs on the label.

This does not mean fish oil supplements are inherently bad. It means that freshness, handling, and verification matter more here than in many other categories.

For fish oil, trust should be based on data, not assumptions.