Omega-3 is among the most extensively researched supplements in the world. The evidence for EPA and DHA in cardiovascular health, inflammation, and cognitive function is substantive and consistent across decades of study. This is one of the supplements where the science genuinely supports supplementation for a large proportion of people — particularly those with low oily fish consumption.
The problem is not whether omega-3 is worth taking. The problem is that a meaningful proportion of omega-3 products on the market are rancid — oxidised to a degree that not only neutralises the health benefits but may actively work against them. And the label will not tell you.
Fish oil oxidises. That’s the central issue.
Omega-3 fatty acids are polyunsaturated — their chemical structure includes multiple double bonds that are chemically reactive. This reactivity is part of why they’re biologically useful, but it also makes them vulnerable to oxidation when exposed to heat, light, and oxygen. When fish oil oxidises, it produces a range of compounds including aldehydes and lipid peroxides — compounds that, in sufficient quantities, carry their own health concerns rather than benefits.
Oxidation begins at the point of manufacture and continues through processing, packaging, shipping, retail storage, and sitting in your kitchen. Most fish oil products are not tested for oxidation at point of sale. When they are, the results are often concerning.
“Several independent analyses of fish oil products on retail shelves have found that a substantial proportion exceed recommended oxidation thresholds — sometimes by a significant margin. The label gives no indication of this.”
A 2015 study published in Nature Scientific Reports tested 47 New Zealand fish oil supplements and found that over 80% exceeded at least one oxidation marker threshold. The products appeared normal on the label. Several smelled rancid when opened.
The smell test — and why most people miss it
Fresh, high-quality fish oil has a mild, clean smell. Slightly fishy, but not offensive. Oxidised fish oil smells significantly worse — rancid, sharp, unpleasant in a way that’s difficult to mistake once you know what you’re looking for.
The catch: many manufacturers add flavouring (lemon, orange, mint) specifically to mask the smell of oxidised oil. A fish oil capsule that smells pleasantly of citrus is telling you nothing about its oxidation status — only that the manufacturer anticipated you might smell it and added flavouring accordingly.
The better test: bite into or puncture a softgel and smell the oil directly. Fresh oil is mild. Oxidised oil will be obviously unpleasant regardless of flavouring. This takes five seconds and is far more informative than any front-of-pack claim.
Triglyceride vs ethyl ester form — why it matters
Fish oil supplements come in two primary forms: natural triglyceride (rTG) and ethyl ester (EE). This distinction is rarely disclosed clearly on labels, and it affects both absorption and oxidation stability.
Ethyl ester is the most common form in supplements. It’s produced during molecular distillation — a purification process that concentrates EPA and DHA by converting the natural triglycerides into ethyl esters. This is cheaper and allows higher EPA/DHA concentrations. However, ethyl esters are not a form found in nature, and research consistently shows lower bioavailability (roughly 50–70%) compared to the triglyceride form. Ethyl esters are also inherently less stable and more prone to oxidation.
Re-esterified triglyceride (rTG) is produced by taking ethyl esters and converting them back to triglyceride form. The result more closely resembles the form found in oily fish, has superior bioavailability (roughly equal to eating fish), and better oxidation stability. It costs more to produce.
Labels rarely state “ethyl ester” explicitly — because it doesn’t read well for marketing purposes. Indicators that a product is in triglyceride form include: the brand explicitly stating “triglyceride form,” “rTG,” or “natural triglyceride form” on the label or website, and the product not having a flavouring added to mask the smell.
A product that simply says “concentrated EPA/DHA” with a high combined omega-3 percentage and no form disclosure is almost certainly in ethyl ester form. If a product was in the more expensive triglyceride form, the manufacturer would almost certainly say so.
Reading the numbers that matter
The key figures on an omega-3 label are EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid) in milligrams per serving. Total “fish oil” milligrams is not the relevant number — a 1000mg fish oil capsule might contain only 180mg EPA and 120mg DHA, with the rest being other fats with no particular omega-3 benefit.
Research on cardiovascular and inflammatory outcomes typically uses 1000–3000mg of combined EPA+DHA per day. Most standard fish oil capsules provide 300–500mg combined per capsule, meaning the “recommended” one-capsule serving often falls well short of the doses used in research. Check the EPA and DHA figures specifically, not the total fish oil figure.
What to look for — and what to avoid
The storage question
Even a high-quality fish oil product can become oxidised through poor storage. Fish oil should be stored in a cool, dark place — ideally refrigerated after opening. Heat and light are the primary accelerants of oxidation. A product left on a sunny kitchen counter or in a warm cupboard for weeks is oxidising faster than the same product kept cold. This is a consumer responsibility, not a labelling failure — but it’s worth knowing.
Open your current fish oil. Smell it directly — bite through the softgel if it’s a capsule. If it smells sharp, rancid, or unpleasant despite flavouring, throw it away. Check the label for EPA and DHA figures — not total fish oil. Look for triglyceride form disclosure. If none is present, assume ethyl ester. And if the price seems very low for a high EPA/DHA claim, that combination is almost never sustainable with a premium-form product.
Is your omega-3 actually working — or working against you?
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