Eighty-three percent of fish oil supplements tested in New Zealand exceeded recommended oxidation limits. Only 8% met all international standards for freshness. More than two-thirds contained less than 67% of the EPA and DHA claimed on the label—eicosapentaenoic acid and docosahexaenoic acid, the two omega-3 fatty acids that give fish oil its purported health benefits—and some products contained as little as one-third.
These findings, published in Scientific Reports by researchers at the University of Auckland, examined every encapsulated fish oil supplement available in the New Zealand market. The products were purchased from retail stores, stored under optimal conditions, and tested well within their best-before dates. Cost provided no protection: expensive supplements were as oxidized as cheap ones. Country of origin made no difference. Even products requiring naturopathic consultation showed excess oxidation.
The study identified a telling correlation: the more EPA and DHA missing from a product, the higher its markers of oxidative damage. The omega-3s hadn’t vanished—they had degraded into something else entirely.
To understand why this happens, and why it matters, requires understanding what polyunsaturated fats actually are and what happens when they break down.
The Chemistry That Determines Everything
Fats are chains of carbon atoms with hydrogen atoms attached. What distinguishes one fat from another is the structure of that carbon chain—specifically, whether it contains double bonds.
Saturated fats have no double bonds. Every carbon atom holds as many hydrogen atoms as possible. The chain is stable, resistant to oxidation, and solid at room temperature. Think butter, tallow, coconut oil.
Monounsaturated fats have one double bond. This single point of unsaturation makes them slightly less stable but still relatively resistant to oxidation. Olive oil is the familiar example.
Polyunsaturated fats have multiple double bonds. Each double bond is a site where oxygen can attack the molecule. The more double bonds, the more vulnerable the fat is to oxidation—and the faster it degrades when exposed to heat, light, or oxygen.
This is not a minor technical detail. It is the central fact that determines whether a fat remains beneficial or becomes harmful.
Linoleic acid, the omega-6 fat that dominates industrial seed oils like soybean, corn, canola, and sunflower oil, has two double bonds. These two points of vulnerability make linoleic acid prone to oxidation during processing, cooking, and storage. When it oxidizes, it fragments into lipid peroxides, aldehydes, and other reactive compounds that damage cell membranes, denature proteins, and create oxidative stress throughout the body.
This is why health-conscious people have learned to avoid seed oils. The double bonds that make polyunsaturated fats biologically active also make them biologically dangerous when they degrade.
Now consider fish oil.
EPA, one of the two main omega-3 fatty acids in fish oil, has five double bonds.
DHA, the other, has six double bonds.
If two double bonds make linoleic acid unstable enough to warrant removing seed oils from your kitchen, what happens when you subject fats with five and six double bonds to industrial extraction, bulk shipping across oceans, chemical refining, and months of shelf storage in room-temperature pharmacies?
The same oxidation chemistry applies—only multiplied. EPA and DHA are not slightly more fragile than linoleic acid. They are dramatically more vulnerable to oxidative degradation. They are, in fact, among the most oxidation-prone molecules in the human diet.
When omega-3 fatty acids oxidize, they don’t simply disappear. They fragment into the same categories of harmful compounds that make rancid seed oils dangerous: lipid peroxides, aldehydes like malondialdehyde and 4-hydroxynonenal, and other reactive oxygen species. These molecules are chemically reactive—they don’t sit inert in your body. They attack cell membranes, damage mitochondria, denature proteins, and trigger inflammatory cascades. They are implicated in cardiovascular disease, neurodegeneration, and accelerated aging. In other words, oxidized fish oil may contribute to the very conditions it is marketed to prevent.
The fish oil supplement industry has taken the most oxidation-prone fats in nature and subjected them to standard commercial processing and storage conditions that reliably degrade them. The rancidity statistics from New Zealand, North America, Australia, and South Africa are not surprising. They are what the chemistry predicts.
The Three-Legged Stool
Before any medical intervention, three questions must be satisfied:
Necessity: Is this intervention needed?
Safety: Is it safe?
Effectiveness: Does it work?
If any leg fails, the stool falls over. You don’t proceed.
Why does one failure invalidate the whole? Consider each leg in turn. If an intervention isn’t necessary, you’re accepting risk—however small—for no benefit. Even a perfectly safe and effective treatment for a disease you don’t have and won’t get is a bad trade. If an intervention isn’t safe, effectiveness becomes irrelevant; you’re simply exchanging one harm for another. And if it doesn’t work, you’re taking on risk while receiving nothing in return. The three legs are not independent criteria to be weighed against each other. They are sequential gates. Fail any one and the analysis ends.
Fish oil supplements fail on all three.
Leg One: Necessity
The case for fish oil supplementation rests on a comparison. Traditional populations eating whole fish have lower rates of cardiovascular disease than Western populations eating industrial diets. The conclusion drawn: we need more omega-3s.
But this framing obscures what actually changed. Traditional diets weren’t high in omega-3s so much as they were low in omega-6s. The ratio mattered. When Western diets flooded with soybean oil, corn oil, and other industrial seed oils—with US per-capita linoleic acid consumption increasing an estimated 25-fold between 1865 and the present, according to dietary reconstruction studies—the balance collapsed.
The proposed solution was to add omega-3 supplements to counteract the omega-6 excess. This is like treating chronic poisoning by adding a second substance rather than removing the first. The necessity for fish oil supplementation evaporates once you recognize that eliminating industrial seed oils addresses the underlying problem.
People eating whole foods from animals raised on pasture, avoiding industrial vegetable oils, and occasionally consuming fatty fish have no demonstrated need for omega-3 supplementation. The “necessity” was manufactured by comparing a damaged modern diet to traditional baselines and concluding we need a product to fix the damage—rather than simply stopping the damage.
The necessity leg does not hold.
Leg Two: Safety
The biochemistry predicts that fish oil supplements will be oxidized. The testing confirms it.
Independent surveys across multiple countries tell the same story. A survey of 171 over-the-counter products in North America found approximately half exceeded voluntary oxidation limits on at least one measure, with 27% having more than twice the recommended lipid peroxide levels. In South Africa and New Zealand, more than 80% of supplements tested exceeded recommended oxidation thresholds. An Australian study of products from Sydney pharmacies found roughly one in three exceeded international limits for total oxidation.
The Auckland researchers measured three markers: peroxide value (reflecting primary oxidation products), anisidine value (reflecting secondary oxidation products like aldehydes), and total oxidation value. Eighty-three percent exceeded recommended peroxide values. Twenty-five percent exceeded anisidine thresholds. Fifty percent exceeded total oxidation limits. The correlation between missing omega-3 content and elevated oxidation markers revealed the mechanism: the EPA and DHA hadn’t failed to be included—they had oxidized into other compounds.
The processing chain makes this difficult to avoid at industrial scale. Crude fish oil is extracted from small pelagic fish—anchovies, sardines—caught primarily off the coast of South America. The extraction often involves heating and chemical solvents. The oil goes into bulk tanks and ships across oceans, frequently to China for further processing. Less than a quarter is refined for human consumption; the rest becomes aquaculture feed.
Refining involves degumming, neutralization, bleaching, and repeated high-temperature deodorization to strip the aldehydes and ketones responsible for rancid odor. Each step exposes EPA and DHA—with their five and six double bonds—to heat, oxygen, light, and trace metals. These are precisely the conditions that accelerate oxidative degradation. Antioxidants are added, but they slow rather than prevent oxidation. The chemistry cannot be reliably outrun at scale.
The finished product goes into capsules engineered with thick shells specifically designed to trap odor. Consumers cannot smell what they’re swallowing. Cut open a fish oil capsule and smell it. You may encounter anything from a mild fishy odor to an unmistakable rancid stench—but absence of obvious rancidity doesn’t mean the oil is fresh. Early oxidation products are often odorless; the damage may be invisible to your nose. The capsule prevents any sensory evaluation at all. It’s not protecting the oil from oxidation. It’s protecting the sale from your senses.
Animal studies show oxidized lipids cause organ damage, growth impairment, and accelerated atherosclerosis. One short-term human trial fed highly oxidized fish oil to healthy volunteers and found no changes in standard oxidative-stress markers over several weeks—but this tells us only that the markers used were insensitive to early damage in healthy people over a brief period, not that chronic intake of lipid peroxides is benign. There are no long-term trials designed to test whether daily ingestion of oxidized fish oil increases hard endpoints like cardiovascular events or neurodegeneration. In a context where oxidized lipids are known to cause pathology and retail products are demonstrably oxidized, the burden of proof lies on those selling the capsules, not on consumers to prove harm.
A note on bleeding: many consumers believe fish oil thins blood dangerously. A 2024 meta-analysis in the Journal of the American Heart Association examined this question across 120,643 patients in 11 randomized trials. Omega-3 supplementation showed no increased risk of overall bleeding, hemorrhagic stroke, intracranial bleeding, or gastrointestinal bleeding. The bleeding concern is largely unfounded. The oxidation concern is not.
The safety leg does not hold.
Leg Three: Effectiveness
The cardiovascular evidence deserves honest engagement. A 2019 meta-analysis published in the Journal of the American Heart Association, incorporating 13 randomized controlled trials and over 127,000 participants, found that marine omega-3 supplementation was associated with statistically significant reductions in myocardial infarction (8%), CHD death (8%), and total cardiovascular disease (3-7%).
These are real findings from peer-reviewed research. They cannot be dismissed.
But context matters.
First, the effect sizes are modest—and understanding why requires distinguishing between relative and absolute risk. Relative risk describes the percentage change from your baseline. Absolute risk describes the actual difference in outcomes. These are not the same thing, and the distinction matters enormously.
An 8% relative risk reduction sounds meaningful. But if your baseline risk of heart attack over ten years is 5%, an 8% relative reduction brings it to 4.6%—a difference of 0.4 percentage points. To prevent one heart attack, you would need to treat 250 people with fish oil for ten years. The other 249 receive no benefit. This is what “statistically significant” can look like in practice. The finding is real; the clinical relevance is marginal.
Second, and critically: clinical trials use pharmaceutical-grade products with controlled oxidation states. The supplements consumers actually buy are not what the trials tested. When 69% of retail products contain less than two-thirds of labeled EPA and DHA content, and 83% exceed oxidation thresholds, consumers are not replicating trial conditions. They’re swallowing something substantially different.
The correlation in the Auckland study between missing omega-3 content and elevated oxidation markers tells the story: consumers pay for omega-3s and receive their degradation products. Whatever intact EPA and DHA might do for cardiovascular health, lipid peroxides and aldehydes do something else entirely.
If oxidized, degraded fish oil supplements show any cardiovascular benefit in trials, this might reflect residual intact omega-3s surviving the processing and storage gauntlet. It does not validate the products consumers actually purchase months after manufacture from room-temperature pharmacy shelves.
Earlier meta-analyses were less favorable. A 2012 analysis published in JAMA found no significant association between omega-3 supplementation and major cardiovascular events or mortality. Another 2012 meta-analysis focused on secondary prevention found no significant benefit for recurrent cardiovascular events. The evidence has never been consistent—which is itself telling for an intervention promoted as unambiguously beneficial. The heterogeneity in outcomes likely tracks differences in product quality, dosing, and baseline diets, which underscores the central point: whatever modest benefit appears in tightly controlled trials does not reliably translate to the oxidized, under-dosed retail capsules on pharmacy shelves.
The effectiveness leg is, at best, cracked. At worst, it fails entirely once you account for the gap between trial products and retail reality.
What To Do Instead
If the three-legged stool has collapsed, what remains standing?
Eat whole fish, not supplements. Omega-3 fatty acids in fish tissue are protected within cell membranes, stabilized by naturally occurring antioxidants, and consumed fresh or properly preserved. Sardines, mackerel, salmon, and anchovies deliver EPA and DHA in the biological context that keeps them intact. The fats arrive at your cells as nature packaged them, not as industrial processing degraded them.
Reduce omega-6 intake rather than trying to balance it. The necessity for omega-3 supplementation dissolves when you stop flooding your body with industrial seed oils. The goal is not to add more polyunsaturated fats to counteract other polyunsaturated fats. The goal is to return to a dietary pattern where the ratio corrects itself—by eliminating the industrial oils that distorted it.
Recognize the limits of isolated nutrients. Fish oil supplements are a case study in how industrial processing destroys what made the original food valuable. A sardine is not a delivery vehicle for EPA and DHA molecules. It is a complete food with fats, proteins, minerals, and cofactors in biological relationship. The supplement paradigm—isolate the “active ingredient,” concentrate it, encapsulate it, sell it—repeatedly fails because it misunderstands what makes food nourishing. Fish oil joins a pattern: beta-carotene supplements increased lung cancer in smokers; vitamin E megadoses failed to deliver expected benefits and may have caused harm. Whole foods work. Isolated, industrially processed extracts consistently disappoint or backfire.
The Capsule’s Purpose
Fish oil capsules are engineered with thick gelatin shells. The industry describes this as protecting the oil from oxidation. But the capsules are oxygen-permeable. They don’t prevent oxidation—it proceeds despite the shell.
What the thick shell does accomplish is blocking sensory evaluation. You cannot smell the oil before you swallow it. Whether fresh or degraded, mild or rancid, you have no way to know. With cooking oil, your nose would warn you. With encapsulated oil, that warning system is bypassed entirely.
This is worth sitting with. The product design eliminates the consumer’s ability to assess quality. The shell is not protecting you from oxidation. It is protecting the manufacturer from your judgment.
Conclusion
Fish oil supplements fail the three-legged stool test that should govern any medical intervention.
Necessity: The need for supplemental omega-3s is manufactured by a dietary context flooded with industrial omega-6s. Remove the seed oils and the necessity dissolves.
Safety: The products consumers actually purchase are oxidized beyond recommended limits. EPA and DHA, with their five and six double bonds, are among the most oxidation-prone fats in nature—and standard industrial processing reliably degrades them. Consumers receive lipid peroxides and aldehydes in smell-proof capsules.
Effectiveness: Clinical trials used pharmaceutical-grade products. Retail supplements don’t match those conditions. The modest benefits observed in some studies may not apply to what consumers actually swallow.
If you’re currently taking fish oil, you were trying to do something good for your health. The marketing was convincing and the logic seemed sound. But the evidence says the products don’t contain what labels claim, arrive oxidized beyond safe thresholds, and hide their rancidity behind thick capsules you can’t smell through.
The industry knows this. The testing has been done. The data is published. The capsules keep selling.
Eat fish. Skip the pills.
Source: https://open.substack.com/pub/unbekoming/p/degraded-fats-in-smell-proof-capsules
References
Albert, B.B., et al. (2015). Fish oil supplements in New Zealand are highly oxidised and do not meet label content of n-3 PUFA. Scientific Reports, 5:7928.
Hu, Y., et al. (2019). Marine Omega-3 Supplementation and Cardiovascular Disease: An Updated Meta-Analysis of 13 Randomized Controlled Trials Involving 127,477 Participants. Journal of the American Heart Association, 8:e013543.
Javaid, M., et al. (2024). Bleeding Risk in Patients Receiving Omega-3 Polyunsaturated Fatty Acids: A Systematic Review and Meta-Analysis of Randomized Clinical Trials. Journal of the American Heart Association, 13:e032390.
Bagge, A., et al. (2018). High-dose omega-3 fatty acids have no effect on platelet aggregation or coagulation measured with static and flow-based aggregation instruments and Sonoclot. Scandinavian Journal of Clinical and Laboratory Investigation, 78:7-8, 539-545.
Rizos, E.C., et al. (2012). Association Between Omega-3 Fatty Acid Supplementation and Risk of Major Cardiovascular Disease Events: A Systematic Review and Meta-analysis. JAMA, 308(10):1024-1033.
Ottestad, I., et al. (2012). Oxidised fish oil does not influence established markers of oxidative stress in healthy human subjects: a randomised controlled trial. British Journal of Nutrition, 108(2):315-326.
Jackowski, S.A., et al. (2015). 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, 4:e30.
Heller, M., et al. (2019). Oxidation of fish oil supplements in Australia. Food Chemistry, 275:638-643.
Cameron-Smith, D., et al. (2015). Fishing for answers: is oxidation of fish oil supplements a problem? Journal of Nutritional Science, 4:e36.