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Is Fish Oil Really Bad for You?

This article is a companion piece to a video. To access the video, click here. Before watching that video, you may wish to watch my comprehensive review video on fish oil supplementation, which you can access here.


Recently, overzealous journalists have carelessly spread the rumor that fish oil may be harmful for cardiovascular health. Journalists appear to have developed this opinion from a recent meta-analysis[1] describing increased risk of atrial fibrillation with the consumption of N-3 fatty acids. In fact, it has long been known that although fish oil supplementation dramatically lowers the risk of heart attacks in people with cardiovascular disease[2], it has a less consistent effect on stroke and arrythmia incidence and mortality.

This is not the first time public educators have attempted to warn against the use of N-3 fatty acids. The contrarian pedagogue, Ray Peat, has long been warning against their use because of the relatively higher potential of polyunsaturated fats (PUFAs, as compared to saturated fats) to oxidize. Generally, Peat has suggested that the abundantly evident anti-inflammatory effect[3] of high-quality fish oil may not hold after it is ingested and oxidized in the human body.

In this article, we will review the evidence for the dangers of oxidized fish oil. In particular, we will learn to what degree high quality fish oil may turn pro-oxidant in the human body, we will learn whether fish oil brands commonly contain oxidized fish oil already, and we will review the evidence for fish oil supplementation for cardiovascular disease.


Oils can be oxidized in mammals in vivo, either enzymatically or nonenzymatically. Nonenzymatic production occurs through lipid peroxidation of membrane PUFAs, initiated by reactive oxygen and nitrogen species, and is enhanced in pathological states and when oxidative stress is high[4]. In particular, cytochrome P450-modulated ROS overproduction as a response to mitochondrial dysfunction is known to produce reactive aldehydes like HNE[5]. In vitro studies indicate that even circulating lipoprotein particles may be oxidized to produce reactive aldehydes[6].

Oils oxidize when exposed to oxygen, producing primary oxidation products (e.g. peroxide) and eventually secondary oxidation products (e.g. p-anisidine).

N-3 PUFAs produce aldehydic end-products such as 4-hydroxyhexenal (HHE) in storage, cooking, and through gastrointestinal digestion.

The Products of N-3 and N-6 Fatty Acid Oxidation

Oxidation of unsaturated fats is called peroxidation. Peroxidation leads to the generation of genotoxic and cytotoxic[7] molecules such as the aldehydic end-product 4-hydroxy-2-alkenals. The 4-hydroxy-2-alkenals include 4-Hydroxy-2-hexenal (HHE) and 4-hydroxy-2-nonenal (HNE), derived as the major end-products from n-3 and n-6 PUFAs, respectively[8].

Can Consuming N-3 PUFAs Increase HHE Levels in Animals?

Though endogenous hydroxy-2-alkenals are known to be associated with disease, the peroxidation danger from consumption of n-3 PUFAs remained unclear. For the first time, a 2012 rodent study showed that plasma HHE levels increased after consumption of oxidized n-3 PUFAs and that consumption of dietary HHE also raised plasma HHE levels, indicating that plasma HHE levels may be influenced by oxidized fish oil consumption in humans[9]. Oral consumption of HNE was also shown to increase excreted and stored HNE in rodents subsequently[10].

An in vitro study on the digestion of beef and salmon with equal fat composition found that while digestive fluids contained a similar amount of HNE at maximum (2 uM micromolar), intestinal digestion of salmon oil produced more HHE (3.5 uM) than that of the minced beef (2 uM)[11].

HNE and HHE in Humans

Free plasma HNE ranges between 3-125 nM in healthy people, though it can reach over 100 nM and up to the micromolar magnitude with aging and the diseases of aging, including cardiovascular disease, Alzheimer’s, and arthritis. Though free plasma HHE is less studied, it appears to average 6 nM in healthy subjects and up to 15-17 nM in subjects with arthritis or encephalitis[12].

Can Supplemental N-3 PUFAs Increase HHE Levels in Humans?

A 2-week human study using 200, 400, 800, or 1600 mg/day of DHA found that while 200 and 400 mg left plasma HHE unchanged, 800 and 1600 mg/day of DHA produced progressive increases in HHE but none of HNE[13].

Are Fish Oil Products Already Oxidized?

A 2015 study sought to determine the primary (peroxide), secondary (anisidine) and total oxidation value of 171 n-3 PUFA supplements from 49 brands available over the counter in Canada. Voluntary international standards recommend peroxide, anisidine, and TOTOX values below 5 mEq/kg, 20, and 26, respectively. Though 89% of the products contained an added antioxidant, 17% of the products exceeded peroxide levels of 5 mEq/kg, 41% exceeded the limits for anisidine, and 39% exceeded the TOTOX value limits. Interestingly, products with flavor additives had higher anisidine and TOTOX values and most of the children’s products were of this nature[14].

In a 2018 study surveying 26 fish oil supplements available in Australia, 38% exceeded the limit for primary oxidation (peroxide), 25% exceeded the limit for secondary oxidation (anisidine), and 33% exceeded the total oxidation limit[15]. In a Syrian survey of 3 brands purchased from 3 local pharmacies, only one brand remained below the agree limits for oxidation through the study period[16].

Fish Oil Products in America

In 2013, a study of the top 16 selling EPA/DHA products found that 31% exceeded 5 meq O2/kg, exceeding the limit for primary oxidation[17]. In 2017, it was shown that two American fish oils and one algal oil exceeded primary oxidation limits[18].

In a 2020 study[19] of the 48 most widely sold retail n-3 PUFA brands, 48% of the products contained less EPA/DHA than claimed, though most remained within the legal range (at least 80% for “Class II nutrients”). 40.5% had an EPA + DHA content of between 80-100% of the label while 59.5% had a content between 100-138.7% of the label. Re-esterified triglyceride products, when containing below stated EPA + DHA, were closer to the label (95-99.3%).

They found major inter-laboratory differences in assessing oxidative status, as indicated by the 2015 Canadian study that reviewed previous values. Overall, their results reflected previous findings, that about a third of US products fail primary oxidation standards. Interestingly, they found a wide variety of peroxide values between encapsulated forms, indicating that encapsulated forms are not always prone to more oxidation. Krill oil forms had the least oxidative stress, in line with previous reports of its stability[20]. They also observed that ethyl ester forms had higher oxidation than triglyceride forms, in line with previous thinking[21].

The authors found that measuring anisidine in colored and flavored oils was impossible to do accurately. The 25 tested flavored and colored oils averaged 63.2 p-AV while nearly all non-flavored products complied with industry limits. Overall, 85.4% of products with the highest sales in the US passed primary oxidation standards while 95.7% of non-colored or flavored products passed secondary oxidation standards.

How Can Fish Oil Products be Prepared Better?

The activation energy for the combination of oxygen and lipid radicals is near zero, meaning that even small amounts of oxygen will produce peroxidation. A recent study[22] found that fortifying the fish oil with an antioxidant that can reduce oxygen more than 80% and storing it in a cool temperature may work. Using lipid hydroperoxide and TBAR assays, tert-butylhydroquinone (TBHQ) and Trolox (water-soluble analogue of vitamin E) were able to suppress n-3 PUFA oxidation during a two-day period while tocopherols were unable to (at a level of 500 ppm in the oil).

Could this Affect Mothers?

A 2016 in vivo study on pregnant rodents gave rodents either highly oxidized fish oil, unoxidized fish oil, or nothing and continued to give them the fish oil while they weaned. The mothers consuming oxidized fish oil had a 13.7x more likelihood of mortality compared to the unoxidized fish oil group and an 8.3x more likelihood of mortality compared to the control group. Further, the oxidized fish oil produced insulin resistance in the mothers while their offspring weaned[23].

Does Fish Oil Uniformly Lower Inflammation?

Although fish oil reduces cytokine production by T cells and reduces their motility[24], it can also have pro-inflammatory effects by increasing TNF-a production in macrophages that may be governed by prostaglandin synthesis[25]. For this reason, a rodent study that compared the effects of fish oil, olive oil, and coconut oil on damage from ozone exposure found that while only fish oil could protect rodents from the vasoconstriction due to ozone exposure, it also induced pulmonary complications arising from the inhibition of lipid and cholesterol transporters and increased TNF-a and IL-6 levels[26].

Nonetheless, most studies[27] indicate fish oil supplementation broadly reduces inflammatory markers, and parenteral treatment with n-3 PUFAs has recently been proposed for the treatment of Covid-19[28].

Do Fish Oil Products Varyingly Affect Health?

A 2017 RCT of 54 human subjects was conducted for 7 weeks with three arms: 1.6 g/day of oxidized fish oil, unoxidized fish oil, or high-oleic sunflower oil (HOSO). Unoxidized fish oil reduced IDL and large, medium and small LDL particles compared with oxidized fish oil and HOSO. It also reduced concentrations of total lipids, phospholipids, total cholesterol, cholesterol esters, and free cholesterol in IDL and large, medium, and small LDL compared to oxidized fish oil and HOSO. Fish oil also reduced LDL-C and non-HDL-cholesterol compared to oxidized fish oil and HOSO. LDL-C was raised by 15% with HOSO, raised by 19% with oxidized fish oil, and lowered by 6% with high quality fish oil. High quality fish oil was also associated with a reduction in ApoB, leading to larger LDL sizes.

The authors found that changes in the expression of cholesterol ester transfer protein (CETP) may modulate the changes in lipoprotein subclasses, as its expression varied with their change. As HDL did not increase, they speculated an improvement in reverse cholesterol transport was at play. Previous research supports both the improvement in reverse cholesterol transport[29] and n-3 PUFA’s inhibitory effect on CETP[30].

Does Fish Oil Not Preserve Cardiovascular Function?

Despite the findings on arrythmias and strokes, fish oil supplementation was found to produce a 10% reduction in death from coronary heart disease in a 2017 meta-analysis of 20 randomized control trials (RCTs)[31].

A 2019 meta-analysis of 13 RCTs, also produced by the American Heart Association, found that even when the REDUCE-IT trial was excluded from the analysis, fish oil supplementation lowered the risk of heart attacks by 8%, lowered the risk of death from coronary heart disease by 14%, lowered the incidence of coronary heart disease by 9%, lowered the risk of death from cardiovascular disease by 7%, and lowered the incidence of cardiovascular disease by 3%[32].

A 2021 meta-analysis of 40 studies, published by the Mayo Clinic Proceedings, found that fish oil supplementation reduced the risk of heart attacks by 13%, coronary heart disease events by 10%, and coronary heart disease mortality by 9%[33].

A 2021 pooled analysis using data from 17 studies, published by Nature, found that people in the highest quintile of blood N-3 fatty acid levels were 15-18% less likely to die from all causes than those in the lowest quintile[34].


Oxidized, low-quality fish oil may be harmful for human health. While high-quality fish oil can be oxidized in the human body, it still produces an overall anti-inflammatory effect in humans, contrary to the thinking of Ray Peat. The majority of fish oil products on the market are not already oxidized.

Fish oil supplementation consistently and dose-dependently reduces the risks of heart attacks and coronary heart disease events in humans. It also consistently reduces mortality from cardiovascular disease and can sometimes be shown to reduce all-cause mortality.

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