Eicosatetraenoic acid

Eicosatetraenoic Acid (ETA, 20:4 ω-3)

1. Identity: Chemical Name, Isomers, and Structural Characteristics

Eicosatetraenoic acid (ETA, 20:4) designates any straight-chain tetra-unsaturated 20-carbon fatty acid. These compounds are classified as polyunsaturated fatty acids (PUFA). ETA refers to the family of polyunsaturated, long-chain fatty acids with a 20-carbon backbone and four double bonds, and there are eight total isomers of ETA.

Two isomers are of particular biological relevance:

• All-cis-5,8,11,14-eicosatetraenoic acid (20:4 ω-6) is an omega-6 fatty acid with the trivial name arachidonic acid. It is formed by desaturation of dihomo-gamma-linolenic acid (DGLA, 20:3 ω-6).
• All-cis-8,11,14,17-eicosatetraenoic acid (20:4 ω-3) is an omega-3 fatty acid. It is an intermediate between stearidonic acid (18:4 ω-3) and eicosapentaenoic acid (EPA, 20:5 ω-3).

Almost all writings in biology, medicine, and nutrition limit the use of the term "arachidonic acid" to all-cis-5,8,11,14-eicosatetraenoic acid (ω-6). In nutritional supplement science and in most contemporary discussions of marine-derived dietary compounds, the abbreviation ETA most commonly refers to the omega-3 isomer, all-cis-8,11,14,17-eicosatetraenoic acid (C20:4 ω-3), which is the focus of this article.

ETA (C20:4ω3) possesses four double bonds and is suggested to have greater cardio-protective and anti-inflammatory effects than eicosapentaenoic acid (EPA). In pure form, these compounds are encountered rarely and are colorless oils.

Arachidonic acid, the omega-6 isomer, is the most prevalent and well-studied member of the ETA family. Though uncommon, some ETAs are essential fatty acids found in certain fungi, fish oils, and human and animal fat tissue.

2. Natural Sources

2.1 Marine Animal Sources

ETA (20:4 n-3), a long-chain omega-3 polyunsaturated fatty acid, occurs in small amounts in various marine-derived foods. Primary sources include fatty fish such as herring and sardines, where ETA typically comprises 0.4–0.7% of total fatty acids in the lipid profile.

The most concentrated natural source of the omega-3 ETA is the New Zealand green-lipped mussel (Perna canaliculus). ETA is an omega-3 fatty acid found almost exclusively in New Zealand green-lipped mussel oil. Standard fish oil contains EPA and DHA but no ETA. ETA is found almost exclusively in New Zealand green-lipped mussel (Perna canaliculus) — it is not present in salmon, sardine, or other fish oil sources.

Marine-based green-lipped mussel lipid extract contains numerous sterol esters, sterols, polar lipids, triglycerides, and at least 91 different fatty acids, including eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), and eicosatetraenoic acid (ETA).

2.2 Microalgal Sources

A microalgal strain (HDMA-20) identified from Lake Chengfeng, China, was found to accumulate high content of ω-3 PUFAs (63.4% of total lipid), with alpha-linolenic acid (ALA) and eicosatetraenoic acid (ETA, C20:4ω3) accounting for 35.4% and 9.6% of total lipid, respectively. HDMA-20 not only represents an additional source of ALA but also a totally new source of ETA.

In engineered cyanobacterial strains, the rare anti-inflammatory n-3 LC-PUFA eicosatetraenoic acid (ETA, 20:4 n-3) was synthesized alongside stearidonic acid (SDA), with approximately 99% of SDA and ETA complexed to bioavailable monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) species.

2.3 Fungal and Other Sources

ETA is found in the green-lipped mussel and appears to inhibit the oxygenation of arachidonic acid by both the cyclooxygenase (COX) and lipoxygenase pathways. A mutant of the fungus Mortierella alpina 1S-4, employed for producing arachidonic acid, produces larger amounts of ETA due to the expression of an ω-3-desaturase gene.

3. Biosynthesis and Metabolic Pathway

Alpha-linolenic acid can be converted to stearidonic acid (18:4n-3) by delta-6 desaturase, and then stearidonic acid can be elongated to eicosatetraenoic acid (20:4n-3), which can be further desaturated by delta-5 desaturase to yield eicosapentaenoic acid (20:5n-3; known as EPA).

In mammalian cells, ALA is the substrate of a series of elongation and desaturation reactions to generate long-chain ω-3 PUFAs. Eicosatetraenoic acid (ETA, C20:4ω3) is an intermediate metabolite in the ω-3 pathway. The 8,11,14,17-eicosatetraenoic acid (ETA) is a natural substance to the extent that it is formed in the body as an intermediate in the alpha-linolenic acid metabolic pathway.

The n-3 fatty acid alpha-linolenic acid (18:3 n-3) is converted to stearidonic acid (18:4 n-3) and then eicosatetraenoic acid (20:4 n-3) to form eicosapentaenoic acid (EPA, 20:5 n-3), using the same series of enzymes as those used to synthesize arachidonic acid.

ETA may also be synthesized by stoichiometric hydrogenation of pure eicosapentaenoic acid (C20:5ω3).

4. Common Forms and Preparations

ETA is not available as a standalone commercially marketed supplement. ETA is not sold as a standalone supplement — it is naturally concentrated in New Zealand green-lipped mussel oil. For those seeking an ETA supplement, the primary option is a high-quality mussel oil product.

Preparations containing ETA include:

• Lipid extract of green-lipped mussel (e.g., Lyprinol®/PCSO-524): A commercial stabilized lipid extract from Perna canaliculus. Total lipid extracts of P. canaliculus moderately inhibited COX-1 and COX-2 pure enzymes in vitro. Lyprinol® (a commercial extract from P. canaliculus) exhibited strong inhibition of both COX isoforms.
• Whole greenshell mussel powder: Dried whole mussel ground into powder form for encapsulation. Both whole powder and lipid extract forms have been studied in clinical trials.
• Free acid or ester preparations: The compound may be in the form of the free acid or in the form of any pharmaceutically acceptable salt or ester.

Powdered green-lipped mussel is reported to be more allergenic than lipid extracts.

5. Traditional and Historical Use

Eicosatetraenoic acid (ETA, 20:4 n-3), an intermediate in the omega-3 polyunsaturated fatty acid biosynthetic pathway, was first identified in the mid-20th century as part of early research on lipid metabolism in animal tissues and fish oils. During the 1950s, studies on polyunsaturated fatty acids in marine sources highlighted ETA's presence alongside longer-chain omega-3s like eicosapentaenoic acid (EPA).

Historically, ETA has been present in the diets of indigenous populations, especially those who consumed oily fish and certain marine plants. These populations exhibited notably lower incidences of inflammatory and cardiovascular diseases, which led to the early recognition of the health-promoting properties of ETA and related fatty acids.

The green-lipped mussel (Perna canaliculus), the richest known dietary source of ETA, has particular historical significance. Green-lipped mussel — known as kūtai (or kuku) in te reo Māori — is a large bivalve shellfish found only in the coastal waters of New Zealand. It is not farmed anywhere else in the world. The Māori people of New Zealand consumed kūtai as a traditional food staple over many generations, and epidemiological observations of lower rates of joint disorders in coastal Māori communities who consumed the mussel regularly formed part of the early impetus for scientific investigation.

In traditional remedies, ETA-rich sources such as fish oil and green-lipped mussel extracts were often employed to alleviate symptoms associated with joint discomfort, arthritis, and skin conditions. These natural treatments harnessed the anti-inflammatory effects of ETA, providing relief for ailments that modern science now understands to be influenced by chronic inflammation.

6. Key Constituents, Active Compounds, and Mechanisms of Action

6.1 The Eicosanoid System and the Role of ETA

Eicosanoids, a diverse family of signaling molecules, are produced by oxygenation of polyunsaturated eicosatetraenoic acids. These eicosanoids, working in tandem, contribute to a lipid signaling complex widely responsible for inducing an inflammatory immune response. Common signs of inflammation include visible redness, pain, swelling, and the sensation of heat — many of these an effect of varying eicosanoid species.

ETAs directly impact a variety of physiological states, and their impact is largely effected by their derivatives, which are bioactive compounds including epoxide eicosatrienoic acids (EETs), lipoxins (LXs), and hydroxyeicosatetraenoic acids (HETEs). Variants of each of these molecules are associated with cancers, cardiovascular diseases, inflammation, and other conditions.

6.2 Dual COX and LOX Pathway Inhibition

The most distinctive and pharmacologically significant proposed mechanism of the omega-3 ETA isomer is its ability to simultaneously inhibit both major enzymatic pathways of arachidonic acid metabolism:

ETA is found in green-lipped mussel and appears to inhibit the oxygenation of arachidonic acid by both the cyclooxygenase (COX) and lipoxygenase pathways. ETA is clinically significant because it acts as a dual inhibitor of arachidonic acid metabolism — it inhibits both the cyclooxygenase (COX) pathway and the lipoxygenase (LOX) pathway. The LOX pathway produces leukotrienes, which are potent inflammatory mediators involved in conditions such as arthritis and asthma.

Previous studies have demonstrated ETA's capacity to serve as a dual inhibitor of cyclooxygenases (COX1 and COX2) and lipoxygenases that can block the production of several classes of pro-inflammatory eicosanoids including leukotrienes.

ETA can compete with arachidonic acid (ARA) in blocking the COX-2 and LOX systems, although its structural differences may affect its efficacy in converting to PGE-3.

6.3 Competitive Substrate Inhibition of Arachidonic Acid

Incubation of Lyprinol in the absence of exogenous arachidonic acid showed the appearance of alternate prostaglandin metabolites, confirming Lyprinol PUFA as a competitive substrate inhibitor of arachidonic acid (AA) metabolism.

ETA has also been demonstrated to compete with arachidonic acid at the arachidonoyl-CoA synthetase step, thereby preventing arachidonic acid uptake. Having a structure very similar to arachidonic acid — insofar as chain length and number of double bonds — ETA would be expected to fill the active sites of the enzyme and thus compete with and thereby inhibit the activity of the enzyme on true arachidonic acid.

6.4 Pro-Resolving Mediator Activity

ETA possesses pro-resolving or anti-inflammatory activities analogous to the well-described EPA-, DPA-, and DHA-derived resolvins, protectins, and maresins. These resolvins, protectins, and maresins are products of dual lipoxygenase activities, and it is reported that ETA is also a substrate for the generation of such metabolites.

6.5 The Omega-6 Isomer: Arachidonic Acid (ARA)

The omega-6 ETA isomer, arachidonic acid, operates through contrasting pathways. These isomers play critical roles as precursors to eicosanoids, a family of signaling molecules that mediate inflammation, vascular tone, and immune responses through enzymatic pathways such as cyclooxygenase (COX), lipoxygenase (LOX), and cytochrome P450. Arachidonic acid is released from membranes by phospholipases upon cellular activation and is metabolized into pro-inflammatory prostaglandins, thromboxanes, and leukotrienes, influencing processes like platelet aggregation, fever, and pain sensation.

Immune cells involved in rheumatoid arthritis usually contain a high proportion of arachidonic acid (AA) and low proportions of other 20-carbon polyunsaturated fatty acids, with AA considered to be the major substrate for synthesis of eicosanoids. Eicosanoids produced by both the COX and LOX pathways are found in the synovial fluid of patients with active RA.

7. Scientific Evidence by Area of Use

7.1 Inflammatory Joint Disease: Osteoarthritis and Rheumatoid Arthritis

The most studied area of ETA's application is musculoskeletal and joint inflammation, primarily through research on green-lipped mussel (GLM) extracts that contain ETA as a key component alongside EPA and DHA.

Systematic review evidence (osteoarthritis): Intervention studies using New Zealand greenshell mussel (Perna canaliculus) extract in osteoarthritis (OA) patients have shown effective pain relief. A systematic review summarising the efficacy of GSM extracts was conducted by searching EMBASE, MEDLINE, and Scopus up to March 2020. Inclusion criteria were clinical trials measuring the effect of supplementation of whole or lipid extract from GSM on pain and mobility outcomes in OA patients. A total of nine clinical trials were included. Pooled results showed that GSM extracts (lipid extract or whole powder) provide moderate and clinically significant treatment effects on a visual analogue scale (VAS) pain score (effect size: −0.46; 95% CI −0.82 to −0.10; p = 0.01).

Earlier systematic review evidence: A systematic review evaluated the efficacy of the nutritional supplement Perna canaliculus (green-lipped mussel, GLM) in the treatment of OA, adding to previous work by focusing solely on GLM use in OA and providing re-analysis of original trial data. Four RCTs were included — three placebo-controlled, the fourth a comparative trial of GLM lipid extract vs. stabilized powder extract. All four studies assessed GLM as an adjunctive treatment to conventional medication for a clinically relevant time in mild to moderate OA. All trials reported clinical benefits in the GLM treatment group, but findings from two studies could not be fully included because of possible un-blinding and inappropriate statistical analysis.

Rheumatic diseases — broader omega-3 context: A total of 20 clinical trials have been carried out in RA, of which 16 exhibited significant improvements in multiple disease clinical outcomes. Nine clinical trials have been completed in SLE and lupus nephritis, of which 6 exhibited significant improvements in one or more clinical outcomes. A total of 4 clinical trials have been conducted in OA, of which 3 exhibited significant improvements in at least one clinical parameter. These results, while applying to omega-3 PUFAs broadly, provide the mechanistic and clinical backdrop against which ETA-specific research is conducted.

Important limitation: Most human research on ETA comes from studies using green-lipped mussel extracts, which naturally contain ETA along with other bioactive lipids. This means that attributing efficacy specifically to ETA, as opposed to the combined action of EPA, DHA, ETA, and other compounds in the extracts, remains methodologically difficult. No published human trial has isolated and administered pure omega-3 ETA as a single agent.

7.2 Respiratory: Asthma

Asthma is a chronic inflammatory disease of the airways mediated, at least in part, by leukotrienes and other lipid mediators. Experimental studies have shown that lipid extract of New Zealand green-lipped mussel, Perna canaliculus, is effective in inhibiting 5'-lipoxygenase and cyclooxygenase pathways responsible for production of eicosanoids, including leukotrienes and prostaglandins. The aim of one study was to assess the effect on symptoms, peak expiratory flow (PEF), and hydrogen peroxide (H₂O₂) in expired breath condensate as a marker of airway inflammation in patients with steroid-naïve atopic asthma in a double-blind, randomised, placebo-controlled clinical trial.

In the Emelyanov et al. (2002) trial, 46 patients with atopic asthma received two capsules of lipid extract (Lyprinol®) or placebo twice daily for 8 weeks. Each capsule of lipid extract contained 50 mg ω-3 polyunsaturated fatty acids and 100 mg olive oil, whereas placebo capsules contained only 150 mg olive oil.

In the Emelyanov trial, daytime wheezing was reduced in corticosteroid-naïve patients with mild to moderate atopic asthma who received stabilized mussel extract, and morning peak expiratory flow (PEF) was increased in those receiving mussel extract (Lyprinol) compared with those receiving placebo. However, mean forced expiratory volume in the first second of expiration (FEV1) and evening PEF did not differ between the two groups.

Lipid extracts from New Zealand green-lipped mussel have been shown to have benefits in patients with atopic asthma. Evidence is promising but limited by small sample sizes and the inability to isolate ETA's individual contribution.

7.3 Dermatology: Psoriasis

One of the few studies examining ETA directly — rather than through GLM extracts — is a population-based analysis specifically investigating ETA dietary intake and psoriasis risk.

Publicly available data with 50,938 individuals from NHANES 2003–2006 and 2009–2014 were analyzed to determine the relationship between daily dietary n-3 PUFA intakes and the risk of psoriasis. Individuals with responses to the psoriasis question and complete dietary data, aged 18–60 years, were selected, and a total of 15,733 participants were included in the study.

Multivariable logistic regression analysis, trend tests, subgroup analysis, and interaction tests were used to evaluate the associations of ETA, EPA, and DHA intake with the risk of psoriasis. In the optimal multivariate-adjusted model, the odds ratio (OR) with 95% confidence interval (CI) of psoriasis were 0.30 (0.12, 0.88), 1.92 (0.78, 4.74), and 1.28 (0.72, 2.27) for daily dietary ETA, EPA, and DHA intake, respectively. Trend tests showed a dose-effect relationship between daily dietary ETA intake and the lower risk of psoriasis. Subgroup analysis and tests for interaction showed that the association was stable in different subgroups.

Comparing the 24-hour dietary recall survey between participants with and without psoriasis showed differences in ETA intake only, not EPA or DHA. Psoriasis patients were not only more likely to eat fewer ETA but also the odds ratio of ETA consumption and psoriasis risk was 0.36, indicating that consumption of ETA may be associated with a decreased risk of psoriasis. Interestingly, this was observed in a dose-effect manner, with greater consumption associated with less reported psoriasis.

An analysis of data from the NHANES found a potential association between daily dietary intake of eicosatetraenoic acid and a lower risk of psoriasis among U.S. adults. This was a cross-sectional observational study, and causality cannot be established from this design. The finding is considered hypothesis-generating rather than definitive.

7.4 Oncology: Breast Cancer Cell Research

Research into ETA's role in oncology is at an early, preclinical stage.

In a study examining the survival and phospholipid fatty acid composition of tumorigenic and non-tumorigenic breast cells, n-3 intermediates stearidonic acid (SDA) and eicosatetraenoic acid (ETA) reduced (P < 0.05) the growth of tumorigenic but not non-tumorigenic cells. All treatments resulted in a higher PUFA and saturated content and a lower monounsaturated content in tumorigenic cells.

In a systematic review of omega-3 PUFAs in breast cancer research, eicosatetraenoic acid (ETA) was used as a test compound in 7.4% of included studies. This reflects ETA's emerging but still marginal role in preclinical cancer research. No human clinical trials have evaluated ETA as an isolated agent in oncology.

7.5 Cardiovascular Health

ETAs directly impact a variety of physiological states through their bioactive derivatives, including epoxide eicosatrienoic acids (EETs), lipoxins (LXs), and hydroxyeicosatetraenoic acids (HETEs). The ETA-derivative LXA4 plays a protective role during myocardial ischemia. Further studies exploring the ETA metabolic pathways are needed and may lead to new targets for cardiovascular disease treatment.

ETA is suggested to have greater cardio-protective and anti-inflammatory effects than eicosapentaenoic acid (EPA). This suggestion is based predominantly on mechanistic and in vitro data. Human clinical trial evidence specifically attributing cardiovascular benefit to ETA as an isolated agent is not available in the published literature.

8. Body Systems and Health Areas Associated with ETA

• Musculoskeletal system: Joint inflammation, osteoarthritis, and rheumatoid arthritis — the most studied area, primarily through GLM extract trials.
• Respiratory system: The LOX pathway produces leukotrienes, which are potent inflammatory mediators involved in conditions such as arthritis and asthma, providing a mechanistic rationale for ETA in respiratory inflammation.
• Immune system: In vitro modulation of leukotrienes, cytokines, and immunoglobulin has been demonstrated.
• Integumentary system (skin): Association between ETA dietary intake and reduced psoriasis risk in observational data.
• Cardiovascular system: Mechanistic evidence and population-level epidemiological associations; no isolated human clinical trial evidence.
• Oncology: Preclinical in vitro evidence for growth inhibition in tumorigenic breast cells; no clinical trial data for ETA specifically.

9. Dosage Forms and Reported Dosages

Because ETA is not marketed as a standalone supplement, dosage information is derived from studies using GLM lipid extracts that contain ETA as one of multiple active fatty acids.

• Emelyanov et al. (2002) asthma trial: Forty-six patients received two capsules of lipid extract (Lyprinol®) or placebo twice daily for 8 weeks. Each capsule of lipid extract contained 50 mg ω-3 polyunsaturated fatty acids and 100 mg olive oil.
• PCSO-524 (Lyprinol/Omega XL) exercise-induced bronchoconstriction trial: A small double-blind, randomized, placebo-controlled, crossover trial enrolled 20 adults with mild to moderate asthma. PCSO-524 (comprised of 50 mg of omega-3 [n-3] fatty acids, including EPA 72 mg and DHA 48 mg plus olive oil 100 mg) or placebo (olive oil 150 mg) was administered for 3 weeks, with a 2-week washout between treatments.

No established daily intake recommendation, tolerable upper intake level, or standardized therapeutic dose exists for ETA as an isolated compound from any regulatory authority, including the NIH Office of Dietary Supplements, the European Food Safety Authority (EFSA), or WHO, as of the current evidence base. Individual ETA content in commercially available GLM products varies and is not always disclosed by manufacturers.

10. Safety Considerations and Notable Interactions

10.1 General Tolerability

Green-lipped mussel appears to be generally well tolerated in non-allergic people.

10.2 Shellfish Allergy

Patients with allergy to mussel or other shellfish (mollusks, crustaceans) are at risk. Powdered green-lipped mussel is reported to be more allergenic than lipid extracts. Use in allergic patients is considered potentially unsafe.

10.3 Hepatic Disorders

Green-lipped mussel preparations are considered likely unsafe when used in patients with hepatitis or other hepatic disorders; possible toxic hepatitis has been associated with the Seatone brand of GLM in several case reports.

10.4 Neurotoxicity Risk

The B4 analog of brevetoxin B (BTXB4) has been associated with neurotoxic shellfish poisoning in some mussel sources. Use in patients with neurotoxicity is considered potentially unsafe.

10.5 Pregnancy and Lactation

Use when pregnant or breastfeeding is considered potentially unsafe based on available reports.

10.6 Oxidative Stability

Since ETA is a polyunsaturated fatty acid, it is easily subject to oxidation; any pharmaceutical or supplement composition containing ETA should also contain a suitable antioxidant.

10.7 Potential Drug Interactions

As a long-chain polyunsaturated fatty acid with demonstrated COX and LOX inhibitory activity, ETA (delivered through GLM extracts) may theoretically interact with anticoagulant or antiplatelet medications by augmenting bleeding tendency, a concern well established for other omega-3 fatty acids as a class. No specific pharmacokinetic interaction studies for isolated ETA have been published in the peer-reviewed literature. The clinical significance of this theoretical concern remains uncharacterized.

11. Limitations of the Current Evidence Base

While not yet as extensively studied as EPA or DHA, ETA shows particular promise for joint health and inflammatory conditions, especially when delivered through green-lipped mussel extracts. The following limitations characterize the current state of research:

• No published human clinical trial has administered isolated, purified omega-3 ETA (20:4 n-3) as a single agent.
• Most human research on ETA comes from studies using green-lipped mussel extracts, which naturally contain ETA along with other bioactive lipids, making it impossible to attribute observed effects exclusively to ETA.
• The NHANES-derived psoriasis study, while large (n = 15,733), is cross-sectional and observational and cannot establish causality.
• A rigorous systematic review noted that updated and more conclusive evidence is needed to evaluate the efficacy and safety of GSM supplements in clinical practice.
• Preclinical (in vitro and animal) data on ETA's anti-inflammatory mechanisms are more extensive than human clinical evidence, and extrapolation from these findings to clinical practice requires caution.

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