Stearidonic Acid (SDA): A Comprehensive Reference
1. Identity and Chemical Characterization
Chemical Names and Properties
Stearidonic acid (SDA: C18H28O2; 18:4, n-3) is an ω-3 fatty acid, sometimes called moroctic acid. It is an essential polyunsaturated fatty acid (PUFA) with four cis (Z) double bonds, with the first double bond from the methyl end in the omega-3 (n-3) position, giving it the shorthand notation 18:4n-3. Its molecular weight is 276.41 g/mol, its molecular formula is C18H28O2, and its IUPAC name is (6Z,9Z,12Z,15Z)-octadeca-6,9,12,15-tetraenoic acid; it is registered under CAS number 20290-75-9 and PubChem CID 5312508. Other synonyms include moroctic acid, cis-6,9,12,15-octadecatetraenoic acid, and all-cis-6,9,12,15-octadecatetraenoic acid.
Position in the Omega-3 Metabolic Pathway
Stearidonic acid constitutes the first metabolite of α-linolenic acid (ALA, 18:3n-3) in the metabolic pathway leading to C20–22 PUFAs, such as eicosapentaenoic acid (EPA, 20:5n-3) and docosahexaenoic acid (DHA, 22:6n-3), which have received much attention because of their various physiological functions in the human body. SDA is biosynthesized from alpha-linolenic acid (ALA: C18H30O2; 18:3, n-3) by the enzyme delta-6-desaturase, which removes two hydrogen atoms.
2. Natural Sources
Plant Sources
Natural sources of SDA are the seed oils of hemp (between 0.16 and 1.54% of the oil), blackcurrant (between 2.5 and 4.5%), Buglossoides arvensis (corn gromwell), and Echium plantagineum, and the cyanobacterium Spirulina.
Buglossoides arvensis (Ahiflower®): Ahiflower® oil extracted from Buglossoides arvensis seeds is the richest known non-genetically modified source of dietary SDA. SDA is found in refined Buglossoides arvensis oil (RBO) at 17–21% of fatty acids. In addition to SDA, the seed oil also contains 42–48% ALA and 4.5–8% γ-linolenic acid (GLA), and it has approximately 80–90% PUFA with an ω-6/ω-3 ratio of less than 1.
Echium plantagineum (Purple Viper's Bugloss / Paterson's Curse): Upon mechanical extraction, oleaginous seeds of Echium plantagineum may yield around 30% oil that consists of about 15% oleic acid, 10% GLA, 13% linoleic acid, 30–35% ALA, and 14–16% stearidonic acid; this makes it a unique vegetable oil as an in vivo precursor of EPA. Echium plantagineum is a winter annual native to Mediterranean Europe and North Africa.
Echium species (Boraginaceae family): Seeds from 30 species, mainly of Boraginaceae and Primulaceae, were analysed in order to identify potential new sources of SDA; Echium species (Boraginaceae) contained the highest amount of total n-3 PUFA (47.1%), predominantly ALA (36.6%) and SDA (10.5%), combined with high GLA (10.2%). Further Boraginaceae species rich in both SDA and GLA included Omphalodes linifolia (8.4% SDA), Cerinthe minor (7.5% SDA), and Buglossoides purpureocaerulea (6.1% SDA).
Blackcurrant Seed Oil (Ribes nigrum): The SDA content of blackcurrant seed oil is 2–4% of fatty acids.
Hemp Seed Oil (Cannabis sativa): Omega-6 and omega-3 polyunsaturated fatty acids such as GLA and stearidonic acid account for about 80% of the composition of hempseed oil.
Other botanical and microbial sources: SDA has been isolated from Lithospermum officinale and fish oils. A limited number of foods contain small amounts of SDA, including certain fish such as sardines and herring, and algae.
Transgenic Crop Sources
The oil from the transgenic soybean MON 87769 contains approximately 20–30% stearidonic acid (% of total fatty acids); SDA-enriched soybean oil can be used for the production of margarine, mayonnaise, shortenings, salad dressings, ready-to-eat foods, and other food categories. Monsanto developed this SDA-producing soybean using recombinant DNA techniques to introduce two coding sequences: Pj.D6D, derived from Primula juliae (Juliana primrose), which encodes a Δ6 desaturase, and Nc.Fad3, derived from Neurospora crassa (fungus), which encodes an ω3 desaturase. Conventional canola (Brassica napus) oil has also been modified to contain as much as 23% of its fatty acid pool as SDA, accomplished by generating transgenic canola lines using expression vectors containing Δ6 and Δ12 desaturases derived from the fungus Mortierella alpina and the Δ15 desaturase from canola.
Common Forms and Preparations
SDA is commercially available primarily in the following forms derived from peer-reviewed and regulatory literature:
- Cold-pressed or refined seed oils from Echium plantagineum, Buglossoides arvensis (Ahiflower®), and blackcurrant (Ribes nigrum), consumed as liquid oils or in soft-gel capsules.
- Purified SDA ethyl esters, used principally in controlled clinical research settings.
- SDA-enriched soybean oil developed via transgenic means, which has received regulatory review in several jurisdictions.
- In 2007, the US Food & Drug Administration declared refined Echium oil as a novel food for the market, and oil from E. plantagineum is available as a food ingredient.
- The US FDA affirms self-determined GRAS (Generally Recognised as Safe) status for B. arvensis seed oil. The European Union has granted novel food status for the oil and approved daily intake levels in dietary supplements up to 500 mg SDA/day or 2.5 g of oil/day.
3. Traditional and Historical Use
Stearidonic acid as a chemically defined entity is a product of modern lipid science and was not identified or isolated as such in any historical ethnobotanical tradition. However, the plant species that are its richest sources — particularly members of the Boraginaceae family including Echium and Borago — have longstanding histories of use in European herbal traditions.
Echium has a long history of use as a garden plant with some herbal medicine applications. Echium plantagineum contains pyrrolizidine alkaloids, yet it has traditional uses as a diaphoretic. The use of borage (Borago officinalis), another SDA-containing Boraginaceae member, in Mediterranean European traditional medicine is documented, primarily for the seed oil containing GLA and SDA. SDA occurs as a glycerol ester in the leaf lipids of borage (Borago officinalis L., of the Boraginaceae) and in blackcurrant seed oil (Ribes nigrum, of the Saxifragaceae).
Blackcurrant seeds and berries have been consumed in northern European cuisines for centuries, though the specific identification of SDA as a constituent of blackcurrant seed oil did not occur until the era of modern fatty acid research. There is no documented historical use in any medical tradition specifically targeting SDA as a therapeutic compound; its investigation is entirely a product of 20th- and 21st-century nutritional biochemistry.
4. Key Constituents and Mechanisms of Action
Bypassing the Rate-Limiting Step in n-3 PUFA Synthesis
SDA (18:4n-3) is emerging as a sustainable omega-3 source with an efficient conversion rate to n-3 LC-PUFA, especially to EPA, as it bypasses the Δ6-desaturase rate-limiting step. There are many non-fish food sources of the essential plant-derived ω-3 fatty acid α-linolenic acid, but conversion from this to longer-chain EPA and especially to DHA is poor. Because SDA is itself the Δ6-desaturase product of ALA, its dietary consumption circumvents this bottleneck.
Metabolic Pathway from SDA to EPA and DHA
In the ω-3 Δ6 desaturase/Δ6 elongase pathway: (1) ALA is converted to SDA by the action of a Δ6 desaturase; (2) SDA is converted to eicosatetraenoic acid (ETA) by the action of a C18/20 elongase; and (3) ETA is converted to EPA by the action of a Δ5 desaturase. Dietary and de novo synthesized stearidonic acid is efficiently converted to eicosapentaenoic acid or EPA and docosapentaenoic acid or DPA by the subsequent actions of elongase and Δ5-desaturase, making its use a potentially valuable tool for increasing EPA and DHA tissue concentrations. The further metabolism to DHA depends again on Δ6-desaturase (followed by a β-oxidation), and for this reason this conversion is limited.
Relative Efficiency of Conversion to EPA
Several studies indicated that dietary SDA increases EPA more efficiently than ALA; thus vegetable oils containing SDA may become a dietary source of n-3 fatty acids that is more effective in increasing tissue n-3 PUFA concentrations than the current ALA-containing vegetable oils. Results from previous studies suggested that SDA, relative to EPA, increases RBC EPA, with reported efficiencies ranging from approximately 16 to 30%.
Anti-Inflammatory Mechanisms
Plant n-3 PUFAs that are precursors of EPA and DHA (e.g., ALA and SDA) appear to share some of the physiological and functional attributes of very long-chain n-3 PUFA. These actions appear to derive from their conversion to the biologically active EPA. Therefore, because this conversion is limited, although greater for SDA than ALA, the biological potency of the plant n-3 PUFA is less than that of the very long-chain n-3 PUFA.
Increased membrane content of EPA and DHA (and decreased arachidonic acid content) results in a changed pattern of production of eicosanoids and probably also of resolvins and protectins. Because SDA is a precursor to EPA, these downstream effects are attributed, at least in part, to SDA via its conversion to EPA in tissues.
Studies in human volunteers and cell cultures suggest that SDA increases EPA levels more efficiently than an equimolar amount of ALA. SDA is also a precursor to N-acylethanolamine (NAEs).
5. Scientific Evidence by Health Area
5.1 EPA Enrichment and the Omega-3 Index (Cardiovascular Biomarker)
The omega-3 index is defined as the percentage of EPA plus DHA in red blood cell (RBC) membranes, and it is regarded as an emerging marker of cardiovascular risk.
Two clinical trials have demonstrated that SDA-enhanced soybean oil can significantly improve an emerging marker of cardiovascular health, the omega-3 index (RBC EPA+DHA), and omega-3 fatty acids have been reported to have a variety of cardiovascular and neuropsychiatric benefits.
The purpose of one randomized, controlled, parallel group study was to characterize the relationships between dosages of SDA and EPA, and incorporation of EPA into red blood cell (RBC) membranes over time. A randomized, single-blind, controlled, parallel group study in healthy men and women characterized the relationships between intakes of SDA and EPA and EPA enrichment of RBC membranes over a 12-week period; %EPA in RBC membranes was greater after EPA (0.44, 1.3, or 2.7 g/d, respectively) and SDA (1.3, 2.6, or 5.2 g/d, respectively) consumption compared to a safflower control.
A number of clinical trials have investigated the impact of dietary SDA ethyl ester, echium oil, or SDA soybean oil on tissue fatty acid composition and have shown that tissue EPA content, but not DHA, is significantly elevated following the consumption of dietary SDA.
Evidence strength: There are consistent findings across multiple randomized controlled trials that SDA supplementation raises circulating and RBC EPA levels. However, these studies measured biomarkers rather than clinical cardiovascular outcomes (such as myocardial infarction or cardiovascular mortality), so the clinical significance of the EPA elevation remains to be directly demonstrated in outcome trials.
5.2 Serum Triglycerides
The rate-limiting step for the conversion of ALA to EPA is the Δ6-desaturation of ALA into SDA. Providing oils rich in SDA may increase endogenous synthesis of EPA, which may subsequently lower serum triacylglycerol concentrations, an effect frequently observed after EPA supplementation; researchers studied the effects of Echium oil on serum triacylglycerol concentrations and the omega-3 index, which correlate negatively with the risk for coronary heart disease (CHD).
Animal experiments and direct human supplementation studies show that Echium seed oil (ESO) can bring about beneficial changes in biomarkers of chronic diseases, such as lowering triacylglycerol (TAG) in plasma and tissue.
In an animal study, after 12 weeks, obese rodents fed modified fish or SDA diets had lower serum lipids and hepatic fat content compared to control diet animals.
Evidence strength: The triglyceride-lowering effect associated with SDA consumption has been observed in animal models and is supported by human supplementation data using Echium oil; however, the clinical evidence specifically and exclusively attributable to SDA (rather than to the complex lipid matrix of Echium oil, which also contains high GLA and ALA) remains difficult to fully disentangle. Evidence is preliminary to moderate.
5.3 Immune Modulation and Inflammation
In a randomized, double-blind, placebo-controlled clinical trial in healthy subjects, production of interleukin-10 (IL-10) was increased in the 100% Ahiflower oil group compared to the 100% high oleic sunflower oil group (p < 0.05). IL-10 production was also increased in lipopolysaccharide-stimulated M2-differentiated THP-1 macrophage-like cells in the presence of 20:4n-3 or EPA. Overall, this indicates that consumption of Ahiflower oil is associated with an anti-inflammatory phenotype in healthy subjects.
Recent studies highlight the efficient conversion of SDA to EPA, suggesting that SDA may offer similar health benefits to EPA, including immune, joint, cognitive, and gut microbiome modulation (with distinct SDA-derived metabolites).
Evidence strength: Modest and preliminary in humans. IL-10 upregulation is an anti-inflammatory signal, but the study period was short (28 days) and the population was healthy, limiting generalizability. Mechanistic evidence is largely indirect and relies on SDA's conversion to EPA.
5.4 Inflammatory Arthritis
Gamma-linolenic acid and omega-3 fatty acids alpha-linolenic acid and stearidonic acid from black currant seed oil (BCSO) have been investigated for their therapeutic activity in rheumatoid arthritis. Approximately 10.5 g of BCSO was given to RA patients in double-blind fashion with soybean oil as placebo for 24 weeks continuously. This study assessed a complex oil containing multiple fatty acids, of which SDA was only one component; isolating the specific effect of SDA from that of GLA and ALA co-present in the oil is not straightforward.
Marine n-3 fatty acids and γ-linolenic acid both have anti-inflammatory effects and may be useful to help treat inflammatory diseases. Their effects alone or combined were examined in patients with arthritis in a randomized controlled trial; patients with rheumatoid arthritis or psoriatic arthritis were randomized into four groups in a double-blind, placebo-controlled parallel designed study, receiving either 3.0 g n-3 LC-PUFA/day, 3.2 g γ-linolenic acid/day, a combined dose, or olive oil for twelve weeks. While this was not an SDA-specific trial, it illustrates the relevant inflammatory context.
Evidence strength: The evidence specifically for isolated SDA in inflammatory arthritis remains indirect. Studies in this area have used complex seed oils, making it difficult to attribute effects specifically to SDA.
5.5 Buglossoides arvensis (Ahiflower) Oil — Phase I Safety and Efficacy Trial
A parallel-group, randomised, double-blind, comparator-controlled phase I clinical trial investigated the safety and efficacy of Ahiflower oil. Diets of 40 healthy subjects were supplemented for 28 days with 9.1 g/day of Ahiflower oil (46% ALA, 20% SDA) or flax seed oil (59% ALA). Blood and urine chemistries, blood lipid profiles, hepatic and renal function tests, and haematology were measured as safety parameters. No clinically significant changes in safety parameters were measured in either group. Tissue ALA and EPA content increased in both groups compared with baseline, but EPA accrual in plasma and in all cell types was greater in the Ahiflower group.
A further randomized, double-blind, placebo-controlled clinical trial investigated the effects of three Ahiflower oil dosages (healthy subjects, n=88, consuming 9.7 mL per day for 28 days of: 100% high oleic sunflower oil; 30% Ahiflower + 70% HOSO; 60% Ahiflower + 40% HOSO; or 100% Ahiflower). No clinically significant changes in blood and urine chemistries, blood lipid profiles, hepatic and renal function tests, or haematology were measured. Plasma and mononuclear cell EPA levels increased from baseline at day 28 in all Ahiflower groups, and the increase was greater in all Ahiflower groups compared to control; similar results were obtained for ALA, eicosatetraenoic acid (ETA), and docosapentaenoic acid (DPA), but not DHA.
5.6 DHA Synthesis — Consistently Negative Finding
Three studies in humans support the view that SDA is more readily converted than ALA into EPA, but little conversion to DHA has been observed. Neither EPA nor SDA affected DHA in RBCs in these studies. This is not surprising because the same Δ6-desaturase that is rate-limiting in the reaction of ALA to SDA has been shown to catalyze the conversion of 24:5n-3 to 24:6n-3, the step prior to the β-oxidation step that produces DHA.
Evidence strength: The finding that SDA does not meaningfully raise DHA levels is consistent and replicated across multiple human studies. This is a well-established limitation of SDA as an omega-3 supplement for purposes where DHA specifically is the goal.
6. Body Systems and Health Areas of Association
- Cardiovascular system: The consumption of EPA and DHA has been associated with reduced risk for cardiovascular disease morbidity and mortality; mean intakes of EPA and DHA in the United States and elsewhere are below levels recommended by health authorities. SDA raises RBC EPA and the omega-3 index, biomarkers inversely associated with cardiovascular risk, though direct outcomes data in humans are lacking for SDA specifically.
- Lipid metabolism: Providing oils rich in SDA may increase endogenous synthesis of EPA, which may subsequently lower serum triacylglycerol concentrations.
- Immune and inflammatory systems: The consumption of Ahiflower oil is associated with an anti-inflammatory phenotype in healthy subjects, as evidenced by upregulated IL-10 production.
- Cell membrane composition: SDA enriches erythrocyte membranes, polymorphonuclear cells, and mononuclear cells with EPA, which results in a changed pattern of production of eicosanoids and probably also of resolvins and protectins, influencing cellular function.
- Hepatic lipid metabolism: In rodent models of obesity, SDA diets reduced hepatic fat content compared to control diets.
7. Dosage Forms and Dosages Reported in Studies
The following dosage information is drawn directly from published clinical research; these are not recommendations.
- SDA ethyl esters (encapsulated), dose-response RCT (12 weeks): In a randomized, single-blind, controlled, parallel group study, participants (n = 137) received SDA at 1.3, 2.6, or 5.2 g/day as encapsulated ethyl esters over 12 weeks, compared to EPA doses of 0.44, 1.3, or 2.7 g/day and a safflower oil control.
- Ahiflower oil (containing ~20% SDA), Phase I RCT (28 days): 40 healthy subjects were supplemented for 28 days with 9.1 g/day of Ahiflower oil (46% ALA, 20% SDA) or flax seed oil (59% ALA).
- Ahiflower oil, dose-ranging RCT (28 days): 88 healthy subjects consumed 9.7 mL per day for 28 days of: 100% high oleic sunflower oil (control); 30% Ahiflower oil + 70% HOSO; 60% Ahiflower + 40% HOSO; or 100% Ahiflower oil.
- Black currant seed oil (containing SDA alongside GLA and ALA), RA study (24 weeks): Approximately 10.5 g of BCSO was given to RA patients in double-blind fashion with soybean oil as placebo for 24 weeks.
- EU-approved daily intake (regulatory, not clinical study): The European Union has approved daily intake levels in dietary supplements up to 500 mg SDA/day or 2.5 g of Buglossoides arvensis oil/day.
8. Safety Considerations
Clinical Trial Safety Data
In the Phase I Ahiflower trial, blood and urine chemistries, hepatic and renal function tests, and haematology were measured, and no clinically significant changes in safety parameters were measured in either the Ahiflower or flax oil groups. In the dose-ranging trial (88 healthy subjects, 9.7 mL/day for 28 days), no clinically significant changes in blood and urine chemistries, blood lipid profiles, hepatic and renal function tests, or haematology were measured.
Regulatory Status
In 2009, Monsanto obtained Generally Recognized as Safe (GRAS) status from the FDA for SDA-enriched omega-3 soybean oil. The US FDA affirms self-determined GRAS status for B. arvensis seed oil. The European Union has granted novel food status for the oil and approved daily intake levels in dietary supplements up to 500 mg SDA/day or 2.5 g of oil/day. Health Canada notified Monsanto Canada Inc. that it has no objection to the food use of SDA-producing soybean MON 87769, following a comprehensive assessment according to its Guidelines for the Safety Assessment of Novel Foods.
Pyrrolizidine Alkaloids (PAs) — A Key Consideration for Echium-Derived Products
Echimidine is the main pyrrolizidine alkaloid of Echium plantagineum, a plant domesticated in many countries. Because of echimidine's toxicity, this alkaloid has become a target of the European Food Safety Authority regulations, especially in regard to honey contamination.
Pyrrolizidine alkaloids (PAs) are naturally occurring secondary metabolites of plants. To date, more than 660 types of PAs have been identified from an estimated 6000 plants, and approximately 120 of these PAs are hepatotoxic. As a result of PAs being found in spices, herbal teas, honey, and milk, PAs are considered contaminants in foods, posing a potential risk to human health.
Pyrrolizidine alkaloids occur as free-base/tertiary forms or their N-oxides; both forms are hepatotoxic and genotoxic. Maximum levels for PAs were established at the European level and are currently regulated under Commission Regulation (EU) 2023/915, which sets limits based on the sum of 21 pyrrolizidine alkaloids and their corresponding N-oxide forms.
Refined Echium oil intended for human consumption is processed to reduce or remove pyrrolizidine alkaloid content. Consumers using crude or poorly refined Echium-derived products should be aware of this concern. SDA is more stable than EPA/DHA in food matrices, which is a notable technological advantage.
Absence of Meaningful DHA Effect
As noted consistently in human studies, tissue EPA content, but not DHA, is significantly elevated following the consumption of dietary SDA. Individuals supplementing SDA specifically for the purpose of raising DHA levels should be aware that SDA does not appear to be an effective pathway for doing so, and preformed DHA sources would be required.
Oxidative Stability
As SDA has fewer double bonds than EPA (20:5) and DHA (22:6), SDA soybean oil is more stable to oxidation compared with fish oils. This property reduces the risk of lipid peroxidation products forming during storage and food processing, which is a practical safety and formulation advantage relative to fish-oil derived supplements.
Populations Not Studied or Understudied
The available human clinical trials have been conducted primarily in healthy adult volunteers over periods of 28 days to 12 weeks. Long-term safety data in vulnerable populations (pregnant women, children, individuals with hepatic disease) are lacking in the peer-reviewed literature specifically for SDA-rich oils. The short trial durations represent a limitation of the safety evidence base.
References
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