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Linoleic acid

Linoleic acid is a polyunsaturated omega-6 fatty acid with the molecular formula C₁₈H₃₂O₂ and the IUPAC name (9Z,12Z)-octadeca-9,12-dienoic acid, featuring an 18-carbon chain with two cis double bonds at positions 9 and 12.^[1] As an essential fatty acid, it cannot be synthesized by the human body and must be obtained from dietary sources, where it serves as a precursor to longer-chain polyunsaturated fatty acids like arachidonic acid.^[2] It plays critical roles in maintaining cell membrane integrity, supporting skin barrier function, and regulating inflammation through the production of eicosanoids.^[3] Linoleic acid is vital for human physiology, with deficiency—rare in modern diets—leading to symptoms such as dry skin, hair loss, and impaired wound healing.^[2] Primary dietary sources include vegetable oils (e.g., soybean, sunflower, corn, safflower), nuts, and seeds, contributing 5–10% of total energy intake in typical Western diets.^[2] Adequate intake is associated with reduced cardiovascular disease risk, potentially through lowering low-density lipoprotein cholesterol,^[4] though excessive consumption relative to omega-3 fatty acids may promote pro-inflammatory pathways.^[3] Recent meta-analyses as of 2025 reinforce links to lower risks of type 2 diabetes and improved cardiometabolic health.^[5] Ongoing research explores its roles in metabolic and neurological functions.^[6]

Chemical properties

Structure and nomenclature

Linoleic acid is a straight-chain carboxylic acid with the molecular formula \ce{C18H32O2}.^[1] Its systematic IUPAC name is (9Z,12Z)-octadeca-9,12-dienoic acid, reflecting the 18-carbon chain (octadeca-) with double bonds at positions 9 and 12 in the Z (cis) configuration.^[7] The common name "linoleic" derives from the Greek word linon (flax) and oleic (relating to oil), as it was first isolated from linseed oil, the oil extracted from flax seeds.^[8] As a polyunsaturated fatty acid (PUFA), linoleic acid features multiple carbon-carbon double bonds along its hydrocarbon chain, specifically two nonconjugated cis double bonds separated by a methylene group (methylene-interrupted), which imparts flexibility and reactivity distinct from saturated fatty acids lacking such unsaturations.^[1] The molecule consists of a carboxyl group (-COOH) at carbon 1, followed by a chain of 17 methylene (-CH2-) and methyl (-CH3) groups, with the double bonds positioned between carbons 9-10 and 12-13 when numbered from the carboxyl end; from the methyl end, the terminal double bond starts at position 6, classifying it as an omega-6 fatty acid.^[9] This omega-6 designation highlights its role within the family of essential fatty acids, where the position of unsaturation influences metabolic pathways.^[1] Linoleic acid exists in various isomeric forms differing in double bond positions, configurations (cis or trans), or conjugation. The standard form is nonconjugated, with isolated double bonds separated by two single bonds, whereas conjugated linoleic acid (CLA) comprises geometric and positional isomers featuring conjugated double bonds (separated by a single bond), such as (9Z,11E)-octadeca-9,11-dienoic acid, which alters chemical stability and biological activity compared to the nonconjugated parent compound.^[10]

Physical characteristics

Linoleic acid appears as a colorless to straw-yellow liquid at room temperature.^[1] It possesses a mild fatty odor.^[11] The compound has a melting point of -5 °C and a boiling point of 230 °C at reduced pressure (16 mmHg).^[12] Its density is 0.902 g/cm³ at 25 °C, and the refractive index is 1.466 at 20 °C (sodium D line).^[12] Linoleic acid is insoluble in water (solubility approximately 0.14 mg/L at 25 °C) but exhibits good solubility in organic solvents such as ethanol, chloroform, acetone, benzene, and ethyl ether.^[1] Due to its two unconjugated double bonds, linoleic acid is susceptible to oxidation, which can lead to rancidity and the formation of peroxides upon exposure to air, light, or heat.^[13] To maintain stability and prevent peroxidation, it should be stored in a cool, dark place under an inert atmosphere such as nitrogen.^[14]

Synthesis and reactions

In plants, linoleic acid is biosynthesized through the desaturation of oleic acid (18:1 Δ9) by the endoplasmic reticulum-localized fatty acid desaturase 2 (FAD2), also known as Δ12-desaturase, which introduces a double bond at the Δ12 position to yield linoleic acid (18:2 Δ9,12). This enzymatic step occurs primarily in the desaturation-elongation cycle of fatty acid metabolism within plant cells, with FAD2 expression regulated by factors such as temperature and developmental stage to optimize membrane fluidity and seed oil composition. Commercially, linoleic acid is obtained through partial chemical processing of high-linoleic vegetable oils, such as safflower oil, which contains up to 75-80% linoleic acid as triglycerides.^[15] The process involves mechanical pressing or solvent extraction of the oil followed by hydrolysis to free fatty acids, urea complexation or distillation for purification, and fractional distillation to isolate linoleic acid, yielding products of 95% or higher purity for industrial use.^[16] Linoleic acid undergoes several key chemical reactions due to its polyunsaturated nature. Hydrogenation, typically catalyzed by nickel or palladium under hydrogen gas, reduces the double bonds to produce saturated derivatives like stearic acid (18:0) or partially hydrogenated intermediates such as oleic acid, a process widely used to solidify oils for margarine production.^[17] Esterification with glycerol forms linoleic acid triglycerides via acid- or enzyme-catalyzed reactions, while reaction with lysophospholipids or diacylglycerols incorporates it into phospholipids, essential for lipid assembly in biological and food applications.^[18] Oxidation of linoleic acid proceeds via a free radical chain mechanism during autoxidation, where molecular oxygen abstracts a hydrogen from the allylic methylene (positions 11 or 14), leading to peroxyl radical formation and subsequent addition of O₂ to produce hydroperoxides. The primary products are 9-hydroperoxy-10E,12Z-octadecadienoic acid and 13-hydroperoxy-9Z,11E-octadecadienoic acid, as represented by the simplified equation:

\ce{(CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH + O2 ->[free radical] hydroperoxy derivatives}

This peroxidation is a major pathway in lipid rancidity and oxidative stress. Under alkaline conditions or thermal catalysis, linoleic acid isomerizes to conjugated linoleic acid (CLA) isomers, such as 9Z,11E-octadecadienoic acid, by shifting the double bonds from the skipped (methylene-interrupted) to conjugated positions through proton abstraction and reprotonation. This reaction, often performed with sodium hydroxide in propylene glycol, produces a mixture of CLA isomers used in nutraceuticals.^[19]

Occurrence and sources

Natural occurrence

Linoleic acid is a ubiquitous polyunsaturated fatty acid found in high concentrations in the seeds and oils of numerous plant species, where it serves as a primary component of storage lipids. In safflower oil, it comprises 70-78% of total fatty acids, while sunflower seed oil contains approximately 68-70% linoleic acid. These elevated levels contribute to the oils' roles in seed germination and plant development.^[20]^[21]^[22] In animal tissues, linoleic acid occurs at relatively low levels, particularly in non-ruminants, where it is not endogenously synthesized but incorporated from dietary sources and subsequently elongated or desaturated to form longer-chain polyunsaturated fatty acids such as arachidonic acid. Concentrations in non-ruminant meats, such as pork or poultry, typically range below 20% of total fatty acids in muscle lipids, reflecting dependence on plant-based feed. In contrast, microorganisms like the oleaginous fungus Mortierella alpina produce substantial amounts, with linoleic acid accounting for up to 42% of its lipid profile under optimal conditions.^[23]^[24]^[25] Linoleic acid plays key roles in plant physiology, primarily by enhancing membrane fluidity through its incorporation into phospholipids, which allows for proper protein function and adaptation to environmental stresses. It also participates in signaling pathways, serving as a precursor for oxylipins involved in defense responses against pathogens and abiotic factors. In ecosystems, linoleic acid contributes to environmental cycling via its presence in soil lipids, where it forms part of the organic matter derived from plant roots and microbial activity, and in algae, such as blue-green algae species, comprising variable but significant portions (up to 10-20%) of their fatty acid content to support photosynthetic membrane integrity.^[26]^[27]^[28] Non-edible natural sources further illustrate its widespread distribution, including wood resins where linoleic acid is a representative fatty acid extractive (typically 5-15% of lipid fraction in coniferous species) aiding in structural protection, and insect cuticles, where it integrates into the lipid barrier (around 10-30% in some species) to regulate permeability and prevent desiccation.^[29]^[1]^[30]

Dietary sources

Linoleic acid is abundant in various vegetable oils, which serve as primary dietary sources for this essential fatty acid. Safflower oil contains approximately 74 g of linoleic acid per 100 g, while corn oil provides about 54 g per 100 g. Nuts and seeds also contribute significantly; for example, sunflower seeds offer around 23 g per 100 g. Meats, particularly poultry, provide smaller amounts, with chicken containing 0.2–1.5 g per 100 g depending on the cut and preparation. According to WHO/FAO guidelines, linoleic acid should provide at least 2.5% of total energy intake (approximately 5–6 g/day for adults assuming 2000–2500 kcal diets) to prevent deficiency.^[31] Western diets typically meet or exceed this level through widespread use of processed foods and vegetable oils, though excessive intake from ultra-processed sources may pose risks.^[31] Many commercial products are fortified with linoleic acid to enhance nutritional profiles. Margarines often incorporate vegetable oils rich in linoleic acid to replace trans fats, providing 10–20 g per 100 g in some formulations.^[32] Infant formulas are similarly fortified to mimic breast milk composition, typically including 7–12% of total fatty acids as linoleic acid to support early development.^[33] Dietary patterns vary regionally, influencing linoleic acid consumption. In Mediterranean diets, intake is moderated at around 3–4% of energy from sources like olive oil blends and nuts, balancing omega-6 with higher monounsaturated fats.^[34]

Industrial production

Linoleic acid is primarily produced on an industrial scale through solvent extraction from high-linoleate vegetable oils, such as soybean oil, which typically contains about 50-51% linoleic acid by weight. The process begins with mechanical pressing or solvent extraction of oilseeds using hexane, a non-polar solvent that efficiently dissolves lipids from the seed matrix, followed by distillation under vacuum to separate the crude oil and recover the solvent. This method yields a fatty acid mixture with approximately 50% linoleic acid purity from soybean sources, making it cost-effective for large-scale operations due to the abundance of oilseed crops.^[35]^[20]^[36] Global production of linoleic acid exceeds 1 million tons annually, primarily derived from oilseed crops like soybeans and sunflowers, with major producers including the United States and China, which together account for over 45% of output. In the United States, soybean processing contributes significantly, while China dominates through extensive vegetable oil refining. These volumes support applications in cosmetics, pharmaceuticals, and food additives, driven by the economic viability of plant-based extraction.^[37]^[38] For higher-purity isolates required in specialized applications, purification techniques such as urea complexation and chromatography are employed on the crude extracts. Urea complexation exploits the differential solubility of saturated and monounsaturated fatty acids in urea solutions, allowing polyunsaturated linoleic acid to remain in the non-complexed fraction; optimal conditions using a urea-to-fatty acids ratio of 0.94 in 95% ethanol achieve up to 87.8% purity with 83.4% recovery from sunflower oil. Chromatography, including silver-ion or preparative methods, further refines the product to over 95% purity, as demonstrated in separations from crude tall oil yielding 95.2% linoleic acid.^[39]^[40] Alternative biotechnological approaches involve microbial fermentation using engineered organisms to produce linoleic acid, offering potential sustainability benefits over traditional extraction. Engineered yeasts, such as Saccharomyces cerevisiae with overexpressed desaturases and lipases, have achieved enhanced linoleic acid accumulation through metabolic pathway modifications. Similarly, oleaginous algae like Scenedesmus species can be optimized for linoleic acid production via controlled fermentation, though these methods remain in research stages and are not yet commercially dominant.^[41]^[42]

Biological role

Essentiality as a nutrient

Linoleic acid is classified as an essential fatty acid because humans and other mammals cannot synthesize it endogenously and must obtain it from the diet.^[2] It serves as the primary precursor for the omega-6 family of polyunsaturated fatty acids, which are crucial for maintaining cell membrane integrity, supporting eicosanoid production, and facilitating various physiological processes.^[43] The essentiality of linoleic acid stems from the human body's inability to introduce double bonds at positions beyond the delta-9 carbon in fatty acid chains. Specifically, humans lack the Δ12-desaturase enzyme required to convert oleic acid (18:1 n-9) into linoleic acid (18:2 n-6), necessitating a dietary supply to meet physiological needs.^[44] Deficiency of essential fatty acids, including linoleic acid, was first demonstrated in historical animal studies by George and Mildred Burr in 1929–1930, where rats fed fat-free diets exhibited symptoms such as scaly dermatitis, growth retardation, hair loss, impaired wound healing, and reproductive failure.^[45] In humans, similar manifestations occur with prolonged inadequate intake, including dry, scaly skin (often resembling phrynoderma), alopecia, poor wound healing, and growth retardation in children, particularly in cases of malabsorption or parenteral nutrition without lipid supplementation.^[2]^[46] To prevent deficiency, the minimum dietary requirement for linoleic acid is approximately 1% of total energy intake for infants and 0.5–2% for adults, though adequate intake recommendations are higher to support optimal health.^[47]^[48] Biomarkers of deficiency include a triene-to-tetraene ratio exceeding 0.2 in plasma phospholipids and linoleic acid levels below 0.5% of total plasma fatty acids, indicating biochemical impairment.^[2]^[49]

Metabolism in humans

Linoleic acid, an essential omega-6 polyunsaturated fatty acid, is primarily absorbed in the human small intestine through a process involving mixed micelles formed by bile salts, phospholipids, and dietary lipids, which solubilize the fatty acid and facilitate its uptake by enterocytes via passive diffusion and protein-mediated transport, such as CD36 and fatty acid-binding proteins.^[50]^[51] Once inside enterocytes, linoleic acid is re-esterified into triglycerides and incorporated into chylomicrons, which are then secreted into the lymphatic system and enter the bloodstream via the thoracic duct for distribution to tissues.^[52]^[53] Following absorption, linoleic acid undergoes enzymatic metabolism primarily in the liver and other tissues, where it serves as a precursor for longer-chain polyunsaturated fatty acids. The initial step involves delta-6 desaturase (encoded by FADS2), which introduces a double bond to convert linoleic acid (18:2 n-6) to gamma-linolenic acid (GLA, 18:3 n-6); this is followed by elongation via elongase enzymes to produce dihomo-gamma-linolenic acid (DGLA, 20:3 n-6).^[54]^[55]

\text{Linoleic acid (18:2 n-6)} \xrightarrow{\Delta^6\text{-desaturase}} \gamma\text{-linolenic acid (18:3 n-6)}

^[54] This pathway is rate-limited by delta-6 desaturase activity and competes with omega-3 fatty acids for the same enzymes.^[56] Linoleic acid and its metabolites are distributed throughout the body, with high incorporation into phospholipids of cell membranes, where it contributes to membrane fluidity and signaling, and storage in adipose tissue as triglycerides, reflecting long-term dietary intake.^[3]^[57] Several factors influence linoleic acid metabolism in humans, including age, which is associated with declining delta-6 desaturase activity and reduced conversion efficiency, leading to lower levels of downstream metabolites.^[58]^[59] Genetic variations, particularly polymorphisms in FADS1 and FADS2 genes, significantly modulate desaturase activity and thus the efficiency of linoleic acid conversion to GLA and DGLA, with certain variants resulting in lower endogenous production.^[60]^[61] Additionally, high dietary intake of omega-3 fatty acids competes with linoleic acid for delta-6 desaturase, potentially reducing its metabolic flux.^[56]^[62]

Bioactive derivatives

Linoleic acid undergoes enzymatic conversion to arachidonic acid through alternating desaturation and elongation reactions, positioning it as a central precursor for bioactive eicosanoids involved in cellular signaling. The process begins with Δ6-desaturation of linoleic acid (18:2 n-6) to γ-linolenic acid (18:3 n-6), followed by elongation to dihomo-γ-linolenic acid (20:3 n-6), and concludes with Δ5-desaturation to arachidonic acid (20:4 n-6). This pathway, primarily occurring in the liver and other tissues, relies on fatty acid elongases (e.g., ELOVL5) for the carbon chain extension step.^[63] Arachidonic acid is subsequently metabolized via two major pathways to generate eicosanoids. The cyclooxygenase (COX) pathway, catalyzed by COX-1 and COX-2 enzymes, produces prostaglandins such as prostaglandin E2 (PGE2) and thromboxane A2 (TXA2). In parallel, the lipoxygenase (LOX) pathway, involving enzymes like 5-LOX and 15-LOX, yields leukotrienes (e.g., LTB4) from arachidonic acid, while 15-LOX directly oxidizes linoleic acid to 13-hydroxyoctadecadienoic acid (13-HODE), an oxylipin derivative. The elongation in the upstream pathway can be represented as:

\text{γ-linolenic acid (18:3 n-6)} + 2\text{C} \xrightarrow{\text{elongase (ELOVL)}} \text{dihomo-γ-linolenic acid (20:3 n-6)}

Followed by desaturation to arachidonic acid.^[64]^[65] These bioactive derivatives exert regulatory effects on key physiological processes. For instance, prostacyclin (PGI2), derived from arachidonic acid via COX, promotes vasodilation by relaxing vascular smooth muscle cells. Thromboxane A2 facilitates platelet aggregation to support hemostasis, while prostaglandins like PGE2 modulate cytokine production, influencing immune cell activation and inflammatory signaling.^[66]^[67]^[68]

Health effects

Cardiovascular and metabolic impacts

Linoleic acid (LA), an essential omega-6 polyunsaturated fatty acid, has been extensively studied for its role in modulating cardiovascular risk factors, particularly through improvements in lipid profiles. Meta-analyses of prospective cohort studies indicate that higher dietary LA intake is inversely associated with coronary heart disease (CHD) events and mortality; for instance, each 5% increase in energy from LA replacing saturated fats or carbohydrates is linked to a 9% reduction in CHD events and a 13% lower risk of CHD death. Similarly, circulating levels of LA show a dose-response relationship with reduced atherosclerosis progression, as LA-enriched diets lower low-density lipoprotein (LDL) cholesterol by promoting its clearance and reducing oxidation in arterial walls. A 2023 meta-analysis confirmed that dietary LA supplementation decreases total cholesterol and LDL-C levels, with the effect more pronounced in younger individuals and those with lower body mass index, thereby mitigating plaque formation and vascular inflammation.^[69]^[4]^[70] In metabolic health, LA contributes to enhanced insulin sensitivity and a lower incidence of type 2 diabetes (T2D) by altering cell membrane composition. Incorporation of LA into phospholipids increases membrane fluidity, facilitating greater insulin receptor binding and downstream signaling for glucose uptake. A 2021 meta-analysis of cohort studies found that higher LA intake correlates with a 6% reduced risk of T2D per standard deviation increase, supported by randomized trials showing improved glycemic control and insulin resistance indices in LA-rich diets. This protective effect is evident in recent prospective data from the 2020s, where moderate LA consumption (around 5-7% of total energy) was associated with better fasting glucose regulation and reduced T2D progression in diverse populations.^[71]^[72]^[5] The balance between omega-6 and omega-3 fatty acids is crucial for these benefits, with the omega-6/omega-3 ratio influencing lipid metabolism and cardiovascular outcomes. An optimal ratio of approximately 4:1 has been linked to a 70% decrease in total mortality in secondary prevention trials, while ratios exceeding 10:1—common in Western diets—are associated with dyslipidemia, elevated triglycerides, and heightened CHD risk due to pro-inflammatory eicosanoid imbalances. Maintaining a lower ratio enhances LA's favorable effects on endothelial function and lipid homeostasis.^[73]^[74] Key clinical evidence includes the Sydney Diet Heart Study from the 1960s, which tested LA-enriched safflower oil replacement of saturated fats in post-heart attack patients and showed lowered serum cholesterol but increased all-cause mortality (17.6% vs. 11.8%; HR 1.62) and cardiovascular mortality (17.2% vs. 11.0%; HR 1.70) compared to control, indicating potential harm from high LA doses without omega-3 balancing. In contrast, recent cohort studies and meta-analyses from the 2020s, such as those analyzing large U.S. and European datasets, confirm cardiovascular benefits at moderate LA levels (2-5% energy intake), with up to 15-20% lower CVD event rates when substituting saturated fats, underscoring the importance of dietary context.^[75]^[5]^[70]

Inflammatory and immune responses

Linoleic acid (LA), an essential omega-6 polyunsaturated fatty acid, plays a dual role in inflammation through its conversion to arachidonic acid (AA), which serves as a precursor for both pro-inflammatory and pro-resolving eicosanoids. AA is metabolized via the 5-lipoxygenase (5-LOX) pathway to produce leukotriene B4 (LTB4), a potent chemoattractant that amplifies neutrophil recruitment and cytokine release during acute inflammation.^[64] In contrast, balanced LA metabolism can lead to anti-inflammatory derivatives, such as lipoxins generated from AA via the lipoxygenase pathway, which actively promote resolution by inhibiting further leukocyte influx and enhancing macrophage phagocytosis.^[76] This duality underscores LA's context-dependent effects, where excessive AA-derived pro-inflammatory signaling predominates in dysregulated states, while equilibrated pathways support inflammation resolution.^[77] Studies in disease models highlight LA's variable impact on inflammatory cytokines. In cystic fibrosis models, elevated LA intake increases AA conversion, elevating pro-inflammatory eicosanoids and cytokines like interleukin-8, exacerbating airway inflammation.^[78] Similarly, high LA-derived AA has been linked to heightened cytokine production in arthritis-like conditions, where omega-6 enrichment amplifies joint inflammation through LTB4-mediated pathways.^[79] Conversely, LA exhibits protective effects in allergic responses; its metabolites modulate T helper 2 (Th2)-biased immunity, reducing hypersensitivity by altering long-chain polyunsaturated fatty acid consumption during allergic inflammation and shifting toward Th1 dominance.^[80] LA incorporation into immune cell membranes enhances T-cell functionality and adaptive immunity. By integrating into phospholipid bilayers, LA improves CD8+ T-cell metabolic fitness, boosting oxidative phosphorylation and preventing exhaustion, which in turn promotes memory-like differentiation and antitumor cytotoxicity.^[81] This membrane modulation also supports antibody production indirectly through optimized T-cell help to B cells, as seen in enhanced humoral responses in LA-supplemented models. Recent post-2020 meta-analyses confirm no net elevation in chronic inflammation markers at recommended dietary intakes (around 12-17 g/day), with higher LA levels associated with reduced all-cause and cardiovascular mortality, suggesting overall anti-inflammatory benefits in balanced diets.^[82] However, in contexts of ultra-processed foods—where LA from seed oils combines with high glycemic loads—oxidized LA metabolites accumulate, potentially heightening inflammatory risks through mitochondrial dysfunction and oxidative stress.^[3] Emerging research as of 2025 suggests potential risks from excessive linoleic acid intake, including associations with higher cancer incidence through mechanisms like oxidative stress and pro-inflammatory eicosanoids, and polycystic ovary syndrome (PCOS)-like changes in rodent models, though human clinical evidence remains limited and debated. Conversely, other 2024-2025 reviews reaffirm cardiometabolic benefits at moderate levels.^[83]^[84]^[85]

Skin and other physiological effects

Linoleic acid plays a critical role in maintaining skin barrier function through its incorporation into ceramides, particularly the ester-linked omega-hydroxy sphingosine (EOS) ceramides that form the lipid lamellae of the stratum corneum. These ceramides, esterified with linoleic acid, provide structural integrity and prevent transepidermal water loss (TEWL), with deficiencies leading to impaired barrier recovery and conditions like essential fatty acid deficiency (EFAD) dermatitis.^[86] Topical application of linoleic acid, often at concentrations around 2-3% in formulations, has been shown to normalize follicular hyperkeratinization in acne vulgaris by reducing comedone formation and improving lesion severity, while also accelerating barrier repair in eczema when combined with ceramides.^[87]^[88] For instance, linoleic acid-enriched moisturizers reduce TEWL by up to 20-30% in atopic dermatitis patients, enhancing hydration and alleviating itch without significant adverse effects.^[89] In reproduction, linoleic acid contributes to sperm membrane fluidity as a major polyunsaturated fatty acid (PUFA) component, facilitating capacitation and acrosome reaction necessary for fertilization; higher sperm membrane linoleic acid levels correlate with improved motility and fertility outcomes in animal models and human studies.^[90] During fetal development, maternal linoleic acid intake supports placental transfer of n-6 PUFAs, aiding embryonic membrane formation and early organogenesis, with deficiencies linked to increased risks of preterm birth and low birth weight.^[91] Neurologically, linoleic acid serves as a precursor to n-6 docosapentaenoic acid (DPA), which is incorporated into brain phospholipids and myelin sheaths, potentially supporting neuronal insulation and signaling; however, human data remain limited, with most evidence from rodent models showing elevated n-6 DPA in brain lipids under linoleic acid supplementation. Recent studies from the 2020s highlight linoleic acid's potential in wound healing via promotion of angiogenesis and collagen deposition, as seen in topical walnut oil applications that accelerate full-thickness burn recovery by stimulating fibroblast proliferation.^[92] Additionally, emerging research suggests linoleic acid influences the gut-skin axis by modulating microbial-derived metabolites that affect skin inflammation, though clinical trials are ongoing. In cosmetics, linoleic acid is commonly incorporated into lotions at 1-5% to bolster barrier function and hydration, often derived from oils like sunflower or soybean for anti-aging and emollient effects.^[93]

History and research

Discovery and early characterization

Linoleic acid was first isolated in 1844 from linseed oil by F. Sacc in the laboratory of Justus von Liebig, where it was identified as an unsaturated fatty acid based on its chemical properties and reactivity.^[94] The name "linoleic acid" originates from the Latin words linum (flax) and oleum (oil), highlighting its derivation from flaxseed (linseed) oil.^[1] In 1929, George O. Burr and Mildred M. Burr conducted pioneering experiments on rats fed a fat-free diet, observing a deficiency syndrome characterized by growth retardation, skin lesions, and reproductive failure, which they attributed to the absence of specific unsaturated fatty acids.^[95] They demonstrated that supplementing the diet with linoleic acid prevented and cured this syndrome, establishing linoleic acid as an essential nutrient required in small amounts for normal health and growth.^[96] This work marked a paradigm shift, proving that certain fatty acids are indispensable dietary components, as mammals cannot synthesize them de novo.^[96] During the 1930s and 1940s, the chemical structure of linoleic acid was elucidated through a series of analytical techniques, including hydrogenation, bromination, and ozonolysis. Researchers such as T. P. Hilditch and colleagues confirmed the positions of the two double bonds at carbons 9 and 12 (counting from the carboxyl end), establishing it as all-cis-9,12-octadecadienoic acid.^[97] Ozonolysis, in particular, proved instrumental in cleaving the double bonds to yield identifiable fragments like azelaic acid and pelargonaldehyde, providing direct evidence for the double bond locations.^[98] These studies also revealed early nomenclature variations, with linoleic acid initially described by its delta notation (Δ9,12) before being recognized as the parent compound of the omega-6 fatty acid family due to the position of its terminal double bond (counted from the methyl end).^[95] The omega-6 designation, formalized in subsequent decades, underscored its role in the biosynthetic pathway leading to longer-chain polyunsaturated fatty acids like arachidonic acid.^[2]

Modern studies and controversies

In the mid-20th century, several large randomized controlled trials examined the impact of increasing linoleic acid intake through vegetable oils on cardiovascular disease outcomes, yielding mixed results that fueled ongoing debates. The Minnesota Coronary Experiment (1968–1973), involving 9,423 participants in Minnesota nursing homes and mental hospitals, tested replacing saturated fats with corn oil providing about 13% of energy from linoleic acid; while serum cholesterol decreased by 13.8% in the intervention group, there was no reduction in coronary heart disease mortality, and all-cause mortality was 22% higher for each 30 mg/dL cholesterol reduction.^[99] Similar findings emerged from the Sydney Diet Heart Study (1966–1973), where linoleic acid enrichment (15% of energy) lowered cholesterol but increased cardiovascular mortality by 77% (HR 1.77) compared to the control group.^[99] These results, published or reanalyzed in the 1980s and 1990s, prompted caution against excessive omega-6 polyunsaturated fatty acid consumption, influencing dietary guidelines to emphasize balance with omega-3s rather than unrestricted linoleic acid intake.^[99] Advancements in the 2000s shifted focus to genetic influences on linoleic acid metabolism, revealing interpersonal variability in its conversion to bioactive derivatives. Polymorphisms in the FADS gene cluster, particularly rs174537 near FADS1 and FADS2, were strongly associated with desaturase activity; carriers of minor alleles exhibited up to 20–30% lower plasma levels of arachidonic acid derived from linoleic acid, affecting cardiometabolic responses to dietary intake.^[100] These insights informed personalized nutrition research. Paralleling this, meta-analyses from the 2010s onward affirmed cardiovascular benefits at moderate levels: a pooled analysis of 32 studies found that each 5% increase in energy from linoleic acid correlated with a 15% reduction in coronary heart disease events, supporting intakes of 5–10% of total energy without adverse effects.^[101] Controversies surrounding linoleic acid intensified in the 2010s, with claims that its abundance in industrialized seed oils promotes inflammation and diseases like cancer, though recent evidence has largely refuted these. Pro-inflammatory hypotheses, based on arachidonic acid's role in eicosanoid production, were challenged by a 2012 systematic review of 15 randomized trials (referenced in a 2018 analysis), which found no increase in inflammatory markers (e.g., C-reactive protein or interleukin-6) with linoleic acid supplementation up to 10% of energy.^[102] For cancer, data from the European Prospective Investigation into Cancer and Nutrition (EPIC) cohort, involving over 500,000 participants, showed no positive association; a meta-analysis incorporating EPIC results indicated a modest, non-significant reduction in breast cancer risk (RR 0.97 per 5 g/day higher linoleic acid intake), with neutral or inverse links for other sites.^[103] Persistent concerns focus on seed oil processing, where high-heat extraction may generate oxidized linoleic acid metabolites that damage endothelium and exacerbate atherosclerosis, as proposed in mechanistic reviews linking these to coronary heart disease progression.^[104] Significant research gaps hinder a complete understanding of linoleic acid's long-term implications. Human trials on intakes exceeding 10% of energy are scarce, with most data limited to 1–5 years, leaving uncertainties about chronic effects on oxidative stress or insulin sensitivity.^[5] Interactions with the gut microbiome remain underexplored, though rodent studies suggest high-linoleic diets alter endocannabinoid signaling and microbial composition, potentially heightening colitis risk without clear human translation.^[105] Sustainability issues in seed oil production, including high water use and land conversion for crops like soybeans, underscore needs for eco-friendly sourcing. Emerging vegan supplement research, such as fortified algal oils providing balanced omega-6 without animal-derived arachidonic acid precursors, shows promise for optimizing intake in plant-based diets, but lacks longitudinal efficacy data.^[106] As of 2025, recent reviews and cohort studies continue to support that higher linoleic acid intake is associated with reduced risks of cardiovascular disease and type 2 diabetes, with no evidence of increased inflammation.^[5]^[107]

References

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It has a role as a plant metabolite, a Daphnia galeata metabolite and an algal metabolite. It is an omega-6 fatty acid and an octadecadienoic acid.
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In particular, the antioxidative and anti-inflammatory properties of these PUFA may help protect neurons, promote synaptic plasticity, and limit cellular death.
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Nov 16, 2005 · Average Molecular Weight, 280.4455 ; Monoisotopic Molecular Weight, 280.240230268 ; IUPAC Name, (9Z,12Z)-octadeca-9,12-dienoic acid ; Traditional ...
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The term conjugated linoleic acid (CLA) refers to a mixture of linoleic acid positional and geometric isomers, characterized by having conjugated double ...
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linoleic acid, 60-33-3 - The Good Scents Company
It is a pale, oily liquid with low odor, and is highly resistant to discoloration on exposure to heat and light. Because of the near absence of saturated fatty ...
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Linoleic Acid - an overview | ScienceDirect Topics
Due to multiple double bonds, polyunsaturated fatty acids (principally linolenic acid and to a lesser degree linoleic acid) are also oxidatively unstable.
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Lipid oxidation in foods and its implications on proteins - PMC - NIH
Jun 15, 2023 · Lipid oxidation is one of the leading causes of food spoilage. It refers to how unsaturated fatty acids in fats are slowly oxidized when exposed ...
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and Double-Fractionated Olein of Safflower Oil Produced by Low ...
Among all sources of vegetable oils, safflower oil (SO) possesses the maximum linoleic acid content. To boost the industrial applications of SO, two ...
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linoleic acid - Organic Syntheses Procedure
The linoleic acid rises to the surface as a clear layer. The water layer is siphoned off; the acid is washed once with hot water and then dried over anhydrous ...
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Catalytic hydrogenation of linoleic acid to stearic acid over different Pd
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Nov 7, 2009 · The triglycerides were synthesized by esterification of glycerol and conjugated linoleic acid (CLA) in a no-solvent system using lipase ...
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Isomers of Conjugated Fatty Acids. I. Alkali-isomerized Linoleic Acid
Direct Isomerization of Linoleic Acid to Conjugated Linoleic Acids (CLA) using Gold Catalysts. Chemical Engineering & Technology 2009, 32 (12) , 2005-2010 ...
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Oils Rich in Linoleic Acid - News-Medical
Linoleic acid (alongside oleic acid) represents major unsaturated fatty acid present in practically all oils. Peanut, cotton and corn oils display higher ...
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The major fatty acid of safflower oil is linoleic acid, which accounted for 55.1-77.0% in oils, with a mean value of 70.66%.
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Being non-ruminant animals, pork and horse meats showed c9,t11 CLA levels lower than 1 mg/g fat, in agreement with previous studies (Schmid et al., Citation2006) ...
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Linoleic acid (LA) (n−6) and α-linolenic acid (ALA) (n−3) are essential ... Chemical structure of gallic acid. IUPAC name: 3, 4,5-trihydroxybenzoic acid.Missing: classification | Show results with:classification
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Examples of extractive components in wood – fatty acids represented by linoleic acid, resin acids represented by abietic acid, and the esterified compounds ...
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Linoleic Acid, Vitamin E and Other Fat-Soluble Substances in the ...
Without linoleic acid there is no emergence, with very little linoleic acid the cuticles separate, but the scales remain stuck to the wrong cuticle, and ...
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[PDF] Fats and fatty acids in human nutrition
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Nov 29, 2021 · ... diet with 5.3 percent linoleic acid or to a so-called Mediterranean diet with 3.6 percent linoleic acid. [18] The diets differed in many ...
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May 31, 2008 · The separation and purification of linoleic acid (LA) from sunflower seed oil by urea complex fractionation was studied.
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Signs include scaly dermatitis, alopecia, thrombocytopenia, and, in children, intellectual disability.
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Dihomo-γ-linolenic acid (DGLA, 20:3n-6) is a polyunsaturated fatty acid (PUFA) that is usually present in low proportions in mammals but has recently emerged as ...
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Quantifying conversion of linoleic to arachidonic and other n-6 ...
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[PDF] Appendix E‐2.43: Saturated Fat and Risk of CVD Evidence Portfolio
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Dietary Linoleic Acid and Risk of Coronary Heart Disease
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Indeed, the essential fatty acid linoleic acid (LA; C18:2n-6) is of particular significance to skin health, as it contributes to the formation of ceramides ...
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It has been shown that topical linoleic acid rich-phosphatidylcholine seems to be effective in normalization of follicular hyperkeratinization, ...
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The discovery of essential fatty acids is one of the great advances in lipid research. It reversed the commonly held belief that dietary fat was merely a ...
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More research is needed on the long-term effects of low erythrocyte and adipose EPA and DHA and chronic disease risk in both VGNs and omnivores. 8 ...