Linoleic Acid (LA): A Comprehensive Reference
1. Identity
Chemical and Botanical Names
Linoleic acid (LA) is a polyunsaturated fatty acid belonging to the omega-6 (n-6) family.
Its molecular formula is C₁₈H₃₂O₂, corresponding to a monocarboxylic acid composed of a straight chain with 18 carbon atoms and two double bonds in the cis configuration, at positions 9 and 12.
It is accordingly designated as 18:2(n-6) in lipid shorthand.
In systematic nomenclature, linoleic acid can also be called cis-9,cis-12-octadecadienoic acid.
Linoleic acid was first isolated in 1844 from flaxseed (linseed) oil, which also inspired its name: linum meaning "flax" in Latin, and oleic meaning "pertaining to oil."
Natural Sources
Linoleic acid is a polyunsaturated essential fatty acid found mostly in plant oils. It is known as the parent fatty acid of the omega-6 series, and it is essential for human nutrition because it cannot be synthesized by the human body.
It is the primary omega-6 fatty acid in most diets; the major source is vegetable oils such as sunflower, safflower, and corn oils.
It accounts for 40–50% of the total PUFAs in beans, 75–85% in beef and chicken meats, exceeds 80% in eggs, and reaches approximately 88% in soybean oil. Currently, soybean oil accounts for approximately 45% of dietary linoleic acid in the US diet.
Common Forms and Preparations
Linoleic acid (LA) is a polyunsaturated fatty acid commonly found in nutraceutical supplements and used to fortify foods.
Formulations for oral administration include tablets, capsules, and liquids.
Formulations also exist for topical (dermal) administration.
For commercial supplement production, linoleic acid is obtained from the hydrolysis of vegetable oils rich in linoleic acid, mainly sunflower and soybean oils, and the obtained fatty acid is purified by further distillation.
LA is found in various plant-based oils, including soybean, corn, sunflower, and safflower, and these oils can be used to fortify food products such as bread, pasta, and cereal to enhance their nutritional value.
2. Traditional and Historical Use
Pre-Scientific Dietary Use
Ancient diets that included nuts, seeds (like flax, sesame, and sunflower), whole grains, and pressed oils naturally provided linoleic acid — though people had no knowledge of its chemical structure.
Until the 20th century, people easily obtained the amount they needed from foods such as nuts, grains, meat, eggs, and dairy products.
Up until the late 1930s, linoleic acid consumption averaged around 1 to 2% of daily calories — but when agricultural shifts led to increased production of soybean oil and corn oil (both high in linoleic acid), those levels rose to the current intake of over 7% of calories.
Ayurvedic and Traditional Oilseed Cultures
In Ayurveda, unrefined sesame oil (rich in LA) was used internally and externally to balance the body, though this usage predates any recognition of LA as a discrete nutrient.
More broadly, the pressing and culinary use of oilseed crops — sunflower, safflower, sesame, and flax — across ancient civilizations in the Middle East, South Asia, and the Mediterranean meant that dietary LA intake was woven into traditional food culture for millennia, even without awareness of its chemical identity.
Scientific Discovery: The Burr Era (1929–1930)
Dietary fat was recognized as a good source of energy and fat-soluble vitamins by the first part of the 20th century, but fatty acids were not considered essential nutrients because they could be synthesized from dietary carbohydrate. This well-established view was challenged in 1929 when George Burr reported that dietary fatty acid was required to prevent a deficiency disease that occurred in rats fed a fat-free diet. He concluded that fatty acids were essential nutrients, and he showed that linoleic acid prevented the disease.
Burr found that oils containing linoleic acid and methyl linoleate were effective against the deficiency, and stated: "We were driven to the conclusion that the only thing that could be missing from the diet was linoleic acid." In March or April 1930, he wrote a paper announcing linoleic acid as an essential fatty acid — and that term was born.
Burr subsequently demonstrated that linoleic acid was converted to arachidonic acid and that linolenic acid, the omega-3 analog, was also an essential fatty acid.
These findings were controversial initially because of the prevailing opinion that fatty acids were only a concentrated source of calories, but the effectiveness of linoleic acid was confirmed in many other laboratories, and there was general agreement by 1940 that linoleic acid is an essential nutrient for animals.
20th Century Industrial Expansion
Per capita consumption of soybean oil (containing about 55% linoleic acid) increased 1,000-fold between 1909 and 1999, along with a simultaneous increase in consumption of other industrial seed oils rich in omega-6 fatty acids that were not widely available before the 20th century.
Linoleic acid is the only essential fatty acid whose intake has changed dramatically over the last century, rising from 1 to 2% of daily calories in the late 1930s to over 7% today due to agricultural shifts that led to increased production of soybean oil and corn oil.
3. Key Constituents and Mechanisms of Action
Chemical Character
Linoleic acid is an essential fatty acid that plays a significant role in maintaining human health. It cannot be synthesized in the human body and must be obtained through dietary sources; it is a vital structural component for membrane fluidity and serves as a precursor for γ-linolenic acid, arachidonic acid, and eicosanoids.
Metabolic Conversion Pathway
Arachidonic acid is synthesized from linoleic acid (LA) via a process starting with the conversion of LA into gamma-linolenic acid (GLA), effected by Δ6 desaturase.
Linoleic acid is then converted to arachidonic acid by a series of fatty acid desaturase (FADS) and elongase (ELVOL) enzymes.
This pathway proceeds: LA (18:2 n-6) → GLA (18:3 n-6) → DGLA (20:3 n-6) → Arachidonic Acid / AA (20:4 n-6).
These metabolites — GLA, DGLA, and arachidonic acid — are the long-chain derivatives in the omega-6 pathway.
Eicosanoid Production
The arachidonic acid pathway plays a key role in cardiovascular biology, carcinogenesis, and many inflammatory diseases. Esterified AA on the inner surface of the cell membrane is hydrolyzed to its free form by phospholipase A2 (PLA2), which is in turn further metabolized by cyclooxygenases (COXs) and lipoxygenases (LOXs) and cytochrome P450 (CYP) enzymes to a spectrum of bioactive mediators including prostanoids, leukotrienes, epoxyeicosatrienoic acids, and lipoxins.
Eicosanoids are typically not stored within cells but are synthesized as required. They derive from the fatty acids that make up the cell membrane and nuclear membrane, and must be released from membrane sites and then metabolized through various pathways to yield bioactive eicosanoids.
Cell Membrane Structure and Fluidity
Along with other omega-6 and omega-3 fatty acids, arachidonic acid (derived from LA) contributes to the structure of cell membranes. When incorporated into phospholipids, omega fatty acids affect membrane properties such as permeability and the activity of enzymes and cell-signaling mechanisms.
LA may exert its LDL-C lowering properties by increasing membrane fluidity, which increases LDL receptor activity and consequently decreases LDL apoB and increases LDL catabolism.
Incorporating unsaturated fats may also improve cell membrane fluidity and functions such as GLUT translocation, insulin receptor binding and affinity, cell signaling, and ion permeability, collectively improving insulin sensitivity.
Skin Ceramide and Barrier Function
LA can be oxidized to produce oxylipins, including HODEs and oxo-ODEs, which function in cell proliferation, differentiation, and inflammatory processes. It is of particular importance that LA can be incorporated into ceramides, thereby playing a vital role in the formation of the skin barrier.
Regulation of Gene Expression and Fat Metabolism
LA might also affect the balance between fat oxidation and synthesis by regulating related gene expression (such as SREBP1), which could explain the reduced hepatic fat contents of obese participants consuming a diet containing a large amount of LA during a 10-week intervention.
4. Scientific Evidence by Area of Use
4.1 Cardiovascular Disease and Lipid Profiles
Mechanism: In experimental studies, LA has been shown to lower serum levels of low-density lipoprotein cholesterol (LDL-C), especially when substituted for saturated fatty acids.
LA also increases CYP7 activity, thereby converting cholesterol to bile acids in the liver, which indirectly increases LDL receptor production.
Linoleic acid consumption has also been reported to increase HDL levels due to an increase in apolipoprotein A1 (ApoA1) expression, which triggers cholesterol esterification that converts cholesterol to a form that can be fully integrated into HDL and subsequently transported to the liver.
Cohort evidence: A systematic review and meta-analysis of prospective cohort studies searched MEDLINE and EMBASE databases through June 2013 and identified 13 published and unpublished cohort studies with a total of 310,602 individuals and 12,479 total CHD events, including 5,882 CHD deaths.
In observational studies in which dietary intake or serum content of LA were either cross-sectionally or prospectively related to risk for CHD, higher LA intakes or serum levels have usually been associated with reduced risk.
Intervention trial evidence — conflicting results:
Dietary intervention studies have shown that replacing 5% of the dietary energy derived from saturated fat with n-6 PUFA reduces LDL-C by up to 10%, consequently resulting in a significant reduction of CVD risk.
However, in the Sydney Diet Heart Study cohort, substituting dietary linoleic acid in place of saturated fats increased the rates of death from all causes, coronary heart disease, and cardiovascular disease; an updated meta-analysis of linoleic acid intervention trials showed no evidence of cardiovascular benefit.
This represents a significant controversy in the field, and the overall human evidence on CHD outcomes from LA substitution trials is mixed.
4.2 Type 2 Diabetes and Glycemic Control
Dietary fat quality impacts the risk of many chronic diseases, including type 2 diabetes (T2DM). A pooled analysis involving 20 studies from around the world revealed that higher linoleic acid biomarker is associated with dose-dependent decreases in the incidence of T2DM.
This corroborates earlier cross-sectional studies and intervention trials showing that biomarkers of LA intake are associated with reduced risk of T2DM and better glycemic control and/or insulin sensitivity.
Potential mechanisms linking LA intake and type 2 diabetes have not been fully elucidated, but one proposed explanation is that incorporating unsaturated fats may improve cell membrane fluidity and functions including GLUT translocation and insulin receptor binding, collectively improving insulin sensitivity.
A high-LA diet may also reduce abdominal fat, which is an established risk factor for type 2 diabetes.
A note of caution: findings from pooled biomarker analyses of linoleic acid and incident T2DM should be interpreted with caution, as linoleic acid and arachidonic acid in serum are derived from the diet but levels are controlled by the rate of peroxidation to bioactive metabolites; serum linoleic acid status can be strongly affected by alcohol intake or smoking, which promote the formation of such metabolites.
Evidence in this area is rated moderate — consistent from cohort data but mechanistic pathways are not fully characterized.
4.3 Skin Health — Barrier Function, Acne, and Atopic Dermatitis
LA can be incorporated into ceramides, thereby playing a vital role in the formation of the skin barrier. Abnormalities in the metabolism of LA have been linked to the development of various skin diseases, including acne, atopic dermatitis, and psoriasis. Studies have revealed that the topical application of LA or LA-rich vegetable oils exerts regulatory effects on skin health and hair growth.
Essential fatty acid deficiency and skin: Essential fatty acid (EFA)-deficient rodents demonstrate abnormal epidermal permeability barrier function and differentiation — defects which can be corrected by either topical or systemic administration of linoleic acid.
Dietary deficiency of linoleic acid results in a scaly and pruritic skin disorder similar to atopic dermatitis in hairless mice.
Acne: In acne patients, a significant portion of LA may undergo β-oxidation, thus reducing its content in the skin; acne patients also exhibit elevated total sebum secretion, which may dilute the proportion of LA in sebum. Despite these hypotheses, the precise mechanisms underlying the reduced LA levels in acne pathogenesis remain unclear and require further investigation.
Atopic dermatitis: Atopic dermatitis (AD) is one of the most prevalent chronic inflammatory dermatological conditions. Elevated LA levels in serum have been observed in AD patients, accompanied by reduced levels of its metabolites, such as γ-linolenic acid (GLA) and arachidonic acid (AA).
Studies based on animal or skin cell models suggest that LA or LA-rich vegetable oils, topically applied, exhibit diverse biological activities, including the repair of the skin barrier, promotion of wound healing, skin whitening, photoprotection, anti-inflammatory effects, and stimulation of hair growth.
The evidence for topical LA in skin conditions is preliminary to moderate, coming largely from animal and in vitro models, with limited randomized controlled clinical trial data specifically on LA itself.
4.4 Essential Fatty Acid Deficiency
Linoleic acid intake at levels from 1 to 2% of total dietary calories is sufficient to prevent both biochemical and clinical evidence of deficiency in several animal species and in humans.
Insufficient linoleic acid in the diet can lead to adverse health effects, including hair loss and poor wound healing.
The essentiality of fatty acids was determined by the Burrs in the 1920s, and it is commonly accepted that provision of linoleic acid (LA) and alpha-linolenic acid (ALA) prevents and reverses essential fatty acid deficiency (EFAD).
The evidence here is well-established and is the basis for the Adequate Intake designation.
4.5 Physical Function in Older Adults
A pilot study found that a final analytical sample of 317 participants aged 74.4 ± 2.8 years consumed 18.9 ± 11.4 g/day of linoleic acid, with 24.6% of participants having low daily linoleic acid intake. Persons with low daily linoleic acid intake had 2.58 times (95% CI: 1.27–5.24) greater odds for a limitation in climbing stairs. The pilot investigation found that low daily linoleic acid intake could be associated with physical function in older adults.
This evidence is very preliminary (pilot observational study) and requires replication.
4.6 Inflammation — Mixed and Contested Evidence
In epidemiologic studies, there is little evidence that suggests linoleic acid contributes to cardiovascular disease, cancer, or inflammation, and inverse correlations may exist.
Conversely, at the mechanistic level, emphasis has been placed on the potential pro-inflammatory properties of some n-6 PUFA metabolites (e.g., some eicosanoids deriving from arachidonic acid) and on the competition between LA and ALA as substrates for the same metabolic pathway, possibly leading to reduced levels of ALA-derived n-3 long-chain PUFA in organs and tissues.
The 12/15-lipoxygenase enzyme catalyzes the insertion of molecular oxygen into linoleic acid; 9- and 13-hydroxy-octadecadienoic acids (HODEs) are its products, all of which are considered pro-inflammatory.
Overall, the relationship of LA with systemic inflammation in humans is not resolved and the evidence is genuinely mixed.
5. Body Systems and Health Areas
- Cardiovascular system: Higher LA intake reduces LDL cholesterol, promotes insulin sensitivity, and reduces risk of hypertension.
- Metabolic / endocrine system: Membrane incorporation of LA may improve insulin receptor binding and cell signaling, collectively improving insulin sensitivity.
- Integumentary system (skin and hair): Skin diseases including acne, atopic dermatitis, and psoriasis are associated with disordered LA metabolism.
- Immune system: LA and its metabolites are involved in maintenance of the stratum corneum permeability barrier, inhibition of proinflammatory eicosanoids, elevation of the sunburn threshold, inhibition of proinflammatory cytokines (TNF-α, IFN-γ, and IL-12), and promotion of wound healing.
- Neurological system: Arachidonic acid — a major downstream metabolite of LA — is one of the most abundant fatty acids in the brain and is involved in the early neurological development of infants.
- Musculoskeletal system: Low daily linoleic acid intake has been associated with physical function limitations in older adults in pilot data.
6. Dosage: Adequate Intakes and Study-Reported Amounts
Dietary Reference Values
The dietary reference intakes for linoleic acid report that the adequate intakes (AIs) for women and men between the ages of 19 and 50 years are 12 g/d and 17 g/d, respectively; the AI is based on approximate median intakes of healthy individuals in the US population.
These amounts are modified to 11 g/d and 14 g/d for women and men, respectively, between the ages of 51 and 70.
The American Academy of Pediatrics (AAP, 1985) recommended that infant formulas provide at least 2.7% of energy as linoleic acid.
For the average adult, a minimally adequate intake of linoleic acid is 3 to 6 g/day.
The American Heart Association recommends an Acceptable Macronutrient Distribution Range (AMDR) for LA of 5–10% of total energy.
Population-level reports showed typical intakes of LA around 12.6 g/day by adult women and 16.0 g/day by adult men in 2007, corresponding to 5.5% and 6.0% of total average energy intake, respectively.
No upper limit (UL) has been set for linoleic acid because of a lack of a defined intake establishing adverse effects.
Nevertheless, consumption above recommended intakes should be carefully considered because there are equally insufficient data to adequately evaluate adverse effects at these higher levels.
Amounts in Clinical Studies
Current intake of LA in the US is approximately 12–17 g/d or about 6% of energy, provided largely by seed oils or foods that contain them.
In a pilot physical function study, participants aged approximately 74 years consumed an average of 18.9 ± 11.4 g/day of linoleic acid.
An intervention study by Bjermo et al. compared n-6 PUFAs with saturated fatty acids on liver fat and lipoproteins, with Summers et al. demonstrating that substituting dietary saturated fat with polyunsaturated fat changes abdominal fat distribution and improves insulin sensitivity.
7. Safety Considerations and Interactions
Oxidative Susceptibility
One unique concern about linoleic acid involves its susceptibility to oxidation. LA's two double bonds make it chemically unstable compared to saturated or monounsaturated fats.
Omega-6 and omega-3 polyunsaturated fatty acids can be non-enzymatically oxidized and are also substrates for oxidation by lipoxygenases, cyclooxygenases, and epoxygenases; 9- and 13-hydroxy-octadecadienoic acids (HODEs), the 12/15-lipoxygenase products of linoleic acid, are pro-inflammatory oxidation products.
Competition with Omega-3 Fatty Acids (ALA/EPA/DHA)
An LA intake that is too high can impair endogenous synthesis of eicosapentaenoic acid (EPA) from alpha-linolenic acid (ALA), but the threshold at which this becomes clinically relevant is not known.
High intakes of LA competitively interfere with the endogenous conversion of alpha-linolenic acid (ALA) to EPA and DHA; high somatic levels of LA/low ALA indicate a decreased ALA conversion to EPA and DHA.
Notably, the omega-6 to omega-3 PUFA ratio alone is not informative and does not shed meaningful insight about the amount of individual fatty acids in each class needed to confer health benefits.
High-Intake Controversy
More than a century after linoleic acid was first described as an essential nutrient, there is concern that current intake levels are unhealthy, and it has been suggested that high dietary linoleic acid intake may pose risks.
Some research indicates that linoleic acid serum concentrations (as opposed to percent of fatty acids) are higher in patients with coronary artery disease.
However, other recent evidence supports the hypothesis that higher intakes of LA are associated with improvements in relevant biomarkers and with lower risk for developing cardiometabolic diseases.
This area represents one of the most actively debated topics in nutritional science, and a firm consensus has not been reached.
Absence of an Established Upper Limit
No upper limit (UL) has been set for linoleic acid because of a lack of a defined intake establishing adverse effects.
Updating dietary reference intakes (DRIs) for LA and ALA is needed; however, there are insufficient data to establish Recommended Dietary Allowances (RDAs) for these fatty acids.
Deficiency Symptoms
Insufficient linoleic acid in the diet can lead to adverse health effects, including hair loss and poor wound healing.
Linoleic acid intake at levels from 1 to 2% of total dietary calories is sufficient to prevent both biochemical and clinical evidence of deficiency in several animal species and in humans.
References
Volver