Linoleic Acid

Linoleic acid (LA, 18:2n-6) is an 18-carbon essential fatty acid with two cis double bonds, located between C9-C10 and C12-C13. As the parent omega-6 fatty acid, LA is the dietary precursor for the entire omega-6 eicosanoid cascade, including arachidonic acid and its downstream prostaglandins, thromboxanes, and leukotrienes. Linoleic acid is one of two essential fatty acids (along with alpha-linolenic acid) that humans must obtain from diet.

Essentiality and requirements

Humans lack the delta-12 desaturase enzyme needed to introduce a double bond at the omega-6 position of a fatty acid. Linoleic acid must therefore come from the diet. Frank linoleic acid deficiency was demonstrated in the classical fat-free diet experiments of the 1950s-60s, producing dermatitis, growth retardation, and reproductive impairment, reversible by linoleic acid reintroduction. The NAM 2005 Adequate Intake is 14 g/day for adult men and 11 g/day for adult women, derived from median US intake rather than a deficiency threshold — frank deficiency is extraordinarily rare in the modern food supply.

Metabolism

Ingested linoleic acid is incorporated into membrane phospholipids, stored as triglyceride, or converted stepwise to longer-chain omega-6 fatty acids: delta-6 desaturase produces gamma-linolenic acid (GLA, 18:3n-6); elongase-5 extends to dihomo-gamma-linolenic acid (DGLA, 20:3n-6); delta-5 desaturase yields arachidonic acid (AA, 20:4n-6). Conversion rates are modest — approximately 0.3-6% of linoleic acid is converted to arachidonic acid, depending on measurement approach and metabolic state.

Intake and food sources

Typical US linoleic acid intake: 12-20 g/day, approximately 6-8% of total energy. Major sources per USDA FoodData Central: soybean oil 53% LA (1 tablespoon ~7 g), corn oil 54% (~7 g), sunflower oil 60% standard / 30% high-oleic, safflower oil 75% standard, walnuts 38% of fat as LA, grapeseed oil 72%. Whole foods contributing substantial LA include nuts, seeds, and products made with vegetable oils.

Cardiovascular outcomes

Farvid et al. (Circulation 2014) conducted a systematic review and meta-analysis of 13 prospective cohorts comprising over 300,000 participants and found a 15% reduction in coronary heart disease mortality for the highest versus lowest quintile of linoleic acid intake. Randomized trial evidence aligned: Mozaffarian et al. (2010) found cardiovascular event reductions with PUFA-for-SFA substitution, predominantly driven by linoleic acid. The AHA 2009 science advisory on omega-6 fatty acids concluded that at least 5-10% of energy from omega-6 PUFA is associated with reduced CHD risk.

Inflammatory claims

Popular narratives frame linoleic acid as "inflammatory" via its role as arachidonic acid precursor. The human evidence does not support this framing: controlled feeding studies of LA intake up to 20% of energy show no consistent increase in inflammatory markers (CRP, IL-6, TNF-alpha) or in tissue arachidonic acid content, because LA-to-AA conversion is tightly regulated and rate-limited by delta-6 desaturase. The AHA 2009 advisory explicitly addressed this point.

Oxidation and linoleic acid

Linoleic acid is susceptible to peroxidation, producing lipid hydroperoxides, malondialdehyde, and 4-hydroxynonenal. In vitro and animal models at high exposure doses show cellular stress responses. In humans at normal dietary intakes, with adequate antioxidant status (vitamin E, selenium, polyphenols from whole foods), oxidative damage from LA is not a demonstrated clinical concern. Industrial high-temperature frying with oil reuse is a separate exposure pathway where oxidation products reach higher concentrations.