Linoleic acid (LA) is a polyunsaturated ω-6 fatty acid. It is a colorless liquid at room temperature. In physiological literature, it has a lipid number of 18:2 cis,cis-9,12. From the chemistry perspective, linoleic acid is a carboxylic acid with an 18-carbon chain and two cis double bonds; with the first double bond located at the sixth carbon from the methyl end.
Two main fatty acids essential in the diet are Linoleic (or ω-6) fatty acid and α-Linolenic (or ω-3) acid. Both of them are polyunsaturated fatty acid, which means that they possess two or more double bonds and lack several hydrogen atoms that are found in saturated fatty acids. Linoleic acid belongs to one of the two families of essential fatty acids, which means that the human body cannot synthesize it from other food components.
Linoleic acid keeps the skin impermeable to water, but to exert other effects the compound must undergo specific metabolism. First step is conversion to γ-Linolenic acid by δ-6-desaturation. γ-Linolenic acid is subsequently converted to dihomo-γ-Linolenic acid, which is in turn converted to Arachidonic acid.
Arachidonic acid can form prostaglandins and thromboxanes – hormone-like lipids that promote blood clotting, induce inflammation and cause smooth muscle contraction. In alternative pathway it can also form Leukotrienes, which are one of the most potent inflammatory agents in the human organism.
The necessity of metabolism is reflected by the increasing potency of each substance in the form of an essential fatty acid, as it moves down the chain from Linoleic to Arachidonic acid; hence to achieve full range of activities, Linoleic acid must be metabolized to other substances. Therefore it can be considered as analogous to proVitamines.
In infants, δ-6-desaturase is too immature to provide the desired metabolism of Linoleic acid, which is a reason why human milk contains γ-Linoleic acid, dihomo-γ-Linoleic acid and Arachidonic acid. In contrast, conventional infant formula milks have only Linoleic and α-Linolenic acid, which can lead to a deficiency state in formula-fed infants.
The word "Linoleic" derived from the Greek word Linon (flax). Oleic means "of, relating to, or derived from oil of olive" or "of or relating to oleic acid" because saturating the ω-6 double bond produces oleic acid.
