5  Overview of the n-3 polyunsaturated fatty acids
The long-chain n-3 polyunsaturated fatty acids (n-3 PUFAs) are essential fatty acids obtained from both dietary and non-dietary sources. The simplest n-3 PUFA is α-linolenic acid (ALA, 18:3 n-3). Once inside the body, ALA can be converted through a series of elongation and desaturation reactions into other n-3 PUFAs. For instance, ALA is metabolized into eicosapentaenoic acid (EPA, C20:5n-3) which can be further metabolized into docosahexaenoic acid (DHA, C22:6n-3), the two most abundant n-3 PUFAs in mammalian tissues (Wiktorowska-Owczarek, Berezinska, & Nowak, 2015). Mammals lack the necessary enzymes (delta-12 and delta-15 desaturase) required to synthesize ALA de novo. As such, these fatty acids are described as “essential” and must be obtained from the diet such as fish, other marine sources, plants or supplements (Burdge & Calder, 2015; Sprecher, 1981). Conversely, linoleic acid (LA, 18:2 n-6) is considered the primary source of the essential n-6 PUFAs. LA can be further metabolized into arachidonic acid (AA, 20:4n-6) by the same series of elongase and delta-4,-5,-6 desaturase enzymes. As n-3 PUFAs can compete for the same metabolic pathways with n-6 PUFAs, n-3 PUFAs supplementation may reduce the synthesis of n-6 PUFA-derived metabolites, thus, altering the metabolite profile and impacting numerous signaling pathways within the body, including the immune system, leading to disparate effects (Arterburn, Hall, & Oken, 2006).