**5. Essential fatty acids**

The capability of an organism to metabolically introduce double bonds in certain positions of a fatty acid or the inability to do this, determines the existence of the so-called nonessential or essential fatty acids (EFAs). According to this capability, mammals, including primates and humans, can introduce a double bond only at the C9 position of a saturated fatty acid (according to "ω" nomenclature) and to other carbons nearest to the carboxyl group, but not at carbons nearest the C1 position [38]. This is the reason why OA is not an EFA. In contrast, mammals can not introduce double bonds at C6 and C3 positions, being the reason why AL and ALA are EFAs. By derivation, the AA is formed by the elongation and desaturation of LA, and EPA and DHA, which are formed by elongation and desaturation of ALA, become also essential for mammals when their respective precursors (LA and ALA, respectively) are nutritionally deficient [39]. Figure 3 shows the chemical structure of a SAFA, such as the stearic acid (C18:0), AO, LA and ALA, exemplifying the "ω" notation of each and indicating the essential condition in relation to the position of their unsaturated bonds.

**Figure 3.** The chemical structure of a SAFA, such as the stearic acid (C18:0), AO, LA and ALA, exemplifying the "ω" notation of each and indicating the essential condition in relation to the position of their unsaturated bonds
