**1.2 Omega-3 (ω-3) versus omega-6 (ω-6) PUFAs**

The Greek letter omega (ω) is used in the systemic nomenclature of the polyunsaturated fatty acids (PUFAs). The PUFAs that have a C=C double bond between the 6th and 7th carbon position counting from the terminal methyl end are called ω-6 and those with the double bond between the 3th and 4th carbon are called ω-3 PUFAs. The letter 'n' is also used to denote the position of the double bond. The locations of double bonds in the PUFAs confer huge differences in their physical, biochemical, and physiological properties. The essential fatty acid (EFA) linoleic acid (C18:2) is of ω-6 series, while the EFA α-linolenic acid is the member of ω-3 series. Some of the beneficial effects overlap between the ω-3 and ω-6, while many effects are antagonistic to each other. ω-6 PUFAs can be found in vegetable oils and seeds, whereas ω-3 PUFA is found more in fish/marine animals, walnuts, and canola oil.

#### **Figure 1.**

*The straight chain structural features of the most common fatty acids. PLA = palmitic acid, STA = stearic acid, OLA = oleic acid, LLA = linoleic acid, LLN = α-linolenic acid, AA = arachidonic acid, EPA = eicosapentaenoic acid, DHA = docosahexaenoic acid. Omega (ω) is used to denote the position of double bonds from the methyl end of the fatty acid. Colored curved arrows = biological conversion is possible from the precursor by the actions of elongase/desaturase enzymes. Black arrow = indicates the position(s) of double bond.*

**13**

*Fatty Acids: From Membrane Ingredients to Signaling Molecules*

The fatty acids, which mammals cannot synthesize in their body, are known as essential fatty acids (EFAs); they must be obtained by the mammals in a preformed condition, that is, from the exogenous dietary sources. EFAs were originally designated as vitamin F, until it was realized that they must be classified with fats [4]. Of all the 18-C UFAs, two unsaturated fatty acids are found to be essential fatty acids (EFAs): they are linoleic acid (**Figure 1D**) and α-linolenic acid (**Figure 1E**). Both of them can act as precursors of very long chain polyunsaturated fatty acids (LPUFAs), such as ω-6 linoleic acid acting as the precursor of arachidonic acid (C20:4, ω-6) and ω-3 α-linolenic acid acting as the precursor of eicosapentaenoic acid (EPA, C20:5, ω-3) and docosahexaenoic acid (DHA, C22:6, ω-3). The rest are nonessential. Some examples are (common names): stearic (C18:0), oleic (C18:1), palmitic (C16:0), myristic (C14:0), and lauric acid (C12:0). Being nonessential does not actually mean that they are not important. Our body does need them to function properly; it, however, can synthesize them without receiving them directly

**1.4 AA (C20:4, ω-6) versus DHA (C22:6, ω-3) or EPA (C20:5, ω-3)**

AA is referred to as a conditionally essential fatty acid for animals [5–7], including humans, that experience persistent deficiencies of linoleic acid (LLA, C18:2, ω-6), or during prematurity and growth, or if there is a limited capacity to convert LLA to AA [5]. However, consumption of vegetable-based oils, with large amounts of LLA, and an adequate capacity to convert LLA to AA, can eliminate the need for exogenous supply of AA, excluding it thereby from the list of essential fatty acids.

Both EPA and DHA are the members of ω-3 PUFA family. Both can be biosynthesized from the precursor α-linolenic acid (C18:3, ω-3, LLN). However, they are believed to act differently in different organs. For example, the differential roles of EPA and DHA have been studied in lymphocytes [8], macrophages [9], vascular smooth muscle cells [10], and endothelial cells [11]. Their differential roles have also been seen in the brains. EPA constitutes a tiny part in the unsaturated fatty acid pool of the brain. DHA, however, constitutes >17% by weight of the total fatty acids in the brain of adult rats and >33% of the total fatty acids in the retina [12]. DHA is thus referred to as essential for the growth and development of the brains, and animals have to take it in preformed form. The brain has a limited capacity to convert αLNN to DHA because of the lack of synthesizing enzymes [13, 14]. DHA plays an important role in the learning-related memory of animals, including humans.

Monounsaturated ω-7 and ω-9 fatty acids are also considered to be nonessential, as majority of them are obtained from dietary sources (**Figure 2**). They can also be biosynthesized in the body. The most common ω-7s are palmitoleic acid (PA) and *cis* and *trans*-vaccenic acid (VA) (11-cis-octadecenoic acid). The most common ω-9s include oleic acid (OA), erucic acid (EA) and mead acid (it is a triunsaturated fatty acid). Since the human body can create ω-9 unsaturated fatty acids, there is no need to include them in diet. Full-fat grass-fed dairy, wild-caught salmon, olives,

sprouted nuts, etc. are the sources of ω-7 and ω-9 unsaturated fatty acids.

*DOI: http://dx.doi.org/10.5772/intechopen.80430*

from food.

**1.3 Essential versus nonessential fatty acids**

**1.5 EPA (C20:5, ω-3) and DHA (C22:6, ω-3)**

**1.6 ω-7 and ω-9 Monounsaturated fatty acids (MUFAs)**
