**1. Introduction**

The term lipid is used to classify a large number of substances having very different physical - chemical characteristics, being its solubility in organic non-polar solvents the common property for their classification. Lipids are composed of carbon, hydrogen and oxygen atoms, and in some cases contain phosphorus, nitrogen, sulfur and other elements. In this context, fats and oils are the main exponents of lipids present in foods and in nutritional processes [1,2], being diverse fatty acids and cholesterol the most representative molecules due their important metabolic and nutritional functions [3,4]. The structural, metabolic and nutritional importance of lipids in the body is supported by numerous investigations in different biological models (cellular, animals and humans). Lipids have been instrumental in the evolution of species, having important role in the growth, development and maintenance of tissues [5,6]. A clear example of this importance is the elevated fatty acid concentration present in nerve tissue, especially very long-chain polyunsaturated fatty acids [7,8].

Fatty acids are, among lipids, of crucial relevance in the structure and physiology of the body because: i) forms an integral part of phospholipids in cell membranes; ii) are the primary source of energy (9 kcal /g or 37.62 kjoules/g); iii) in infants, provide more than 50% of the daily energy requirements; iv) some fatty acids are of essential character and are required for the synthesis of eicosanoids and docosanoids (of 20 and 22 carbon atoms, respectively), such as leukotrienes, prostaglandins, thromboxanes, prostacyclins, protectins and resolvins), and; v) some of them may act as second messengers and regulators of gene expression [9,10]. Besides fatty acids, cholesterol is another lipid that has important functions in the body, among which are: i) together with phospholipids is important in the formation of cell membranes; ii) constitutes the skeleton for the synthesis of steroid hormones (androgens and estrogens); iii) from its structure is derived the structure of vitamin D, and; iv) participates in the synthesis of the bile salts and the composition of bile secretion [11].

© 2013 Valenzuela and Valenzuela, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### 4 Lipid Metabolism

Lipids play a key role in the growth and development of the organism, where the requirements of these molecules (mainly fatty acids) will change depending on the age and physiological state of individuals [12]. Furthermore, lipids have crucial participation both, in the prevention and/or in the development of many diseases, especially chronic noncommunicable diseases [13], affecting the lipid requirements in humans [14]. As food components, lipids are also important because: i) are significant in providing organoleptic characteristics (palatability, flavor, aroma and texture); ii) are vehicle for fat soluble vitamins, pigments or dyes and antioxidants, and; iii) may act as emulsifying agents and/or promote the stability of suspensions and emulsions [15].

Overview About Lipid Structure 5

it is classified as monounsaturated fatty acid (MUFA) and when has two to six unsaturations are classified as polyunsaturated fatty acids (PUFAs) [21]. The presence of unsaturation or double bonds in fatty acids is represented by denoting the number of carbons of the molecule followed by an indication of the number of double bonds, thus: C18:1 corresponds to a fatty acid of 18 carbons and one unsaturation, it will be a MUFA. C20: 4 correspond to a molecule having 20 carbons and four double bonds, being a PUFA. Now, it is necessary to identify the location of the unsaturations in the hydrocarbon chain both in MUFAs and

According to the official chemical nomenclature established by IUPAC (International Union of Practical and Applied Chemistry) carbons of fatty acids should be numbered sequentially from the carboxylic carbon (C1) to the most extreme methylene carbon (Cn), and the position of a double bond should be indicated by the symbol delta (Δ), together to the number of the carbon where double bonds begins. According to this nomenclature: C18: 1, Δ9, indicates that the double bond is between carbon 9 and 10 [23]. However, in the cell the metabolic utilization of fatty acids occurs by the successive scission of two carbon atoms from the C1 to the Cn (mitochondrial or peroxisomal beta oxidation). This means that as the fatty acid is being metabolized (oxidized in beta position), the number of each carbon atom will change, creating a problem for the identification of the metabolic products formed as the oxidation progress. For this reason R. Holman, in 1958, proposed a new type of notation that is now widely used for the biochemical and nutritional identification of fatty acids [24]. This nomenclature lists the carbon enumeration from the other extreme of the fatty acid molecule. According to this notation, the C1 is the carbon farthest from the carboxyl group (called as terminal or end methylene carbon) which is designed as "n", "ω" or "omega". The latter notation is the most often used in nutrition and refers to the last letter of the Greek alphabet [25]. Thus, C18: 1 Δ9 coincidentally is C18: 1 ω-9 in the "ω" notation, but C18: 2 Δ9, Δ12, according to this nomenclature ω would be C18: 2 ω-6. What happens with fatty acids having more than a double bond? Double bonds are not randomly arranged in the fatty acid structure. Nature has been "ordained" as largely incorporate them in well-defined positions. Most frequently double bonds in PUFAs are separated by a methyl group (or most correctly methylene group) forming a **-C=C-C-C=C**structure which is known as "unconjugated structure", which is the layout of double bonds in most naturally occurring PUFAs [26]. However, although much less frequently, there are also present "conjugated structures" where double bonds are not separated by a methylene group, forming a **-C=C-C=C-** structure. This particular structural disposition of double bonds, i.e conjugated structures, is now gaining much interest because some fatty acids having these structures show special nutritional properties, they are called "conjugated fatty acids". Most of them are derived from the unconjugated structure of linoleic acid

For the application of the "ω" nomenclature and considering the "order" of double bonds in unsaturated fatty acids having unconjugated stucture, it can be observed that by

PUFAs [22].

(C18:2 ω-6) [27,28].

**3. Nomenclature of fatty acids** 

Fats and oils, the most common lipids in food, are triacylglyceride mixtures, i.e. structures formed by the linking of three different or similar fatty acids to the tri-alcohol glycerol [16]. A fat is defined as a mixture of triacylglycerides which is solid or pasty at room temperature (usually 20 °C). Conversely, the term oil corresponds to a mixture of triglycerides which is liquid at room temperature. In addition to triacylglycerides, which are the main components of fats and oils (over 90%), these substances frequently contain, to a lesser extent, diacylglycerides, monoacylglycerides, phospholipids, sterols, terpenes, fatty alcohols, carotenoids, fat soluble vitamins, and many other minor chemical structures [17,18]. This chapter deals with the general aspects of lipids, especially those related to the chemical structure and function of these molecules.
