**1. Introduction to lipids and fatty acids**

Before going into the chemical structure and properties of fatty acids, it is important to mention that they are merely one component of the major nutrient group commonly known as lipids. Lipids are biological compounds, which are soluble only in nonpolar solvents. They are typically known as fats and oils as well. However, fats and oil differ from each other based on their physical characteristics. The term "fats" is used to refer solid lipids at room temperature such as lard and butter, while "oils" are liquid lipids at room temperature such as sunflower oil, olive oil, etc. The classification of lipids is shown in **Figure 1**. Fatty acids appear under "triglycerides" since it is a component of this particular category of lipids.

To provide a brief introduction on fatty acids at a very basic level, they are the building blocks of the fat, which is physiologically present and obtained from the food we eat. During digestion, the body breaks down fats in the food products into fatty acids, which are subsequently absorbed into the blood. Upon absorption, fatty acid molecules are typically joined in groups of three, forming a molecule called a triglyceride. It has to be noted in this instance that triglycerides can even be made up from the carbohydrates in the food that we consume. There are several important functions of fatty acids in the body, including being a medium of storing energy and being involved in the cellular composition in the forms of phospholipids and

**Figure 1.**

*Major types of lipids: fatty acids come under triglycerides according to this classification.*

cholesterol esters. When glucose is unavailable for generation of energy in the cellular mechanism, the body uses fatty acids as fuel instead.

In terms of the chemical structure, a fatty acid is a carboxylic acid. It has an aliphatic chain, which is either saturated (having only single bonds) or unsaturated (having double bonds). Most of the naturally occurring fatty acids do not have branches in the aliphatic chain and contain an even number of carbon atoms (i.e., from 4 to 28). The long nonpolar tails of the fatty acids are responsible for the hydrophobic characteristics of fats and oils, while the carboxyl group of the fatty acids is polar or hydrophilic. Because of having both the hydrophobic and hydrophilic nature, when fatty acids are placed in an aqueous solution, they form spherical clusters or micelles. The micelles are arranged in such a way that the nonpolar straight chain is extended toward the interior of the structure and away from the water, while the polar carboxyl groups face outward in contact with the water.

There are two major types of classification of fatty acids, among many others. One classification is based on the length of the aliphatic chain, which is as follows:


Another classification is based on the level of saturation:


Unsaturated fatty acids can be divided into *cis-* and *trans*-fatty acids. A *cis* configuration means that the two hydrogen atoms adjacent to the double bond appear on the same side of the aliphatic chain, whereas *trans* configuration means that the adjacent two hydrogen atoms lie on the opposite sides of the chain. Unsaturated fatty acids can also be divided based on the number of double bonds, where monounsaturated fatty acids would have only one double bond in the aliphatic tail and polyunsaturated fatty acids would have two or more double bonds. Polyunsaturated fatty acids may have the double bonds next to each other as conjugated double bonds or alternatively between single bonds as nonconjugated double bonds.

The *cis* configuration of unsaturated fatty acids will create a bend in the fatty acid chain that is not found in saturated fatty acids. These bends prevent unsaturated fatty acids from packing closely together. As a result, they form less London dispersion forces between the fatty acids. This leads to *cis-*fatty acids having lower melting points. *Trans*-fatty acids, on the other hand, are obtained via hydrogenation of polyunsaturated fatty acids, e.g., margarine. *Trans*-fatty acids will not pack as well as saturated fatty acids, but do not produce a bend as in *cis-*fatty acids. Therefore, the melting points of *trans*-fatty acids are between the melting points of saturated fatty acids and *cis*-fatty acids of the same carbon length.

**5**

and stroke [14–19].

of human health.

**3. Short-chain fatty acids (SCFAs)**

*Introductory Chapter: Fatty Acids in Modern Times DOI: http://dx.doi.org/10.5772/intechopen.82440*

Fatty acids have gained much attention over the past few decades owing to its implications on human health. Much of this is due to the fact that the western diet has changed with the advent of "fast" and "convenient" foods [1]. These food products are energy dense, have a low dietary fiber content, and produce a comparatively lower satiety and satiation signals than low-energy-dense foods [2]. This newly introduced diet is markedly different to the historical diet of humans that the gut was adapted to over several millennia [3]. According to current evidence for most of the history on the human lineage, the diet consisted of more indigestible plant material, such as grasses, sedges, and tubers, than the present and is therefore likely to have contained a larger nondigestible component [4]. Recent systematic reviews of randomized trials [5, 6] and prospective cohort studies [7] have called for the re-evaluation of dietary guidelines based on these dietary transitions, primarily in terms of the intake, and a reappraisal of the effects of fatty acids on health. It is heartening to observe, nevertheless, that public health efforts to remove

*trans* fats from the food supply in several countries have intensified [8].

Saturated fatty acids contribute to approximately 10% of energy to the North American diet [9, 10]. According to De Souza et al. [8], the main sources of fatty acids in the food supply of North Americans are animal-based food products, such as butter, cows' milk, meat, salmon, and egg yolks, and a few plant products such as chocolate and cocoa butter, coconut, and palm kernel oils. Despite attempts to completely remove *trans* fats from the diet, they evidently contribute to approximately 1–2% of energy in the North American diet [8, 11–13] and are primarily produced through industrial processing such as partial hydrogenation of liquid plant oils in the presence of a metal catalyst, vacuum, and high heat. Production of *trans* fats can also occur naturally in meat and dairy products, where ruminant animals biohydrogenate unsaturated fatty acids via bacterial enzymes [8]. According to De Souza et al. [8], the major industrially produced *trans*-fatty acids in the food supply are elaidic acid isomers, while the major ruminant-derived *trans*-fatty acid is vaccenic acid. Both these chemical compounds share the characteristic of having at least one double bond in the *trans*-configuration. Present dietary guidelines recommend that saturated fats should be limited to <10% (5–6% for those who would benefit from lowering of low-density lipoprotein, LDL cholesterol) and *trans* fats to <1% of energy or as low as possible in view of reducing the risk of ischemic heart disease

There is increased attention on SCFAs given its importance in the human gut microbiota. The human microbiota is primarily the collection of microbes that live on and in our body, where the largest and most diverse cluster of microorganisms inhabits the gut [20]. Evidence has surfaced that the gut microbiota has coevolved with the host, which provides the microbes with a stable environment, while the microbes provide the host with a broad range of functions such as digestion of complex dietary macronutrients, production of nutrients and vitamins, defense against pathogens, and maintenance of the immune system [20]. Interactions between the microbiota and the distal gut are currently considered as fundamental determinants

As previously mentioned, SCFAs are a subset of saturated fatty acids containing six or less carbon molecules which include acetate, propionate, butyrate, pentanoic

**2. Fatty acids and health**
