List of common saturated and unsaturated fatty acids.
In recent years, polyunsaturated fatty acids (PUFA) have received considerable attention in both human and animal nutrition. As a mean of increasing the low consumption of long chain n-3 PUFA by humans consuming diets, there has been some interest in the enrichment of poultry meat with these fatty acids for people seeking healthy lifestyles. Dietary supplementation with n-3 PUFA, such as these found in fish oil and linseed oil, were found to have nutritional benefits in humans. Modulation of fatty acid profiles as a result of n-3 PUFA incorporation is well documented in humans, rodents, and poultry. The current chapter focuses on enriching poultry meat with these beneficial fatty acids to increase its consumption by human beings.
- n-3 fatty acids
- polyunsaturated fatty acids
Recently, PUFA have received considerable attention in both human and animal nutrition, particularly those of the n-3 family; which are distinct due to the placement of the first double bond onto the third carbon atom from the methyl end of the fatty acid molecule. Long-chain fatty acids primarily those with more than 18 carbon atoms, derived mainly from fish oils are consumed quite less along with the other PUFAs. In order to increase their consumption through human diets, has led to studies for enriching the poultry meat infused with these fatty acids and thus enabling people to live healthier lifestyles. Dietary supplementation with n-3 PUFA, such as these found in fish oil and linseed oil, were found to have nutritional benefits in humans [1, 2, 3, 4, 5].
This chapter will shed light on the overview, sources, and metabolism of PUFA, their incorporation into cell membrane structure, their involvement in health and clinical problems, enrichment of poultry products with PUFA, and their involvement in immune system.
2. Overview of fatty acids
All fatty acids are carboxylic acids characterized by a chain-like structure with a carboxyl group (COOH) at one end, and a methyl group (CH3) at the other end. The rest of the chain consists of carbon atoms varying in length from 2 to 20 or more with hydrocarbon bonds (CH2). Fatty acids (FA) differ in the number of hydrogen atoms and the number and location of the double bonds between adjacent carbon atoms if hydrogen atoms are removed. If a fatty acid chain is fully loaded with hydrogen atoms, the FA is termed saturated. Consequently, saturated fatty acids form straight chains as there are no double bonds between carbon atoms. These usually contain between 12 and 24 carbon atoms. This kind of FA is abundantly present in adipose tissues of animals, including poultry and used as a source of energy if needed. An example of a saturated FA is stearic acid (C18:0). This is one way to name a fatty acid (C:D) where C is the number of carbon atoms in the fatty acid and D is the number of double bonds in the fatty acid. Sources of saturated FA include meat, dairy products, palm oil, coconut oil and vegetable shortening .
If a pair of hydrogen atoms is removed under the influence of specific enzymes, a double bond is formed between adjacent carbon atoms and the saturated FA becomes monounsaturated. An example of a monounsaturated FA is oleic acid (18:1), an n-9 FA that constitutes 74% of total FA in olives. n-x is a nomenclature of fatty acids where a double bond is located on the xth carbon▬carbon bond, counting from the terminal methyl carbon (designated as n). Other sources of monosaturated FA are avocados, rapeseed, peanuts and soybeans . If two or more double bonds are formed due to removal of more than a pair of hydrogen atoms, the FA is termed polyunsaturated. The more double bonds a fatty acid has, the more unsaturated it is [8, 9, 10]. The main sources of PUFA are seeds and seed oils, oily fish and fish oils .
Moreover, the orientation of the fatty acid chain at the site of a double bond determines and characterizes a fatty acid. For example, a FA called cis-configured when both segments of the molecule lie at the same side. On the other hand, in the trans configuration, the two parts of the molecule face opposite with respect to the bond directions (see Figure 1). Most PUFA in plants and sea foods are of cis configuration .
The two major types of PUFA which play a crucial role in the biological functioning of both, humans and animals are the n-3 and n-6 PUFA. The n-3 PUFA consists of linolenic acid (LNA, C18:3), eicosapentaenoic acid (EPA, C20:5) and docosahexaenoic acid (DHA, C22:6) whereas the n-6 PUFAs comprise mainly linoleic acid (LA, C18:2) and arachidonic acid (AA, C20:4). LA and α-LNA are classified as essential fatty acids (EFA) due to their inability to be synthesized by the body. However, these EFAs should be consumed through the diet because of shortage of specific desaturation enzymes. AA can be synthesized in from LA when the diet is consumed. In a similar manner, EPA along with DHA can be synthesized from α-LNA although synthesis between them is inadequate in most conditions. Due to the absence of specific desaturase enzymes, the n-3 and n-6 fatty acids are not inter-convertible. On the other hand, saturated FA such as palmitic acid (C16:0) and stearic acid (C18:0) and most monounsaturated FA such as oleic acid (C18:1 n-9) can be synthesized in the human body from precursors such as glucose or amino acids [12, 13]. Table 1 shows a list of the common saturated and unsaturated fatty acids.
|Common name||FA name|
|c10 Pentadecanoic acid||C15:1|
|c11, 14 Eicosadienoic||C20:2|
|Erucic acid||C22:1 n9|
|Eicosapentaenoic acid (EPA)||C20:5n3|
|Docosapentaenoic acid (DPA)||C22:5n3|
|Docosahexaenoic acid (DHA)||C22:6n3|
3. Sources and metabolism of fatty acids
General speaking, there are small amounts of AA in fish. However Brown et al.  have reported that there is 4.8–14.3% AA in some Australian fish species. However, fish oil contains high amounts of EPA and DHA. These fatty acids are synthesized by phytoplankton that are consumed by fish. Some fish species may contain more than 30% n-3 PUFA about 50% of the FA in fish is PUFA, of which about 30% are n-3 FA [15, 16].
Conversely, the presence of α–LNA in seafood is almost nil; although plant sources like chia, linseed, rapeseed, perilla and blackcurrant possess high amounts of this FA, this is because these plant sources have Δ12-desaturase that converts oleic acid into LA, this is further converted into α-LNA under the influence of Δ 15-desaturase . Linseed is one of the richest know sources of α-LNA, as it contains almost 60% of this fatty acid in its oil .
Some algal oil and algal biomass obtained from marine regions are known to be good sources of DHA and EPA and thus can be used as a means to enrich meats and eggs using these long chain fatty acids. This has proved to be successful and is well documented in literature, even though DHA is mostly obtained from these algal biomasses [18, 19, 20, 21, 22, 23, 24, 25, 26].
In addition, echium oil from the plant Echium plantagineum has been recognized as an ideal source of stearidonic acid (C18:4n-3) that is naturally converted to the important long-chain n-3 fatty acid, EPA, when metabolized in the body [27, 28]. In addition, there are considerable amounts of α-linolenic acid and γ-linolenic acid in the echium oil as well. Rymer et al.  showed that γ-linolenic acid is accumulated as stearidonic acid increases in the chicken’s diet.
N-3 PUFA, particularly EPA and DHA, are reported to compete with AA for incorporation in the phospholipid bilayer of cell membranes of all body cells, especially erythrocytes, platelets, neutrophils, monocytes and liver cells [30, 31]. Both AA and EPA are parent precursors of different kinds of eicosanoids that play a crucial role in the inflammatory responses in both humans and animals, including poultry.
Initially, the dietary essential fatty acid α-LNA is converted to EPA and DHA while LA is converted to AA by elongation and desaturation reactions [32, 33, 34]. These conversion reactions are mediated in humans by three desaturases, Δ9, Δ6, and Δ5. The desaturases work by introducing a double bond at a specific position of the carbon backbone. Nakamura and Nara  have reported that desaturases in mammals are regulated at the transcriptional level and their transcription is genetically controlled. However, regulation of Δ9 desaturase differs from Δ6 and Δ5 desaturases because the Δ 9-desaturase converts the nonessential stearic acid (18:0) to oleic acid (18:1 n-9). Oleic acid can go through the same steps of desaturation and elongation as LA and α-LNA, resulting in the synthesis of the fatty acids 20:3 n-9 and 22:4 n-9. Consequently, the Δ 9-desaturation provides an alternative to Δ6 and Δ5 desaturation when the cell is subject to essential fatty acid deficiency. However in the case of availability of sufficient amounts of essential fatty acids, AA and EPA act as precursors for eicosanoid synthesis, although EPA metabolism predominates [32, 33, 36, 37]. When sources rich in stearidonic acid (SDA) such as echium oil are consumed, the body deposits EPA directly in tissues such as plasma, blood leukocytes, liver, breast and legs of human, rodents and chicken because SDA does not require Δ6 desaturase activity to form EPA [28, 38, 39, 40, 41, 42, 43].
Under the influence of Δ6 desaturase, free α-LNA is converted to SDA (18:4 n-3) then to eicosatetraenoic acid (20:4 n-3) by an elongase. Next, Δ5 desaturase acts on eicosatetraenoic acid and converts it into EPA (20:5 n-3). Elongase converts EPA into the FA (24:5 n-3) that is converted into the FA (24:6 n-3) by the action of Δ6 desaturase. Then, oxidation of (24:6 n-3) by β-oxidase produces DHA. During this metabolic pathway, eicosanoids such as leukotriene 5-series, prostaglandins E3 and thromboxane A3 are derived from EPA [37, 41, 44, 45, 46, 47, 48, 49, 50]. Figure 2 shows the metabolic pathway of the long chain n-3 and n-6 PUFA .
4. Incorporation into cell membrane structure
Cell membranes consist of a variety of molecules that enable cells to survive via various biological interactions. Proteins and lipids are the main elements of cell membranes. Different cell types have different cell membrane lipids and proteins that reflect different biological functions and specializations of cells.
Lipids in the cell membranes are arranged in a bilayer structure with the hydrophobic moieties in the center of the membrane and the hydrophilic heads at the two surfaces, facing the inner cytoplasm and the outside surrounding. There are three main types of lipids in the cell membranes, namely: phospholipids, glycolipids, and steroids. Both saturated and unsaturated FA are attached to the glycerol moiety in the cell membrane, with the saturated FA attached to the first carbon atom in the glycerol backbone (sn-1), while PUFA occupy the sn-2 position . Membrane fluidity is highly affected by the length and the degree of unsaturation of FA chains. Lipid moieties within the cell membrane determine different biological cellular functions such as intracellular pathways and receptors formation. In humans, EPA, DHA, AA and oleic acid are the main PUFA incorporated into the cell membranes. Interestingly, changes in these lipid moieties leads to changes in biological functions of different cell types due to the production of different cellular intermediates such as leukotrienes, prostacyclins and prostaglandins. These intermediates are involved in the immunomodulatory effect of PUFA [42, 49, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60].
5. Involvement in health and clinical problems
Vitality of living cells depends profoundly on dietary lipids that are incorporated into phospholipid layers of cellular membranes as a result there is a constant competition between the omega-3 fatty acids; eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), with arachidonic acid (AA) for this incorporation. As AA controls the upregulation of eicosanoids such as leukotrienes, this competitive inhibition downregulates inflammation responses related to man, which are associated to numerous diseases and disorders such as cardiovascular disease, increased triglycerides, blood pressure, thrombosis, atherosclerosis, stress, mental problems, asthma and rheumatoid arthritis [21, 50, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79]. These benefits of an optimal ratio of n-3/n-6 PUFAs on health are just a few examples of a wide range of clinical problems that are improved by consumption of the very long chain n-3 fatty acids.
6. n-3 enrichment of poultry diet
Traditionally, fish and fish oil are the main sources of essential, long chain n-3 PUFA that induce modifications in the lipid composition of poultry products because marine sources in general contain high levels of EPA and DHA PUFA. Of less nutritional importance are plant sources such as linseed that is rich in α-linolenic acid (α-LNA). α-LNA is an 18 carbon n-3 fatty acid that is the precursor to the long chain n-3 PUFA, but because the efficiency of conversion is so low in humans, the accumulation of α-LNA is of little real nutritional benefit.
In chickens, there are number of studies that investigated effects of PUFA on fatty acid profile of different tissues, if sources rich in these fatty acids are added to the poultry feed. Bou et al.  reported that supplementing the diet of broilers with 2.5% fish oil produced double the amount of EPA and DHA in their carcass than diets supplied with 1.25% fish oil. In another study, Ratnayake et al.  fed broiler chickens increasing levels of redfish meal (40–120 g/kg) for a period of 42 days. The effect of this dietary manipulation on fatty acid composition of breast and thigh muscles was investigated. Authors of this study observed a linear relationship between the level of the dietary fish meal and the proportions of DHA, DPA and EPA in the meat muscles. Givens and Rymer  also conducted an experiment to investigate the effect of poultry species and genotype on the efficiency of incorporation of n-3 PUFA in poultry meat. The two genotypes of turkeys (Wrolstad and BUT T8) and broilers (Ross 308 and Cobb 500) were fed one or four diets that contained 50 g/kg added oil; either vegetable oil (control), partially replaced with linseed (20 or 40 g/kg), FO (20 or 40 g/kg), or mixture of linseed and FO (20 g linseed and 20 g FO/kg diet). It was observed that on replacement of the control diet with either low or high levels of FO caused a significant increase in the concentration of EPA and DHA in all the meats whereas feeding linseed-enriched diet significantly increased the concentration of α-linolenic acid. No significant difference was noted with the incorporation of n-3 PUFA between the two broiler genotypes. Turkey genotypes were only different in the case of α-linolenic acid incorporation. It was also seen that there was a greater incorporation of DHA in white than in dark meat. In order to confirm the effect of dietary fatty acid modulation in broiler chickens, another study was conducted by Lopez-Ferrer et al. . Here, a diet enriched with 8.2% FO was fed to broilers for duration of 5 weeks, after this it was replaced by diets containing 8.2% linseed or rapeseed in three different periods: the last week before slaughtering, the last 2 weeks and throughout the experiment. The end results for the fatty acid analysis of thigh and breast showed that the total amounts of n-3 PUFA were significantly decreased after removal of FO diet. Upon replacement of FO with the linseed diet caused a substantial increase in α-linolenic acid, furthermore there was an increase in the total amounts of n-6 PUFA and a decrease in the DHA proportions due to its limited conversion to longer n-3 PUFA. When FO was replaced by rapeseed there was an increase in the total amounts of monounsaturated fatty acids, especially oleic acid.
Recently, Zelenka et al.  studied the effect of increasing levels of linseed oil in the diets of chickens and its influence on the fatty acid content in breast and thigh meat of chickens. Linseed oil at levels of 1, 3, 5 or 7% were fed to broiler chickens from 25 to 40 days of age. Oils were derived from the linseed cultivar Atalante with a high content of α-linolenic acid or the cultivar Lola with a high content of linoleic acid. Results showed that feeding a diet with a high content of α-linolenic acid significantly increased all n-3 PUFA, decreased n-6 PUFA and decreased the ratio of n-6/n-3 PUFA. On the contrary, when the birds were fed a diet with a high content of linoleic acid, this caused a significant increase in the levels of all n-6 PUFA in thigh and breast of chickens. Similarly, a study by Kartikasari et al.  showed that feeding broilers on diets with a high content of α-linolenic acid, while keeping a constant linoleic acid level, significantly increased the incorporation of all n-3 PUFA into breast and thigh meat by 5 and 4-fold compared to chickens fed low α-linolenic acid content. In another experiment , the authors fed broiler chickens on diets with constant level of α-linolenic acid (2.1%) and different levels of linoleic acid, which included 2.9–4.4%, and consisted of pure or blended vegetable oils such as macadamia, flaxseed and sunflower oils. The overall lipid content was kept at a constant of 5%. Post analysis it was observed that chickens when fed diets the lowest linoleic acid content (2.9%) contributed towards higher incorporation of total n-3 PUFA in the breast by 16% compared with feeding the highest linoleic acid content (4.4%). When the chickens were fed with a diet with a high content of linoleic acid, this resulted in a significant reduction in EPA levels in both thigh and breast tissues. The levels for DPA and DHA were not affected by dietary linoleic acid. Authors suggested that this could be due to fact that linoleic acid competes with α-linolenic acid for Δ6 desaturase. In other words, high dietary level of linoleic acid might reduce the conversion of α-linolenic to n-3 PUFAs. In a further study , the authors fed broiler chickens on diets containing 0, 2, or 4% linseed oil plus tallow to make 8% added fat throughout 38 growth period. The total amounts of saturated and monounsaturated fatty acids were significantly decreased after feeding increased levels of linseed. Conversely, the total amounts of PUFA were significantly increased. A recent study  showed that upon supplementing n-3 PUFA, in the form of linseed oil (3/100 g mixed feed), in the diet of laying hens resulted in a significant increase in α-linolenic of the plasma. The same study also revealed that, FO administration (same dose as linseed) caused a significant increase in the proportion of plasma EPA and DHA.
7. Involvement in avian immune function
The immunomodulatory effect of PUFA in broiler chickens occurs by affecting intercellular communications and signals that change the reactivity of leukocytes upon antigenic stimulation. This effect is highly associated with down-regulation or up-regulation of different cytokines that are believed to affect the avian immune function such as IL-1β, IFNγ, MGF, IL-1, IL-4, IL-2 [89, 90, 91, 92].
There is some concern that diets enriched with n-3 PUFA have detrimental effects on chicken immunity and impair resistance to infection. However, it is not clear whether this concern is justified, since some studies show no effect , some show a detrimental effect  while some show an improvement [89, 90, 93, 95, 96, 97] in chicken immune response following feeding of n-3 PUFA.
Consumption of omega-3 fatty acids should be increased in human diets to get the beneficial effects of these fatty acids. One way to achieve this goal is by enriching poultry meat and eggs with omega-3 fatty acids, which is proved to be very successful. This role of poultry production in enhancing health aspects of human needs more research and interest from nutritionists and poultry producers.
The authors would like to extend their gratitude and appreciation to the management of Kuwait Institute for Scientific Research for their continuous technical and financial support of scientific research.
Conflict of interest
There is no conflict of interest related to the current work.