Introduction

There are two families of polyunsaturated fatty acids (PUFA) that are of importance in affecting biological functions in both human and animals. These are the n-3 and n-6 PUFA, whereby n-3 PUFA include alpha-linolenic acid (α-LNA, C18:3), eicosapentaenoic acid (EPA, C20:5) and docosahexaenoic acid (DHA, C22:6). The main n-6 PUFAs are linoleic acid (LA, C18:2) and arachidonic acid (AA, C20:4). Two PUFAs, α-LNA and LA are essential fatty acids because they cannot be synthesised by the body, but must be supplied through the diet due to lack of specific desaturation enzymes. AA can be synthesised from LA in the consumed diet. Similarly, EPA and DHA can be synthesised from α-LNA, but their synthesis is normally insufficient under most conditions. Interconversion between α-LNA and LA is not possible due to lack of the specific desaturase enzymes. Metabolic pathways of the fatty acids are controlled by elongation and desaturation reactions which are mediated by desaturase enzymes (Voss et al., Reference VOSS, REINHART, SANKARAPPA and SPRECHER1991; Mohammed et al., Reference MOHAMMED, SANKARAPPA, GEIGER and SPRECHER1995).

The vitality of living cells depends on dietary lipids that are incorporated into phospholipid bilayers of cellular membranes. n-3 PUFA, especially EPA and DHA, are essential for normal growth and development and are important in the prevention of certain diseases. EPA and DHA are reported to compete with AA for incorporation into cell membranes. Since AA is responsible for upregulation, of the immune eicosanoids, such as leukotrienes, this competitive inhibition down-regulates inflammation responses related to many diseases and disorders such as cardiovascular disease, increased triglycerides, blood pressure, thrombosis, atherosclerosis, stress, mental problems, asthma and rheumatoid arthritis (Bavelaar et al., Reference BAVELAAR, HOVENIER, LEMMENS and BEYNEN2004; Cyrus, Reference CYRUS1999; Calder, Reference CALDER2002; Calder et al., Reference CALDER, YAQOOB, THIES, WALLACE and MILES2002; Volker, Reference VOLKER2000; Arm et al., Reference ARM, HORTON and MENCIA-HUERTA1988; Reference ARM, HORTON, SPUR, MENCIA-HUERTA, LEE and AM1989; Simopoulos, Reference SIMOPOULOS2002; Reference SIMOPOULOS2003; Sellmayer et al., Reference SELLMAYER, SCHREPF, THEISEN and WEBER2004; Schwalfenberg, Reference SCHWALFENBERG2006; Nettleton, Reference NETTLETON1995; Leaf and Kang, Reference LEAF and KANG2001; Connor, Reference CONNOR1994; Budowski, Reference BUDOWSKI1988; Woods et al., Reference WOODS, BRULL, HUMPHRIES and MONTGOMERY2000; Connor et al., Reference CONNOR, NEURINGER and LIN1990; Nkondjock et al., Reference NKONDJOCK, SHATENSTEIN, MAISONNEUVE and GHADIRIAN2003; Prasad et al., Reference PRASAD, KUMAR, YAN, HANSON and COLE2003; Ge et al., Reference GE, CHEN, CLUETTE-BROWN, LAPOSATA and KANG2002; Koller et al., Reference KOLLER, SENKAL, KEMEN, KONIG, ZUMTOBEL and MUHR2003).

There has been interest in the enrichment of poultry meat and eggs with long chain n-3 PUFA to increase consumption in humans, which is much lower today because of the decrease in eating fish and the industrial production of animal feeds and meat, which is rich in n-6 and poor in n-3 fatty acids. Modulation of fatty acid profiles as a result of feeding n-3 PUFA sources in the diet is well documented in chickens. Examples of these sources are marine sources such as fish and algal biomass and plant sources such as echium and linseed.

The capacity of enriching poultry meat with n-3 PUFA varies depending on the dietary source used. Traditionally, fish and its oils are the main sources of essential, long chain n-3 PUFA. These induce modifications in the lipid composition of poultry products because marine sources in general contain high levels of the very long chain n-3 PUFA, especially EPA and DHA, which are initially synthesised by phytoplankton consumed by fish, and thereby accumulate in the aquatic food chain (Napolitano et al., Reference NAPOLITANO, RATNAYAKE and ACKMAN1988). About 50% of the FA in fish is PUFA, of which approximately 30% are n-3 FA (Kris-Etherton et al., Reference KRIS-ETHERTON, HARRIS and APPEL2002; Kartal et al., Reference KARTAL, KURUCU, ASLAN, OZBAY, CEYHAN, SAYAR and CEVHEROGLU2003). High concentrations of fish oil supplementation decrease the saturated and monosaturated FA and increase the very long chain PUFA in poultry meat (Lopez-Ferrer et al., Reference LOPEZ-FERRER, BAUCELLS, BARROETA and GRASHORN2001b). Bou et al. (Reference BOU, GUARDIOLA, TRES, BARROETA and CODONY2004), reported that supplementing broiler diets with 25 g/kg fish oil produced double the amount of beneficial EPA and DHA than diets supplied with 12.5 g/kg fish oil. In addition, Ratnayake et al. (Reference RATNAYAKE, ACKMAN and HULAN1986) fed broiler chickens increasing levels of redfish meal (40 to 120 g/kg) for 42 days and investigated the effect of this dietary manipulation on fatty acid composition of breast and thigh muscles. They observed a linear relationship between the level of the dietary fish meal and the proportions of DHA, DPA and EPA in the meat muscles. Interestingly, Givens and Rymer (Reference GIVENS and RYMER2006) conducted a study to investigate the effect of poultry species and genotype on the efficiency of incorporation of n-3 PUFA in poultry meat. These researchers fed two genotypes of turkeys (Wrolstad and BUT T8) and broilers (Ross 308 and Cobb 500) on one or four diets that contained 50 g/kg added oil, which was either vegetable oil (control), partially replaced with linseed (20 or 40 g/kg), fish oil (20 or 40 g/kg), or mixture of linseed and fish oil (20 g linseed and 20 g fish oil/kg diet). Results showed that replacing the control diet with low or high levels of fish oil significantly increased the concentration of EPA and DHA in all the meats while feeding linseed-enriched diet significantly increased the concentration of α-linolenic acid. There was no significant difference in the incorporation of n-3 PUFA between the two broiler species. Wrolstad dark meat was slightly more responsive than BUT T8 to increased dietary α-linolenic acid concentrations. There were no other significant differences between the genotypes. Results of this study revealed that white meat incorporated significantly higher amount of DHA than dark meat. To further clarify the effect of dietary fatty acid modulation in broiler chickens, a study was conducted by Lopez-Ferrer et al. (Reference LOPEZ-FERRER, BAUCELLS, BARROETA and GRASHORN1999b), in which a diet enriched with 8.2% fish oil was fed to broilers for five weeks and then it was replaced by diets containing 8.2% linseed or rapeseed in three different periods: the last week or the last two weeks before slaughtering or throughout the experiment. Results of fatty acid analysis of thigh and breast showed that the total amounts of n-3 PUFA were significantly decreased after withdrawal of the fish oil. Replacing fish oil by linseed diet substantially increased α-linolenic acid, increased total amounts of n-6 PUFA and decreased DHA proportions due its limited conversion to longer n-3 PUFA. Replacing fish oil by rapeseed increased the total amounts of monounsaturated fatty acids, especially oleic acid.

Although very successful in enriching poultry meat with EPA and DHA, fish oil and its meal may increase the degree of unsaturation and, as a result, increase susceptibility to oxidation. This may lead to off-flavours and off-odours in the final product and, consequently, lower consumer acceptability. It was reported that using fish oils at concentrations above 2% in poultry diets may cause sensory problems that compromise meat quality and acceptability (Hargis and van Elswyk, Reference HARGIS and VAN ELSWYK1993; Miller and Robisch, Reference MILLER and ROBISCH1969). Unacceptable odours have been detected in carcasses of chickens fed fish oil at levels of 4% (Dansky, Reference DANSKY1962) and 2% (Edwards and May, Reference EDWARDS and MAY1965), and hence Fry et al. (1965) recommended that no more than 1.5% of fish oil should be fed.

To overcome the obstacle of the fishy flavour of poultry meat when dietary fish or fish oils are used, tocopherols (vitamin E) or α-tocopheryl acetate (α-TA) may be supplemented in the diet to increase the α-tocopherol content of chicken tissues. The antioxidant effect of tocopherols in poultry meat prevents the formation of primary and secondary oxidation products and reduces total undesired volatiles (Lauridsen et al., Reference LAURIDSEN, BUCKLEY and MORRISSEY1997; Grau et al., Reference GRAU, GUARDIOLA, GRIMPA, BARROETA and CODONY2001).

Some algal oils and its derivates of marine origin can be used as a rich source of DHA and EPA, to enrich eggs and meat with this long chain n-3 FA. This enrichment has been successful and well documented in many studies, although most algal biomass are sources of DHA mainly, and EPA to a much lesser extent. Rymer et al. (Reference RYMER, GIBBS and GIVENS2010) investigated the suitability of algal biomass as an alternative to fish oil in enrichment of poultry meat with the long chain n-3 PUFA. In this study, algal biomass was enriched as low, medium and high doses (2, 4, and 6 g DHA/kg feed). Fresh fish oil and encapsulated fish oil were used at 4 g DHA/kg feed. All the treatments significantly increased DHA content of breast and leg meat. Interestingly, the breast content of DHA was linearly increased as the algal biomass inclusion rate increased. Results of this study confirm that algal biomass enrichment of broiler meat can be used as efficiently as fish oil for enrichment of poultry meat with n-3 PUFA, especially DHA.

Conversely, alpha-linolenic acid (α-LNA) is rarely found in large quantities in seafood; rather, plant sources such as linseed, rape, chia, perilla and blackcurrant seeds contain high quantities of this FA because these plants contain Δ¹²-desaturase that converts oleic acid into LA, which is in turn converted into α-LNA under the influence of Δ ¹⁵-desaturase (Nettleton, Reference NETTLETON1991). Linseed is the richest source of α-LNA, containing approximately 60% of this FA in its oil (Alexander, Reference ALEXANDER1998). α-LNA can be converted into EPA and DHA, although this conversion is quite limited. This limitation is due to the fact that both linoleic acid and α-linolenic acid share the same desaturation and elongation enzymes that convert them to AA and EPA, respectively. Since there are naturally more levels of linoleic acid than α-linolenic acid in animal cells, then conversion of α-LNA to EPA is limited by high competition for linoleic acid conversion to AA. Only 1-5% of α-linolenic acid molecules are converted into EPA (Bioriginal, 2010; Brenna, Reference BRENNA2002). Rymer and Givens (Reference RYMER and GIVENS2005) showed a weak relationship between dietary α-LNA and meat content of EPA, and there was no relationship between dietary α-LNA and meat DHA in broiler chickens. Interestingly, Zelenka et al. (2008), investigated the effect of increasing levels of linseed oil in the diet on the content of fatty acids in breast and thigh meat of chickens. 1, 3, 5 or 7% of linseed oil were fed to broiler chickens from 25 to 40 days of age. Oils made either of seeds of the linseed cultivar Atalante with a high content of α-LNA or of the cultivar Lola with a high content of LA. Results showed that feeding a diet with a high content of α-LNA significantly increased all n-3 PUFA, decreased n-6 PUFA and decreased the ratio of n-6/n-3 PUFA. However, feeding a diet with a high content of LA significantly increased levels of all n-6 PUFA in thigh and breast meat. Similarly, a study by Kartikasari et al. (2009) demonstrated that feeding broiler chickens on diets with a high content of α-LNA, while keeping LA levels constant, significantly increased the incorporation of all n-3 PUFA into breast and thigh meat by five and four-fold compared to chickens fed low α-LNAcontent. In a later study by Kartikasari et al. (2010) broiler chickens were fed diets with a constant α-LNA (2.1%) but varying levels of LA, which ranged from 2.9 to 4.4%, using pure or blended vegetable oils of macadamia, flaxseed and sunflower oils. The total fat content was kept constant at about 5%. Results indicated that feeding chickens on diets with the lowest LA content (2.9%) resulted in higher incorporation of total n-3 PUFA in the breast (16%) compared with feeding the highest LA content (4.4%). Feeding a diet with a high content of LA caused a significant reduction in EPA level in both thigh and breast tissues. DPA and DHA levels were not affected by dietary LA. The researchers explained this effect based on the fact that LA competes with α-LNA for Δ6 desaturase availability. Stated the other way around, high dietary level of LA might depress the conversion of α-LNA to n-3 PUFAs. In another study by Lopez-Ferrer et al. (2001a), the authors fed broiler chickens on diets containing 0, 2, or 4% linseed oil plus tallow (8% added fat) throughout a 38 day growing 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 by Svedova et al. (2008) revealed that the administration of n-3 PUFA, in the form of linseed oil (3 g/100g mixed feed), in the diet of laying hens resulted in a significant increase in α-LNA in plasma. In the same study, fish oil administration (same dose as linseed) resulted in a significant increase in the proportion of plasma EPA and DHA. Chickens fed linseed oil deposit significant amounts of α-LNA in the egg yolk (in laying hens) and meat (in broilers) (Raju et al., 2005; Lopez-Ferrer et al., 2001a; 1999a; 1999b).

The effect of other types of oils on fatty acid modulation in broilers have been investigated. For example, Crespo and Esteve-Garcia (Reference CRESPO and ESTEVE-GARCIA2001) studied the modulatory effect of tallow, olive, sunflower, and linseed oils at two levels of fat inclusion (6 or 10%), in male or female broilers. Broilers fed sunflower and linseed oils showed significantly better feed efficiency and lower abdominal fat and cholesterol content of their thigh muscles compared to those fed tallow or olive oil. In females, abdominal fat increased with level of oil inclusion only in birds fed tallow or olive oil, but was constant in case of feeding sunflower or linseed oil. Monounsaturated fatty acids were higher in abdominal fat, whereas muscle fat contained higher polyunsaturated fatty acids. These results suggest that polyunsaturated fatty acids produce lower abdominal fat deposition than saturated or monounsaturated fatty acids. In a later study of Crespo and Esteve-Garcia (Reference CRESPO and ESTEVE-GARCIA2002), the authors investigated the effect of using 10% of tallow, olive, sunflower, and linseed oils from 28 to 48 d of age on efficiency of energy, fat, nitrogen, and fatty acid deposition in broiler females. Results showed that broilers fed linseed oil had significantly less abdominal fat than those fed tallow. Absorbed fat losses were slightly higher for birds fed linseed oil, and nitrogen efficiency was significantly lower in those fed tallow. Fatty acid balance showed the highest values of fatty acid oxidation in broilers fed linseed oil (48.2 g), followed by those fed sunflower oil (23.2 g). These results suggest that PUFA are effective in improving the efficiency and the quality of broilers.

In addition, echium oil from the plant Echium plantagineum has been recognised as an ideal source for enriching poultry meat with n-3 PUFA. Echium is a good source of stearidonic acid which is naturally converted to EPA when metabolised in the body (Miller et al., Reference MILLER, NICHOLS and CARTER2007; Kitessa and Young, Reference KITESSA and YOUNG2009). In addition, there are considerable amounts of α-LNA and γ-LA in echium oil. Rymer et al. (Reference RYMER, GIBBS and GIVENS2010) showed that γ-linolenic acid was accumulated as stearidonic acid (SDA) increased in the chicken's diet. SDA in echium oil overcomes the problem of Δ⁶ desaturase limitation of α-linolenic metabolism because it is the immediate metabolic derivative of α-LNA, as it is located after α-LNA in the n-3 metabolic pathway. So, Δ⁶ desaturase is not needed to synthesise EPA from SDA.

Conversion of SDA to EPA was estimated in human and animal studies to be approximately 17-30%, which is substantially higher than α-LNA (Yamazaki et al., Reference YAMAZAKI, FUJIKAWA, HAMAZAKI, YANO and SHONO1992; Guil-Guerrero, Reference GUIL-GUERRERO2007; Whelan, Reference WHELAN2009; Yang and O'Shea, Reference YANG and O'SHEA2009). Kitessa and Young (Reference KITESSA and YOUNG2009) conducted a study in chickens to compare the effect of echium oil and rapeseed in enriching poultry meat with n-3 PUFA. They fed two groups of chickens (21 days of age) on either echium or rapeseed oil diets for 35 days. Results of the study showed that feeding echium oil significantly increased α-LNA, SDA, EPA and DPA in thigh and breast compared with feeding rapeseed oil. There was a 17% increase in EPA by using echium oil instead of rapeseed oil. DHA was not affected by echium oil enrichment.

It can be concluded that plant oils containing SDA may offer a suitable and sustainable alternative to fish oil, which may not be preferred by some people because of the fishy taste, and to plant oils containing α-LNA, where high consumption is needed to achieve noticeable increases in EPA (Whelan, Reference WHELAN2009; Kew et al., Reference KEW, MESA, TRICON, BUCKLEY, MINIHANE and YAQOOB2004). One limitation is that the total and the long chain n-3 PUFA content (and the SDA content) of these plant oils is relatively low and the conversion to long-chain n-3 PUFA in the body is clearly not sufficient to enrich tissues with these fatty acids to the same degree as fish oil.

Overall, it can be summarised that enrichment of poultry meat with n-3 fatty acids can be achieved by using sources rich in these beneficial fatty acids and that the long chain n-3 PUFA marine sources (algal biomass and fish oil) are more effective than the 18-carbon plant-based n-3 fatty acid sources (echium and linseed oils) in increasing DHA, EPA, and total n-3 content in broiler chickens.