Growth rate and feed efficiency

Opinions in the literature are divided about the nutritional effects of L-carnitine on feed consumption and weight gain in poultry. Some researchers reported beneficial effects of L-carnitine in poultry diets, while others found no effect on broiler performance. Tufan et al. (Reference TUFAN, ARSLAN, DURNA, ÖNK, SARI and ERMAN2015) reported no significant effect of L-carnitine supplementation on feed consumption, live weight and live weight gains and feed conversion ratio in Japanese quail. Parizadian et al. (Reference PARIZADIAN, AHANGARI, SHAMS and SARDARZADEH2011) reported that L-carnitine (125 and 250 mg/kg feed) did not produce any significant difference in feed intake in quails. However, Parizadian et al. (Reference PARIZADIAN, AHANGARI, SHAMS and SARDARZADEH2011) and Sarica et al. (Reference SARICA, CORDUK, ENSOY, BASMACIOGLU and KARATAS2007) reported improved FCR in Japanese quail by L-carnitine. Daskiran et al. (Reference DASKIRAN, ONOL, CENGIZ, TATLI and SARI2009) reported no significant difference in feed intake in layers supplemented with L-carnitine. On the other hand, Nouboukpo et al. (Reference NOUBOUKPO, TONA, AGBONON, GBEASSOR, BUYSE and DECUYPERE2010) reported increased weight gain in broiler after supplementation of L-carnitine in drinking water. Buyse et al. (Reference BUYSE, JANSSENS and DECUYPERE2001) reported an increased weight gain of broiler chicks by L-carnitine supplementation, although Corduk et al. (Reference CORDUK, CEYLAN and ILDIZ2007) found no significant difference in feed consumption and weight gain in broiler chicks. Sarica et al. (Reference SARICA, CORDUK, ENSOY, BASMACIOGLU and KARATAS2007) supplemented L-carnitine at various dose levels but did not report any positive effect on feed intake of quail in a 28-day experiment. Celik and Ozturkcan (Reference CELIK and OZTURKCAN2003) found significantly increased feed intake in broiler chicks under heat stress.

However, some research trials have reported positive outcome due to the supplementation of L-carnitine in broiler chicks (Rabie et al., Reference RABIE, SZILAGYI and GIPPERT1997a; Reference RABIE, SZILAGYI, GIPPERT, VOTISKY and GERENDAD1997b; Rabie and Szilagyi, Reference RABIE and SZILAGYI1998; Kita et al., Reference KITA, KATO, YAMAN, OKUMURA and YOKOTA2002). The beneficial effects of L-carnitine on growth are considered due to the secretion of insulin-like growth factors (IGFs), which is responsible for growth (Beccavin et al., Reference BECCAVIN, CHEVALIER, COGBURN, SIMON and DUCLOS2001; Kita et al., Reference KITA, KATO, YAMAN, OKUMURA and YOKOTA2002). Increasing the rate of supplementation of L-carnitine in an experiment using 0 to 1000 mg/kg in feed increased plasma IGFs concentration and ultimately led to increased weight gain. In addition, different factors, such as composition of basal diet, sparing effect of L-carnitine on its precursor, age, sex and other managemental differences could be the possible reasons of the conflicting results on the growth performance.

Carcass traits and abdominal fat

Excessive fat accumulation in broiler chicks is a problem because of consumers’ preference and health concerns. Excessive fat is accumulated in the birds’ body due to many factors including as genetics and nutrients. Determining the weight of abdominal fat is considered the yardstick of total body fats in poultry (Sonaiya, Reference SONAIYA1985). Supplementation with L-carnitine reduces both abdominal and the mesenteric fat percentage. L-carnitine accelerates the rate of oxidation of fatty acid on one hand, and on the other, decreases the availability of fatty acids for conversion into triacylglycerols and subsequent storage in the adipose tissues (Xu et al., Reference XU, WANG, MAO, ZHAN and HU2003). Recently, improved carcass quality was reported by Asadi et al. (Reference ASADI, SADEGHI, EILA and AMINAFSHAR2016).

Provision of L-carnitine in the diet generates adenosine tri-phosphate (ATP) by accelerating the β-oxidation of fatty acid (Neuman et al., Reference NEUMAN, LIN and HESTER2002; Corduk et al., Reference CORDUK, CEYLAN and ILDIZ2007). In a recent study, Hrnčár et al. (Reference HRNČÁR, VERGULIAKOVÁ, SVORAD, WEIS, APRÁŠOVÁ, MINDEK, FIK and BUJKO2015) reported that supplementation of L-carnitine had a significant effect on carcass weight, leg meat and breast muscle in broiler chicks. Similarly, Parsaeimehr et al. (Reference PARSAEIMEHR, AFROUZIYEH and HOSEINZADEH2014) reported that L-carnitine supplementation in the diet enhanced the weight of thigh and breast.

Lower abdominal fat and liver weight, and higher breast and thigh muscles in broilers were reported by Zhang et al. (Reference ZHANG, JIA, JI, MA, HUANG, YIN and LIU2014) through supplementation of acetyl -L-carnitine and lipoic acid. Michalczuk et al. (Reference MICHALCZUK, LUKASIEWICZ, NIEMIEC, WNUK and MROCZEK2012) reported that dressing percentage in response to supplementation of L-carnitine in broilers was significantly improved. Similarly, Zhang et al. (Reference ZHANG, MA, BAI, ZHAO, WANG and JI2010) reported that L-carnitine increased dressing percentage. Xu et al. (Reference XU, WANG, MAO, ZHAN and HU2003) found that L-carnitine increased breast muscle yield and decreased abdominal fat. Similar results were reported by Rabie and Szilagyi (Reference RABIE and SZILAGYI1998) in broilers.

Lien and Horng (Reference LIEN and HORNG2001) and Corduk et al. (Reference CORDUK, CEYLAN and ILDIZ2007) did not report a significant difference in carcass yield and the proportion of carcass sections. Insufficient dietary supplementation causes excess abdominal fat in broilers (Lien and Horng, Reference LIEN and HORNG2001), while supplementation of L-carnitine helps the body turn fat into energy by accelerating β-oxidation (Kazemi-Fard et al., Reference KAZEMI-FARD, YOUSEFI, DIRANDEH and REZAEI2015).

Meat composition

Arslan et al. (Reference ARSLAN, CITIL and SAATCI2004a) reported that L-carnitine supplementation enhanced total saturated fatty acid and decreased polyunsaturated fatty acid concentration in abdominal fat of geese at the age of 12 weeks. In another study, Arslan et al. (Reference ARSLAN, CITIL and SAATCI2003) concluded that total saturated fatty acid content of abdominal fat of the duck decreased, while mono and polyunsaturated fatty acids contents were not affected by L-carnitine supplementation. Sarica et al. (Reference SARICA, CORDUK and KILINC2005) reported that supplementation did not affect crude protein, ether extract contents, dry matter and moisture of total edible meat in Japanese quail. Some researchers did not report any significant effect of L-carnitine on dry matter, crude protein and ether extract, crude fat of breast and thigh meat (Barker and Sell, Reference BARKER and SELL1994; Rabie et al., Reference RABIE, SZILAGYI, GIPPERT, VOTISKY and GERENDAD1997b; Rabie and Szilagyi, Reference RABIE and SZILAGYI1998; Celik and Ozturkcan, Reference CELIK and OZTURKCAN2003). Conversely, Xu et al. (Reference XU, WANG, MAO, ZHAN and HU2003) found that crude fat was significantly higher in breast of male broilers in response to supplementation of 50 or 75 mg/kg of diet.

Egg production and quality

Kazemi-Fard et al. (Reference KAZEMI-FARD, YOUSEFI, DIRANDEH and REZAEI2015) reported that 100 and 150 mg/kg L-carnitine resulted in an increase of egg production and egg mass. Nofal et al. (Reference NOFAL, SAMAK, MARIEY and MAHAMUD2006) found that provision of L-carnitine at 100 mg/kg in laying hen feed significantly increased egg weight and egg mass. Similarly, supplementation of L-carnitine in diets increased fertility (Nofal et al., Reference NOFAL, SAMAK, MARIEY and MAHAMUD2006). Thiemel and Jelinek (Reference THIEMEL and JELINEK2004) reported that the hatchability of eggs was increased by 8.89% with L-carnitine supplementation. Rabie et al. (Reference RABIE, SZILAGYI and GIPPER1997c) and Celik et al. (Reference CELIK, TEKELI and ÖZTÜRKCAN2004) reported that improved albumen quality with L-carnitine supplementation may be linked with faster metabolic rate and increased secretion from the shell gland. It has been suggested that L-carnitine supplementation increases the synthesis and secretion of β-ovomucin in the magnum (Daskiran et al., Reference DASKIRAN, ONOL, CENGIZ, TATLI and SARI2009) and promotes the gelatinous characteristics of the albumen (Kita et al., Reference KITA, NAKAJIMA and NAKAGAWA2005). In other studies, it was reported that albumen height and Haugh unit of eggs were improved when L-carnitine was included in feed (Kita et al., Reference KITA, NAKAJIMA and NAKAGAWA2005; Rezaei et al., Reference REZAEI, DEHGHANI, GHAFFARI and HAGHNAZAR2008; Ghods-Alavi et al., Reference GHODS-ALAVI, SAMIE and JAHANIAN2010).

Antioxidant effects

Free radicals produced in the body ultimately result in the formation of oxidative damage of the cell membrane and the organelle of the cell, if not neutralised (Khan et al., Reference KHAN, NAZ, NIKOUSEFAT, TUFARELLI, JAVADANI, RANA and LAUDADIO2011). The body is dependent on the antioxidant capacity for quenching the oxidants (Khan et al., Reference KHAN, RAHMAN, JAVED and MUHAMMAD2013). Oxidative stress may develop if antioxidants in feed fail to meet requirements for correct antioxidant status (Khan, Reference KHAN2011).

It has been proposed that L-carnitine exhibits antioxidant properties by acting as a metal chelator to reduce free Fe⁺² ions (Muthuswamy et al., Reference MUTHUSWAMY, VEDAGIRI, GANESAN and CHINNAKANNU2006). A number of radicals such as superoxide, hydroxyl and hydrogen peroxide have been documented to be neutralised by the presence of L-carnitine (Derin et al., Reference DERIN, IZGUT, ALICIGUZEL and DEMIR2004). It has been reported that supplementation of L-carnitine in feed decreased melanodialdehyde (MDA) concentrations in broiler (Yousefi et al., Reference YOUSEFI, KHAJALI, HASSANPOUR and KHAJALI2013; Wang et al. (Reference WANG, NING, PENG and GUO2013). Similarly, the supplementation of L-carnitine improved liver antioxidative capacity in broilers (Geng et al., Reference GENG, GUO and YUAN2004). In addition, L-carnitine increases the antioxidant status of poultry through promoting antioxidant enzymes, such as superoxide dismutase and glutathione peroxidase. L-carnitine has buffering effect on excess acetyl-CoA, inhibits microsomal peroxidation and increasing oxygen consumption and maintains low level of fatty acids inside the cell (Adabi et al., Reference ADABI, COOPER, CEYLAN and CORDUK2011).

Blood biochemistry

It has been documented that supplementation of L-carnitine decreases a number of blood metabolites such as liver enzymes, cholesterol, triglyceride and low-density lipoprotein in broiler (Rezaei et al., Reference REZAEI, DEHGHANI, GHAFFARI and HAGHNAZAR2008; Zhai et al., Reference ZHAI, NEUMAN, LATOUR and HESTER2008a; Hassan et al., Reference HASSAN, YOUSSEF and EL-BAHY2011; Parizadian et al., Reference PARIZADIAN, AHANGARI, SHAMS and SARDARZADEH2011; Hosseintabar et al., Reference HOSSEINTABAR, DADASHBEIKI, BOUYEH, SEIDAVI, VAN DEN HOVEN and GAMBOA2015). Reduced serum cholesterol in L-carnitine supplemented broiler diets has been well established (Lien and Horng, Reference LIEN and HORNG2001; Nofal et al., Reference NOFAL, SAMAK, MARIEY and MAHAMUD2006; Zhang et al., Reference ZHANG, MA, BAI, ZHAO, WANG and JI2010). The reduction in serum cholesterol may be due to a stimulatory effect of L-carnitine on lipid metabolism (Parizadian et al., Reference PARIZADIAN, AHANGARI, SHAMS and SARDARZADEH2011). In addition, supplementation of L-carnitine decreased egg yolk cholesterol and triglycerides (Zhai et al., Reference ZHAI, NEUMAN, LATOUR and HESTER2008b; Parizadian et al., Reference PARIZADIAN, AHANGARI, SHAMS and SARDARZADEH2011; Hassan et al., Reference HASSAN, YOUSSEF and EL-BAHY2011). However, other studies (Arslan et al., Reference ARSLAN, CITIL and SAATCI2004b) did not report any significant effect on serum cholesterol. The reduction in serum triglyceride is probably due to the increased transportation of fatty acid into the mitochondria for oxidation (Parizadian et al., Reference PARIZADIAN, AHANGARI, SHAMS and SARDARZADEH2011).

L-carnitine increases the activities of lipase and reduces lipoprotein lipase leading to the conversion of triglyceride into fatty acids and glycerol (Zhang et al., Reference ZHANG, MA, BAI, ZHAO, WANG and JI2010). A reduction in serum triglyceride in broilers fed a diet containing L-carnitine was reported by Rezaei et al. (Reference REZAEI, ATTAR, GHODRATNAMA and KERMANSHAHI2007) and Zhang et al. (Reference ZHANG, MA, BAI, ZHAO, WANG and JI2010). Other studies did not report any significant reduction in serum triglyceride in Japanese quail and duck when L-carnitine was included in feed (Arslan et al., Reference ARSLAN, CITIL and SAATCI2003; Reference ARSLAN, CITIL and SAATCI2004a; Reference ARSLAN, CITIL and SAATCI2004b). Reduced serum and egg yolk low density lipoprotein (LDL) were reported when L-carnitine was supplemented in the diet of laying hens (Rezaei et al., Reference REZAEI, DEHGHANI, GHAFFARI and HAGHNAZAR2008; Hassan et al., Reference HASSAN, YOUSSEF and EL-BAHY2011). It has been suggested that L-carnitine reduces LDL due the decreased activity of HMG-co reductase (the limiting enzyme for cholesterol synthesis) in the liver (Hassan et al., Reference HASSAN, YOUSSEF and EL-BAHY2011). L-carnitine has the capacity to remove the short and medium chain fatty acids from the mitochondria, which are accumulated due to abnormal metabolism.

Immunity

Poultry has both cell-mediate and humoral immunity (Khan et al., Reference KHAN, RAHMAN, NIKOUSEFAT, JAVDANI, TUFARELLI, DARIO, SELVAGGI and LAUDADIO2012a; Reference KHAN, NAZ, JAVADANI, NIKOUSEFAT, SELVAGGI, TUFARELLI and LAUDADIO2012b). A number of studies have reported improved humoral immune response against different infectious diseases such as Newcastle, avian influenza and ascites when birds were supplemented with L-carnitine (Mast et al., Reference MAST, BUYSE and GODDEERIS2000; Geng et al., Reference GENG, LI and GUO2007; Hassan et al., Reference HASSAN, YOUSSEF and EL-BAHY2011; Wang et al., Reference WANG, NING, PENG and GUO2013; Parsaeimehr et al., Reference PARSAEIMEHR, AFROUZIYEH and HOSEINZADEH2014; Asadi et al., Reference ASADI, SADEGHI, EILA and AMINAFSHAR2016). Some studies have reported higher weight of the lymphoid organs in broilers when L-carnitine was supplemented in broiler diets (Deng et al., Reference DENG, WONG and NOLAN2006; Hassan et al., Reference HASSAN, YOUSSEF and EL-BAHY2011; Parsaeimehr et al., Reference PARSAEIMEHR, AFROUZIYEH and HOSEINZADEH2014). The exact mechanism through which L-carnitine improves the immune profile in bird is not fully understood. One simple reason forwarded for higher immune status is the heavier weight of the lymphoid organs (Deng et al., Reference DENG, WONG and NOLAN2006; Hassan et al., Reference HASSAN, YOUSSEF and EL-BAHY2011). One of the most cited reasons of the positive effects of L-carnitine is the inhibition of apoptosis of the immune cells. Some researchers suggest that the antioxidative action of L-carnitine is responsible for its immune modulating effect (Liu et al., Reference LIU, HEAD, KURATSUNE, COTMAN and AMES2004). It has been suggested that endogenous L-carnitine is a major determinant of improved immune and inflammatory responses towards invading pathogens (Famularo et al., Reference FAMULARO, DE SIMONE, TRINCHIERI and MOSCA2004).