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Effects of Dietary Conjugated Linoleic Acid on the Fatty Acid Profile and Cholesterol Content of Egg Yolks from Different Breeds of Layers¹

Ministry of Agriculture Feed Industry Center, College of Animal Science and Technology, China Agricultural University, Beijing, China, 100094

³ Corresponding author: defali{at}public2.bta.net.cn

	ABSTRACT

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Brown Dwarf hens and White Leghorn hens were fed corn- and soybean meal-based diets containing 0, 2.5, or 5.0% conjugated linoleic acid (CLA) for 56 d to explore the effects of dietary CLA on the fatty acid profile and cholesterol content of egg yolks from laying hens of different breeds. Four hens were placed in 1 cage, and 3 cages were grouped as 1 replicate, resulting in 6 replicates per treatment. After feeding the experimental diets for 11 d, eggs were collected to determine the fatty acid composition of the egg yolks. From d 54 to 56, eggs were collected to measure the cholesterol content of yolks, and on d 56, a hen was selected randomly from each replicate and bled to determine the cholesterol content in plasma. There was a significant effect of layer breed on layer performance and egg composition. Concentrations of stearic, arachidonic, and docosahexaenoic acids were higher in the yolks of Brown Dwarf hens than in those of White Leghorn hens. Enrichment of cis-11, trans-13 was higher in the yolks of White Leghorns, but cis-10, cis-12 was higher in those of Brown Dwarf hens. In contrast, feed intake and egg weight, as well as yolk weight and its ratio to egg, were decreased by the 5% dietary CLA treatment. Egg production and feed efficiency were not affected by dietary CLA. Concentrations of total CLA and CLA isomers in the yolk lipids increased (P < 0.01) with increasing dietary CLA. Furthermore, yolk cholesterol was increased with increasing dietary CLA (P < 0.01), but this was significantly decreased in Brown Dwarf hens (P < 0.01) by feeding 2.5% CLA. There was no apparent correlation between yolk cholesterol content and serum cholesterol content. In conclusion, Brown Dwarf layers had the breed-specific characteristics of enrichment of CLA isomers and fatty acids in yolk lipids in response to dietary CLA.

Key Words: conjugated linoleic acid • hen • egg yolk • fatty acid profile • cholesterol

	INTRODUCTION

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Conjugated linoleic acid (CLA) is a mixture of linoleic acid isomers with conjugated double bonds. The consumption of CLA by human beings has been shown to result in many favorable health benefits, such as reducing carcinogenesis, atherosclerosis, diabetes, and body fat mass and modulating immune function (Belury, 2002). However, to obtain this health benefit, the effective concentration of CLA for daily consumption by a 70-kg human is approximately 1.5 to 3.0 g of CLA (Decker, 1995), which is significantly greater than the estimated consumption of 0.5 to 1.0 g of CLA per person daily (Ip et al., 1991). Consequently, there is considerable interest in supplementing CLA in animal feed to increase the CLA content in animal products for human consumption.

Eggs from CLA-fed layers are a good source of CLA in the human diet, and a large egg (60 g) from White Leghorn hens fed 5% CLA would provide 0.3 to 1.0 g of CLA (Chamruspollert and Sell, 1999; Du et al., 1999; Raes et al., 2002; Sun et al., 2003). Nevertheless, increased dietary CLA will contribute to more negative effects on layer production and egg quality (Shang et al., 2004; Suksombat et al., 2006). How to balance the side effects of CLA becomes a key point before using CLA in layer diets becomes practical (Kim et al., 2007). Conjugated linoleic acid cosupplemented with other fatty acids was reported to prevent apparent adverse effects on egg quality and egg production (Kim et al., 2007). However, the lipid metabolism of Dwarf hens differs from that of White Leghorns (Guillaume, 1976). Therefore, layers of different breeds may have different traits regarding CLA metabolism. Burghelle-Mayeur et al. (1990) reported that compared with White Leghorns, Dwarf hens were much more adapted to utilizing dietary fat to compose the yolks. Scheideler et al. (1998) also observed similar results. Furthermore, Huang and Yang (2001) reported that the disposition of fatty acids in yolks was affected by the dw gene and that the concentrations of linolenic, arachidonic, eicosapentaenoic, and docosahexaenoic acids of yolk lipids from Brown Dwarf hens were higher than those from White Leghorn hens when they were fed the same diets. However, there have been no reports on the effect of the dw gene on enrichment of CLA in yolks. In addition, the effect of dietary CLA on the cholesterol content of yolks remains unclear. Sun et al. (2003) reported that the cholesterol concentration of egg yolks was significantly decreased by feeding 5% CLA for 5 wk. However, Szymczyk and Pisulewski (2003) indicated that the cholesterol content was not affected by dietary CLA. In this research, we explored the effects of dietary CLA on the fatty acid profile and cholesterol content of egg yolks from different breeds of layers.

	MATERIALS AND METHODS

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Dietary Treatments

In the current study, we adopted a 2 x 3 factorial random block design using 50-wk-old Brown Dwarf hens (1.53 ± 0.07 BW) and White Leghorn hens (1.65 ± 0.12 BW). Layers were fed corn- and soybean meal-based diets containing 0, 2.5, or 5.0% CLA (Table 1). The experimental diets were formulated as described in our previous study (Shang et al., 2005). The CLA was supplied by Aohai Biotech Co. Ltd. (Qingdao, Shandong Province, China) in the form of free fatty acids. To equalize the concentrations of total fat, the CLA source was substituted for soybean oil on an equal-weight basis. Fresh diets were prepared each week, and ethoxyquin (0.015% of feed) was added to all diets to avoid fat oxidation.

Two hundred sixteen 50-wk-old birds of each breed were housed in wire-floored cages measuring 45 x 40 cm with a height of 45 cm. Four hens were placed in 1 cage, and 3 cages were grouped as 1 replicate, resulting in 6 replicates per treatment. All hens were allowed free access to their respective diets and water. The photoperiod was set at a 16L:8D cycle throughout the 56-d experiment. Room temperature was maintained at 25 ± 5°C. Egg production and feed consumption for each replicate were recorded daily or weekly. Hens were weighed on d 0, 28, and 56 of the experiment, and rates of egg production and feed efficiency (kilograms of feed used to produce a kilogram of eggs) were calculated at the end of the experiment. On d 28 and 56, eggs were weighed and broken open, and the yolks and albumens were separated. The weights of the egg, albumen, yolk, and shell were recorded.

Feed Analysis

Feed samples from each diet were collected after mixing and stored at 4°C until analyzed. The diets were analyzed for CP by the Kjeldahl method (N x 6.25; AOAC, 2000; method 954.01). Total lipids in the diets were determined by the method of Folch et al. (1957), and the fatty acid composition of lipids was analyzed as described below. The CP, total lipids, and fatty acid composition of the diets are presented in Table 2.

Chamruspollert and Sell (1999) reported that the maximum level of CLA in egg yolk lipids appeared on d 10 to 11 after CLA feeding was initiated. Therefore, on d 11, 4 eggs randomly sampled from each replicate (a total of 24 eggs per treatment) were broken open, and the yolks and albumens were separated to determine the fatty acid composition of the egg yolks. Eggs were collected from d 54 to 56 to measure the cholesterol content of the yolks. Hens were selected randomly from each replicate and were bled on d 56 via heart puncture by using 9-mL vacuum tubes (Becton Dickinson Vacutainer Systems, Franklin Lakes, NJ). Serum was separated by centrifuging at 1,500 xg for 10 min and then stored at –20°C to determine the serum contents of cholesterol and triglycerides.

Fatty Acid Analysis

The fatty acid composition of egg yolks was analyzed according to the method described by Raes et al. (2002). Heptadecaenoic acid (17:0) was used as an internal standard. Fatty acid methyl esters were analyzed by gas chromatography (GC) on an HP6890 GC system (Hewlett-Packard, Wilmington, DE) installed with a Chrompack capillary column (CP-Sil 88 column, 100 m x 250 µm x 0.25 µm, Varian Inc., Palo Alto, CA). The chromatography conditions were as follows: 250°C injector temperature; 250°C detector temperature; helium as the carrier gas, 1:40 split ratio; temperature program set to 180°C for 45 min, followed by an increase of 10°C/min to 215°C, and then maintained for 17 min. Peaks were identified by comparison of retention times with those of the corresponding standards (Sigma, St. Louis, MO; Matreya Biochemicals, State College, PA). Identification of the peaks included fatty acids between 14:0 and 24:1 and 6 different CLA isomers. The trans-8, cis-10 CLA isomer was probably present in the CLA source, but its peak overlapped with that of cis-9, trans-11 CLA and could not be detected.

Yolk Cholesterol Analysis

Cholesterol analysis of yolks was carried out according to the method described by Jiang et al. (1998). 5--Cholesterol was added as an internal standard. The cholesterol esters were analyzed by GC on an HP6890 GC system (Hewlett-Packard) installed with a Chrompack capillary column (HP column, 30 m x 320 µm x 0.25 µm, Varian Inc.). The GC conditions were as follows: 250°C injector temperature; 300°C detector temperature; nitrogen as the carrier gas, 1:74 split ratio; temperature program set to 250°C for 8 min, followed by an increase of 1°C/min to 260°C, and then maintained for 15 min. Cholesterol and triglycerides in serum were determined by using commercial kits (Beijing Biological Technology Co., Beijing, China) according to the manufacturer’s instructions.

Statistical Analysis

All data were analyzed by 2-way ANOVA to determine the effect of dietary CLA, layer type, and their interaction by using the statistics software SPSS 11.0 (SPSS Inc., Chicago, IL). P < 0.05 was considered a significant difference.

	RESULTS AND DISCUSSION

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Layer Performance and Egg Composition

Layer performance and egg composition were significantly affected by layer type (Tables 3 and 4). Brown Dwarf hens had lower feed consumption, a lower egg production rate, egg weight, and BW gain, and better feed efficiency than White Leghorn layers. For both Brown Dwarf and White Leghorn hens, feed consumption and egg weight were decreased by 5% dietary CLA (P < 0.05). Body weight gain was decreased by feeding even 2.5% dietary CLA. Egg production and feed efficiency were not altered by increasing amounts of dietary CLA (2.5 and 5.0%), which is in conflict with the report by Jones et al. (2000) that egg production was significantly decreased even with levels as low as 0.5 to 1.0% CLA in the diet. This may be due to the different laying stages of layers or the diet composition, which warrants further study.

Dietary CLA has been reported to reduce feed intake in most research in poultry (Ahn, et al., 1999; Szymczyk and Pisulewski, 2003), pigs (Dugan et al., 1997), and mice (West et al., 1998), which is consistent with the results of this study. Because feed consumption is positively related to egg weight, BW gain, and yolk weight, its reduction may partly account for the decrease in egg weight, BW gain, and yolk weight in this experiment. Nevertheless, the reason albumen weight did not change was not clear. In our previous study, yolk weight was decreased by 3% dietary CLA, but albumen weight was not decreased until the dietary CLA level increased to 4%. The divergent alteration of yolk and albumen weights induced by dietary CLA may contribute to an understanding of the modulation of egg composition by dietary CLA (Shang et al., 2004). There was no interaction between layer breed and CLA concentration in the diet and on layer performance or egg composition (Tables 3 and 4).

Yolk Fatty Acid Profile of Different Breeds of Layers

The yolk fatty acid profile was affected by layer breed as shown in Tables 5 and 6. Compared with White Leghorns, Brown Dwarf hens had greater contents of stearic, arachidonic, and docosahexaenoic acids in the yolk, whereas the yolk had less palmitic and palmitoleic acids, consistent with the report by Huang and Yang (2001). All fatty acids (except myristoleic acid) in yolk lipids were significantly altered by dietary CLA supplementation (Tables 5 and 6). As compared with the control group, feeding 2.5% or more of dietary CLA increased the contents of myristic, palmitic, and stearic acids in the yolk lipids, but decreased the contents of palmitoleic, oleic, linoleic, linolenic, arachidonic, docosahexaenoic, and nervonic acids. Concurrently, total saturated fatty acids (SFA), polyunsaturated fatty acids (PUFA), and total CLA were increased (P < 0.01), whereas the contents of monounsaturated fatty acids (MUFA) and non-CLA PUFA were decreased (Table 6). The results of the study were similar to previous reports (Schafer et al., 2001; Cherian et al., 2002; Yang et al., 2002; Szymczyk and Pisulewski, 2003, Shang et al., 2004, 2005).

There was an interaction between breed and dietary concentrations of SFA, MUFA, PUFA, and non-CLA PUFA in egg yolks (Table 6). White Leghorn hens seemed to be more sensitive than Brown Dwarf hens to dietary CLA in reducing MUFA and non-CLA PUFA in the yolk. Furthermore, the alterations of SFA and PUFA induced by increasing dietary CLA were also different between White Leghorn and Brown Dwarf layers, respectively.

Enrichment of Dietary CLA in Egg Yolk Lipids of Different Breeds of Layers

The isomers of CLA-enriched yolks have been extensively investigated in recent years. In this study, total CLA and its isomers in yolk lipids of layers of both breeds were enhanced with increasing dietary CLA (P < 0.01), which was consistent with other reports (Ahn et al., 1999; Cherian et al., 2002; Szymczyk and Pisulewski, 2003). Furthermore, results of the study showed that the concentration of individual CLA isomers in egg yolks did not completely reflect those in the diet (Table 7). The percentages of cis-9, trans-11 and trans-10, cis-12 in the CLA source were similar, at 43.23 and 40.30%, respectively, whereas their percentages in the yolk lipids differed at 54.35 and 27.50%, respectively, which was consistent with the report of Chamruspollert and Sell (1999). Szymczyk and Pisulewski (2003) also observed that compared with other isomers of CLA, the cis-9, trans-11 isomer of CLA was preferentially incorporated into yolks, whereas the incorporation of trans-10, cis-12 was less efficient. However, in the current research, the cis-11, trans-13 isomer of CLA in yolk lipids was the same as that in the diets, a result which differed from other reports (Szymczyk and Pisulewski, 2003). The cis-11, trans-13 isomer of CLA has been shown to be preferentially accumulated into cardiac lipids, in particular, the major phospholipid in the inner mitochondrial membrane (Kramer et al., 1998), whose appearance in cardiolipin may suggest a functional basis.

Yolk Cholesterol Content of Different Breeds of Layers

The effect of dietary CLA on the contents of yolk cholesterol of different breeds of layers is shown in Table 8. There was no difference between Brown Dwarf and White Leghorn layers in yolk cholesterol and other serum lipids, except for serum high-density lipoprotein (HDL) cholesterol. However, the cholesterol content in yolks was significantly altered by dietary CLA, and there was an interaction between breed and dietary CLA level (P < 0.01). Yolk cholesterol was increased as CLA in the diet increased, although it was decreased by 2.5% dietary CLA in Brown Dwarf layers. The results of the study differ from the report by Sun et al. (2003), in which feeding 5.0% CLA to White Leghorn hens for 5 wk decreased yolk cholesterol concentrations. In contrast, other studies have shown that the cholesterol content of egg yolks was not affected by dietary CLA concentrations (Raes et al., 2002; Szymczyk and Pisulewski, 2003). The divergence among these studies may be due to the various laying stages of hens and the amounts of CLA supplemented in the diets, which warrants further study. The concentrations of low-density lipoprotein cholesterol and triglycerides in serum were not altered by layer breed, dietary CLA concentration, or their interaction. However, total cholesterol and HDL-cholesterol in the serum were affected by the level of dietary CLA and the interaction between layer breed and dietary CLA concentration. High-density lipoprotein cholesterol can clean the surplus cholesterol out of plasma, transport it, and excrete it by the feces. In the present study, the increase in total cholesterol induced by dietary CLA was mainly derived from the enhancement of HDL-cholesterol in serum.

Ahn et al. (1999) showed that dietary CLA caused yolks to harden. In previous studies, it was concluded that the greater egg firmness of CLA-fed hens might be related to changes in pH, water content, and ion concentration during refrigerated storage (Ahn et al., 1999; Shang et al., 2004). In the current study, we did not observe a difference in egg quality between the 2 breeds of layers.

Collectively, Brown Dwarf layers, compared with White Leghorn layers, had the breed-specific characteristic of enrichment of CLA isomers and fatty acids in the yolk in response to dietary CLA. The effect of CLA on the yolk cholesterol content warrants further study, and the breeds and laying stages of the hens must be taken into account. It is feasible to add CLA to the diets of Brown Dwarf layers to produce eggs rich in CLA. However, the adverse effects of dietary CLA on egg quality must be alleviated.

	FOOTNOTES

 
¹ Supported by the New Century Excellent Talent in University and National Natural Science Foundation of China.

² Contributed equally to this work.

Received for publication June 1, 2007. Accepted for publication October 22, 2007.

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