Abstract
: It has been well documented from animal and human studies that conjugated linoleic acid (CLA) has numerous beneficial effects on health. In chickens, CLA exerts many effects on performance ranging from egg quality and yolk lipids to meat quality. Although there are several CLA isomers available, not all CLA isomers have the same incorporation rates into egg yolk: cis-9,trans-11 and trans-10,cis-12 CLA isomers are more favorably deposited into egg yolk than other isomers investigated, but of the two isomers, the former has a higher incorporation rate than the latter. CLA alters the amounts and profiles of lipids in plasma, muscles and liver. Furthermore, increased liver weight was reported in chickens fed dietary CLA. As observed in egg yolk, marked reduction in intramuscular lipids as well as increased protein content was observed in different studies, leading to elevation in protein-to-fat ratio. Inconsistency exists for parameters such as body weight gain, feed intake, feed conversion ratio, egg production rate and mortality, depending upon experimental conditions. One setback is that hard-cooked yolks from CLA-consuming hens have higher firmness as refrigeration time and CLA are increased, perhaps owing to alterations in physico-chemistry of yolk. Another is that CLA can be detrimental to hatchability when provided to breeders: eggs from these breeders have impaired development in embryonic and neonatal stages, and have increased and decreased amounts of saturated fatty acids and monounsaturated fatty acids (MUFAs), respectively. Thus, both problems can be fully resolved if dietary sources rich in MUFAs are provided together with CLA. Emerging evidence suggests that CLA exerts a critical impact on stress and immune functions as it can completely nullify some of the adverse effects produced by immune challenges and reduce mortality in a dose-dependent manner. Finally, CLA is a key regulator of genes that may be responsible for lipid metabolism in chickens. CLA down-regulates both expression of the gene encoding stearoyl-CoA desaturase-1 and its protein activity in the chicken liver while up-regulating mRNA of sterol regulatory element-binding protein-1.
Highlights
Conjugated linoleic acid (CLA), derivatives of linoleic acid with anticancer properties found during cooking ground beef (Ha et al, 1987), is a collective term which indicates geometric and positional isomers (7,9; 8,10; 9,11; 10,12; 11,13; 12,14) of cis-9,cis-12octadecadienoic acid (C18:2n-6) (Figure 1)
For the content of Na, K, and Mg there was a linear increase in egg yolks with storage time but a decrease in Na content in albumen, which was unrelated to dietary CLA. These results suggest that the greater firmness of egg yolks from CLA-fed hens is likely to be due to changes in pH, water content, and/or ion concentrations during refrigerated storage and to increased content in saturated fatty acids (SFAs) (Shang et al, 2004)
These findings indicate that there are age differences in responses of laying hens to dietary CLA, as shown in elsewhere (Nielsen, 1998)
Summary
Conjugated linoleic acid (CLA), derivatives of linoleic acid with anticancer properties found during cooking ground beef (Ha et al, 1987), is a collective term which indicates geometric (cis, trans) and positional isomers (7,9; 8,10; 9,11; 10,12; 11,13; 12,14) of cis-9,cis-12octadecadienoic acid (C18:2n-6) (Figure 1).
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