Enrichment of conjugated linoleic acid (CLA) in the eggs of laying hens fed pure isomers of CLA or vaccenic acid.

Conjugated linoleic acid (CLA) is a complex mixture of positional and geometric conjugated isomersof linoleic acid. Dairy products are the major dietary source of CLA and cis-9, trans-11 CLA (9,11CLA) is the predominant isomer resulting from the specific metabolism of feed derived fatty acids inthe rumen. Recent studies suggest endogenous synthesis of CLA by action of Δ-9 desaturase on trans-11 18:1 fatty acid (vaccenic acid, VA) is more important than ruminal production. In view of potentialhuman health benefits, several studies have attempted to enrich eggs with CLA by supplementing thediets of laying hen with mixtures of CLA. This has resulted in marked enrichment but also negativeeffects on egg quality and loss of hatchability of the eggs with concurrent increase in saturated fattyacid content in egg yolk. Such changes are consistent with Δ-9 desaturase inhibition especially associatedwith the presence of the trans-10, cis-12 CLA isomer (10,12 CLA) in the CLA mixtures. It istherefore worthwhile to determine which of the major CLA isomers present in the CLA mixtures, 9,11CLA or 10,12 CLA, is associated with the negative effects on egg quality. This was done by supplementingthe diets of laying hen with relatively pure forms of 9,11 CLA and 10,12 CLA. Furthermore,using relatively pure forms of 9,11 CLA and 10,12 CLA allowed us to determine the transfer efficiencyof 9,11 CLA and 10,12 CLA into egg yolk. In addition, endogenous conversion of VA to 9,11CLA in hens and deposition in egg yolk was examined.Experimental treatments consisted of diets supplemented with 0, 0.5, 1.0 and 1.5 g/d of CLA orVA. CLA isomer content (% of total fatty acids) of the materials was 83 and 7% (9,11 CLA), 9 and85% (10,12 CLA), 35 and 35% (Mix CLA) of 9,11 CLA and 10, 12 CLA, respectively. VA was suppliedin the form of dehydrated hydroxy stearic acid with trans-11 C18:1 content of 28%. The othermajor isomers in this product were cis-11, trans-12 and cis-12 C18:1 (28, 11 and 11%, respectively).There were three hens in each treatment and experimental diets were fed for 21 days. When dietswere supplemented with 9,11 CLA, 10,12 CLA and Mix CLA the transfer efficiencies (percentageof dietary CLA recovered in the egg) of 9,11 CLA and 10,12 CLA in egg yolk total lipids were 21 and<1%, 24 and 12%, 20 and 9%, respectively. Dietary VA was efficiently converted to 9,11 CLA anddeposited in the yolk. The slope of linear regression of VA plus 9,11 CLA versus 9,11 CLA in the eggyolk total lipids represents an estimate of the average conversion over the range of VA intakes studied.In this study 74% of VA available for desaturation was converted to 9,11 CLA. The transfer efficiencyof VA (percentage of dietary VA recovered as VA plus 9,11 CLA in the egg) was 14%. Dietary vaccenicacid in hens is efficiently converted to 9,11 CLA, but the overall efficiency of dietary VA use forCLA enrichment is quite low. Further studies examining the limitations of VA transfer to the liver andavailability for conversion by Δ-9 desaturase should be examined.