Effect of lipid supplementation on the endogenous synthesis of milk cis-9,trans-11 conjugated linoleic acid in dairy sheep and goats: A tracer assay with 13C-vaccenic acid

A major proportion of milk rumenic acid (RA; cis-9,trans-11 CLA) is synthesized through mammary Delta(9)-desaturation of vaccenic acid (VA; trans-11 18:1). Diet composition may determine the relative contribu-tion of this endogenous synthesis to milk RA content, with effects that might differ between ruminant species. However, this hypothesis is mostly based on estimated values, proxies of stearoyl-CoA desaturase (SCD) activ-ity, and indirect comparisons between publications in the literature. With the aim of providing new insights into this issue, in vivo Delta(9)-desaturation of C-13-labeled VA (measured via milk C-13-VA and-RA secretion) was directly compared in sheep and goats fed a diet without lipid supplementation or including 2% of lin-seed oil. Four Assaf sheep and 4 Murciano-Granadina goats were used in a replicated 2 x 2 crossover design to test the effects of the 2 dietary treatments during 2 consecutive 25-d periods. On d 22 of each period, 500 mg of C-13-VA were i.v. injected to each animal. Dairy performance, milk fatty acid profile, including isotope analysis, and mammary mRNA abundance of genes coding for SCD were examined on d 21 to 25 of each period. Supplementation with linseed oil improved milk fat concentration and increased the content of milk VA and RA. However, the isotopic tracer assay suggested no variation in the relative proportion of VA desaturated to milk RA, and the percentage of this CLA isomer deriving from SCD activity would remain constant regardless of dietary treatment. These results put into question a major effect of lipid supplementa-tion on the endogenous synthesis of milk RA and sup-port that mammary Delta(9)-desaturation capacity would not represent a limiting factor when designing feeding strategies to increase milk RA content. The lack of diet-induced effects was common to caprines and ovines, but inherent interspecies differences in mammary li-pogenesis were found. Thus, the higher proportions of VA desaturation and endogenous synthesis of milk RA in sheep supported a greater SCD activity compared with goats, a finding that was not associated with the similar mRNA abundance of SCD1 in the 2 species. On the other hand, transfer efficiency of the isotopic tracer to milk was 37% higher in caprine than in ovine, suggesting a greater efficiency in mammary fatty acid uptake from plasma in caprine.

Automated summary

The research compared the Delta(9)-desaturation in sheep and goats fed different diets and found that lipid supplementation did not have a significant effect on the endogenous synthesis of milk RA. Inherent interspecies differences were found in mammary lipogenesis, with sheep showing greater SCD activity compared to goats despite similar mRNA abundance of SCD1. Additionally, caprines had a higher transfer efficiency of isotopic tracer to milk compared to ovines, suggesting a greater efficiency in mammary fatty acid uptake from plasma in caprine.