Abstract
A major proportion of milk rumenic acid (RA; cis-9,trans-11 CLA) is synthesized through mammary Δ9-desaturation of vaccenic acid (VA; trans-11 18:1). Diet composition may determine the relative contribution 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) activity, and indirect comparisons between publications in the literature. With the aim of providing new insights into this issue, in vivo Δ9-desaturation of 13C-labeled VA (measured via milk 13C-VA and -RA secretion) was directly compared in sheep and goats fed a diet without lipid supplementation or including 2% of linseed oil. Four Assaf sheep and 4 Murciano-Granadina goats were used in a replicated 2 × 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 13C-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 supplementation on the endogenous synthesis of milk RA and support that mammary Δ9-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 lipogenesis 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.
Highlights
Rumenic acid (RA; cis-9,trans-11 18:2) is the most abundant CLA isomer in milk and has attracted a great deal of attention in dairy science due to its potential benefits for human health (Dilzer and Park, 2012; Shingfield et al, 2013)
Because detailed kinetics of [1-13C]vaccenic acid (VA) secretion have already been reported in goats and sheep (Bernard et al, 2010; Hervás et al, 2020), this study focused on examining the endogenous synthesis of milk RA for the 0- to 72-h p.i. period, within which the enrichment of [1-13C]fatty acid (FA) in milk would have reached its maximum and subsequently declined to very low but still detectable levels using GC-C-IRMS
The addition of linseed oil to the diet decreased the concentrations of FA with 10 to 17 carbon atoms, cis-11 18:1, 18:2n-6, and the sums of C20–22 n-6 PUFA and odd- and branched-chain FA, whereas it had a positive effect on the proportions of 18:3n-3, most 18-carbon intermediate metabolites, and 18:0 in milk (P < 0.05)
Summary
Rumenic acid (RA; cis-9,trans-11 18:2) is the most abundant CLA isomer in milk and has attracted a great deal of attention in dairy science due to its potential benefits for human health (Dilzer and Park, 2012; Shingfield et al, 2013). Toral et al.: ENDOGENOUS MILK CLA IN DAIRY SHEEP AND GOATS broad range of endogenous RA data in the literature (approximately 46–100% of total milk RA; Kay et al, 2004; Taugbøl et al, 2008; Hervás et al, 2020) may be determined, at least in part, by diet-induced variations in mammary SCD activity Another decisive factor in the contribution of endogenous synthesis to milk RA content might be the ruminant species, given the potential metabolic differences in ruminal and mammary fatty acid (FA) metabolism (Shingfield et al, 2013; Toral et al, 2016a; Nudda et al, 2020). Between-species responses to feeding strategies aimed at modulating milk FA profile have been described (Toral et al, 2016a; Fougère and Bernard, 2019; Nudda et al, 2020), the existing information is still too scarce to offer a full picture of the situation and, to our knowledge, there is no direct comparison of mammary RA synthesis between ruminant species
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
