Fatty Acid Composition and Expression of Lipid Metabolism-Related Genes in Longissimus Dorsi Muscle of Grazing Beef Heifers Offered Supplements Containing Either Safflower Oil or Ruminally-Protected Fish Oil

There is increasing interest in enhancing the nutritional value of ruminant-derived foods for human consumption. Beef from grass-fed cattle contains more fatty acids considered to benefit human health including the n-3 fatty acids eicosapentaenoic acid (EPA, 20:5n-3), docosahexaenoic acid (DHA, 22:6n-3) and cis-9, trans-11 conjugated linoleic acid (CLA) than beef from concentrate-fed cattle. The objective of this study was to determine the effect of long-term supplementation of grazing beef heifers with rumen protected fish oil (PFO) or safflower oil (SAFF) on longissimus dorsi muscle concentrations of EPA + DHA and CLA, respectively, and associated effects on the expression of selected genes coding for proteins involved in muscle lipid metabolism. Spring-born Aberdeen Angus * Friesian (AA, n=24) and Belgian Blue * αFriesian (BB n=24) heifers were assigned (n=8/breed), at four months old, to either a standard grass-based production system (CON), or within that system to supplementation with the experimental oils. Heifers were slaughtered from pasture at 21 months of age and muscle tissue collected. The fatty acid profile was measured by gas chromatography while mRNA was extracted and gene expression analysed by RT-qPCR. The proportion of EPA + DHA in muscle was 4.6, 2.5, 19.0, 8.0, 4.2 and 24.1 (sed 1.77) g/kg fatty acids for AACON, AASAFF, AAPFO, BBCON, BBSAFF and BBPFO, respectively. The corresponding proportion of CLA was 8.6, 27.0, 6.1, 8.0, 23.4 and 5.5 (sed 1.28) g/kg fatty acids. Expression of acetyl-CoA carboxylase and stearoyl-CoA desaturase (SCD) genes was higher in BB (P=0.013; P= 0.055, respectively), while fatty acid synthase (FAS) gene expression was higher in AA (P=0.007). FAS and SCD expression were increased by PFO supplementation (P=0.006; P=0.074, respectively) compared to CON. There was an interaction between breed and diet for peroxisome proliferator-activated receptor α (PPARα; P= 0.006) and adiponectin (ADIPOQ; P=0.055) gene expression. Thus, PPARα expression was reduced by PFO in AA and increased in BB; ADIPOQ expression was increased by SAFF in AA and deceased in BB. The expression of delta-6-desaturase, delta-5-desaturase, fatty acid elongase 5, activated protein kinase α, G protein-coupled receptor, adipose differentiation-related protein, sterol regulatory element-binding protein, peroxisome proliferator-activated receptor γ, statin 5 and glucose transporter 4 genes did not differ. In conclusion, provision of PFO to heifers for 17 months enhanced muscle EPA+ DHA content, whereas provision of SAFF effectively increased muscle CLA. In general the effects of oil supplementation on gene expression were small with some evidence of breed specificity.