Endogenous synthesis of milk oleic acid in dairy ewes: In vivo measurement using 13C-labeled stearic acid

Abstract

The use of stable isotopes is a reliable and risk-free alternative to radioactive tracers to directly examine in vivo fatty acid (FA) metabolism. However, very limited information is available in ruminants, and there is none in sheep. Therefore, we conducted an experiment in dairy ewes to determine, for the first time in this species, the uptake, Δ9-desaturation and secretion of 13C-labeled stearic acid (SA) into milk, with the aim of measuring in vivo endogenous synthesis of milk oleic acid (OA) and stearoyl-CoA desaturase (SCD) activity. Six lactating Assaf ewes fed a total mixed ration (forage:concentrate ratio 30:70) received an intravenous injection of 2 g of 13C-labeled SA. At −24, −15, 0, 4, 8, 12, 16, 20, 24, 36, 48, 60 and 72 h postinjection (p.i.), milk yield was recorded and milk samples were collected to examine fat concentration and FA composition, including compound-specific isotope analysis of SA and OA by gas chromatography-combustion isotope ratio mass spectrometry. Over the p.i. period, the SA proportion ranged from 7.6 to 8.3% of total FA, with a maximum 13C enrichment of 1.9%, whereas OA was more abundant (14.3-15.4% of total FA) and had lower 13C enrichments (up to 0.69%). On average, 15% of the isotopic tracer was transferred to milk

within 72 h p.i., and 47-50% of the SA taken up by the mammary gland would have been desaturated to OA. The proportion of oleic acid being synthesized endogenously was estimated to represent between 48% and 57% of the amount secreted in milk. Further research under different dietary conditions would be now recommended.