Journal of dairy science

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

PMID 28527806


The use of stable isotopes is a reliable and risk-free alternative to radioactive tracers for directly examining in vivo fatty acid (FA) metabolism. However, very limited information is available in ruminants, and none is available in sheep. Therefore, we conducted an experiment in dairy ewes to determine, for the first time in this species, the uptake, Δ(9)-desaturation, and secretion of (13)C-labeled stearic acid (SA) into milk with the aim of measuring in vivo endogenous synthesis of milk oleic acid (OA) and stearoyl-CoA desaturase activity. Six lactating Assaf ewes fed a total mixed ration (forage:concentrate ratio = 30:70) received an intravenous injection of 2 g of (13)C-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 (13)C enrichment of 1.9%, whereas OA was more abundant (14.3-15.4% of total FA) and had lower (13)C enrichments (up to 0.69%). On average, 15% of the isotopic tracer was transferred to milk within 72 h p.i., and 47 to 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 is recommended.