Endogenous synthesis of milk cis-9,trans-11 conjugated linoleic acid in dairy ewes: Quantification using 13C-labeled vaccenic acid and comparison with estimates based on cobalt administration.

Isotopic tracers are used to directly quantify the effect of mammary Δ9-desaturation on milk fatty acid (FA) composition, but very few studies have applied this method to measuring the endogenous synthesis of rumenic acid (RA; cis-9,trans-11 conjugated linoleic acid) in cows and goats, and no publications exist in ewes. In sheep, knowledge about the contribution of stearoyl-coenzyme A desaturase (SCD) to milk FA secretion is derived mostly from indirect estimates based on inhibition of the enzyme by oral administration of cobalt, a cost-effective method that has not been validated to date. To fill both gaps, we conducted an isotopic tracer assay in sheep to quantify the proportion of endogenous RA in milk for the first time in this species. We then compared the results with estimates derived from a Co administration assay performed on the same animals. First, 5 lactating ewes received an intravenous injection of 200 mg of [1-13C]trans-11 18:1 (vaccenic acid, VA), the precursor for RA production by SCD activity. At -24, -15, 0, 9, 24, 33, 48, 57, 72, 81, and 96 h post-injection, we recorded milk yield and collected milk samples to examine fat concentration and FA profile. We conducted compound-specific isotope analysis of VA and RA by gas chromatography-combustion isotope ratio mass spectrometry. Afterward, in the Co administration assay, ewes received a daily dose of 7 mg of Co/kg of body weight for 5 d. We analyzed milk samples for composition before and on the last days of cobalt dosing. On average, 17% of the injected amount of [1-13C]VA was transferred to milk within 96 h post-injection, and up to 29% of the VA taken up by the mammary gland was desaturated to milk RA. Under our conditions, the mean proportion of this conjugated linoleic acid isomer deriving from Δ9-desaturation represented 82 to 90% of the amount secreted in milk. However, the proportion estimated in the Co assay with calculations based on individual FA concentrations was lower (on average, 46%). When we calculated the same estimates based on changes in Δ9-desaturation ratios after Co dosing, the higher values of endogenous RA (75%) did not differ from the results of the isotopic tracer assay. Nevertheless, correlation analysis between the results obtained through [1-13C]VA or Co administration revealed no significant relationship, which would prevent acceptance of the latter as a reliable alternative to isotopic labeling to examine mammary Δ9-desaturation in dairy ewes.