Stable Carbon Isotope Composition of c 9, t 11-Conjugated Linoleic Acid in Cow's Milk as Related to Dietary Fatty Acids

This study explores the potential use of stable carbon isotope ratios (δ13C) of single fatty acids (FA) as tracers for the transformation of FA from diet to milk, with focus on the metabolic origin of c9,t11-18:2. For this purpose, dairy cows were fed diets based exclusively on C3 and C4 plants. The FA in milk and feed were fractionated by silver-ion thin-layer chromatography and analyzed for their δ13C values. Mean δ13C values of FA from C3 milk were lower compared to those from C4 milk (−30.1‰ vs. −24.9‰, respectively). In both groups the most negative δ13C values of all FA analyzed were measured for c9,t11-18:2 (C3 milk=−37.0±2.7‰; C4 milk −31.4±1.4‰). Compared to the dietary precursors 18:2n-6 and 18:3n-3, no significant 13C-depletion was measured in t11-18:1. This suggests that the δ13C-change in c9,t11-18:2 did not originate from the microbial biohydrogenation in the rumen, but most probably from endogenous desaturation of t11-18:1. It appears that the natural δ13C differences in some dietary FA are at least partly preserved in milk FA. Therefore, carbon isotope analyses of individual FA could be useful for studying metabolic transformation processes in ruminant