This study explores the potential use of stable carbon isotope ratios
(delta C-13) 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 C-3 and C-4 plants. The FA in milk and feed were
fractionated by silver-ion thin-layer chromatography and analyzed for
their delta C-13 values. Mean delta C-13 values of FA from C-3 milk were
lower compared to those from C-4 milk (-30.1aEuro degrees vs. -24.9aEuro
degrees, respectively). In both groups the most negative delta C-13
values of all FA analyzed were measured for c9,t11-18:2 (C-3 milk =
-37.0 +/- A 2.7aEuro degrees; C-4 milk -31.4 +/- A 1.4aEuro degrees).
Compared to the dietary precursors 18:2n-6 and 18:3n-3, no significant
C-13-depletion was measured in t11-18:1. This suggests that the delta
C-13-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 delta C-13
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 ruminants.