To provide further insights into ruminant lipid digestion and
metabolism, and into cis9, trans-11 18:2 synthesis, 12 growing Engadine
lambs grazing either mountain pasture (2,250 m above sea level; n = 6)
or lowland pasture (400 m above sea level; n = 6) were studied. Both
pastures consisted exclusively of C-3 plants. Before the experiment, all
animals grazed a common pasture for 6 wk. Grasses and perirenal adipose
tissues of the sheep were analyzed for fatty acids by gas
chromatography. Stable C-isotope ratios (delta C-13 values in % vs. the
Vienna Pee Dee Belemnite standard) were determined in the composite
samples by elemental analysis-isotope ratio mass spectrometry. The delta
C-13 of the individual fatty acids were measured by gas
chromatography-combustion-isotope ratio mass spectrometry. The delta
C-13 value of the entire mountain pasture grass was -27.5% (SD 0.31),
whereas that of the lowland pasture grass was -30.0% (SD 0.07). This
difference was reflected in the perirenal adipose tissues of the
corresponding sheep (P < 0.05), even though the delta C-13 values were
less in the animals than in the grass. The delta C-13 values for cis-9
16:1 and cis-9 18:1 in perirenal fat differed between mountain and
lowland lambs (P < 0.05). The 16:0 in the adipose tissue was enriched in
C-13 by 5% compared with the dietary 16:0, likely as a result of partly
endogenous synthesis. The d13C values of cis-9, trans-11 18:2 (cis-9,
trans-11 CLA) in the adipose tissue were smaller than those of its
dietary precursors, cis-9, cis-12 18:2 and cis-9, cis-12, cis-15 18:3;
conversely, the delta C-13 values of trans-11 18:1 were not, suggesting
that large proportions of perirenal cis-9, trans-11 18:2 were of
endogenous origin and discrimination against C-13 occurred during
Delta(9)-desaturation. The same discrimination was indicated by the
isotopic shift between 16:0 and cis-9 16:1 in the mountain grazing
group. Furthermore, the delta C-13 values of cis-9, trans-11 18:2 were
smaller relative to the precursor fatty acids in the mountain lambs
compared with the lowland group. This result suggests a reduced extent
of biohydrogenation in lambs grazing on mountain grass in comparison
with those grazing on lowland grass. This was supported by the smaller
cis-9, trans-11 18:2 concentrations in total fatty acids found in the
adipose tissues of the lowland lambs (P < 0.001). The results of this
study demonstrate that natural differences between delta C-13 values of
swards from different pastures and the adipose tissue fatty acids could
be used as tracers in studies of lipid metabolism in ruminants.