octadecenoic acid (vaccenic acid), which
subsequently is hydrogenated to stearic
acid. However, small amounts of trans
isomers and CLA escape hydrogenation
and are absorbed from the small intestine.
These isomers are incorporated into milk
and meat, which explains their relatively
high content in ruminant products.
Eight positional isomers of CLA are
possible, but the predominant product in
the rumen is the cis-9, trans-11 isomer.
Research has demonstrated that this
compound has widespread effects in a
number of biological systems, including
inhibition of carcinogenesis, decreased
body fat accumulation, modulation of the
immune system and prevention of athero-
sclerotic lesions (Belury, 1995). Factors
that increase the accumulation of CLA in
the rumen and its subsequent absorption
into milk and meat currently are the
subject of intense research effort.
Rumen microorganisms also synthe-
size fatty acids, most of which are incor-
porated into cell membrane phospholipids.
Bacteria synthesize odd-chain fatty acids
containing 15–17 carbons, as well as
branched-chain fatty acids, which also are
relatively unique to ruminant fats (Doreau
and Chilliard, 1997). As a consequence of
the unique actions of the rumen microbes,
85% of the lipids entering the small
intestine of ruminants are free fatty acids,
which are predominantly saturated and
adsorbed to the surface of small feed
particles. At the prevailing pH in the
rumen, most of the fatty acids are present
as salts of sodium, potassium or calcium.
The remaining 15% of lipids reaching the
duodenum consist mostly of bacterial
phospholipids.
Pancreatic juice and bile enter the
duodenum through the common bile duct
and are essential for lipid digestion and
absorption in the small intestine.
Ruminants secrete more taurine-conjugated
bile salts than glycine-conjugated bile salts
because the former are more soluble at the
low pH found in the ruminant small
intestine (Moore and Christie, 1984). Bile
salts are essential to dissociate fatty acids
adsorbed to feed particles and enable
micelle formation. Phospholipase A 2
secreted in pancreatic juice becomes active
in the upper jejunum where the pH is more
favourable and hydrolyses fatty acids from
the sn-2 position of phospholipids. The
major phospholipid in the intestine is
phosphatidylcholine (lecithin), which
enters in bile, pancreatic juice and digesta
from the abomasum. The resultant product
of phospholipase-catalysed hydrolysis is
lysophosphatidylcholine (lysolecithin),
which is an excellent detergent for forma-
tion of mixed micelles from the highly
saturated fatty acids in the ruminant small
intestine.
Bile salts and lysolecithin promote
micelle formation from free fatty acids. The
mixed micelle associates with the brush
border of the intestinal epithelium and
facilitates transfer of the hydrophobic fatty
acids across the unstirred water layer at the
surface of the brush border membranes.
Fatty acids and lysolecithin then diffuse
across intestinal cell membranes into the
cells. The bile salts are not absorbed in the
jejunum, but continue to form micelles.
Most bile salts are absorbed in the ileum
and are returned to the liver to be reincor-
porated into bile.
Within the small intestinal cells, fatty
acids are re-esterified to glycerol-3-
phosphate to form triacylglycerols. The
glycerol-3-phosphate is formed from blood
glucose via glycolysis. Pre-ruminants con-
suming milk triacylglycerol function like
non-ruminants and absorb large amounts of
2-monoacylglycerol that can be re-esterified
to form triacylglycerol. Along with apo-
lipoproteins (B48, AI and AIV), cholesterol
and phospholipids, the triacylglycerols are
packaged into lipoprotein particles that are
secreted from the cells and enter the
lacteals to be carried into the lymph and
reach the peripheral circulation. These
particles are analogous to chylomicrons in
non-ruminants but are classified more
correctly as VLDLs because of their small
size in functioning ruminants. This is a
factor of the highly saturated nature of the
triacylglycerols, the low dietary fat content
of ruminant diets and the constant flow of
digesta into the intestine as compared withLipid Metabolism 101