Digestion and absorption of lipids
in ruminantsRuminant and non-ruminant animals differ
with respect to strategies for lipid diges-
tion, primarily because of the nature of the
dietary lipids and the microbial processes
within the rumen (Moore and Christie,
1984). In forage-fed ruminants, dietary
lipids consist primarily of galactolipids
and other glycolipids that are rich in
linolenic acid (18:3). Cereal grains and
other concentrate ingredients contribute
triacylglycerols that are high in linoleic
acid (18:2). Oilseeds and animal fats
contribute triacylglycerols. A variety of
commercial fat products are available,
including calcium soaps of long-chain fatty
acids, saturated or protected triacyl-
glycerols, or mostly saturated free fatty
acids. Phospholipids are smaller com-
ponents of both grains and forages.
Bacteria and protozoa in the rumen
hydrolyse complex lipids (glycerides) into
their constituent long-chain fatty acids,
sugars, organic bases (choline, ethanol-
amine, serine) and glycerol. Thus, the
rumen is the primary site of complex lipid
hydrolysis, rather than the small intestine
as in non-ruminants and pre-ruminants.
The glycerol and sugars are fermented
rapidly to volatile fatty acids (mainly
acetic, propionic and butyric).
Unsaturated fatty acids are hydro-
genated extensively to saturated fatty acids
(stearic, 18:0 and palmitic, 16:0) by
ruminal bacteria and protozoa. The process
of hydrogenation, which only occurs on
free fatty acids, requires a mixed popula-
tion of microbial species (Doreau and
Chilliard, 1997). The biohydrogenation of
linoleic acid occurs through the sequence
of reactions shown in Fig. 5.1. The first
isomerization reaction converts the cis-9,
cis-12 linoleic acid to the cis-9, trans-11
form, known as conjugated linoleic acid
(CLA). Most of the CLA that is produced in
the rumen is hydrogenated to trans-11100 J.K. Drackley
Fig. 5.1.General pathway for ruminal microbial hydrogenation of linoleic acid (cis-9, cis-12 18:2) through
conjugated linoleic acid (CLA; cis-9, trans-11 18:2) and trans-vaccenic acid (TVA; trans-11 18:1) to stearic
acid (18:0).