Lipid Digestion
Unless supplemental fats or oils are added, dairy diets contain
low concentrations of lipid—2-3% of dietary dry matter is
typical. Because cattle digest lipids very inefficiently, dietary
concentrations should never exceed 5-6%.
Lipid digestion in the rumen begins with cleavage of
glycerides into glycerol and free fatty acids. Glycerol is used by
rumen microbes for the synthesis of propionic acid.
Unsaturated fatty acids are hydrogenated to make them
completely saturated.
TRIGLYCERIDE
Lipids are composed of fatty acids (FA) in
free form (FFA) and as di- and triglycerides.
The FA ‘backbone’ is a long chain of 12-22
carbon atoms. A saturated FA has 2 hydrogen (H) atoms attached to each carbon (the
end carbon always has 3). When a single
pair of H atoms are missing from adjacent
carbons, the FA is monounsaturated
(MUFA); when more than one pair are
missing it is termed polyunsaturated
(PUFA). In the diagram on the right, 16:0 is
a 16-carbon (C16) FA with zero H atoms
missing—it is saturated. 18:2 is a polyunsaturated C18 FA with 2 pairs of H atoms
missing.
16:0
FFA
The saturated fatty acids then move through the digestive
tract to the small intestine where they are mixed with bile salts
and pancreatic juices. Once they have been thoroughly
emulsified, the fatty acids are absorbed into the lymph system
then slowly released into to the bloodstream for delivery to
muscles, adipose (fat) tissue, the liver and the mammary gland
for further use.
18:1
18:2
12:0
OXYGEN
CARBON
HYDROGEN
MISSING HYDROGEN
source: Dairy Production Primer - Feeds & Feeding Management
©2004 Western Dairy Science Inc.