Page 99 - Small Animal Clinical Nutrition 5th Edition

Macronutrients 99

portal circulation for metabolism by the liver.
VetBooks.ir Hepatic Handling of Dietary Lipid

The liver determines the fate of dietary lipid under the direc-
tion of hormonal signals related to energy balance.The primary
fuel source for hepatocytes is provided via β-oxidation of
NEFA whether dietary or endogenous in origin (Figure 5-8).
The fate of dietary lipid from chylomicron origin can be traced
as follows (Figure 5-19) (Brody, 1994b):
• Chylomicron remnants attach to receptors on liver cells.
• Triglycerides are hydrolyzed to glycerol and CoA esterified
Figure 5-17. Triglycerides are the main storage forms of fatty
fatty acids.
acids, each molecule of which is composed of a three-carbon glyc-
• CoA esterified fatty acids may be either shunted to β-oxi-
erol nucleus and three fatty acids (R , R , R ).
2
3
1
dation in mitochondria or repackaged into triglycerides and
then VLDL for use by peripheral tissues (storage or fuel).
Their fate depends on the energy status of the animal.
• Glycerol is converted to 3-P-glycerol (only in liver) and
enters the carbohydrate metabolic pathway.
Although fat storage is easily accomplished by de novo syn-
thesis (production of fat from carbohydrate or protein), it is
more energy efficient for animals to deposit dietary fat than to
synthesize it. When fat is deposited from foods, the fatty acid
profile tends to reflect the type of fat consumed. When fat is
synthesized, stored fat composition reflects the fat synthetic
enzyme activity of the animal.

Lipoprotein Metabolism
Lipoproteins are relatively large conglomerates of protein and
lipid that are necessary to transport hydrophobic lipids effective-
ly through the aqueous medium of physiologic solutions. Table
5-20 gives a generic relative composition of the different
lipoprotein classes observed in mammals (Chapter 28). As the
protein component increases, the relative density of the lipopro-
tein increases reflecting dilution of the buoyant density of fat.
Lipoproteins are made only in the liver (VLDL, HDL) and
enterocytes (chylomicrons). The protein fraction, before it is
integrated with the lipid component, is termed apolipoprotein.
Lipoprotein metabolism is very complex with species varia-
tion. However, a general introduction is necessary to under-
Figure 5-18. Diagram of metabolic pathways (elongation and
stand lipoprotein metabolic disorders discussed in later chap-
desaturation) for essential fatty acids.
Table 5-19. Essentiality and biologic function of common fatty acids.
Structure Common name Essential Biologic function
14:0 Myristic No Energy use and storage, acylation of proteins
16:0 Palmitic No Energy use and storage, acylation of proteins
16:1 Palmitoleic No Energy use and storage
18:0 Stearic No Energy use and storage, membrane fluidity
18:1n-9 Oleic No Energy use and storage, phospholipid structure
18:2n-6 Linoleic Yes Energy use and storage, arachidonic acid precursor (20:4n-6)
18:3n-3 Alpha-linolenic Yes Energy use and storage, eicosapentaenoic acid precursor (20:5n-3)
18:3n-6 Gamma-linolenic Yes Energy use and storage, arachidonic acid precursor
20:4n-6 Arachidonic Yes (cat) Energy use and storage, synthesis of cytokines and eicosanoids, synthesis of
No (dog) steroid hormones, membrane fluidity, competitor of eicosapentaenoic acid
(20:5n-3)
20:5n-3 Eicosapentaenoic Probably Energy use and storage, synthesis of cytokines and eicosanoids, retinal and
nervous tissue development, membrane fluidity, competitor of arachidonic
acid (20:4n-6)
22:6n-3 Docosahexaenoic Probably Energy use and storage, retinal and nervous tissue development, membrane
fluidity, competitor of omega-6 fatty acids