Page 100 - Small Animal Clinical Nutrition 5th Edition

100 Small Animal Clinical Nutrition

ters. An important point to remember is lipoprotein structure is
Box 5-12. Hormone Sensitive Lipase.
VetBooks.ir Hormone sensitive lipase (HSL) catalyzes the reaction of triglyc- not static and each class of lipoprotein has characteristics that
may overlap with other classes. The following outline and
Figure 5-20 depict lipoprotein metabolism from the origin of
erides to nonesterified fatty acids (NEFA) and glycerol in
adipocytes. Under the influence of insulin (following a meal), lipoproteins in enterocytes or hepatocytes (Brody, 1994b):
HSL is modified to a very low activity, which favors deposition of • Chylomicrons are formed in enterocytes.
triglycerides in adipose tissue. Under the influence of glucagon • HDL transfers apolipoprotein C-II and E (lipoprotein
(fasting) or epinephrine (flight or fight mechanism), HSL is high- lipase cofactors) to chylomicrons and VLDL.
ly active and the result is an efflux of NEFA and glycerol into the • Lipoprotein lipase in peripheral tissue hydrolyzes triglyc-
blood (bound to albumin) for transport to other tissues as an erides in chylomicrons (Box 5-10).
energy source. The NEFA that arrive back at the liver are catab- • Chylomicron remnants bind to receptors in the liver.
olized or repackaged to triglycerides. In some pathologic condi- • Liver hydrolyzes, reforms triglycerides and combines new
tions, triglycerides may accumulate in the liver (feline hepatic
lipidosis) because the necessary repackaging materials for very apolipoproteins to form VLDL or HDL.
low-density lipoprotein synthesis are not available in the liver. • VLDL triglycerides are hydrolyzed by lipoprotein lipase in
the periphery to form VLDL remnants.
• VLDL remnants are taken up by the liver or converted to
LDL for uptake by peripheral tissues.
• HDL made in the liver (major) and enterocytes (minor)
Table 5-20. Composition of general lipoprotein classes in
mammals. transport excess cholesterol from the periphery to the liver
for excretion in bile salts (Box 5-11).
Lipoprotein Acronym Protein:lipid (%)
Chylomicron CM 1:99
Very low-density lipoprotein VLDL 10:90 Storage
Low-density lipoprotein LDL 25:75 Excess energy intake is stored in adipocytes as triglycerides.
High-density lipoprotein HDL 50:50
The triglycerides in adipocytes are formed directly from
excess dietary fat or from de novo synthesis of fat in the liver
under appropriate metabolic signals. The fat energy store in
human adipocytes is capable of supporting life for one to two
months. The key enzyme for release of energy from
adipocytes is hormone sensitive lipase, which is under the
control of hormonal signals (Box 5-12).

Lipid Function
Energy
Although providing dietary fat is an excellent way to meet an
animal’s energy requirement, this requirement can also be theo-
retically met by the protein and carbohydrate content of a food.
On a per weight basis, the energy value of dietary fat is approx-
imately 2.25 times that of protein or carbohydrate. Additionally,
direct use of dietary fat for storage in adipocytes or use in func-
tional lipid requires less energy for assimilation and storage
when compared with de novo synthesis from protein or carbo-
hydrate in food. In other words, fat stored directly from dietary
fat has 10 to 15% more energy than fat made from excess dietary
carbohydrate or protein because of the inherent loss of efficien-
cy in de novo fat synthesis. This increased efficiency of fat use
results in an increased energy value for dietary fat that animals
may use to meet energy requirements or store as adipose tissue.

Essentiality
Fatty acids of the omega-6 family have functionally distinct
effects compared with those of fatty acids of the omega-3 fam-
Figure 5-19. Digestion, absorption and fate of dietary triacylglyc- ily. The addition of lipid-containing arachidonic acid to foods
erides in mammals. See text for details. Key: TAG = triglyceride, containing no arachidonic acid results in increased feed effi-
MAG = monoacylglyceride, NEFA = nonesterified fatty acid, ciency during growth and enhanced skin condition including
LC-NEFA = Long-chain NEFA, CM = chylomicron, VLDL = very reduced epidermal water loss. Arachidonic acid also allows
low-density lipoprotein. processes requiring eicosanoids to occur such as reproduction