Page 96 - Small Animal Clinical Nutrition 5th Edition

96 Small Animal Clinical Nutrition

the main protein source of the food may be limiting.
VetBooks.ir Methionine and lysine are typically used to fortify pet foods.

LIPIDS

Definition
Lipids are high energy compounds that supply nutritional and
functional requirements in mammals. Generally, lipids share
the physiochemical property of being insoluble (hydrophobic)
in polar solvents such as water. Lipids that are solid at room
temperature are commonly called fats whereas those that are
liquid at room temperature are referred to as oils. In a nutrient
analysis, the ether extract of a food contains primarily lipids and
represents the crude fat content (Figure 5-3).

Structure
The structure of lipids ranges from simple to complex although
any one classification scheme is difficult to impose. The basic
subunits of lipids are hydrocarbon molecules linked by covalent
bonds in various manners to themselves and other molecules in
a vast assortment of permutations that result in the myriad of
functions and structures observed in nature. Table 5-18 and
Figures 5-16 and 5-17 will be helpful for reference throughout
this chapter and the text.

Function
Dietary intake of lipids benefit the animal by supplying energy,
essential fatty acids (EFA) and a positive environment for fat-
soluble vitamin absorption. Dietary fats are the most concen-
trated forms of energy sources in pet foods, supplying 2.25
times the metabolizable energy of proteins and carbohydrates.
Excess dietary lipids may be assimilated and stored as fat in
adipocytes, whereas additional lipids are incorporated into
functional lipid or catabolized for fuel, depending on the ener-
gy status of the animal. Triglycerides compose the majority of
fat found in adipocytes, which may be synthesized de novo
from nonfat precursors such as carbohydrate or protein during
periods of positive energy balance.
Some lipids required for adequate physiologic function, such
as certain long-chain fatty acids, cannot be synthesized de novo
and are thus required in food. These fatty acids are called EFA
Figure 5-16. Fatty acids consist of hydrocarbon chains with a car-
because a lack of them in foods results in classic signs of defi-
boxylic acid group (COOH) on one terminus and a methyl group on
ciency. A small amount of lipid (1 to 2% of total food) of no
the opposite terminus. Fatty acids with no double bonds in the
specific structure is also required in foods for proper absorption
hydrocarbon chain are referred to as saturated, one double bond
as monounsaturated and more than one double bond, polyunsatu- of fat-soluble vitamins (A, D, E and K).
rated. The carbon on the opposite terminus from the carboxylic
acid group is designated the number one carbon and given the Fatty Acids
symbol n-1. Nomenclature specifies the number of carbons and the Structure
location and number of double bonds. For example, an 18-carbon Nonesterified fatty acids (NEFA) consist of hydrocarbon chains
polyunsaturated fat with three double bonds, the first of which is
ranging from two to 24 carbons or more, with a carboxylic acid
between carbons 6 to 7 is designated 18:3n-6 (γ-linolenic).
group on one terminus and a methyl group on the opposite ter-
minus. NEFA that contain 14 to 24 carbons are classified as
long chain,eight to 12 medium chain and two to six short chain.
Figure 5-17 shows fatty acid chemistry and structure.
Lipid may be either in liquid or solid state depending on