Page 57 - Small Animal Clinical Nutrition 5th Edition

Macronutrients 57

tive equation reasonably provides precision for foods containing
VetBooks.ir traditional ingredients, it may be inadequate for currently avail-
able commercial foods due in part to the use of new food addi-
tives/agents and diverse ingredients (Yamka et al, 2007). These
methods for calculating ME can be applied to cat and dog
foods and assume an average apparent digestibility of 80% for
protein, 90% for crude fat and 84% for carbohydrate. The
digestion coefficients are then multiplied by energy values of
4.4, 9.4 and 4.15 kcal/g for protein, fat and carbohydrate,
respectively. The resulting values of 3.5, 8.5 and 3.5 kcal/g are
reasonable estimates of the ME derived from protein, fat and
carbohydrate in typical commercial pet foods, which range in
digestibility from 75 to 85%.The method assumes no energy is
derived from crude fiber, water or ash. This method overesti-
mates the ME content of foods high in fiber or ash or foods
with very low protein, fat and carbohydrate digestibility. This
overestimation inadequately predicts ME for growth, lactation
or trauma recovery in dogs. Likewise, the equation underesti-
mates the ME content of highly digestible and low-fiber foods.
Underestimation of the ME content could result in obesity and
obesity related disorders (e.g, diabetes mellitus and arthritis)
simply because the pet owner is following recommended feed-
ing instructions for what he or she believes to be a lower calo-
rie food (Yamka et al, 2007). Because of the inconsistency of
the Atwater equation to predict ME, many researchers have
attempted to identify better and more accurate methods for
predicting the energy content of pet foods.
Recent research has focused on the fiber portion of the food
to find a more accurate method for determining ME (Earle et
al, 1998; Kienzle, 2002). This method increases ME predic-
tion; however, the crude fiber content of most commercial dog
foods accounts for approximately 3 to 5% of the food.
Therefore, it becomes practical to focus on other components
of the food, such as crude protein (20 to 30% of the dietary Figure 5-8. This diagram depicts metabolism of long-chain fatty
DM) or carbohydrate (20 to 50% of the dietary DM), which acids for energy production. Entry of long-chain fatty acids into
mitochondria is facilitated by carnitine-acyl transferase enzymes.
would contribute more to total energy content and, thus, may
Medium-chain and short-chain fatty acids bypass this important
have a greater impact on the prediction of ME (Yamka et al,
regulatory step and enter the mitochondria unaided. β-oxidation is
2007). The crude protein fraction represents numerous com-
an energy-yielding catabolic process involving fatty acids that
pounds that can broadly be classified as amino acid (TAA)
occurs within the mitochondrial matrix. The resultant products are
and non-amino acid (NAA: e.g., nucleic acids, amines, acetyl-CoA, which enters the TCA cycle for further metabolism (to
amides, etc.) compounds (Yamka et al, 2007). Digestibility CO , water and ATP) and reduced coenzymes (NADH/FADH ) for
2
2
(Giesecke et al, 1982; Yamka et al, 2005) and relative contri- ATP production by oxidative phosphorylation.
bution to urinary nitrogen energy losses (Blaxter, 1989) differ
among these compounds. Nucleic acids have low digestibility
(approximately 57%) in dogs (Giesecke et al, 1982). from the exchange of oxygen and carbon dioxide (indirect or
Numerous studies have demonstrated that amino acids have respiratory calorimetry) (Kleiber, 1972; Blaxter, 1989) (Box 5-
high digestibility. (See Protein section.) With this knowledge, 4). Partitioning NE into NE m and NE involves estimating
p
it becomes important to identify protein quality to further the energy retained by the animal and will be discussed below.
characterize crude protein. The NAA portion of crude pro-
tein has a negative impact on ME whereas the TAA influ- Energy Requirements
ences ME directly (Yamka et al, 2007). As a result of defining Knowledge of energy requirements is needed to determine how
protein composition (TAA:NAA), predicting ME has great- much food to feed to an animal. Determining energy require-
ly improved for dry dog foods. ments involves measuring energy expenditure of an animal
Estimation of the NE of a food requires measurement of the under a defined set of physiologic and environmental condi-
heat lost. Heat production can be measured directly using an tions. Energy expenditure studies typically involve carefully
animal calorimeter (direct calorimetry) or estimated indirectly accounting for all components of the energy budget of an ani-