Page 55 - Small Animal Clinical Nutrition 5th Edition

Macronutrients 55

VetBooks.ir

Figure 5-6. Schematic of how total gross energy of a food is partitioned into digestible energy, metabolizable energy and net energy. Net
energy can be further partitioned into energy used for maintenance (NE ) and production (NE ).
p
m
Acetyl-CoA is then oxidized in the TCA cycle and ATP is needs energy, it uses glycogen first, fat stores second and final-
generated via oxidative phosphorylation in the electron trans- ly, as a last resort, amino acids from body protein (Chapter 25).
port chain. The number of ATP generated from fatty acid oxi- In fed animals, food energy is primarily used for meeting body
dation depends on the length of the carbon chain and degree of energy needs, thus preserving body tissues by preventing catab-
unsaturation. For example, myristic acid (C14:0) yields 112 olism. Further discussions of use and control of body energy
ATP, palmitic acid (C16:0) 129 ATP, palmitoleic acid (C16:1) stores during exercise can be found in Chapter 18.
127 ATP, stearic acid (C18:0) 146 ATP and oleic acid (C18:1) Differences in the amount of energy consumed and that
144 ATP. expended by the body can clearly result in changes in body
Amino acids obtained from the diet and generated by weight, growth rate and body composition (especially body fat).
endogenous protein breakdown are (re)used for protein syn- Excess energy intake and storage relative to energy expenditure
thesis or oxidized as an energy source. Protein synthesis is a common problem in pet dogs and cats (e.g., obesity). Obese
requires a substantial expenditure of energy. When amino pets are at increased risk for developing a variety of health prob-
acids are oxidized, there are additional energy costs associated lems (Chapter 27). Excess energy intake in growing large- and
with gluconeogenesis and ureagenesis (Flatt, 2001). As a giant-breed puppies may be related to developmental skeletal
result the efficiency to derive energy from protein is consider- abnormalities (Chapter 33). Conversely, inadequate energy
ably less than that from fat or carbohydrate. The amino acids intake relative to expenditure may occur in animals with cancer
used for energy are deaminated or transaminated to yield a and heart disease and resulting in cachexia (Chapters 30 and
carbohydrate moiety and ammonia; the ammonia is then con- 36) and in vigorously exercising dogs such as sled dogs
verted to urea. The carbon skeleton enters the TCA cycle and (Chapter 18).
ATP is formed in the electron transport chain. The number
of ATP generated from oxidation of amino acids to water, Energy Analyses
carbon dioxide and urea varies, but ranges from six ATP from The most accurate determination of the DE or ME content of
glycine to 42 ATP from tryptophan. food (Figure 5-6) is obtained through feeding studies (Yamka
et al, 2007; McDonald et al, 1995; AAFCO, 2007). ME deter-
Energy Storage minations involve the direct measurement of energy intake and
ATP is the usable form of energy for body cells, but not a good energy lost in feces and urine obtained from digestion or
energy storage molecule because it is used quickly after forma- metabolism studies.Energy lost as expired gases (e.g.,methane)
tion. Glycogen and triglycerides are better storage forms of is negligible (McKay and Eastwood, 1984). As a result, energy
energy. Production and use of ATP are equally balanced lost as gases is typically ignored when calculating ME for dogs
through a series of control mechanisms that monitor the and cats. The Association of American Feed Control Officials
amount of ATP available. After a meal containing adequate (AAFCO) has published accepted protocols for the determina-
energy, the total body metabolism is generally anabolic; the tion of ME of dogs and cat foods (2007) (Box 5-3). Because of
body uses energy for synthetic reactions and tissue accretion the laborious nature of determining the actual ME content of
(e.g., growth, reproduction). After the body has enough energy dog foods, calculated ME values are used extensively for diet
to meet demands, the pathways of glycolysis, β-oxidation, formulation and food labeling (Yamka et al, 2007).
transamination, deamination and the TCA cycle are slowed. Currently, AAFCO (2007) and the National Research
The pathways of glycogen and fat synthesis are simultaneously Council (2003) recommend a predictive equation largely based
accelerated and excess dietary energy is stored as glycogen and on fixed energy values and digestibility coefficients for dietary
body fat.These energy stores can then be used to generate ATP components (i.e., crude protein, crude fat and carbohydrate) for
later when needed.Generally,in fasting animals,when the body estimating the ME content of dog foods. Although this predic-