Page 54 - Small Animal Clinical Nutrition 5th Edition

54 Small Animal Clinical Nutrition

and flatulence). However, because methane production is
VetBooks.ir considered to be negligible in dogs and cats (McKay and
Eastwood, 1984), ME can be defined in terms of DE and uri-
nary energy losses.
Animals continuously produce heat as a result of basal
metabolism and physical work. Heat production increases after
a meal. This increase in heat due to food ingestion is called the
heat increment of food (HI). HI consists of the heat of intes-
tinal microbial fermentation and heat produced in intermediary
metabolism as a result of using nutrients. A study in people
showed that 5 to 10% of the energy consumed was lost as heat.
The HI during the postprandial period was 60% greater for the
protein-consuming group when compared to the isocaloric car-
bohydrate-consuming group. This increase in HI was attrib-
uted to the increased protein turnover observed by the protein-
consuming group. The metabolic cost for protein turnover was
approximately 36 and 68% for carbohydrate and protein feed-
ing, respectively (Robinson et al, 2000). The energy of HI is
normally wasted except when the environmental temperature is
below an animal’s critical temperature (i.e., shivering). In this
situation, the HI is used to keep the body warm.
Subtracting HI from the ME gives the net energy (NE) of
food. NE can also be partitioned into the amount used for
maintenance (NE ) and the amount used for production
m
(NE : growth, pregnancy, lactation, exercise). NE values of
p
foods and ingredients are typically used when discussing live-
stock nutrition (beef cattle, dairy cattle, swine), whereas DE
and ME are more typically used in canine and feline nutrition.
Although not commonly measured and used, the NE princi-
Figure 5-5. Schematic of how animals obtain and use energy.
ples of partitioning energy for maintenance and production
Plants use solar energy to produce energy-containing nutrients
separately hold true for dogs and cats.
(i.e., proteins, fats and carbohydrates) via photosynthesis. Animals
eat the energy-containing plant nutrients and other animals. Once
eaten, energy-containing nutrients are digested, absorbed and Energy Use
metabolized by body cells to release energy that fuels the process- The initial biochemical reactions by which energy is derived
es that sustain life. from carbohydrates, fats and amino acids are different.
However, all three nutrients eventually go through a final
common pathway for energy generation (i.e., TCA cycle).
to as being “bulk limited.” Low-energy, bulk-limited foods Glucose derived from dietary carbohydrates is first oxidized
designed for weight loss can be formulated to provide adequate through the glycolysis pathway to yield pyruvate and then
intake of non-energy nutrients (Chapter 27). acetyl-CoA. Acetyl-CoA is oxidized in the TCA cycle pro-
ducing carbon dioxide, and electrons, which are captured by
Energy Metabolism important heme-containing compounds called cytochromes
Digestion, Absorption and Excretion (Figure 5-7). Electrons produced in the TCA cycle are shut-
Digestion and absorption of the energy-supplying nutrients tled by nicotinamide-adenine dinucleotide (NAD) and flavin
(protein, carbohydrate and fat) are discussed in other sections of adenine dinucleotide (FAD) to the electron transport chain
this chapter. The total amount of potential energy in food is where the cytochromes participate in electron transfer through
termed gross energy (GE). Burning the food and measuring valence changes in their heme iron (Figure 5-4). NAD and
the heat produced in a bomb calorimeter determine GE in FAD are synthesized from the vitamins niacin and riboflavin,
food. Animals are unable to use 100% of the GE in foods respectively. The electrons are passed between successive oxi-
because some of the food energy is lost in the form of solid, liq- dation/reduction reactions to the end of the chain where oxy-
uid and gaseous excretions as well as radiant heat. gen accepts the final electrons and is converted to water. ATP
Nutritionists have partitioned dietary energy based on the is formed as the electrons are passed down the chain (oxidative
losses that occur (Figure 5-6). Digestible energy (DE) refers phosphorylation). A net of 36 ATP is generated for each mol-
to the GE content of food minus energy lost in feces (FE). ecule of glucose that is oxidized to carbon dioxide and water.
Typically, ME is defined as DE minus energy lost in urine Fatty acids and glycerol from dietary fats are initially oxidized
and as intestinal gaseous products of digestion (i.e., eructation to acetyl-CoA by the β-oxidation pathway (Figure 5-8).