Page 321 - Small Animal Clinical Nutrition 5th Edition

328 Small Animal Clinical Nutrition

periods of time. Fatty acids are stored in ample supply in adi-
Table 18-4. Percent fast twitch fibers (Type II) by selected pose tissue and within muscle. They are the primary energy
VetBooks.ir canine breeds.* source for long-lasting exercise. Although small amounts of
Muscle
Biceps femoris Greyhound Crossbred Foxhound fatty acids are stored in muscle, this source may contribute up
63.0
67.2
88.6
Semitendinosus 98.9 85.3 69.6 to 60% of the fatty acids oxidized during the first two to three
Triceps (long head) 94.2 77.2 64.9 hours of exercise (Weber et al, 1993).
Vastus lateralis 96.6 61.4 80.7 Amino acids are usually not a primary energy source for exer-
*Adapted from Guy PS, Snow DH. Skeletal muscle fibre compo-
sition in the dog and its relationship to athletic ability. Research cise. Oxidation of amino acids may contribute up to 5 to 15%
in Veterinary Science 1981; 31: 244-248. of the energy used during exercise, depending on the intensity
and duration of the task (Young et al, 1962; Zackin, 1990;
Hickson and Wolinsky, 1989). Most of this energy comes from
the oxidation of the branched-chain amino acids leucine,
Table 18-5. Metabolism, use and storage sites of isoleucine and valine (Miller and Massaro, 1989; Blomstrand et
metabolic fuels.
al, 1988). Most amino acids are structural or functional compo-
nents of proteins and the size of the labile amino acid pool is
Fuel Metabolism Use Storage sites
ATP Anaerobic Primary fuel for Muscle cells very small, making amino acids a less significant fuel source for
synthetic (concentration is exercise in most circumstances.
processes, ion low and highly
pumps and regulated) The proportion of energy substrates and metabolic pathways
muscle used during exercise depends on the intensity and duration of
contraction the exercise. As exercise intensity increases, the power output
Cr-P Anaerobic Regenerate ATP Muscle cells (low
concentration) and the rate of energy metabolism must also increase. As exer-
Glucose Anaerobic Rapidly available Muscle and liver cise duration increases, total substrate availability and energy
(glycolysis) energy source glycogen yield become more important (Box 18-3). High-power activi-
and aerobic (1 to 2% of
(TCA cycle) body weight) ties (e.g., sprinting) rely heavily on anaerobic metabolism,
Fatty acids Aerobic Long-lasting Adipose tissue whereas more prolonged activities require the higher energy
(β-oxidation energy source (2 to 20% of yield provided by oxidation of glucose and fatty acids. As the
and TCA body weight)
cycle) duration of exercise increases, oxidation of fatty acids becomes
Amino Aerobic Small Structural more important (Figure 18-3).
acids (TCA cycle) contribution proteins
to energy (may
contribute up By-Products of Muscular Work
to 5 to 15%) Heat is the primary by-product of muscle contraction; 75 to
Key: ATP = adenosine triphosphate, Cr-P = creatine phosphate, 80% of the energy used during muscular work is converted to
TCA = tricarboxylic acid.
heat. A 10-fold increase in metabolism results in a 10-fold
increase in heat production. Unless the animal is working in a
very cold environment, this heat is a by-product that must be
removed (even some sled dogs overheat). In dogs, the respira-
tory tract is responsible for dissipating most of this heat.
Normal body temperatures of dogs doing physical work are
higher than their normal resting temperatures. During very
intense exercise or exercise in hot environmental temperatures,
heat production exceeds the ability of the respiratory tract to
lose heat, increasing body temperature. The body temperature
of racing greyhounds may increase more than 1°C (1.8°F) after
a 30-second race (Rose and Bloomber, 1989). Pointing breeds
can have normal working temperatures up to 41.1°C (106°F)
(Gillette, 2002). Labrador retrievers can have normal working
temperatures up to 41.6°C (107°F) (Matwichuk et al, 1999).
Figure 18-3. Relative contributions of the four energy-generating Because evaporative heat loss is the primary way dogs dissipate
pathways, depending on exercise intensity and duration. Key: Cr-P heat, ensuring adequate hydration is crucial for maintenance of
= creatine phosphate. normal body temperature.
Metabolic acid is another by-product of energy metabolism
dation of glucose to CO and water, regenerates ATP less rap- that must be eliminated during and after exercise. Aerobic
2
idly (Box 18-3), but results in a much greater yield (36 ATP per metabolism generates ATP by combustion of carbohydrates
molecule of glucose). Because total body glucose stores (glyco- and fats to CO and water. Lactate is the endpoint of anaero-
2
gen) are relatively small (1 to 2% of body weight), even aerobic bic metabolism. Either way, acid is produced that must be elim-
metabolism of glucose cannot sustain exercise for extended inated in some way for exercise to continue. Muscle enzyme