Page 322 - Small Animal Clinical Nutrition 5th Edition

Feeding Working and Sporting Dogs 329

VetBooks.ir Box 18-3. Metabolic Power and Yield.

Metabolic power is the speed with which energy substrates can be converted to ATP, whereas metabolic yield is the amount of ATP that
can be made from energy substrates. High-intensity exercise (e.g., sprinting) requires rapid mobilization of stored energy for a very short
time; therefore, metabolic power is very important. Because duration of exercise is very short for sprinters, metabolic yield is less impor-
tant. Conversely, endurance activities are longer in duration and lower in intensity. For these activities, the rapidity with which ATP is
made from substrates (power) is less important than the amount of ATP made (yield). Tables 1 and 2 show maximum power and yield
from various substrates using aerobic and anaerobic pathways.
Clinically, canine sprint athletes rely heavily on anaerobic metabolism of carbohydrates whereas canine endurance athletes rely more
on oxidation of fats.

CLINICAL EXAMPLE
Compare a 30-kg racing greyhound with a 30-kg sled dog. Assume the racing greyhound runs an 800-m race in about 48 seconds.
The total energy needed for the race is about 24 kcal, whereas the energy use rate or metabolic power is about 30 kcal per minute (an
increase of more than 25 times resting). Total daily energy requirement (DER) is about 1,600 kcal. In contrast, consider a sled dog that
runs 80 km pulling a sled (its share is about 15 kg) for five hours. The sled dog needs about 3,600 additional kcal for the event and
uses them at a rate of 12 kcal per minute. Total DER is about 5,000 kcal (more than 5 x resting energy requirement). To convert to kJ,
multiply kcal x 4.184.
Table 1. Estimated maximum metabolic power output for human skeletal muscle using different substrates and metabolic profiles.*

Process Metabolic power output
(µmole of ATP/g of muscle/min.)
Aerobic metabolism
Fatty acid oxidation 20.4
Glycogen oxidation 30
Anaerobic metabolism
Glycogen glycolysis 60
Creatine phosphate 96-360
and ATP hydrolysis
*Adapted from Hochachka PW. Design of energy metabolism. In: Prosser CL, ed. Environmental and Metabolic Animal Physiology, 4th
ed. New York, NY: Wiley-Liss, 1991; 332.
Table 2. Energy yield using different substrates and metabolic pathways.*

Process Energy yield (moles of ATP/moles of substrate)
Aerobic metabolism
Triglyceride oxidation (glycerol + 3 palmitate) 403
Fatty acid oxidation (palmitate) 129
Glycogen oxidation 38
Glucose oxidation 36
Proline oxidation 21
Lactate oxidation 18
Anaerobic metabolism
Glycolysis (glycogen) 3
Glycolysis (glucose) 2
Creatine phosphate hydrolysis 1
*Adapted from Hochachka PW. Design of energy metabolism. In: Prosser CL, ed. Environmental and Metabolic Animal Physiology, 4th
ed. New York, NY: Wiley-Liss, 1991; 327-329.

The Bibliography for Box 18-3 can be found at www.markmorris.org.

activity is highly pH sensitive. Therefore, if energy metabolism Assuming no other primary acid-base changes, CO and
2
-
and muscle contraction are to proceed optimally, muscle pH bicarbonate (HCO ) increase in parallel because of the fol-
3
must be tightly regulated. Intracellular buffers can blunt some lowing relationship:
of the acute effects of increased concentrations of CO and lac- CO + H O ↔ HCO 3 - + H +
2
2
2
tate. However, elimination of organic acids from muscle cells is The CO load produced during exercise can be eliminated
2
the primary strategy for avoiding deleterious decreases in mus- via two routes: 1) respiratory loss of CO (acute) and 2) renal
2
cle pH. Because it is a weak electrolyte, CO has less effect on excretion of HCO 3 - (long-term). The ability of the kidneys to
2
pH than lactate (a strong salt of lactic acid) and is handled dif- respond acutely may be impaired because of decreased plasma
ferently by the body. volume and renal blood flow during exercise. The respiratory