80 Canine Sports Medicine and Rehabilitation
consumption can be examined when feeding
differing carbohydrate and fat diets (Toll &
Gillette, 2010). A significant amount of work
has been performed examining gradation of
exercise and duration as they relate to Vo2 max
and RQ values (Wagner et al., 1977; Grandjean,
1998). These studies have laid the groundwork
for our understanding of oxygen consumption,
showing that when dogs reach up to 40% of
their Vo2 max, they primarily use fat for energy,
while between 40 and 70% they use a mixture
of glucose and fatty acids. Once an animal
reaches 70% or higher oxygen consumption,
they use primarily glucose for energy (Toll et al.,
1992; Reynolds et al., 1995) (see Figure 4.6).
Animals working at maximal speed during
the first few seconds of exercise rely upon
immediate energy from the phosphocreatine
system that generates ATP through shuttling
inorganic phosphate to ADP. Glycogenolysis
soon ensues, generating energy for typical
sprinting or intermediate exercise. Generation
of pyruvate and eventually lactic acid predomi-
nates if glycolysis is maintained, leading to
pH alteration and intracellular dysfunction.
Pyruvate incorporation into the citric acid
cycle (carbohydrate oxidation) becomes a major
source of energy for long‐term exercise (20 min-
utes to 2 hours) as long as glycogen is present
for glycogenolysis. Eventually protein oxida-
tion will take place if glycogen is depleted in
endurance exercise. As glycogen is depleted a
dog will not be able to sustain oxygen con-
sumption above 50–60% of Vo2 max. Fatty acid
oxidation begins to rise by 30 minutes into an
exercise bout and will be sustained at an oxy-
gen consumption rate of between 30 and 50% of
the maximum as the primary fuel. This pro-
vides acetyl CoA production for the citric acid
cycle at a steady rate, allowing some dogs to
exercise at this low to moderate oxygen con-
sumption rate for multiple hours (Reinhart,
1998; Toll & Gillette, 2010) (Figure 4.7).
This understanding provides the basis for fat
use as a major form of energy in canid diets,
which is contrary to recommendations in human
endurance exercise, which focuses on carbohy-
drate loading (Hargreaves et al., 1984). One
study showed that time to exhaustion during
low‐intensity exercise did not correlate with gly-
cogen depletion in dogs (Downey et al., 1980).
The generation of energy from fat is up to 70% of
the ME during long‐duration exercise, suggest-
ing a propensity for fat utilization, which may be
due to the dog’s high aerobic activity in skeletal
muscle and increased mitochondrial density as
compared to humans (Wakshlag et al., 2004).
Beagles running at low to moderate intensity
increased their time to exhaustion by approxi-
mately 25% when provided diets with 55–81
g/1000 kcal of fat versus 33 g/1000 kcal (Downey
et al., 1980). Kronfeld, Hammel and colleagues
showed that dogs performed equally well on
diets containing absolutely no carbohydrate200150100500
0246810 30 90 150
Duration of exercise (min)210 270Vomax (%) 2Cr-P
Glycolysis
Carbohydrate oxidation
Fat oxidationFigure 4.7 The graph depicts the Vo2 max based on duration of exercise. Note that initially Vo2 max is above 100%,
reflecting anaerobic energy generated by creatine phosphate (Cr‐P) reserves and glycolysis; this eventually plateaus in
2–4 minutes with carbohydrate oxidation. As glycogen stores are depleted beta‐oxidation of fat occurs and sustains the
exercise bout somewhere around 60–90 minutes, making fat the primary fuel for exercise in the endurance athlete.
Source: Picture reproduced from Hand et al., 2010.