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Proceedings of the 5th European Equine
Nutrition & Health Congress
April. 15-16, 2011
Waregem, Belgium
Next EENHC Congress:
6th European Equine Nutrition & Health Congress: Feeding for Gastrointestinal Health
Mar. 1-2, 2013- Ghent , Belgium
Reprinted in IVIS with the permission of the Scientific Committee of
the European Equine Health & Nutrition Congress: http://www.equine-congress.com/
Reproduced in IVIS with the permission of the Scientific Committee of the EENHC
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Energy metabolism of the performance horse
Ingrid Vervuert
Institute of Animal Nutrition, Nutrition Diseases and Dietetics, Faculty of
Veterinary Medicine, University of Leipzig, Germany
E-mail: [email protected]
Introduction
The superior athletic ability of horses is closely related to their high maximal
aerobic capacity, large intramuscular energy stores (glycogen), high
mitochondrial volume in muscle, the ability to increase oxygen-carrying capacity
of blood at the onset of exercise (splenic contraction), efficient thermoregulation
and the efficiency of gait (Hinchcliff & Geor 2008). The great athletic ability of
the equine is summarized by selected variables in Table 1.
Table 1: Selected physiological parameters in the equine and human athlete
(adapted from Gäbel 2010)
Parameter
Rest
Intensive Increase
exercise
rate
Horses
Heart rate (bpm)
30
210-250
7-8
Stroke volume (L)
1.0
1.7
1.7
Respiratory minute volume (L/min)
30
240-425
8-13
Haemoglobin (g/dl)
13
17-24
1.3-1.6
2-4
160-220
40-110
VO2 max (ml O2/kg/min)
Humans
Heart rate (bpm)
45
190-200
4.2-4.4
Stroke volume (L)
0.1
0.2
2
Respiratory minute volume (L/min)
4.5
38-40
8-9
4
50-90
13-23
VO2 max (ml O2/kg/min)
VO2 max = maximal oxygen consumption
The energy expenditure of exercised horses rises due to the energy cost of the
work of the skeletal muscles, respiratory apparatus and cardiovascular system
(Martin-Rosset 2008). Herby, oxygen consumption is supposed to be the best
criterion of energy expenditure. In horses, the rate at which oxygen consumption
can increase with the onset of exercise is superior (40-110-fold increase within 60
seconds, Table 1). The rapid increases in ventilation, cardiac output and
haemoglobin allow for the rapid VO2, implying that the oxygen dependent
processes for energy production within the muscle are able to utilize the O2.
However, intensive exercise in horses, particularly when performed in hot and
humid conditions will result in the depletion of the energy stores like liver and
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muscle glycogen (Hyyppä et al. 1997) and in excessive water and electrolyte
losses (Coenen 2005). These changes impair exercise capacity and result in
cardiorespiratory and metabolic alterations contributing to fatigue (Hodgson &
Rose 1994). The supply, uptake and utilization of substrates for the production of
energy are integral components for the ability to carry out physical exercise.
The primary sources for energy production in the exercising horse are
carbohydrates and fats and, to a much lesser degree, protein (Hiney & Potter
1996). The extent to which each fuel is utilized depends on several factors,
including the intensity and duration of exercise, the availability of fuels, and the
influence of hormones.
Energy for muscular work
Muscular contraction results from the transformation of chemical energy into
mechanical energy. The conversion of chemical energy into the mechanical
energy of muscular movement requires the provision of adenosine triphosphate
(ATP) which is the substrate utilized by the contracting muscle:
myosin ATPase
ATP + H2O
ADP + Pi and kinetic energy
ADP = adenosine diphosphate
However, endogenous ATP stores within the muscles are very limited (~22-28
mmol/kg dry matter) and provide only energy for a few seconds (Hodgson 1985).
To continue muscle contraction, ATP must be continuously resynthesised. The
regeneration of ATP is achieved by two metabolic pathways which can be
basically divided into
1) Anaerobic (without oxygen) phosphorylation and
2) Oxidative (aerobic) phosphorylation.
Anaerobic phosphorylation
Phosphocreatine reaction
Creatine phosphate and ADP are the substrates for this reaction:
creatine kinase
Phosphocreatine + ADP
creatine + ATP
Endogenous phosphocreatine stores within the muscles are limited (58-62
mmol/kg dry matter) and provide only a restricted amount of ATP (Table 2). It is
postulated that phosphocreatine reaction is of importance during the initial stages
of high intensity exercise and during jumping (Snow 1991).
Myokinase reaction
ATP can be synthesized from ADP, and this reaction is catalyzed by myokinase.
myokinase
2 ADP
ATP + AMP
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AMP = adenosine monophosphate
This rapid reaction provides energy during severe exercise and maintains the
muscle ATP: ADP ratio.
Glycolysis
ATP is regenerated from circulating glucose and glycogen stores. In a multiple
step pathway, 2 ATP molecules result from the glycolytic pathway per glucose
(Table 2). The production of lactate is a possible end stage of this anaerobic
pathway.
Oxidative phosphorylation
The major energy source in muscle cells is the oxidation of free fatty acids and
glucose, and to lesser extent amino acids like alanine. The oxidative
phosphorylation pathway needs oxygen (aerobic pathway) provided by the
circulation. The regeneration of ATP requires a series of complex steps requiring
specific enzymes (Krebs cycle, ß oxidation of fatty acids, electron transport
chain). During aerobic phosphorylation fuel is used efficiently (Table 2) but this
pathway is a slower process than anaerobic metabolism with a rate limited by
oxygen transport.
Table 2: Overview over substrate utilisation and resulting ATP production
during exercise
Energy source
Substrate
Pathway
Outcome
ATP/mol
Immediate response to exercise
Muscle
ATP
anaerobic
1
Phosphocreatine
anaerobic
1
Glycogen
anaerobic
2
Blood
Glucose
anaerobic
2
Slow response to exercise
Muscle
Glycogen
Liver
Glucose
aerobic
36-38
Blood
Amino acids
Muscle
Liver
Fatty acids
aerobic
~140
Blood
Triglycerides
Adipose tissue
Integration of anaerobic and aerobic pathways
At the onset of exercise energy is primarily derived from the anaerobic pathways.
At maximal short lasting exercise speeds, energy must be predominately
generated by anaerobic pathways (Figure 1). Herby glycogen serves as the major
fuel source, resulting in an accumulation of lactate (McMiken 1983, Table 3).
However, within ~60 sec after the onset of exercise, there is a clear shift to
aerobic metabolism (Figure 1).
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Anaerobic contribution
Aerobic contribution
100
90
% of total energy
80
70
60
50
40
30
20
10
0
400 m
1000 m
Quarter Horse
Race
1600 m
3200 m
Thoroughbred
Race
1600 m
2400 m
Standardbred
Race
2.Tag
80 km Ritt
Cross
country
Endurance
Figure 1: Energy partitioning during different types of exercise (adapted from Eaton 1994)
26
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Table 3: Maximal lactate concentrations after different types of exercise
(adapted from Pösö et al. 2008)
Type of exercise
Distance (km)
Peak plasma
lactate (mmol/L)
Endurance rides
80 - 100
0.50 ± 0.2
Standardbred races
2.1
24.5 ± 0.7
Thoroughbred races
1.1 – 3.8
33.0 ± 1.9
Three-day event (cross country)
6.2
19.1 ± 4.2
Showjumping
9.00 ± 0.9
Polo
9.20 ± 1.2
At low to moderate exercise intensities, the oxidation of fatty acids provides the
major source of energy. At moderate to high aerobic exercise intensities, the
oxidation of fatty acids decreases and the oxidation of carbohydrates account for
>50% of the amount of substrate utilized (Pösö et al. 2008, Figure 2).
Energy expenditure (%)
Fat
100
90
80
70
60
50
40
30
20
10
0
Glycogen
Glucose
30
60
Exercise intensity (% VO2 max)
Figure 2: Estimated relative contributions of fat, plasma glucose and muscle glycogen
expenditure in horses during different type of exercise (adapted from Pösö et al. 2008)
to
energy
Metabolic properties of muscle fiber types
During exercise, metabolites and oxygen reach skeletal muscle fibers and
muscular contraction results from the transformation of chemical energy into
mechanical energy. Muscle fibers show significant differences in their
morphologic, physiologic and biochemical properties within and between
muscles. These differences represent the basis for the classification of fiber types.
Muscle fibers have been differentiated either histochemically (staining reaction
for myosin ATPase) into type I and type II myofibers (Rivero et al. 2008) or
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immunohistochemically (based on antibodies directed against myosin heavy
chain isoforms, Valberg 2008), hereby type 1, type 2a and 2x myofibers are most
commonly found in the equine limb muscles. The different physiological and
metabolic properties are illustrated in Table 4 and 5.
Metabolic properties of muscle fiber types in horses
(immunohistochemically differentiation, adapted from Valberg 2008)
Type 1
Type 2a
Type 2x
Speed of contraction
Slow
Intermediate
Fast
Fatigue resistance
High
Intermediate
Low
Oxidative capacity
High
Intermediate
Low
Fat content
High
Intermediate
Low
Glycolytic capacity
Low
High
High
Glycogen content
Low
High
High
Table
4:
Table 5: Metabolic properties of muscle fiber types in horses (histochemically
differentiation, adapted from Rivero et al. 2008)
Type I
Type IIA
Type IIAx
Type IIaX
Type IIX
Speed of
Slow
Fast
Intermediate Intermediate Very fast
contraction
Fatigue
High
Intermediate Intermediate Intermediate
Low
resistance
Oxidative
High
High to
Intermediate Intermediate
Low
capacity
intermediate
Fat content
High
Intermediate
Glycolytic Relative
High
Intermediate Intermediate
High
capacity
low
Glycogen
Relative
High
High
content
low
Fiber-type composition varies extremely between muscles depending largely on
function. For example, forelimb musculature consist of postural type I fibers,
while propulsive muscles of the hindlimbs contain a high proportion of fasttwitch type II fibers (Rivero et al. 2008). Additionally, performance is highly
correlated with selected muscle characteristics. Endurance capacity is linked to
high percentages of type I and IIA fibers and high activities of oxidative
enzymes, whereas sprint capacity is related to high percentages of type II fibers
(Essén-Gustavsson 2008, Rivero et al. 2008).
Sources of energy
The major stored fuels for energy production during exercise occur in the form of
carbohydrates and fats and to a lesser extent of proteins. The fat stores represent
herby the largest nutrient reserve whereas the carbohydrate stores are limited.
However, most studies in exercising horses have focussed on glycogen stores,
whereas only a few studies have addressed muscle triglyceride or protein stores.
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Glycogen stores
In horses, resting muscle glycogen stores varied around 550-650 mmol/kg dry
matter (Pösö et al. 2008), whereas in humans, resting muscle glycogen stores are
significant lower (300-400 mmol/kg dry matter, Hultman & Greenhaff 1993).
Following any type of exercise, glycogen stores within muscle and liver decline.
The extent of the decrease will depend on the intensity and duration of exercise
(Figure 3). During short-term intensive exercise, muscle glycogen stores may be
depleted by 20-35%. Prolonged exercise results in the greatest depletion of
muscle glycogen. For example, during endurance rides in the order of 100 km,
muscle glycogen can decline between 50 and 100% (Hodgson & Rose 1994).
However after cessation of exercise, the rate of glycogen repletion is much lower
than in other animal species and in the human athlete. Following complete
glycogen depletion, complete repletion may take up to 92 h. In general, it is
concluded that glycogen resynthesis is a limiting step in recovery after a
strenuous competition exercise.
before exercise
after exercise
Glucose (mmol/kg DM)
700
600
500
400
300
200
100
0
100 km
80 km
Cross country
4 x 620 m
800 m
2000 m
Figure 3: Decline in muscle glycogen concentrations (expressed in glucose mmol/kg dry matter)
during different types of exercise in horses (adapted from Hodgson & Rose 1994)
Fat stores
The different fat depots like skeletal muscle or adipose tissue store triglycerides,
with the largest storage capacity in the adipose tissue. The triglyceride content in
the skeletal muscle varied between 7-128 mmol/kg dry matter with large breed
related differences (Essén-Gustavsson 2008). It is supposed that triglyceride
stores are used to a great extent during prolonged exercise (Essén-Gustavsson
2008), but obviously without depletion of muscle fat even during endurance rides
(Snow et al. 1991). Interestingly, high pre-exercise muscle levels of triglycerides
before exercise seems to favourite the utilisation of muscle triglycerides during
exercise (Essén-Gustavsson 2008). However, the higher utilisation of muscle
triglyceride stores does not necessarily reflect a better exercise performance. It is
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reported that those endurance horses with a greater depletion of muscle glycogen
stores and a lower reduction of muscle triglycerides were among the best during a
50 km ride compared to those horses that finished the competition at lower speeds
(Table 6). The largest store of triglycerides is the adipose tissue. Fatty acids
(hydrolysis of triglycerides in glycerol and fatty acids) are efficiently mobilised
from the adipose tissue into the bloodstream during exercise, and those fatty acids
may be utilised for ATP production in the muscle rather than intramuscular
triglycerides stores. However, the role of muscle triglycerides stores and their
contribution to fat oxidation in energy metabolism during exercise is still open.
Table 6: Muscle (M. gluteus) glycogen and triglyceride reduction of horses after
a 50 km endurance ride (adapted from Essén-Gustavsson 2008)
Racing times (min)
209-231
237-287
Glycogen reduction (mmol/kg DM)
313 ± 118
115 ± 103
Triglyceride reduction (mmol/kg DM)
13 ± 12
37 ± 31
Protein stores
It has been shown that exercise induces changes in the amino acid profile in
blood and muscle. An increase in branched-chain amino acids (BCAA) such as
leucine, isoleucine, and valine is observed during prolonged sub-maximal
exercise in horses (Pösö et al. 1991, Vervuert et al. 2005) which may have been
due to increased output by the liver, where proteolysis has been shown to
accelerate during exercise (Dohm et al. 1987). Furthermore, it is supposed that
certain amino acids are oxidised for energy production in the muscle (Strüder et
al. 1996), although the contribution of protein to energy expenditure in horses
during exercise remains open (approximating to a maximum of 10-16 %).
Take home message
Energy demands increase with exercise, the extent being dependent on the type
and duration of exercise. Maintenance of muscular work during exercise requires
the provision of ATP which is the substrate utilized by the contracting muscle. As
endogenous ATP stores are very limited, carbohydrates, fats and, to a much lesser
degree, protein are utilized for ATP production. The aerobic pathway to
regenerate ATP is the most efficient way but this pathway is a slower process
than the anaerobic metabolism. At the onset of exercise and during high speed
exercise energy is primarily derived from the anaerobic pathways, herby muscle
glycogen serves as the major fuel source. The production of lactate is a possible
end stage of this anaerobic pathway.
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Proceedings of the 5th European Equine Nutrition & Health Congress (EENHC), 2011 - Waregem, Belgium