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Nutrient Metabolism
Dietary Protein Restriction and Fat Supplementation Diminish the
Acidogenic Effect of Exercise During Repeated Sprints in Horses
P. M. Graham-Thiers,*1 D. S. Kronfeld,* K. A. Kline* and D. J. Sklan†
*Virginia Polytechnic Institute and State University, Blacksburg, VA 24601-0606
and †Hebrew University, Rehovot, Israel
ABSTRACT A restricted protein diet supplemented with amino acids and fat may reduce the acidogenic effects
of exercise. Twelve Arabian horses were assigned to a 2 ⫻ 2 factorial experiment: two fat levels: 0 or 10 g/100 g
added corn oil and two crude protein levels: 7.5 g/100 g (supplemented with 0.5% L-lysine and 0.3% L-threonine)
or 14.5 g/100 g. The experiment began with a 4-wk diet accommodation period followed by a standard exercise
test consisting of six 1-minute sprints at 7 m/s. Horses were interval trained for 11 wk followed by another exercise
test with sprints at 10 m/s. Blood samples were taken at rest and during the exercise tests. Plasma was analyzed
for PCO2, PO2, Na⫹, K⫹, Cl⫺, lactate, pH and total protein. Bicarbonate, strong ion difference and total weak acids
were calculated. Data were analyzed using repeated-measures analysis of variance. Venous pH was higher in the
low protein group during the first test (P ⫽ 0.0056) and strong ion difference became higher (P ⫽ 0.022) during
sprints in the low protein group. During the second test, venous pH and bicarbonate were higher for the low protein
high fat group (P ⫽ 0.022 and P ⫽ 0.043, respectively) and strong ion difference became higher (P ⫽ 0.038) at the
end of exercise in the low protein groups. These results show that restriction of dietary protein diminishes the
acidogenic effect of exercise, especially in combination with fat adaptation. J. Nutr. 131: 1959 –1964, 2001.
KEY WORDS:
●
protein
●
acid base
●
fat
●
horses
diet has been shown to reduce the acidity of exercise in
Arabian horses performing repeated sprints resulting in higher
pH and bicarbonate levels (Graham-Thiers et al. 1999b).
Fat adaptation refers to the combined effect of training and
prolonged consumption of a high fat (HF) diet (Kronfeld and
Downey 1981). Fat adaptation has influenced acid-base responses to repeated sprints, mainly by reducing the increase in
PCO2 in venous blood (Kronfeld et al. 1998). An increase in
PCO2 in venous blood has resulted in an acidosis but a concurrent decrease in PCO2 in arterial blood has resulted in an
alkalosis (Taylor and Kronfeld 1995). Other influences on
acid-base balance include strong ion difference (SID), which is
the difference between strong cations and strong anions in the
body, and total weak acids (Atot; Stewart 1981). These factors
may be influenced by a reduction in dietary protein that may
reduce anions. Separating the effects of PCO2, SID and total
weak acids on hydrogen ion concentration revealed that restricted dietary protein reduced hydrogen ion concentration
mainly due to an increase in SID. Restricted dietary protein
reduced the level of anions thus improving SID and lowering
hydrogen ion concentration (Graham-Thiers et al. 1999c).
Fat supplementation may spare protein during energy demanding states, such as food deprivation or during energy
restriction (McCargar et al. 1989). This study tested the hypothesis that fat adaptation may reduce the risk of protein
deficiency when dietary protein is restricted and energy demand increased, such as during strenuous exercise. Addition of
limiting amino acids may also reduce the risk of protein
deficiency. This study also tested the hypothesis that acid-base
An optimal range of dietary protein has not been developed
for exercising horses (Kronfeld et al. 1994). Increased needs for
protein during exercise include maintenance of new muscle
mass, repair of existing muscle mass and replacement of any
nitrogen that would be lost in sweat (Meyer 1987). The upper
limit of dietary protein for exercising horses should minimize
thermogenic, ureogenic and acidogenic effects, but the lower
limit must avoid protein deficiency.
The current dietary protein recommendations are 9.8, 10.4
and 11.4% crude protein (CP)2 for horses in light, moderate and
intense exercise, respectively (National Research Council 1989).
Surveys in Michigan (Gallagher et al. 1992a and 1992b) and
Australia (Southwood et al. 1993) revealed that thoroughbreds
and standardbreds in training were fed an average of 14% CP.
Another study in central North Carolina (Honore and Uhlinger
1994) found that 70% of the horses were being fed protein in
excess of National Research Council recommendations.
Heat production from higher levels of protein may interfere
with exercise by attenuating fatigue (Kronfeld 1996) and an
increased acidity will interfere with glycolysis and muscle fiber
contraction (Mainwood and Renaud 1985). Restriction of
dietary protein may be beneficial during strenuous exercise by
diminishing production of heat and acid. A restricted protein
1
To whom correspondence should be addressed at Campus Box 219, Virginia Intermont College, Bristol, VA 24201. E-mail: [email protected]
2
Abbreviations used: CP, crude protein; HF, high fat; SID, strong ion difference; Atot, total weak acids; HP, high protein; La, lactate assay; LP, low protein;
LF, low fat; SET, standard exercise test; TP, total protein.
0022-3166/01 $3.00 © 2001 American Society for Nutritional Sciences.
Manuscript received 30 June 2000. Initial review completed 12 January 2001. Revision accepted 26 March 2001.
1959
GRAHAM-THIERS ET AL.
1960
responses to repeated sprints will be reduced by the combination of dietary protein restriction and fat adaptation. In another paper, we reported that several indices of protein status
were not diminished by the restricted level of crude protein
(fortified with lysine and threonine) in this experiment (Graham-Thiers et al. 1999a).
TABLE 2
Protocol for interval training of Arabian horses over 11 weeks
after the accommodation period but before the final
standard exercise test (SET)1
Time, min
MATERIALS AND METHODS
Horses. Twelve previously unconditioned Arabian horses (6
mares, 6 geldings, ages 5–11 y) were housed in a dry lot with free
access to water. General health and feed intake were observed daily,
and body weight, body condition and coat condition were monitored
every 2 wk. Body condition was evaluated by a standardized system
(Henneke et al. 1983). Coat was evaluated on shine and smoothness.
The protocol was approved by the Institutional Animal Care and Use
Committee.
Diets. Horses were randomly assigned to four complete feeds
(HPHF, LPHF, HPLF, LPLF) which were formulated to provide 12.5
MJ of digestible energy/kg of dry matter (Table 1). Protein was also
formulated at two levels: high protein (HP; 14.5 g/100 g CP) and low
protein (LP; 7.5 g/100 g CP) supplemented with 0.5% L-lysine and
0.3% L-threonine (Heartland Lysine, Chicago, IL) to match the
amino acids in HP. The National Research Council currently recommends a dietary CP level of 11.4% for horses in intense exercise
(National Research Council 1989). Fat was also at two levels: either
13 g/100 g (10 g/100 g added corn oil; HF) or 3 g/100 g (no fat
supplementation, low fat [LF]). A 4-wk adaptation period began the
experiment. Feed intake was adjusted to maintain body condition.
TABLE 1
Ingredient and chemical composition of experimental diets,
as-fed basis (Association of Official Analytical
Chemists 1990)1
Item
LPLF
HPLF
LPHF
HPHF
g/100 g
Orchardgrass hay
Oat Straw
Oats
Corn oil
Molasses
Soybean meal
Corn grain
Dicalcium phosphate
Limestone
NaCl
Vitamin/mineral mix2
L-lysine HCl3
L-threonine3
Analysis
Moisture, %
Digestible energy, MJ/
kg
Ether extract, %
CP, %
L-lysine,3 %
L-threonine,3 %
DCAD,4 mEq/kg
35
5
7.5
0
10
0
40
0.75
0.75
0.5
0.5
0.5
0.3
35
0
7.5
0
10
15
30
0.75
0.75
0.5
0.5
0
0
40
20
4
10
5
3
15
1.25
0.25
0.25
0.5
0.5
0.3
40
18
0
10
5
24.5
0
0.85
0.55
0.25
0.5
0
0
10.2
10.4
8.5
9.1
12
2.3
9.9
0.54
0.43
345.5
12.2
2.5
14
0.50
0.47
424.1
12.4
13.3
8.3
0.54
0.51
370.0
12.5
10.7
13.6
0.59
0.50
387.3
Phase A
2.5
2.5
4
3
Phase B
3
3
Phase C
3
3
3
2
Gait
Speed, m/s
Slope, %
Walk
Walk
Trot
Canter
1.5
1.5
3.5
7.0
0
6
6
6
Trot
Canter
3.5
8.0
6
6
Trot
Canter
Trot
Walk
3.5
9.0
3.5
1.5
6
6
6
0
1 Phase A was used for 3 wk after the first SET. Phase A was
extended to include phase B for 4 wk after the first SET and was
extended again to include phase C for 4 wk before the final SET.
Conditioning. All horses underwent 11 wk of conditioning using
a high speed treadmill (Mustang 2000; Kagra, Fahrwangen, Switzerland). The protocol consisted of interval training divided into three
phases (Table 2). Horses were interval trained twice a week and were
walked at 1.5 m/s for 30 min on a mechanical walker on rest days.
Standard exercise test (SET). Before conditioning, but after the
dietary accommodation period, all horses performed a standard exercise test (SET-U). The SET consisted of 6 min at the walk (1.5 m/s;
3 min on no slope and 3 min on a 6% slope), a 5-min warm-up at the
trot (3.5 m/s) followed by six 1-minute sprints at 7 m/s on a 6% slope
separated by 4-min walks (1.5 m/s), and a 30-min recovery period at
the walk with no slope. After the conditioning period, two SET were
conducted 7 d apart (Fig. 1). These two SET were similar to SET-U
except that the sprints were at 10 m/s. This exercise testing was
designed to simulate a typical sport horse’s events, often being repetitive in a short period.
Before all SET, horses were deprived of food overnight but had
access to water. All tests were performed in a climate-controlled barn
with temperature set at 24°C and relative humidity ⬃50%. A 14guage, 150-cm polyethelene catheter (Intramedic polyethylene tubing; Becton Dickinson, Sparks, MD) was fed down the left jugular
vein into the pulmonary artery and a 18-guage catheter was placed in
the right carotid artery (Angiocath Deseret Medical Inc., Sandy, UT)
⬃60 min before the SET (Taylor and Kronfeld 1995). Placement of
the end of the tubing in the pulmonary artery was facilitated by using
an oscilloscope attached to a pressure transducer (Propaq 140; Protocol Systems, Beaverton, OR). A thermocouple (Model IT-18;
Physitemp Instruments, Clifton, NJ) was inserted to the catheter tip
in the pulmonary artery.
Sampling and analysis. During the SET, 10 mL of arterial and 30
mL of mixed venous blood were taken at rest, during the last 15 s of
1 n ⫽ 6.
2 Vitamin/mineral premix provided: Ca, 8%; P, 8%; Mg, 1%; S, 1%;
Zn, 0.5%; Cu, 0.12%; Mn, 0.1%; Co, 0.002%; I, 0.002%; Na, 13.65%;
Cl, 21.35%; Se, 0.001%; vitamin A, 1,380,000 IU/kg; vitamin D, 258,000
IU/kg; vitamin E, 26,455 IU/kg; riboflavin, 701 mg/kg; niacin, 3,000
mg/kg; folic acid, 66 mg/kg; thiamin, 1,520 mg/kg; biotin, 42 mg/kg.
3 Heartland Lysine Inc., Chicago, IL.
4 DCAD ⫽ (Na⫹ ⫹ K⫹ ⫹ Ca⫹2 ⫹ Mg⫹2) ⫺ (Cl⫺ ⫹ SO4⫺2 ⫹ PO4⫺2).
FIGURE 1 Experimental timeline for 12 Arabian horses fed low or
high levels of protein and fat (LPLF, LPHF, HPLF, HPHF). The timing of
the SET in relation to the accommodation period (SET-U) and conditioning (SET-2 and SET-3 combined as SET-C) are indicated using
arrows.
PROTEIN AND FAT DIMINISH ACIDITY OF EXERCISE
1961
RESULTS
⫽ 0.0043). There were no differences for pH, PO2, PCO2, SID,
Atot, [La⫺], [Na⫹] or [Cl⫺].
Exercise. During SET-1, arterial PCO2 was decreased (P
⫽ 0.0001). Increases were observed in [La⫺] and [K⫹] (P
⫽ 0.0001), PO2 (P ⫽ 0.001), [Na⫹] (P ⫽ 0.0009) and [Cl⫺] (P
⫽ 0.0007) and pH (P ⫽ 0.0001). Exercise had no effect on
arterial [HCO3⫺] and SID. In venous plasma, decreases were
observed in pH, [Cl⫺] and PO2 during the SET (P ⫽ 0.0001).
Increases were observed in PCO2, [Na⫹], [K⫹], [HCO3⫺] and
[La⫺] (P ⫽ 0.0001). Venous SID was unaffected by exercise.
During SET-2, arterial [HCO3⫺] and PCO2 decreased (P
⫽ 0.0001), along with pH (P ⫽ 0.0086) but [La⫺] increased (P
⫽ 0.0001). Plasma [Cl⫺] increased (P ⫽ 0.0003) along with
[K⫹] (P ⫽ 0.07), [Na⫹] (P ⫽ 0.0048) and PO2 (P ⫽ 0.0007).
Arterial SID was unaffected by exercise. In venous plasma,
decreases were observed in pH and PO2 during the SET (P
⫽ 0.0001). Increases were observed in PCO2, [HCO3⫺], [La⫺]
and [K⫹] (P ⫽ 0.0001). Plasma [Na⫹] also increased (P
⫽ 0.0002) along with SID (P ⫽ 0.0038) and plasma cortisol (P
⫽ 0.0004).
Diet and exercise. During SET-1, venous pH was higher
(P ⫽ 0.0056) in the LP groups (Fig. 2) but was not affected by
fat level. Venous [HCO3⫺] was not affected by fat or protein
level.
Venous PCO2 was also not affected by fat or protein level.
Venous PO2 was higher (P ⫽ 0.019) for the LP groups, especially during recovery, but was not affected by fat level. Plasma
Atot was not affected by fat or protein level.
Venous SID was affected by a time ⫻ protein interaction (P
⫽ 0.022) being higher in the LP groups during the sprints (Fig.
3). Venous [Na⫹], [K⫹] and [Cl⫺] were not affected by protein
and there was not an effect of fat. Venous [La⫺] was higher (P
⫽ 0.023) in the LF groups but was not affected by protein level
(Fig. 4).
Arterial pH and [HCO3⫺] were not affected by fat or
protein levels. Arterial PCO2 was not affected by fat or protein
level. A fat ⫻ protein interaction (P ⫽ 0.0045) occurred for
PO2 contrasting increasing levels in the LPLF group but decreasing levels for the LPHF. Arterial SID, [Na⫹], [K⫹] [La⫺]
and [Cl⫺] were not affected by fat or protein level.
During SET-2, venous pH was affected by a fat ⫻ protein
interaction (P ⫽ 0.022) being higher in the LPHF group and
lower in the LPLF group (Fig. 5). Venous [HCO3⫺] also had
All horses remained in good health throughout the study.
Body weight was 419 ⫾ 15 kg at the start of the study and 459
⫾ 15 kg at the end. The amount of weight gained was not
affected by diet. No signs of protein deficiency (anorexia, loss
of condition, weakness, poor coat) were observed. Feed intake
averaged 8.8 ⫾ 0.3 kg/d with no differences in horses fed the
two diets. Due to differences in dietary protein, protein intake
averaged 711 ⫾ 21 g/d and 1323 ⫾ 45 g/d for horses in LP and
HP groups, respectively.
Rest. Before SET-1, arterial [La⫺] was higher in the LP
groups (P ⫽ 0.0022) but lower in the HF groups (P ⫽ 0.011).
There were no differences in PO2, PCO2, SID, Atot, [Na⫹],
[K⫹], [Cl⫺], pH or [HCO3⫺]. Venous [La⫺] was also higher for
the LP groups (P ⫽ 0.0076) but was not affected by fat level.
No differences were found in PO2, PCO2, SID, Atot, [Na⫹],
[K⫹], [Cl⫺], pH or [HCO3⫺].
Before SET-2, arterial [K⫹] was higher in the LP groups (P
⫽ 0.039) as was [HCO3⫺] (P ⫽ 0.032). No differences were
found in pH, PO2, PCO2, lactate, SID, Atot, [Na⫹], [K⫹] or
[Cl⫺]. Venous [HCO3⫺] was higher (P ⫽ 0.024) in the LP
groups as well as [K⫹] (P ⫽ 0.025). Plasma cortisol was lower
in the HF groups (P ⫽ 0.0008) as well as in the LP groups (P
FIGURE 3 Venous SID of horses fed LP or HP diets at rest (R),
during the sprints (S1, S2 and S6) and at 5, 10, 15 and 20 min of
recovery (R1, R2, R3 and R4). Venous SID was affected by a protein
⫻ time interaction (P ⫽ 0.022), increasing as the SET progressed
(SET-U). Data are means ⫾ SE, n ⫽ 6.
FIGURE 2 Venous pH of horses fed LP or HP diets at rest (R),
during the sprints (S1, S2 and S6) and at 5, 10, 15 and 20 min of
recovery (R1, R2, R3 and R4). Venous pH was affected by protein level
(P ⫽ 0.0056) and by exercise (P ⫽ 0.0001) during the SET that followed
the accommodation period (SET-U). Data are means ⫾ SE, n ⫽ 6.
sprint 1, 2 and 6 as well as at 5, 10, 20 and 30 min of recovery. Blood
samples were taken anaerobically into heparinized syringes (Sherwood Medical, St. Louis, MO) for analysis of PO2, PCO2, pH, [Na⫹]
and [K⫹] (STAT Profile 2; Nova Biomedical, Waltham, MA). Bicarbonate was calculated from pH and PCO2. Additional heparinized
blood was centrifuged to obtain plasma. An aliquot of plasma was
immediately deproteinized in ice-cold perchlorate for lactate (La⫺)
assay (Proc. No. 826-UV; Sigma Diagnostics, St. Louis, MO). The
remaining plasma was frozen for later analysis of total protein (TP;
Proc. No. 541; Sigma Diagnostics), Cl⫺ (Proc. No. 461; Sigma
Diagnostics) and cortisol (Coat-A-Count Cortisol, St. Louis, MO).
Atot were estimated from TP using a conversion factor of 0.21 mEq/L
TP (Stämpfli et al. 1999). SID⫹ was calculated as the algebraic sum
of [Na⫹], [K⫹], [La⫺] and [Cl⫺]. Samples were analyzed in tripicate,
with duplicates being selected that varied by ⬍1%.
Statistics. Data were summarized as least squares means and
standard errors. Analysis of variance with repeated measures was used
to evaluate the effects of time (exercise and/or recovery), fat level,
protein level and interactions in the SET (SAS 1991). Data that were
not significantly different between SET-2 and SET-3 in conditioned
horses were pooled and reported as SET-C. Significance of difference
was set at P ⬍ 0.05.
1962
GRAHAM-THIERS ET AL.
FIGURE 4 Venous lactate of horses fed LF or HF diets at rest (R),
during the sprints (S1, S2 and S6) and at 5, 10, 15 and 20 min of
recovery (R1, R2, R3 and R4). Venous lactate was affected by fat level
(P ⫽ 0.023) and by exercise (P ⫽ 0.0001) during the SET that followed
the accommodation period (SET-U). Data are means ⫾ SE, n ⫽ 6.
a fat ⫻ protein interaction (P ⫽ 0.043) during recovery,
remaining elevated in the LPHF group (Fig. 6).
Venous PCO2 was higher (P ⫽ 0.026) in the LF groups after
the first sprint only as well as in the LP groups during the
sprints (P ⫽ 0.019). Venous PO2 was not affected by fat or
protein level. A fat ⫻ protein interaction (P ⫽ 0.029) was
observed with venous PO2 during recovery with higher levels
observed in the LPLF group. Plasma Atot was not affected by
fat or protein level.
Venous SID had a time ⫻ protein interaction (P ⫽ 0.038)
with higher SID in the LP groups during the later stages of the
sprints (Fig. 7). Venous [Na⫹] was higher (P ⫽ 0.007) in the
LP groups but was not affected by fat. Venous [K⫹] was higher
(P ⫽ 0.026) for the LP groups, especially during the sprints,
but again, was not affected by fat. Venous [Cl⫺] was higher (P
⫽ 0.046) during the sprints in the HF groups. Venous [La⫺]
was higher (P ⫽ 0.05) in the LF groups, especially during the
sprints, but was not affected by protein.
Plasma cortisol levels were lower (P ⫽ 0.006) in the
HF groups and in the LP groups during the sprints (P ⫽ 0.048;
Fig. 8).
FIGURE 5 Venous pH of horses fed low or high levels of protein
and fat at rest (R), during the sprints (S1, S2 and S6) and at 5, 10, 15 and
20 min of recovery (R1, R2, R3 and R4). Venous pH was affected by a
fat ⫻ protein interaction (P ⫽ 0.022) and by exercise (P ⫽ 0.0001)
during the SET that followed the conditioning period (SET-C). Data are
means ⫾ SE, n ⫽ 3.
FIGURE 6 Venous HCO3 in horses fed low or high levels of protein
and fat at rest (R), during the sprints (S1, S2 and S6) and at 5, 10, 15 and
20 min of recovery (R1, R2, R3 and R4). Venous HCO3 was affected by
a fat ⫻ protein interaction (P ⫽ 0.043) and by exercise (P ⫽ 0.0001)
during the SET that followed the conditioning period (SET-C). Data are
means ⫾ SE, n ⫽ 3.
Arterial pH and [HCO3⫺] were not affected by fat or
protein. Arterial PCO2 was higher (P ⫽ 0.04) in the HF groups
with a time ⫻ fat interaction as the difference between the HF
and LF groups became greater during recovery. Arterial PO2
levels were not affected by fat or protein levels. Arterial SID,
[La⫺], [Na⫹] and [K⫹] also were unaffected by fat or protein
levels. Arterial [Cl⫺] was higher (P ⫽ 0.042) in the HF groups
during the sprints but was unaffected by protein.
DISCUSSION
Dietary protein decreased the acidogenic effect of exercise
in unconditioned horses (SET-U). There were no main effects
of fat with the exception of lower plasma lactate during SET-U
and SET-C. However, because the effect of the LP diet during
SET-C was only apparent in combination with the HF diet, it
seems that there is a positive relationship between protein
restriction and fat adaptation. This conclusion is supported by
a lack of difference between horses fed the LPLF and HPLF
diets.
In the comprehensive physico-chemical model of acid-base
status by Stewart (Stewart 1981), the dependent variables,
FIGURE 7 Venous SID in horses fed LP or HP diets at rest (R),
during the sprints (S1, S2 and S6) and at 5, 10, 15 and 20 min of
recovery (R1, R2, R3 and R4). Venous SID was affected by a protein
⫻ time interaction (P ⫽ 0.038), increasing as the SET progressed
(SET-C). Data are means ⫾ SE, n ⫽ 6.
PROTEIN AND FAT DIMINISH ACIDITY OF EXERCISE
FIGURE 8 Plasma cortisol concentrations in horses fed LF or HF
diets (upper panel) or LP or HP diets (lower panel) at rest (R), during the
sprints (S1, S2 and S6) and at 5, 10, 15 and 20 min of recovery (R1, R2,
R3 and R4). Cortisol levels were affected by fat level (P ⫽ 0.006) and
protein level (P ⫽ 0.048) and by exercise (P ⫽ 0.0001) during the SET
that followed the conditioning period (SET-C). Data are means ⫾ SE,
n ⫽ 6.
hydrogen ion and bicarbonate ion, are influenced by changes
in the three independent variables, PCO2, SID and Atot.
During SET-U, venous pH was higher for horses in the LP
group. This difference in pH was attributable mainly to SID
because PCO2 and Atot were not affected by diet. During
SET-C, the lower venous pH in exercising horses fed the two
low fat diets could be explained by the higher PCO2. In studies
of intensely exercising horses, PCO2 was the main determinant
of changes in venous or arterial plasma pH (Kronfeld 1996). In
contrast, the higher venous pH found in horses fed the LPHF
diet could be explained by a combination of reduced PCO2, due
to fat adaptation, and increased SID, due to protein restriction.
The effect of protein restriction on venous plasma pH
observed here confirms previous observations (Graham-Thiers
et al. 1999b). This earlier study also revealed that the higher
pH was influenced through changes in SID (Graham-Thiers et
al. 1999c). The lack of effect on pH and PCO2 in arterial blood
is consistent with arterial blood reflecting pulmonary ventilation more than peripheral metabolism (Carlson 1995)
Dietary protein contains sulfur and phosphorus that become oxidized to sulfate and phosphate. The amino acids that
are oxidized during exercise (leu, iso, val) do not contain sulfur
or phosphorus so dietary protein is not expected to directly
affect the acidogenic effect of sprint exercise. Prolonged restriction of dietary protein is expected to exert a chronic
anti-acidogenic effect. Oxidation of sulfur-containing amino
acids increases the net acid load in the body by 2 mol for every
1963
mole of sulfur-containing amino acid oxidized. Endogenous
acid is also increased by dibasic and phosphorylated amino
acids (Patience 1990). In the present study, venous SID was
higher in the LP group over the course of the SET, compared
with the HP group. Restricting dietary protein may be an
alternative to replacing NaCl with sodium bicarbonate for
reducing exercise acidosis, especially in horses consuming a HF
diet. This would be beneficial for horses working hard in the
heat because HP is thermogenic and will affect water balance
with increased urination and possibly higher sweat loss (Meyer
1987).
In a previous study, sodium bicarbonate administration had
an additive or, perhaps, synergistic effect with fat adaptation in
moderating the blood lactate increase during repeated sprints
(Ferrante et al. 1994). Dietary protein restriction had a similar
interaction with fat adaptation in moderating the acidogenic
effect of repeated sprints in the present experiment.
Dietary fat has also been observed to spare protein, especially under conditions of high energy demand, such as exercise. The present results suggest that protein deficiency may
have been avoided partly because of fat adaptation (in addition to added lys and thr) in exercising horses fed a restricted
protein diet. Fat adaptation has previously been suggested to
improve metabolic regulation as indicated by plasma lactate
responses in repeated sprints and incremental excercise. Moreover, fat adaptation increases fatty acid oxidation, which is
more efficient than amino acid oxidation (Kronfeld 1996).
Lower plasma cortisol levels in the HF group may provide
evidence of less reliance on glucose as fuel and more reliance
on fat metabolism, with a diminished need for gluconeogenisis.
With less amino acid oxidation, less endogenous acid would be
produced in the body (McCargar et al. 1989). This would
partly explain the higher pH and [HCO3⫺] in venous blood of
horses in the LPHF group during SET-C.
In conclusion, the acidogenic effect of sprinting exercise
was diminished by both LP diets, regardless of fat level before
conditioning, but only by the LPHF diet after conditioning.
Thus, the combination of protein restriction and fat adaptation was needed for horses in training to reduce the acidogenic
effect of intense exercise.
LITERATURE CITED
Association of Official Analytical Chemists. (1990) Official Methods of Analysis,
15th ed. Association of Official Analytical Chemists, Arlington, VA.
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