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Keywords:

  • horse;
  • sweat;
  • endurance exercise;
  • Arabian;
  • sodium

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conflicts of interest
  8. Manufacturers' addresses
  9. References

Reasons for performing study: Excessive sweat loss during endurance exercise may lead to electrolyte disturbances and previous research suggests dietary factors may affect hydration status. While investigating the effect of dietary fibre type on hydration status, sweat samples were collected which allowed for the evaluation of sweat composition in horses consuming forage-based, low sodium (Na) rations.

Objective: To investigate sweat composition in Arabian horses performing endurance type exercise while fed forage-based, rations low in Na.

Methods: Six 2-year-old Arabian horses were fed, according to a replicated 3 × 3 Latin square, either grass hay (G), 50:50 grass hay:alfalfa hay (GA), or 50:50 grass hay: chopped fibres (GM) without any additional electrolyte supplementation. After 14 days on each diet, horses performed a 60 km treadmill exercise test. Sweat was collected from sealed pouches on the dorsal thorax after each of four 15 km exercise bouts.

Results: Intake (g/day) of Na (2.5 ± 0.4), Cl (72 ± 16), and Mg (18 ± 3) were not different between diets but K and Ca intakes (g/day) were greater (P<0.05) on GA (246 ± 35; 101 ± 14) than G (176 ± 38; 59 ± 14) or GM (168 ± 33; 62 ± 15). There was no effect of diet on sweat pH (7.65 ± 0.04) or concentrations (mmol/l) of K (46 ± 3), Cl (133 ± 7), Ca (8.5 ± 1.1), or Mg (2.3 ± 0.3); yet diet did influence sweat Na concentration (P<0.05, G 88 ± 5 mmol/l, GA 104 ± 5, GM 96 ± 6). Na and Cl concentrations were lower than those previously reported.

Conclusions: Differences in sweat constituents due to diet were observed, but more importantly both Na and Cl concentration are lower than those previously reported perhaps due to low dietary Na intake or breed of animal.


Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conflicts of interest
  8. Manufacturers' addresses
  9. References

Maintenance of total body water during exercise requires balancing water intake with water loss. In the equine athlete, however, such balance rarely exists and dehydration is a common outcome of prolonged exercise due to water loss in sweat exceeding water intake, despite horses being offered fluid and electrolytes during rest stops. As sweating represents the primary mechanism for thermoregulation in the horse and the horse prioritises thermoregulation over fluid retention, it is not surprising that sweat loss during endurance exercise may approach 5% of bodyweight and is largely affect by ambient temperature and humidity (McCutcheon et al. 1995a). Given equine sweat is generally nearly isotonic to plasma a tremendous electrolyte loss also occurs (McCutcheon et al. 1995a).

The equine large intestine has long been thought to serve as a fluid reservoir and, as such, has been suggested to help attenuate dehydration during exercise. Argenzio et al. (1974) examined digesta passage and water exchange in the large intestine. Results indicated that the large intestine could (re)absorb large quantities of water and electrolytes. More recent research has suggested different dietary fibre types may possess different water holding capacities and thus may differ in their ability to release fluid within the hindgut (Danielsen et al. 1995; Warren et al. 1999). We have previously demonstrated differences in hydration status and physiological parameters in horses fed different dietary fibre types (Spooner et al. 2007). Horses consuming a grass hay:chopped soluble fibre mix diet or grass hay:alfalfa hay diet had greater total body water (TBW) at rest than those consuming only grass hay. Horses consuming the grass hay:fibre mix, while showing a tendency for greater body mass (BM) loss during an endurance exercise test, maintained a lower core body temperature (CT) at the canter. Therefore, a diet containing chopped soluble fibres may provide the horse with a greater ‘pool’ of available fluid for thermoregulation via sweating, thereby attenuating the increase in CT during endurance exercise.

While little research to date has examined the role of diet in sweat constituents, diet may influence electrolyte content of the GI tract. Meyer and Coenen (1989) observed the quantity of sodium in the large intestine of hay-fed ponies was more than twice that of their grain-fed counterparts, while potassium was also greater. It is important to recognise, however, that because water content of the large intestine was also greater, no differences in electrolytes were observed on a concentration basis. Still, a greater amount of electrolytes in hay-fed animals may provide additional ions for absorption and replacement of those lost during exercise.

We have observed anecdotally that diets fed to endurance horses are often higher in forage and lower in concentrate feedstuffs than those provided to other equine athletes, yet no research has been conducted on the effect of such diets on sweat composition and total electrolyte loss. While electrolyte supplementation is often recommended for horses competing in endurance events, commonly such supplementation is typically much less than the animals are observed to lose during the course of exercise. Still, other horses may compete without any such supplementation.

The objective of this study, then, was to investigate sweat composition in horses performing endurance exercise consuming low Na (less than 10% of NRC (2007) recommendation for heavily exercising horses) forage-based rations.

Materials and methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conflicts of interest
  8. Manufacturers' addresses
  9. References

All studies were approved by the Institutional Animal Care and Use Committee at Michigan State University. Six 2-year-old Arabian horses were conditioned to treadmill exercise for a 12 week period. Training consisted of walking (1.6 m/s), trotting (4 m/s) and cantering (8 m/s) on the treadmill at increasing distances thrice weekly until the target of 60 km of endurance exercise per training bout could be met. During the initial training period, all animals had ad libitum access to a spring mixed-grass pasture. No supplemental feeds or minerals (including NaCl) were provided at any time during the study.

After completion of a preliminary exercise test, horses were randomly assigned to a replicated 3 × 3 Latin square study, designed for a concurrent study examining the influence of 3 forage-fibre diets on hydration status, where each horse consumed each treatment diet for at least 14 days before completing a 60 km exercise test. The treatment diets consisted of an all grass hay diet (G), a 50:50 grass hay:alfalfa hay (GA), and 50:50 grass hay:chopped fibre mix (GM) containing chopped alfalfa and timothy hay plus soyahulls, beet pulp and 10% oil. The chopped fibre mix (Research fibre mix)1 was formulated to contain fibre types considered to be highly soluble and highly fermentable. Horses were initially offered 2.5% bodyweight daily, with the amount offered reduced gradually to be just above voluntary intake from the previous feeding so as to reduce sorting and ensure a near 50:50 ratio in the GA and GM diets. Feed was offered twice daily for 4 h. Feed analyses were performed by Equi-Analytical (Ithaca, New York) using standard wet chemistry methods.

The 60 km exercise test used in this study and previously reported (Dusterdieck et al. 1999) was designed to replicate a competitive endurance ride. The test was divided into four 15 km bouts, each lasting 54 min and consisting of alternating walk (1.6 m/s), trot (4 m/s), and canter (8 m/s). Horses were allowed a 2 min rest period and offered water half-way through each bout. After completing the first and third bouts, horses were given a 20 min rest and water break; with a 1 h break with access to water and any remaining morning feed following bout 2.

Horses were instrumented with a Swan-Ganz catheter inserted via the jugular vein and passed into the pulmonary artery for determination of core body temperature and mixed venous blood for a related project previously reported (Spooner et al. 2007). Body mass and core body temperature were also measured and recorded at the initiation and completion of each exercise bout. Core body temperature was recorded at consistent times during the trot and canter of the second half of each exercise bout.

Water intake during the test was recorded. Body mass loss was reported 2 ways. Uncorrected body mass loss was determined as the difference between the starting and ending weight divided by the start value. Body mass loss was also corrected for faecal, urine loss plus water intake during the test and reported as corrected loss. This corrected loss would be equivalent to total fluid loss by the animal during the exercise bout through respiratory and sweat losses, independent of replacement.

Sweat was collected into a sterile syringe immediately following each exercise bout via 2 resealable pouches located bilaterally on the dorsal thorax, as previously described (McCutcheon et al. 1995b). Samples from each pouch were combined into a single aliquot per bout. Sweat was immediately analysed for pH, Na, K, Cl, Ca and Mg concentration with an ion-selective analyser (Stat Profile M)2. Osmolality (Osm) was determined using freezing-point depression (Model 3MO plus Advanced Micro-Osmometer)3 .

All data are presented as mean ± s.e. For the replicated Latin square experiment, changes over time were assessed by 2-way ANOVA, with repeated measures when appropriate. Diet differences were further separated using Tukey's mean separation test. Significance was defined as P<0.05, while trends were considered when P<0.10.

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conflicts of interest
  8. Manufacturers' addresses
  9. References

Diet analyses are provided in Table 1. Feed intake (% BM) was different by diet (P<0.01); GA (2.25 ± 0.07%) was greater than both G (1.91 ± 0.07%) and GM (2.01 ± 0.07%). Intake (g/day) of Cl (72 ± 6.76) and Mg (18 ± 1.67) were not different between diets; Na was greatest (P<0.05) in GM (2.53 ± 0.25), intermediate in G (2.21 ± 0.22) and lowest in GA (1.90 ± 0.20). K and Ca intakes (g/day) were greater (P<0.05) on GA (246 ± 14.1; 101 ± 5.79) than G (176 ± 17.6; 59 ± 5.85) or GM (168 ± 15.1; 62 ± 6.09).

Table 1. Analysis of diets fed to 2-year-old Arabian horses to investigate the role of dietary fibre type on hydration status during prolonged endurance exercise (on a DM basis)
 Grass Hay (G)Grass:alfalfa (GA)Grass:fibre mix (GM)
Dry matter (%)92.591.892.0
Crude protein (%)14.618.112.8
Lignin (%)6.06.97.6
ADF (%)34.533.037.1
NDF (%)49.944.550.4
NFC (%)24.026.622.3
Crude fat (%)2.92.97.7
Ash (%)9.110.27.2
Ca (%)0.771.120.76
P (%)0.300.280.22
Mg (%)0.210.230.21
K (%)2.312.721.77
Na (%)0.0290.0210.032
Estimated DE (mcal/kg)2.292.432.59

As previously reported (Spooner et al. 2007), differences in body mass due to diet were not present at the start of the exercise bout. Uncorrected body mass loss was 2.7 ± 0.5% and was not different by diet (P = 0.25). When corrected for faecal and urine losses and water intake, BM loss (as a representation of respiratory and sweat fluid losses) showed a trend for diet difference during exercise (P = 0.08), decreasing more in GM than G (5.1 ± 0.4% vs. 3.4 ± 0.4%; GA 4.2 ± 0.4%).

Water intake during the course of the exercise test was not different by diet (P = 0.8), but was greater during the second half of the test than the first half (7.8 ± 0.7 l, 4.2 ± 0.7 l, respectively). Water intake during the duration of the exercise test averaged 11.4 ± 1.5 l.

There was no effect of diet or exercise bout on sweat pH (7.65 ± 0.04) or Cl (133 ± 7 mmol/l). Only Na and Osm exhibited a diet difference (Table 2). Differences by exercise test bout were observed for K, Ca, and Mg. Potassium decreased (P<0.05) from 53.9 ± 2.7 mmol/l after the completion of bout 1 (first 15 km) to 45.1 ± 3.0 mmol/l after bout 2 and remained unchanged through the remainder of the test. Similarly, Ca decreased (P<0.001) from 12.0 ± 1.0 mmol/l after bout 1 to 8.5 ± 1.1 mmol/l after bout 2 and remained unchanged through the remainder of the test. Finally, Mg decreased (P<0.001) from 3.3 ± 0.2 mmol/l after bout 1 to 2.3 ± 0.2 mmo/l to after bout 2, further decreasing to 1.6 ± 0.2 mmol/l at the conclusion of the test (after bout 4).

Table 2. Sweat sodium, potassium, chloride, calcium and magnesium concentration and osmolality (Osm) in 2-year-old Arabian horses completing 60 km of endurance exercise, fed either grass hay (G), grass hay:alfalfa hay (GA), or grass Hay:chopped fibre mix (GM)
 GGAGMP value for diet difference
  • a,b

    Values within a row lacking common superscripts differ.

Na (mmol/l)88 ± 5a104 ± 5b96 ± 6a,bP<0.05
K (mmol/l)46 ± 347 ± 348 ± 3P = 0.95
Cl (mmol/l)131 ± 6143 + 7140 + 7P = 0.12
Ca (mmol/l)9.5 ± 18.1 ± 18.8 ± 1P = 0.40
Mg (mmol/l)2.4 ± 0.22.2 ± 0.22.4 ± 0.2P = 0.56
Osm267 ± 16a303 ± 16b266 ± 17aP<0.05

Corrected body mass loss was used to estimate total electrolyte losses during the exercise test, assuming sweat losses represent 80% of corrected body mass loss (Kingston et al. 1997b) and electrolyte-free respiratory loss 20%. Utilising such estimation, Na loss was lower (P<0.05) in diet G (26.1 ± 1.3 g) than GA (29.8 ± 1.3 g) or GM (33.2 ± 1.4 g). Similarly, both K and Cl were lowest (P<0.05) in G (19.6 ± 2.3 g; 50.8 ± 6.3 g), intermediate in GA (24.8 ± 2.3 g; 68.6 ± 6.3 g) and greatest in GM (30.8 ± 2.4 g; 81.5 ± 6.5 g). Both Ca and Mg were lower (P<0.05) in G (4.2 ± 0.4 g; 0.64 ± 0.07 g) and GA (4.4 ± 0.4 g; 0.72 ± 0.07 g) than GM (5.8 ± 0.4 g; 0.95 ± 0.08 g).

Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conflicts of interest
  8. Manufacturers' addresses
  9. References

No adverse effects of diet or signs of illness were observed in any horse at any time. One horse failed to complete an exercise test in period 4 as a result of lameness in a front hoof. Given that these horses had an average body mass of 366 kg, dietary intakes of K, Ca and Mg were at, or greater than, NRC recommendations for all diets, even if recommendations for heavily exercising horses are considered. Dietary Na, at 2.5 ± 0.4 g, was only 6.6% of NRC recommendations (38 g/day) for heavily exercising horses at this bodyweight with the sweat loss observed. Dietary Cl was also lower than NRC recommendations (72 g/day vs. 82 g/day).

As previously reported, horses consuming GM, while having a tendency for greater total body water than horses consuming G, tended to lose more body mass during the exercise bout. We have speculated that greater total body water may allow for greater thermoregulation via sweating, thus resulting in increased body mass loss. This is also supported by the fact that core body temperature at the canter remained lower in GM (Spooner et al. 2007).

In terms of sweat constituents, however, we saw little difference when comparing diets to one another. The only diet differences reported were for Na and Osm. Na concentration and total Na loss was significantly greater in GA than G. This is surprising considering dietary intake of Na was less on GA than G or GM, although total feed intake was greater. Still, it is important to remember equine sweat does not typically become progressively more hypotonic even as exercise progresses, as the sweat gland shows little ability to conserve sodium regardless of body stores. This has been previously demonstrated by Lindner et al. (1983), who demonstrated only minimal reduction in sweat Na concentration in ponies subjected to total body sodium depletion, despite a 75% reduction in ingesta Na content compared to sodium-replete controls. Further, Jansson et al. (2002) demonstrated no effect of acute aldosterone administration on sweat composition, although such administration resulted in reduction in faecal and urinary Na loss. Osmolality was also greater in GA than G or GM. Given that Na is a primary contributor to osmolality, it is not surprising a difference in Na may also result in a difference in Osm.

Alternatively, when discussing the dietary differences observed, we must consider crude protein (CP) differences between diets. Connysson et al. (2006) have reported increased evaporative losses (combined respiratory and sweat losses), water intake, and fecal and urinary water loss in exercising horses consuming a high protein (16.6% CP) diet compared to control (12.5% CP). While, in our study, the higher protein GA diet resulted in greater Na loss and sweat Osm, no difference in water intake or BM loss was observed, making it unlikely that the reported dietary differences are entirely attributable to greater CP intake.

It is also important to note the difference in oil concentration in our diets, the GM diet contained more than twice the crude fat of the other diets. Few studies have investigated the effect of fat supplementation on fluid and electrolyte status. Hoyt et al. (1995) supplemented Thoroughbred horses with 10% dietary fat and found no difference in fluid or electrolyte balance. Sweat production and composition was also unchanged during a SET. A similar study also showed no change in electrolyte balance or aldosterone response to fat-supplemented horses exposed to exercise in a hot environment (Hower et al. 1995). Our results are in agreement with these studies in that we saw no differences in sweat electrolyte concentration attributable to the GM diet.

At the same time, then, dietary differences observed in total electrolyte loss over the course of the exercise bout are more attributable to differences in body mass loss than differences in sweat electrolyte concentration. The estimated electrolyte losses averaging more than 30 g Na, 25 g K and 66 g Cl are substantial for a single bout of exercise, although less than those calculated for horses exercising for 3 h in hot ambient conditions (100 g Na, 45 g K and 198 g Cl; McCutcheon and Geor 1996). This further supports the fact that the horse exhibits a priority for thermoregulation over water and electrolyte balance, as evidenced by heavy sweating rates at the expense of fluid, electrolyte, and body mass loss. Yet, this also demonstrates the tremendous influence ambient conditions have on sweat rate and resulting electrolyte loss. In this trial, our total body mass losses were less than 3% of bodyweight, less than may be experienced in a field setting particularly under hot or humid conditions, and thus total electrolyte loss was less than might be expected in a field setting.

Exercise bout had a significant effect on K, Ca and Mg. Concentrations decreased early on in the exercise test, with significant changes occurring from after bout 1 (the completion of 15 km) to after bout 2 (completion of 30 km); Mg further decreased by the end of the exercise bout. Both sweat Ca and Mg concentrations have previously been shown to decrease over the course of an exercise bout (McCutcheon et al. 1995b). Comparing Ca and Mg to values reported by McCutcheon et al. (1995b), Ca values obtained here are greater (6.6–12 mmol/l vs. 4.8 mmol/l), while Mg is quite similar (1.6–3.3 mmol/l vs. 3.3 mmol/l).

Comparing the sweat concentrations of Na, K, and Cl to those previously reported (Table 3), the Na concentration we observed (88–104 mmol/l depending on diet) in this study was clearly lower than the previously reported range (range 110–249). This may be the result of a difference in collection method, as some of the previous studies technique may have allowed sweat to evaporate which would have falsely increased the concentrations reported (McCutcheon et al. 1995b). Yet, our Na concentration is also lower than that reported by McCutcheon et al. (1995b, 116.7 mmol/l) utilising the same collection technique and under similar ambient conditions. Concentration of K reported here is similar to that reported previously (range 28–53.1 mmol/l), while Cl is slightly lower (133 mmol/l vs. 140–301 mmol/l).

Table 3. Previously reported Na, K, Cl composition (mmol/l) in equine sweat collected from exercising horses
ReferenceConditionsNa+K+Cl−
(McConaghy et al. 1995b) 15939.6194
(Carlson and Ocen 1979) 13253.1174
(McConaghy et al. 1995a) 14437.5182
(Snow et al. 1982) 15932165
(Geor and McCutcheon 1998)Cool dry13928154
Hot dry16733181
(Jansson et al. 1995) 16545 
(Kingston et al. 1997a) 11030–36140
(Rose et al. 1980) 24948301
(Kerr and Snow 1983) 17049200

Feeding Na at only 6.6% of NRC requirements for heavily exercising horses did not result in any apparent negative impact during the 6 month training and experimental period. However, horses were not exercising heavily on a daily basis, completing the exercise test only one time every 14 days, with 2 additional days of moderate exercise. Therefore, this level of recommendation may be above actual requirements for such horses, particularly when compared to estimated electrolyte losses. Sodium balance, however, was not evaluated in this trial. Meyer et al. (1984) reported a transient negative Na balance and clinical signs of dehydration in ponies receiving 5 mg/kg bwt/day, yet over a 30 day period data indicated a positive sodium balance with only 1.6 mg/kg bwt/day, the result of a marked reduction in faecal and urinary Na excretion. For comparison, our horses were receiving 6.8 mg/kg bwt/day. Therefore, while we cannot guarantee our horses were not in a negative sodium balance at some point during the trial, it may be that over the course of the trial, they were able to adapt. Such adaptation, in addition to possible alterations in faecal and urinary Na loss, may have included the lower concentration of Na in sweat observed here.

It may also be that the differences observed in regard to sweat concentration are representative of breed differences. The Arabian horse is long recognised as the most endurance capable horse breed, largely as a result of its predominance of type 1 muscle fibres (Lopez-Rivero et al. 1990). However, having evolved in an arid-environment, perhaps the breed possesses additional advantages in terms of thermoregulation, such as greater adaptation to low-electrolyte diets, or morphological changes in the hindgut altering electrolyte absorption. The majority of research evaluating sweat constituents to date has been conducted utilising Standardbred or Thoroughbred horses and we were unable to obtain any previous data specific to the Arabian. However, the arid-adapted donkey possesses known adaptations to dehydration, including morphological changes in the hindgut and enhanced fluid retention (Sneddon et al. 2006). Therefore, further research should evaluate the potential effect of breed on sweat constituents and electrolyte loss during prolonged endurance exercise.

In all, while some differences in sweat constituents were observed as a result of differences in diet, more significantly, low dietary Na intake may have resulted in lower sweat Na concentration than those previously reported. Still, care should be used in applying this research to horses under field conditions. Additional research should be conducted to further evaluate the effect of diet on sweat constituents and electrolyte loss.

Manufacturers' addresses

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conflicts of interest
  8. Manufacturers' addresses
  9. References

1 Waltham Center for Pet Nutrition, Leics, UK.

2 Nova Biomedical, Waltham, Massachusetts, USA.

3 Advanced Instruments, Norwood, Massachusetts, USA.

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conflicts of interest
  8. Manufacturers' addresses
  9. References
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