Hess et al. (1) conducted an interesting supplementation trial on horses during an endurance race with an international group*. They found that electrolytes with high Sodium and without Potassium, combined with a higher Calcium feed were advantageous to the horses and potentially decreases pull rates.
Clinical signs that are typically seen in horses when they are eliminated during endurance events are associated with increased neuromuscular excitability (which depends on membrane potentials), including slower heart rate recoveries, arrhythmia, muscle cramps, changes in intestinal motility and synchronous diaphragmatic flutter. These changes are related to the Potassium and Calcium levels in the horse’s blood.
The potassium in electrolytes may result in clinical signs related to neuromuscular excitability. During endurance exercise (speeds more than 4m/s) plasma Potassium levels increase, and they hypothesize that an increase in Potassium supplementation could be altering cellular membrane potentials. (Note: A recovery formula with potassium was supplied to the horses that were not supplemented with Potassium during the race and no differences in Potassium levels were found upon recovery.)
Plasma Calcium levels decrease with endurance exercise and horses generally become alkalotic. Diets typically fed to endurance horses have a high DCAB (from high amount of roughage), which may further reduce plasma Calcium levels. Calcium supplementation may help prevent hypocalcaemia.
Sodium supplementation is important in endurance racing as electrolyte losses through sweat can reduce the plasma volume and decrease thirst. Sodium losses are directly related to dehydration and its supplementation helps to restore thirst as it prevents a reduction in plasma osmolality.
The horses given the Potassium-free, Sodium-rich electrolytes had the better hydration rates both throughout and following the race.
Interestingly, for the 80 mile ride they found significant differences already at 27 km between the horses that completed and those eliminated at any point during the ride. Eliminated horses generally had a lower PCO2 (perhaps due to thermoregulation), higher plasma Potassium and lower Calcium concentrations, all of which can lead to increased neuromuscular excitability. Horses that were eliminated at this point had both higher Potassium and lower Calcium concentrations, which may have directly caused arrhythmia (as manifestations of neuromuscular hyperexcitability) that were seen.
At 48 km, the eliminated horses showed a trend towards lower chloride concentrations. Two horses at this point were eliminated for failure to recover heart rate, and one for colic. Clinically these signs can be related to dehydration. Low plasma concentrations of Chloride are related to dehydration through sweat loss.
Although there were no clinical signs related to increased neuromuscular excitability in any of the successful finishers, those without Potassium supplementation showed lower plasma Potassium concentrations, which could help maintain membrane potential and reduce neuromuscular excitability. Another thing to note is that with all groups on electrolyte supplementation, deficits in Sodium Chloride were still seen following the race. The study that attempted to formulate a total recovery formula but both hypernatraemia and hyperchloraemia were induced. But both of those formulas included Potassium supplementation. The ideal amounts of electrolytes for supplementation for endurance racing has not been determined, and an increase of Sodium supplementation without Potassium needs to be researched.
1. Potassium-free electrolytes and calcium supplementation in an endurance race
Comparative Exercise Physiology 2008, 5(1); 33–41
TM Hess, KM Greiwe-Crandell, JE Waldron, CA Williams, MA Lopes, LS Gay, PA Harris and DS Kronfeld
(group includes researchers from *Virginia Polytechnic State University, Colorado State University, Rectortown Equine Clinic, Rutgers University, Universidade Federal de Vic in Brazil, and the Equine Studies Group, Waltham Centre for Pet Nutrition, in the UK.)
Some of the clinical signs seen in horses during endurance races may result from increases in neuromuscular excitability and are related to plasma [Kþ] and [Caþþ]. The present study aimed to test the following hypotheses:
(1) Potassium supplementation will affect plasma [Kþ] and may result in clinical signs related to neuromuscular hyperexcitability during an 80km endurance ride.
(2) Plasma [Caþþ] will reflect dietary cation–anion balance (DCAB) and calcium intake. Feeding with a high DCAB and high dietary calcium content (1.5% total calcium of daily ration) diets would lead to higher plasma [Caþþ] during an endurance race than on feeding high DCAB diets with a moderate dietary calcium content (1% of total calcium of daily ration).
The current study was undertaken during the 80 km endurance research ride in 2002 in Virginia, USA. Forty volunteer rider–horse pairs participated in the race. During the race, electrolyte mixtures with (EM þ K) and without (EM 2 K) potassium were supplied to 18 and 22 horses, respectively. After the race, the horses receiving EM 2 K during the race were supplied with a recovery formula containing potassium (EM-REC). The horses were fed in addition to their own forage (hay and pasture) either their own commercial concentrate (CC; 1% calcium, n ¼ 11) or one of two research-supplied concentrates during 3 months preceding the research ride, one concentrate rich in sugar and starch (SS; 2% calcium, n ¼ 15) and the other rich in fat and fibre (FF; 2% calcium, n ¼ 14). Peripheral blood samples were taken the day before, within 3 min of the arrival at the vet checks at 27, 48 and 80 km, and after 3 h of recovery. Plasma samples were analysed for pH, haematocrit (Hct), [Naþ], [Kþ], [Cl2], [Caþþ], [Mgþþ], total protein (TP) and albumin [alb]. Effects of sampling times, treatments and interactions were evaluated by ANOVA in a mixed model with repeated measures and applied to the 25 horses that completed 80 km. Eliminated horses had their blood sampled before entering the elimination vet check and 3 h after elimination, and were compared with finishing horses by t-test. As the ride progressed, significant increases were found in plasma pH, [Naþ], ½PO2 4 , [TP], [alb], Hct and osmolality; and decreases in [Kþ], [Mgþþ], PCO2, [Caþþ] and [Cl2]. Horses supplied with potassium-free, sodium-rich electrolyte formulae (EM 2 K) had 12.5% lower (P ¼ 0.001) mean plasma [Kþ], 7.8% lower (P ¼ 0.024) TP and 8.4% lower (P ¼ 0.004) albumin at 80 km, and at 3 h after the race they had 6.8% lower (P ¼ 0.045) TP, when compared with EM þ K supplemented horses. Horses fed with SS and FF had higher [Caþþ] at 27 (P ¼ 0.027), 56 (P ¼ 0.006) and 80km (P ¼ 0.022) when compared with horses fed with CC. The lower [Kþ] in the EM 2 K group, and the higher [Caþþ] in the SS- and FF-supplemented horses may help prevent increases in neuromuscular excitability and related clinical signs. The lower TP and albumin indicate less dehydration in the EM 2 K group and could help prevent related disorders.