Rehydration fluid temperature affects voluntary drinking in horses dehydrated by furosemide administration and endurance exercise.

Vet J. 2004 Jan;167(1):72-80

Butudom P, Barnes DJ, Davis MW, Nielsen BD, Eberhart SW, Schott HC 2nd.

Department of Large Animal Clinical Sciences, College of Veterinary Medicine, D-202 Veterinary Medical Center, Michigan State University, East Lansing, MI 48824-1314, USA.

To determine whether temperature of rehydration fluid influences voluntary rehydration by horses, six 2-3-year-old horses were dehydrated (4-5% body weight loss) by a combination of furosemide administration and 30 km of treadmill exercise.

For the initial 5 min following exercise, horses were offered a 0.9% NaCl solution at 10, 20, or 30 degrees C. Subsequently, after washing and cooling out, voluntary intake of water at 10, 20, or 30 degrees C from 20 to 60 min after exercise was measured. Fluid intake (FI) during the first 5 min of recovery was 9.8+/-2.5,12.3+/-2.1 and 9.7+/-2.0L (p>0.05) for saline at 10, 20, and 30 degrees C, respectively.

Although not a significant finding, horses offered 0.9% NaCl at 20 degrees C tended to take fewer (p=0.07), longer drinks than when saline at either 10 or 30 degrees C was offered. Between 20 and 60 min of recovery, intake of water at 20 degrees C (7.7+/-0.8L) and 30 degrees C (6.6+/-1.2L) was greater (p<0.05) than that at 10 degrees C (4.9+/-0.5L). Thus, total FI was 14.7+/-2.5,19.9+/-2.5, and 16.3+/-2.4L for rehydration fluids at 10, 20, and 30 degrees C, respectively (p<0.05, value for 20 degrees C water greater than that for 10 degrees C water).

Although the amount of metabolic heat transferred to the initial saline drink was correlated with the decrease in core temperature during the initial 5 min of recovery, heat transfer to ingested fluid was most likely responsible for the dissipation of, at most, 5% of the heat generated during endurance exercise. In conclusion, following exercise these dehydrated-normothermic horses voluntary drank the greatest amount of fluid at near ambient (20 degrees C) temperature. Although not determined in this study, greater satiation of thirst by oropharyngeal cooling may have contributed to lesser intake of colder (10 degrees C) fluid.

PMID: 14623154 [PubMed – indexed for MEDLINE]

“However, both human and equine athletes fail to replace completely body fluid losses by voluntary drinking during the first few hours of the recovery period, despite free access to various rehydration solutions…

However, both human and equine athletes fail to replace completely body fluid losses by voluntary drinking during the first few hours of the recovery period, despite free access to various rehydration solutions……

However, both human and equine athletes fail to replace completely body fluid losses by voluntary drinking during the first few hours of the recovery period, despite free access to various rehydration solutions…

The influence of water temperature has been fairly well documented in human athletes: following exercise, they both prefer and drink greater volumes of cool water, in comparison to water at temperatures at or above ambient temperature ( [Boulze et al., 1983]; [Sandick et al., 1984]; [Szlyk et al., 1989]). In addition, cool or cold water ingested during and after exercise can act as a heat sink and lower core temperature ( [Wimer et al., 1997])….

2.4. Dissipation of metabolic heat by drinking

In an attempt to determine the heat dissipating effects of the initial period of drinking 0.9% NaCl at 10, 20, or 30 °C, the gain in heat content of the ingested fluid was estimated by assuming that the specific heat of 0.9% NaCl was the same as that of water (1.0 kcal/L/°C) and that all fluid drank was warmed to 38 °C after 5 min of recovery. This heat gain by ingested fluid would be equal to the loss of metabolic heat (MH) by the horse. Next, the total MH produced during the two 15 km exercise bouts was estimated to determine the fraction that could be dissipated into the ingested salt water. MH was estimated by the following formula:

MH=oxygenconsumption (mL/min/kg)×BW (kg)×k (kcal/LO2)×duration (min),

where k, a constant for the amount of heat liberated per litre of O2 consumed, was assumed to be 4 kcal/L O2 ([Hodgson et al., 1993]). Horses were assumed to have a maximal oxygen consumption of 120 mL/kg/min and to work at an average load of 30% of maximal oxygen consumption during the two 52 min exercise bouts. Finally, the role of heat transfer in cooling was evaluated by comparing the decline in Tcore from the end of exercise to 5 min of recovery to the gain in heat content of the ingested fluid….

Mean Tcore ranged from 38.8±0.2 to 39.0±0.2, from 39.1±0.2 to 39.5±0.2, and from 38.2±0.2 to 38.3±0.2 °C during the last canter phases of the first and second exercise bouts and after 1 min of recovery, respectively. Although Tcore increased (p<0.05) from the pre-exercise values during the canter phases of both exercise bouts, values after 1 min of recovery had returned to the pre-exercise values and no differences between treatments were observed (Fig. 4). Depending on both temperature and volume consumed, estimated MH transferred to the fluid imbibed during the initial 5 min of recovery ranged from 48 to 532 kcal. In addition, there was a significant positive linear correlation between the change in Tcore from 0 to 5 min of recovery and the amount of MH transferred to the ingested fluid (Fig. 5). However, exercise at an intensity of 30% of maximal O2 consumption for 104 min produced an estimated MH load ranging from not, vert, similar4300 to not, vert, similar6900 kcal, depending on the BW of the horse. Thus, dissipation of heat by warming of fluid imbibed from 0 to 5 min of recovery accounted for a maximum of not, vert, similar8% of the total MH generated in one horse while in most horses it was responsible for dissipation of less than 5% of the total MH generated during exercise…

As in our previous experiment in which the composition of the initial rehydration solution was varied ([Butudom et al., 2002]), the mean volume of 0.9% NaCl drank by horses in this study during the first 5 min of recovery did not exceed not, vert, similar12 L, irrespective of temperature. This particularly striking finding suggests that neither composition nor temperature of the rehydration solution is the most important factor in determining initial fluid intake. Rather, dehydrated horses appeared to drink until satiated by mechanisms largely unrelated to fluid composition or temperature. Gastric filling has been suggested to be an important factor in satiation of thirst and cessation of drinking in other species ([Towbin, 1949]; [Engstrom and Deaux, 1974]). Indeed, the volumes drunk by these horses were similar to the reported capacity of the equine stomach ( [Pfieffer and MacPherson, 1990]). The suggestion that gastric filling contributes to initial satiation is further supported by the observation that the majority of initial fluid consumption occurred within the first 2 min after completing the exercise bout……

In contrast to laboratory animals that tend to prefer and drink the greatest amount of water at 25–35 °C, in a thermoneutral environment ([Carlisle, 1977]), resting humans appear to prefer water at 20 °C ([Boulze et al., 1983]). This preference for cooler water by adult humans may be a learned behaviour because it is not observed in newborns ( [Boulze et al., 1983]). However, human athletes that become both dehydrated and hyperthermic show a preference for water at 5–15 °C ([Boulze et al., 1983]; [Sandick et al., 1984]; [Szlyk et al., 1989]). Although colder water is preferred following exercise, offering a progressively colder drink (<10 °C) can actually decrease volume consumed ([Boulze et al., 1983]). As a result, hyperthermia, rather than dehydration, has been suggested to be a more important mechanism for the preference for colder water ( [Boulze et al., 1983]) while oropharyngeal cooling has been advanced as a mechanism of greater thirst satiation and decreased intake of colder water ( [Gold et al., 1973]; [Ramsauer et al., 1974]; [Carlisle, 1977]). Further, gastric emptying is slower for cold solutions than for warm solutions ( [Deaux, 1973]). Thus, intake of cold water could prolong satiation by slowing gastric emptying rate. Core temperature of horses in the present study at the end of exercise was not increased from pre-exercise values; thus, lack of hyperthermia could potentially explain why our horses failed to drink greater volumes of fluid at 10 °C. Further, drinking fluid at 10 °C could have produced greater oropharyngeal cooling and may have contributed to more rapid satiation and less intake of fluid at this temperature (supported by the tendency to take shorter drinks of the 10 °C fluid). Although equine endurance athletes cannot rate a fluid preference like their human counterparts, our data suggest that, like dehydrated-normothermic human athletes, dehydrated-normothermic equine athletes exercising under moderate environmental conditions may prefer to drink rehydration fluids at near ambient temperature (20 °C).