Plasma volume, osmolarity, total protein and electrolytes during treadmill running and cycle ergometer exercise

While haemoconcentration due to loss of plasma volume is well established during cycling, the existence of similar changes during running remains contentious. This study compared the changes in plasma volume and associated blood indices during 60 min of running and cycling at the same relative intensity (approximately 65% VO2max), with all changes referenced to blood indices obtained after 30 min seated at rest on a cycle ergometer. Plasma osmolarity increased similarly with both forms of exercise but was less than predicted for water loss alone, such that there was a net loss of sodium during exercise and of potassium postexercise, with essentially no loss of protein. Plasma volume decreased similarly (approximately 6.5%) in both exercise trials, but while that with cycling was initiated by exercise itself and was essentially maximal within 5 min, the reduction in plasma volume in the running trial was induced by adopting the upright posture and was complete before exercise began. These data would indicate that different mechanisms are responsible for the changes in plasma volume induced by running and cycling, while the similarity of change would suggest that there is a lower limit to any reduction in plasma volume, regardless of mechanism. Furthermore, the observation that the changes in plasma volume were complete before or early in exercise, would imply that oral water ingestion during prolonged exercise, which is essential for thermoregulation, may be more concerned with homeostasis of extravascular water rather than plasma volume.     

Effect of blood volume on plasma volume shift during exercise

To assess the relationship between blood volume (BV) and the reduction in plasma volume (PV) during exercise in individual variations, we measured BV and changes in PV in thirteen male volunteers during treadmill exercise until exhaustion. The lactate threshold (LT), as a predictor of aerobic exercise capacity, was calculated from the exercise intensity at the point of plasma lactate concentration buildup to 4 mmol. The relationship of peak VO₂ with BV indicated a significant positive correlation. The strong positive relation between the shifts in PV and total PV, and resulted in a maintenance of the circulating BV.     

A comparison of hydration effect on body fluid and temperature regulation between Malaysian and Japanese males exercising at mild dehydration in humid heat

Background

This study investigated the effect of hydration differences on body fluid and temperature regulation between tropical and temperate indigenes exercising in the heat.

Methods

Ten Japanese and ten Malaysian males with matched physical characteristics (height, body weight, and peak oxygen consumption) participated in this study. Participants performed exercise for 60 min at 55% peak oxygen uptake followed by a 30-min recovery at 32°C and 70% relative air humidity with hydration (4 times each, 3 mL per kg body weight, 37°C) or without hydration. Rectal temperature, skin temperature, heart rate, skin blood flow, and blood pressure were measured continuously. The percentage of body weight loss and total sweat loss were calculated from body weight measurements. The percentage change in plasma volume was estimated from hemoglobin concentration and hematocrit.

Results

Malaysian participants had a significantly lower rectal temperature, a smaller reduction in plasma volume, and a lower heart rate in the hydrated condition than in the non-hydrated condition at the end of exercise (P <0.05), whereas Japanese participants showed no difference between the two hydration conditions. Hydration induced a greater total sweat loss in both groups (P <0.05), and the percentage of body weight loss in hydrated Malaysians was significantly less than in hydrated Japanese (P <0.05). A significant interaction between groups and hydration conditions was observed for the percentage of mean cutaneous vascular conductance during exercise relative to baseline (P <0.05).

Conclusions

The smaller reduction in plasma volume and percentage body weight loss in hydrated Malaysians indicated an advantage in body fluid regulation. This may enable Malaysians to reserve more blood for circulation and heat dissipation and thereby maintain lower rectal temperatures in a hydrated condition.     

Effect of freezing and dehydration on ion and cryoprotectant distribution and hemolymph volume in the goldenrod gall fly, Eurosta solidaginis

Extracellular freezing and dehydration concentrate hemolymph solutes, which can lead to cellular injury due to excessive water loss. Freeze tolerant larvae of the goldenrod gall fly, Eurosta solidaginis, may experience extreme cold and desiccation in winter. To determine whether larvae employ protective mechanisms against excessive cellular water loss we examined the effect of extracellular freezing and dehydration on hemolymph volume, and cryoprotectant and ion levels in the hemolymph. Dehydrated larvae or ones that had been frozen at −5 or −20 °C had a significantly smaller proportion of their body water as hemolymph (26.0-27.4%) compared to controls (30.5%). Even with this reduction in water content, hemolymph osmolality was similar or only slightly higher in frozen or dehydrated individuals than controls (908 mOsm kg⁻¹), indicating these stresses led to a reduction in hemolymph solutes. Hemolymph and intracellular content of ions remained largely unchanged between treatment groups; although levels of Mg⁺⁺ in the hemolymph were lower in larvae subjected to freezing (0.21 ± 0.01 μg mg⁻¹ dry mass) compared to controls (0.29 ± 0.01 μg mg⁻¹ dry mass), while intracellular levels of K⁺ were lower in groups exposed to low temperature (8.31 ± 0.21 μg mg⁻¹ dry mass). Whole body glycerol and sorbitol content was similar among all treatment groups, averaging 432 ± 25 mOsm kg⁻¹ and 549 ± 78 mOsm kg⁻¹ respectively. However, larvae subjected to dehydration and freezing at −20 °C had a much lower relative amount of cryoprotectants in their hemolymph (∼35%) compared to controls (54%) suggesting these solutes moved into intracellular compartments during these stresses. The correlation between reduced hemolymph volume (i.e. increased cellular water content) and intracellular movement of cryoprotectants may represent a link between tolerance of dehydration and cold in this species.     

The influence of the initial state of hydration on endocrine responses to exercise in the heat

This study examines the effect of the initial state of hydration on hormone responses to prolonged exercise in the heat. Five subjects at two initial hydration levels (hypohydrated and hyperhydrated) were exposed to a 36 degrees C environment for 3 h of intermittent exercise. During exercise, the subjects were either fluid-deprived, or rehydrated with water or an isotonic electrolyte sucrose solution (ISO). Both the stress hormones, adrenocorticotropic hormone and cortisol, and the main fluid regulatory hormones, aldosterone, renin activity (PRA) and arginine vasopressin (AVP), were measured in blood samples taken every hour. Prior hyperhydration significantly reduced initial AVP, aldosterone and PRA levels. However, except for AVP, which responded to exercise significantly less in previously hyperhydrated subjects (p less than 0.05), the initial hydration state did not influence the subsequent vascular and hormonal responses when the subjects were fluid-deprived while exercising. Concurrent rehydration, either with water or with ISO, reduced or even abolished the hormonal responses. There were no significant differences according to the initial hydration state, except for PRA responses, which were significantly lower (p less than 0.01) in previously hyperhydrated subjects who also received water during exercise. These results indicate that prior hydration levels influence only slightly the hormonal responses to prolonged exercise in the heat. Progressive rehydration during exercise, especially when extra electrolytes are given, is more efficient in maintaining plasma volume and osmolarity and in reducing the hormonal responses.     

Plasma volume in acute hypoxia: comparison of a carbon monoxide rebreathing method and dye dilution with Evans'' blue

Exposure to acute hypoxia is associated with changes in body fluid homeostasis and plasma volume (PV). This study compared a dye dilution technique using Evans' blue (PV_Evans') with a carbon monoxide (CO) rebreathing method (PV_CO) for measurements of PV in ten normal subjects at sea level and again 24 h after rapid passive ascent to high altitude (4,350 m). Hypobaric hypoxia decreased arterial oxygen saturation to 79 (74–83)% (mean with 95% confidence intervals). The PV_Evans' remained unchanged from 3.49 (3.30–3.68) l at sea level to 3.46 (3.24–3.68) l at high altitude. In contrast PV_CO decreased from 3.39 (3.17–3.61) l at sea level to 3.04 (2.75–3.33) l at high altitude (P < 0.05). Compared with sea level, this resulted in an increase of the mean bias between the two methods [from 0.11 (−0.05–0.27) l at sea level to 0.43 (0.26–0.60) l at high altitude] so that the ratio between PV_Evans' and PV_CO increased from 1.04 (0.99–1.09) at sea level to 1.15 (1.06–1.24) at high altitude (P < 0.05). In conclusion, the two methods were not interchangeable as measures of hypoxia-induced changes in PV. The mechanism responsible for the bias remains unknown, but it is suggested that the results may reflect a redistribution of albumin caused by the combined effects in hypoxia of both an increased capillary permeability to albumin and a decrease in PV. As a result, the small perivascular compartment of albumin beyond the endothelium may increase without changes in the overall albumin distribution volume. Accepted: 31 October 1997     

Potassium changes in trained subjects after potassium loading and during restriction of muscular activity and chronic hyperhydration

The objective of this investigation was to determine whether urinary and plasma potassium changes developed during prolonged hypokinesia (HK) (decreased number of km/d) in endurance-trained subjects could be minimized or reversed with a daily intake of fluid and salt supplementation (FSS). The studies were performed on 30 endurance-trained male volunteers aged 23–26 yr with an average peak oxygen uptake of 65 mL/kg min during 364 d of HK. All volunteers were on an average of 13.8 km/d prior to their exposure to HK. All volunteers were randomly divided into three groups: 10 volunteers were placed continuously under an average of 14.0 km/d (control subjects), 10 volunteers were subjected continuously to an average of 2.7 km/d (unsupplemented hypokinetic subjects), and 10 volunteers were submitted continuously to an average of 2.7 km/d, and consumed daily an additional amount of 0.1 g sodium chloride (NaCl)/kg body wt and 30 mL water/kg body wt (supplemented hypokinetic subjects). During the prehypokinetic period of 60 d and during the hypokinetic period of 364 d, potassium loading tests were performed with 1.5–1.7 mEq potassium chloride/kg body wt, and potassium, sodium, and chloride excretion in urine and potassium, sodium, and chloride in plasma were determined. In the unsupplemented hypokinetic volunteers, urinary excretion of electrolytes and concentrations of electrolytes in plasma increased significantly as compared to the control and supplemented hypokinetic groups of volunteers. It was concluded that daily intake of fluid and salt supplementation had a favorable effect on regulation of urinary and plasma potassium changes in trained subjects during prolonged HK.     

Gastric emptying in soldiers during and after field exercise in the heat measured with the [13C]acetate breath test method

The effects of exercise on gastric emptying remain controversial, with some workers reporting that heavy exercise inhibits it to varying degrees whereas others report no effects up to an intensity of 70% maximal oxygen consumption (O_2max). The state of hydration of the subjects and the environmental conditions may influence the rate of gastric emptying during exercise. To understand further the effects of a 3-h, 16-km walk/run carrying 30 kg of equipment under field conditions at 39°C, we estimated gastric emptying using a [¹³C]acetate breath test method. Breath samples were collected at intervals after giving 150 mg of [¹³C]acetate. The effects of giving a standard volume (530 ml) of water or dextrose (7.5 g · 100 ml⁻¹) with electrolytes or fructose/corn solids (7.5 g · 100 ml⁻¹) at rest before exercise were compared with those of exercise and of recovery after exercise with or without extra fluids (400 ml each 20 min). At rest, after a standard 530-ml load, gastric emptying times [mean (SE)] were: 37 (2) min (water), 46 (3) min (dextrose/electrolytes) and 47 (5) min (fructose/corn solids) and were significantly slower (P < 0.05) than those occurring after extra fluid ingestion, i.e. 32 (3), 39 (2) and 41 (3) min respectively. After a standard 530-ml load, emptying times during exercise were almost identical to those at rest but, during exercise, extra fluid speeded up gastric emptying more than at rest to 24 (2), 26 (1) and 27 (5) min (P < 0.05) respectively. During resting recovery without extra fluids, gastric emptying was significantly slowed to 60 (2), 71 (5) and 78 (3) min, respectively. Although emptying times during recovery from exercise with extra fluid were faster [49 (6), 55 (2) and 58 (4) min, respectively], they were still slower than before exercise. The results suggest that: (1) extra fluid increases gastric emptying more during exercise than at rest, and (2) gastric emptying during resting recovery from exercise is slower than at rest before exercise whether or not fluid has previously been taken. Accepted: 17 June 1996     

Blood volume measurement in baboons: simultaneous estimation of red cell volume and plasma volume

A method is described for frequent sequential blood volume estimation in baboons using 32P for red cell volume measurements and 125I-albumin for simultaneous plasma volume measurements. Values for red cell, plasma, and total blood volumes are reported. Close correlations of the volumes to bodyweight were demonstrated. Circulatory half-lives of the isotopes, determined from disappearance curves, confirmed their suitability for serial measurements in these baboons.     

The effect of heat acclimation on maximal urine osmolality in humans

The purpose of the current study was to determine the effect of 9 days of active heat acclimation on maximal urine osmolality (MUO) in humans. Eight subjects completed 9 days of heat acclimation, which consisted of a daily 90 min exercise session in a heated environmental chamber. Before and following heat acclimation the subjects abstained from drinking fluids for 15 h, and urine samples were collected to measure MUO. The subjects successfully heat acclimated as evidenced by a significant (P<0.05) decrease in the mean±SD exercise core temperature (37.7±0.3 °C on day 1 to 37.4±0.3 °C on day 9) and heart rate (143±17 bpm on day 1 to 128±14 bpm on day 9). The mean pre- and post-heat acclimation MUOs were 868±117 and 846±89 mmol kg⁻¹, respectively, which were not significantly different (P>0.05). Previous studies have shown that prolonged dehydration can increase the MUO in various mammalian species, including humans. In contrast, the results of the current study suggest that 9 days of active heat acclimation, without significant dehydration, does not affect the MUO in humans.     

Hydration effects on thermoregulation and performance in the heat

During exercise, sweat output often exceeds water intake, producing a water deficit or hypohydration. The water deficit lowers both intracellular and extracellular fluid volumes, and causes a hypotonic-hypovolemia of the blood. Aerobic exercise tasks are likely to be adversely effected by hypohydration (even in the absence of heat strain), with the potential affect being greater in hot environments. Hypohydration increases heat storage by reducing sweating rate and skin blood flow responses for a given core temperature. Hypertonicity and hypovolemia both contribute to reduced heat loss and increased heat storage. In addition, hypovolemia and the displacement of blood to the skin make it difficult to maintain central venous pressure and thus cardiac output to simultaneously support metabolism and thermoregulation. Hyperhydration provides no advantages over euhydration regarding thermoregulation and exercise performance in the heat.     

Changes in body core temperatures and heat balance after an abrupt release of lower body negative pressure in humans

Changes in body core temperature (T _cor) and heat balance after an abrupt release of lower body negative pressure (LBNP) were investigated in 5 volunteers under the following conditions: (1) an ambient temperature (T _a) of 20 °C or (2) 35 °C, and (3)T _a of 25 °C with a leg skin temperature of 30°C or (4) 35°C. The leg skin temperature was controlled with water perfusion devices wound around the legs. Rectal (T _re), tympanic (T _ty) and esophageal (T _es) temperatures, skin temperatures (7 sites) and oxygen consumption were measured. The intensity of LBNP was adjusted so that the amount of blood pooled in the legs was the same under all conditions. When a thermal balance was attained during LBNP, application of LBNP was suddenly halted. The skin temperatures increased significantly after the release of LBNP under all conditions, while oxygen consumption hardly changed. The release of LBNP caused significant falls inT _cor s under conditions (1) and (3), but loweredT _cor s very slightly under conditions (2) and (4). The changes inT _es were always more rapid and greater than those ofT _ty andT _re. The falls inT _ty andT _re appeared to be explained by changes in heat balance, whereas the sharp drop ofT _es could not be explained especially during the first 8 min after the release of LBNP. The results suggest that a fall inT _cor after a release of LBNP is attributed to an increase in heat loss due to reflexive skin vasodilation and is dependent on the temperature of venous blood returning from the lower body. It is presumed thatT _es may not be an appropriate indicator forT _cor when venous return changes rapidly.     

Influence of hydration and airflow on thermoregulatory control in the heat

Exercise-heat exposure results in significant sweat losses due to large biophysical requirements for evaporative heat loss. Progressive body water losses will increase plasma tonicity and decrease blood volume (hypertonic–hypovolemia). The result is reduced dry and evaporative heat exchange through alterations in the core temperature threshold for initiation of skin blood flow and sweating as well as changes in the sensitivity of these thermo-effectors. Regulation of reduced sweating conserves body water, which reduces heat loss and increases exercise hyperthermia, but the magnitude of this effect is modified by environmental heat transfer capabilities. The focus of this paper is to (1) examine the major mechanisms by which hypohydration alters thermoregulatory responses in the heat, and (2) illustrate how important differences in environmental airflow characteristics between laboratory and field settings may modify these effects.     

Fluid electrolyte changes in trained subjects after water loading and during restriction of muscular activity and chronic hyperhydration

The objective of this investigation was to determine fluid electrolyte changes after water-loading tests and during hypokinesia (decreased number of km taken per day) and daily intake of fluid and salt supplementation (FSS). The studies during hypokinesia (HK) were performed for 364 d on 30 endurance-trained male volunteers in the age range of 23–26 yr, with an average peak oxygen uptake, POU, of 64 mL/kg/min. All volunteers were divided into three equal groups: 10 volunteers were placed on a continuous regime of exercise of 14.4 km/d and served as control subjects (CS); 10 volunteers were submitted to continuous HK without FSS and were considered as the unsupplemented hypokinetic subjects (UHS); and 10 volunteers were under continuous HK and FSS and were considered as the supplemented hypokinetic subjects (SHS). For the simulation of the hypokinetic effect, the UHS and SHS groups were kept continuously under an average of 2.7 km/d for the duration of the study. Prior to exposure to HK, the two groups of volunteers were on the same exercise regime as the control group. During a 60-d preexperimental period and during the remainder of the study, water-loading tests with a water load of 20 mL/kg body wt/min were performed, and urinary and plasma electrolytes (sodium, potassium, calcium, and magnesium) were measured. In the SHS group, urinary excretion of electrolytes and plasma electrolyte content decreased, while in the UHS these values increased after water loading tests and during HK. Based on the obtained data, it is concluded that chronic hyperhydration may be used to prevent or minimize urinary and plasma electrolyte changes in endurance-trained volunteers after water-loading tests and during prolonged restriction of muscular activity.     

Thermal responses and survival after heat exposure are modulated by maintained differences in body temperature in mice

When individual mice were examined, it was found that the colonic body temperatureT _col of each individual within a genetically heterogeneous population tended to remain either above (warm) or below (cool) the population mean.T _col of warm, but not cool, mice showed circadian variation. When exposed to aT _a of 43° C, theT _col of cool mice increased by as musch as 2.4° C more than that of warm mice for a given 15 min increment of heating at 43°C. Survival of mice after acute lethal heat load (LD₇₅, –45°C) was significantly inversely correlated withT _col. Small persistent differences in body temperature of individuals may indicate differing thermal adaptedness.     

Effect of vitamin C on lipid hydroperoxides and carbonyl groups content of rat plasma depending on age and acute heat exposure

Free radicals produced during hyperthermic stress and aging are thought to play an important role in the degenerative process. To investigate the correlation between oxidative damages caused by acute heat exposure or aging, and the protective effect of vitamin C in vivo, we determined the levels of oxidative protein damage, lipid peroxidation, content of endogenous ascorbic acid, and glutathione in the plasma of young and old Wistar rats, subjected or not-subjected to acute heat stress. The results showed that the level of oxidative protein damage (measured as carbonyl content) in plasma was significantly higher in elderly and in heat-exposed animals. Vitamin C treatment led to inhibition on carbonyl production much more pronounced in young heat-exposed than in aged heat-exposed rats. Aging and acute heat exposure correlated positively with increased production of lipid hydroperoxides in rats plasma, but there were no significant differences in lipid hydroperoxides levels between young and old heat-exposed rats, depending on the treatment with vitamin C. Multiple backward regression analysis showed ascorbic acid to be the only determining variable of lipid hydroperoxides levels in unexposed rats. It was concluded that aging and heat exposure instigate an increase of lipid peroxidation and protein oxidation in rat plasma, while vitamin C supplementation significantly counteracts these changes.     

Seasonal changes in circadian rhythms of body temperatures in humans living in a dry tropical climate

Seven volunteers (3 females and 4 males; 3 Caucasians and 4 Africans) participated in two 24 h sessions during the cool dry (CD) and the hot dry (HD) seasons of the sahelian tropical climate. Body temperatures were taken on portable cassette recorders for 24 h. Rectal (Tre) and mean skin (Tsk) temperatures decreased in the HD compared to the CD conditions, meeting one of the criteria for adaptation to heat. No ethnic differences in thermal responses were found. Males and females differed in their body temperature rhythms and in their reactions to heat. Body temperatures were higher in females than in males. Males reacted to heat with a decrease in Tre, without change in the Tre-Tsk gradient. Females showed a decrease in both Tre and Tsk, more marked for Tsk, with an increase in the Tre-Tsk gradient. It was concluded that males showed seasonal acclimatization to heat via a decrease in metabolism confirmed by a decrease in plasma levels of thyroid stimulating hormone (TSH) in the HD condition. Females showed a mixed metabolic and thermolytic type of acclimatization, with an absence of variation in plasma TSH levels. In conclusion, the steady rise in temperature between the CD and HD conditions was sufficient to trigger an acclimatization to heat similar in Caucasian and African subjects, although exposure to the external climate differed widely.     

Effects of acute hypobaric hypoxia on the appearance of ingested deuterium from a deuterium oxide-labelled carbohydrate beverage in body fluids of humans during prolonged cycling exercise

To determine whether or not acute hypobaric hypoxia alters the rate of water absorption from a carbohydrate beverage ingested during exercise, six men cycled for 80 min on three randomly assigned different occasions. In one trial, exercise was performed in hypoxia (barometric pressure, P(B) = 594 hPa, altitude 4,400 m) at an exercise intensity selected to elicit 75% of the individual's maximal oxygen uptake (VO2max) previously determined in such conditions. In the two other experiments, the subjects cycled in normoxia (P(B) = 992 hPa) at the same absolute and the same relative intensities as in hypoxia, which corresponded to 55% and 75%, respectively, of their VO2max determined in normoxia. The subjects consumed 400 ml of a 12.5% glucose beverage just prior to exercise, and 250 ml of the same drink at 20, 40 and 60 min from the beginning of exercise. The first drink contained 20 ml of deuterium oxide to serve as a tracer for the entry of water into body fluids. The heart rate (HR) during exercise was higher in hypoxia than in normoxia at the same absolute exercise intensity, whereas it was similar to HR measured in normoxia at the same relative exercise intensity. Both in normoxia and hypoxia, plasma noradrenaline concentrations were related to the relative exercise intensity up to 40 min of exercise. Beyond that duration, when exercise was performed at the highest absolute power in normoxia, the noradrenaline response was higher than in hypoxia at the same relative exercise intensity. No significant differences were observed among experimental conditions, either in temporal profiles of plasma D accumulation or in elimination of water ingested in sweat. Conversely, elimination in urine of the water ingested appeared to be related to the severity of exercise, either high absolute power or the same relative power combined with hypoxia. We concluded that water absorption into blood after drinking a 12.5% glucose beverage is not altered during cycling exercise in acute hypobaric hypoxia. It is suggested that the elimination of water ingested in sweat and urine may be dependent on local circulatory adjustments during exercise.     

Relationship of gastric emptying and volume changes after a solid meal in humans

Noninvasive imaging has been developed to measure gastric volumes. The relationship between gastric emptying and volume postprandially is unclear. The aims were to 1) develop a 3-dimensional (3D) single photon emission-computed tomography (SPECT) method to simultaneously measure gastric volume and emptying postprandially, 2) describe the course of gastric volume change during emptying of the meal, and 3) assess a 3D method measuring gastric emptying. In 30 healthy volunteers, we used (111)In-planar and (99m)Tc-SPECT imaging to estimate gastric emptying and volume after a radiolabeled meal. A customized analysis program of SPECT imaging assessed gastric emptying. A Bland-Altman plot assessed the performance of the new SPECT analysis compared with planar analysis. Gastric volume postprandially exceeds the fasting volume plus meal volume. The course of volume change and gastric emptying differ over time. Higher differences in volumes exist relative to fasting plus residual meal volumes at 15 min (median 763 vs. 568 ml, respectively, P < 0.001), 1 h (median 632 vs. 524 ml, P < 0.001), and 2 h (median 518 vs. 428 ml, P < 0.02), in contrast to similar volumes at 3 h (median 320 vs. 314 ml, P = 0.85). Analysis of SPECT imaging accurately measures gastric emptying compared with planar imaging with median differences of 1% (IQR -2.25 to 2.0) at 1 h, 1% (-3.25 to 2.25) at 2 h, and -2.5% (-4 to 0) at 3 h. Gastric volume exceeds meal volume during the first 2 postprandial hours, and simultaneous measurements of gastric volume and emptying can be achieved with a novel 3D SPECT method.     

Fluid replacement beverages and maintenance of plasma volume during exercise: role of aldosterone and vasopressin

Previous experiments have demonstrated that consumption of a glucose polymer-electrolyte (GP-E) beverage is superior to water in minimizing exercise-induced decreases in plasma volume (PV). We tested the hypothesis that elevated plasma concentrations of vasopressin and/or aldosterone above that seen with water ingestion may explain this observation. Six trained cyclists performed 115 min of constant-load exercise (approximately 65% of maximal oxygen consumption) on a cycle ergometer on two occasions with 7 days separating experiments. Ambient conditions were maintained relatively constant for both exercise tests (29-30 degrees C; 58-66% relative humidity). During each experiment, subjects consumed 400 ml of one of the following beverages 20 min prior to exercise and 275 ml immediately prior to and every 15 min during exercise: (1) distilled water or (2) GP-E drink contents = 7% carbohydrate (glucose polymers and fructose; 9 mmol.l-1 sodium; 5 mmol.l-1 potassium; osmolality 250 mosmol.l-1). No significant difference (P > 0.05) existed in mean skin temperature, rectal temperature, oxygen consumption, carbon dioxide production or the respiratory exchange ratio between treatments. Further, no significant differences existed in plasma osmolality and plasma concentrations of sodium, potassium, chloride or magnesium between treatments. Plasma volume was better maintained (P < 0.05) in the GP-E trial at 90 and 120 min of exercise when compared to the water treatment. No differences existed in plasma levels of vasopressin or aldosterone between treatments at any measurement period. Further, the correlation coefficients between plasma concentrations of vasopressin and aldosterone and change in PV during exercise were 0.42 (P < 0.05) and 0.16 (P > 0.05), respectively.(ABSTRACT TRUNCATED AT 250 WORDS)