Exacerbated heat strain during consecutive days of repeated exercise sessions in heat

ObjectivesAn exercise session in a hot environment may increase thermal strain during subsequent exercise sessions on the same and consecutive days. Therefore, this study was conducted to determine lasting physiological strain from moderate-high intensity, intermittent exercise in heat on subsequent exercise.DesignRepeated measures laboratory study.MethodsSeventeen healthy, recreationally active men (age: 22 ± 3 y, maximal oxygen consumption: 54.6 ± 5.3 mL kg⁻¹ min⁻¹) underwent two intermittent moderate-high intensity aerobic exercise sessions separated by 2 h of rest one day, followed by one session 24 h later in a 40 °C, 40% relative humidity environment. Heart rate, rectal temperature, heat stress perception, and environmental symptoms were assessed.Results100%, 35%, and 71% of participants completed the full exercise protocol during the first exercise session, second exercise session, and the following day, respectively. Exercising heart rate and rectal temperature were greater during the second exercise session (189 ± 11 bpm, 38.80 ± 0.47 °C) than the first identical exercise session (180 ± 17 bpm, p = 0.004; 38.41 ± 0.52 °C, p = 0.001), respectively. Immediate post-exercise heart rate, rectal temperature, thirst, thermal sensation, fatigue, and perceived exertion were similar among exercise sessions despite a shorter exercise duration during the second exercise session (93 ± 27 min, p = 0.001) and the following day (113 ± 12 min, p = 0.032) than the first exercise session (120 ± 0 min).ConclusionsModerate-high-intensity intermittent exercise in the heat resulted in greater heat strain during a second exercise session the same day, and exercise the subsequent day.     

Effect of water and electrolyte replacement during exercise in the heat on biochemical indices of stress and performance.

Eight healthy young men exercised on alternate days in a warm, humid environment (32 degrees C, 65% RH) at 50% VO2 max for 2 h while receiving a water supplement (WS), glucose-electrolyte solution (ES), or no fluid supplement (NS). The average weight loss after 2 h of exercise and NS was 2.44 kg with a resultant plasma volume decrease of 17%. This acute period of exercise with no fluid replacement elicited significant increments in serum levels of cortisol, dopamine-B-hydroxylase and uric acid. Alternatively, exercise in the heat for the same duration with water or electrolyte supplement failed to effect significant alterations in any of these physical or biochemical factors compared to preexercise levels. Heart rates under the influence of heat stress and exercise with NS averaged 18% higher at each time period studied compared with WS or ES.     

Water versus carbohydrate-electrolyte fluid replacement during loaded marching under heat stress

This study compared the effectiveness of carbohydrate-electrolyte (CHO-E) fluid replacement versus water (WAT) on hydration status, physiological and subjective responses, and exercise performance during a 3 x 60-minute loaded (14 kg) treadmill walk in 35 degrees C ambient temperature and 55% humidity. CHO-E did not affect urine loss, plasma volume change (WAT = -3.0 +/- 1.6% vs. CHO-E = -1.1 +/- 1.6%), dehydration (WAT = 0.4 +/- 0.3% vs. CHO-E = 0.4 +/- 0.3% of body mass), or core body temperature (Tc) and heart rate (HR) responses. Endurance time was greater but not significantly different with CHO-E (WAT = 134 +/- 9 vs. CHO-E = 146 +/- 9 minutes). CHO-E increased the frequency of task completion (WAT = 21% vs. CHO-E = 50%), elevated blood glucose, and reduced perceived exertion. CHO-E offers potential to enhance exercise capacity by elevating blood glucose and thereby preventing hypoglycemia, maintaining high rates of carbohydrate oxidation, and/or preventing central fatigue; but provided no additional benefits with regard to hydration status and physiological function during loaded walking under heat stress.     

Water and electrolyte requirements during exercise

These studies demonstrate the body's capacity to minimize electrolyte losses during acute and repeated bouts of exercise and dehydration. Although there are marked shifts in water and selected ions in the exercising muscle, only during prolonged exertion is the ratio of intramuscular to extramuscular potassium significantly altered, suggesting that some modifications of the muscle cell membrane may occur. Muscle tissue not engaged in the exercise seems unaffected by the sweat loss during prolonged activity but relinquishes intracellular water shortly after work is terminated. Blood, muscle, sweat, and urine measurements before and following varied levels of dehydration demonstrate that body water loss during exercise in the heat is accomplished at the expense of larger water losses from extracellular and intracellular compartments. Moreover, the loss of ions in sweat and urine have little effect on the potassium content of either plasma or muscle. With repeated days of dehydration and heavy exercise, plasma volume increases in proportion to an increase in body sodium storage. Since red blood cells and hemoglobin are confined to the vascular space, both may decrease significantly as a function of the hemodilution induced by repeated days of exercise and dehydration. This may, in part, explain the apparent anemia reported by sports physicians among athletes undergoing intensive training. It is also possible that such hemodilution may produce low concentrations of plasma potassium, which might be falsely interpreted as suggestive of hypokalemia. In any event, some caution should be used in the clinical interpretation of plasma concentrations of various constituents among endurance-trained athletes. In general, it seems that the large sweat losses incurred during training and competition are adequately tolerated by the athlete, with concomitant adjustments in the water and electrolyte distribution of their fluid compartments. Despite the sizable excretion of ions and sweat, the athlete's large caloric intake and renal conservation of sodium minimize the threat of chronic dehydration or electrolyte deficits, or both.     

Fluid replacement during and after exercise in the heat

This study sought to determine whether ad libitum drinking of a carbohydrate-electrolyte (CE) beverage would minimize the physiological disturbances associated with prolonged (3 h) continuous exercise in the heat (Tdb = 31.5 degrees C, percent RH = 22.3). Seven male subjects performed two bouts of cycle exercise (60% VO2max) drinking either a CE beverage (4.85% polycose, 2.65% fructose) or distilled water. Subjects continued to drink ad libitum for 3 h during recovery in the heat. No significant differences were observed between drinks for rectal temperature, heart rate, or sweat rate during exercise. Subjects tended to drink more (P less than 0.0565) water than the CE beverage during exercise, but plasma volume (percent PV) decreased less (P less than 0.0253) and plasma [( glucose], P less than 0.0001 and [K+], P less than 0.0047) were higher when subjects drank the CE solution. At the end of exercise, plasma osmolality and [Na+] were also higher (P less than 0.05) when subjects drank the CE beverage rather than water. Rating of perceived exertion was higher (P less than 0.0001) when drinking water. In recovery, ingesting the CE beverage, 1) subjects drank more (P less than 0.0012); 2) plasma volume increased to a higher level (P less than 0.0017); 3) plasma osmolality (P less than 0.0001), [Na+] (P less than 0.0001), glucose (P less than 0.0001), and [K+] (P less than 0.0015) were greater; and 4) body weight increased more (P less than 0.0422) than when water was ingested. Thus, in terms of minimizing physiological disturbances, ad libitum drinking of the CE beverage was as effective as drinking water during exercise but was more effective in recovery.     

Sweat electrolyte loss during exercise in the heat: effects of gender and maturation.

Humans may lose large amounts of water and electrolytes from sweat during prolonged exercise in a hot climate. Gender and maturational differences for the total sweat electrolyte losses have not been reported. The purpose of this study was to compare sweat electrolyte losses of prepubescent (PP), pubescent (P) and young adult (YA) males and females, under the same environmental conditions and relative exercise intensities. Twenty-five females (8 PP, 9 P, 8 YA) and 26 males (10 PP, 8 P, 8 YA) cycled for two 20-min bouts at 50% of their peak VO2 in a climatic chamber (42 degrees C, 18% relative humidity). Sweat was collected from a plastic bag attached to the lower back. Total body sweat loss was calculated from the differences in nude body weight corrected for fluid intake, urine, and respiratory water loss. Sweat [Na+] and [Cl-] tended to increase with maturation while sweat [K+] was lower in YA compared with that of PP. Children had a lower sweating rate than YA, even when corrected for body surface area. As a result, total Na+ and Cl- losses per kg body weight from sweat (mEq.kg-1.h-1) were higher in YA compared with those of PP and P; however, no maturational difference was found in K+ losses. Within the same maturational group, there were no gender differences in any of the electrolyte losses. These results may be useful in recommending "optimal" fluid-electrolyte drinks for children exercising in the heat.     

Voluntary dehydration and electrolyte losses during prolonged exercise in the heat

The effects of water temperature (6 degrees, 22 degrees, 46 degrees C) and chlorination on voluntary dehydration (D), sweat electrolyte losses (SEL), and total body electrolyte losses (BEL) were studied in 12 healthy males during 6 h of intermittent treadmill exercise (1.34 m X s-1, 5% grade) in a climatic chamber (40.6 degrees C DB, 25.5 degrees C WB). Body weight (BW), rectal temperature (Tre), mean weighted skin temperature (Tsk), heart rate (HR), plasma osmolality (PO), sweat rate (SR), sweat sodium (Na+), chloride (Cl-), potassium (K+), and magnesium (MG++), urine volume, Na+, and K+ were measured. No differences were found between chlorinated and non-chlorinated treatments except SEL of Mg++. Subjects (Ss) who drank 46 degrees C (-2.1% BW) consumed approximately 50% less water (p less than 0.001), and had D which was 1.050 kg larger (p less than 0.001) than subjects who consumed 6 degrees C (-0.5 %BW). There were no significant between-group PO differences, but Tre and Tsk differed between 46 degrees and 6 degrees C (p less than 0.01), and the HR of 22 degrees and 46 degrees C were both different from 6 degrees C (p less than 0.05). SR of all groups were essentially equal, although differences in total sweat Na+ (p less than 0.02) and Cl- (p less than 0.04) losses were observed between 46 degrees and 6 degrees C. SEL of sweat K+ and Mg++ were not affected by the experimental design. Based on 24 h projections of BEL, it was concluded that K+ depletion is more likely than Na+ depletion because food is often supplemented with sodium chloride.(ABSTRACT TRUNCATED AT 250 WORDS)     

Carbohydrate-electrolyte replacement during a simulated triathlon in the heat

Effects of a 7% carbohydrate-electrolyte drink (CE) or a flavored water placebo (P) on physiological function and performance were compared during a simulated triathlon (ST) in the heat. Ten trained male triathletes performed two STs, consisting of 1.5 km swimming, 40 km cycling, and 10 km running in an environmentally controlled area at self-selected race pace. Subjects consumed 2 ml.kg-1 (130-174 ml) of CE or P following the swim, at 8.0-km intervals during cycling, and at 3.2-km intervals during running. Sweat rate, rectal and mean skin temperatures, perceived exertion, heart rate, plasma osmolality, percent change in plasma volume, total protein, Na+, K+, and lactate were similar during the ST under both drink conditions, but RER and plasma glucose were higher (P less than 0.05) with CE. During the last 4 km of running, VO2 was significantly higher with CE. Mean run time and total ST time were faster with CE (by 1.4 and 1.2 min) although not significantly different (P less than 0.06 and P less than 0.10) from P. Subjects reported no significant difference in nausea, fullness, or stomach upset with CE compared to P. General physiological responses were similar for each drink during 2 h of multi-modal exercise in the heat; however, blood glucose, carbohydrate utilization, and exercise intensity at the end of a ST may be increased with CE fluid replacement.     

Changes in selected blood measures during repeated days of intense training and carbohydrate control

Ten runners were studied to determine whether selected blood measures were useful indices of the metabolic stress associated with intense training and dietary carbohydrate (CHO) deficiency. The runners performed two diet/training regimens, involving 5 repeated days of intense training approximately 80 min/d, approximately 80% VO2max) and dietary CHO control (8.0 g.kg-1.d-1, EQ-CHO; 3.9 g.kg-1.d-1, LO-CHO). Resting blood samples were obtained after a 3-day control period, after 3 and 5 days of intense training, and after 3 days of rest. Resting uric acid levels were significantly higher (P less than 0.05) after 3 and 5 days of training during the LO-CHO vs EQ-CHO regimen (353 +/- 21 vs 309 +/- 24, and 345 +/- 26 vs 302 +/- 26 mol.l-1, respectively). Resting thyroxine (T4) levels were higher (P less than 0.05) after 5 days of training during the LO-CHO vs EQ-CHO regimen (102.2 +/- 6.2 vs 83.7 +/- 4.5 nmol.l-1, respectively). While creatine kinase levels were elevated after both regimens (P less than 0.05), there was no difference between regimens. Serum cortisol (C) levels were reduced by 10% for both regimens (P less than 0.05), possibly due to an expansion in plasma volume (7.6 and 7.3% for the LO-CHO and EQ-CHO regimens, respectively). Resting FFA levels were increased (P less than 0.05) during both regimens, but there was no difference between the regimens.(ABSTRACT TRUNCATED AT 250 WORDS)     

Heat cramps: fluid and electrolyte challenges during tennis in the heat

Sweat losses during tennis can be considerable. And while most players make a genuine effort to stay well hydrated to maintain performance and reduce the risk of heat illness, regular and copious water intake is often not enough. Besides an extraordinary water loss, extensive sweating can lead to a concomitant large electrolyte deficit too--particularly for sodium. Although a variety of other mineral deficiencies and physiological conditions are purported to cause muscle cramps, evidence suggests that, when a tennis player cramps in warm to hot weather, extensive and repeated sweating during the current and previous matches and a consequent sodium deficit are usually the primary contributing factors. Heat cramps often begin as subtle "twitches" or fasciculations in one or more voluntary muscles and, unless treated, can rapidly progress to widespread debilitating muscle spasms that leave an afflicted player on the court writhing in pain. If sufficient preventive measures are taken well before and during play, such cramping is avoidable in most cases. Appropriate and sufficient salt and fluid intake will enhance rehydration and fluid distribution throughout a player's body, so that heat cramps can be completely averted, even during long matches in the most challenging environments.     

Fluid and electrolyte replacement in the ultramarathon runner

We studied three male runners during three separate ultramarathon races. Serial blood samples were tested for electrolytes, renal function, albumin, calcium, phosphorus, and magnesium. Two runners had aldosterone levels measured during the race and one runner had blood lactate levels measured. All runners ingested an isotonic fluid and electrolyte solution during the race. The runners consumed the solution without medical complications and maintained prerace body weights and hydration levels. Each runner maintained normal endocrine/metabolic parameters. We concluded that it is possible to maintain normal hydration and electrolyte states during a prolonged race, provided that sufficient fluid and electrolyte solutions are ingested.     

Fluid replacement during sustained activity in the heat: nutrient solution vs. water

This study examined the thermoregulatory and hydrational status of men during sustained activity in a hot-dry (37 degrees C, 20% rh) environment while they consumed only a nutrient solution (nutrient), or consumed only colored, flavored water (control). Eleven heat acclimated young men attempted 24-h sustained activity experiments. These experiments consisted of alternating 45-min bouts of treadmill walking (410 W, approximately 30% VO2max) and rest (including sedentary activity). Data were analyzed through 13 h (after 13 h subjects began to discontinue testing). No significant differences between trials were observed for metabolic rate, fluid intake, skin or rectal temperature, sweating rate, plasma volume (as indicated by hemoglobin concentration) or plasma glucose concentrations. By the 8th h plasma osmolality was higher and by the 11th h plasma free fatty acids were lower during the nutrient trial compared to the control. In separate experiments with nine different men, the gastric emptying rates of the nutrient solution and water were compared during exercise (55% VO2max) in the heat (35 degrees C, 20% rh). The gastric emptying rates of the nutrient solution and water were similar (approximately 20 ml.min-1). These data indicate that during 13 h of sustained activity in a hot environment, the nutrient solution and water provided similar thermoregulatory and hydrational benefits.     

Effects of exercise mode on muscle glycogen restorage during repeated days of exercise

The purpose of this study was to examine differences in muscle glycogen storage during three successive days of running or cycling. In a crossover design, seven male subjects performed two 3-d trials of either running (trial R) or cycling (trial C) for 60 min at 75% VO2max. Biopsy samples were obtained before and after each day's exercise from the gastrocnemius (trial R) or vastus lateralis (trial C) muscle. Diets in the 2 d preceding and during each trial contained 5 g carbohydrate.kg-1.d-1 and 14,475 +/- 402 kJ.d-1. Mean pre-exercise glycogen content (mmol.kg-1 wet wt.) was significantly reduced in both trials on day 3 (103.4 +/- 6.0) when compared to day 1 and day 2 (119.9 +/- 6.8 and 116.4 +/- 5.7, respectively). Day 1 glycogen reduction was significantly greater in trial C (P less than 0.03), and glycogen restorage was greater (P less than 0.02) only in trial C between the 1st and 2nd d. On day 3, spectrophotometric analysis of PAS strains showed that pre-exercise glycogen content in either muscle group was significantly (P less than 0.01) less in Type I as compared to Type II fibers. This difference in fiber glycogen storage did not appear to be attributable to muscle damage as negligible leukocyte infiltration and low blood CK levels were obtained. No difference between modes were observed for CK values throughout the trials. These data suggest that the depressed glycogen storage before the 3rd d of exercise was due to the moderate carbohydrate intake.     

Water and electrolyte requirements for exercise.

Exercise performance can be compromised by a body water deficit, particularly when exercise is performed in hot climates. It is recommended that individuals begin exercise when adequately hydrated. This can be facilitated by drinking 400 mL to 600 mL of fluid 2 hours before beginning exercise and drinking sufficient fluid during exercise to prevent dehydration from exceeding 2% body weight. A practical recommendation is to drink small amounts of fluid (150-300 mL) every 15 to 20 minutes of exercise, varying the volume depending on sweating rate. Core temperature, heart rate, and perceived effort remain lowest when fluid replacement comes closest to matching the rate of sweat loss. During exercise lasting less than 90 minutes, water alone is sufficient for fluid replacement. During prolonged exercise lasting longer than 90 minutes, commercially available carbohydrate electrolyte beverages should be considered to provide an exogenous carbohydrate source to sustain carbohydrate oxidation and endurance performance. Electrolyte supplementation is generally not necessary because dietary intake is adequate to offset electrolytes lost in sweat and urine; however, during initial days of hot-weather training or when meals are not calorically adequate, supplemental salt intake may be indicated to sustain sodium balance.     

Carbohydrate-electrolyte replacement improves distance running performance in the heat.

The effects of a 7% carbohydrate-electrolyte drink (CE) and an artificially sweetened placebo (P) on performance and physiological function were compared during a 40-km run in the heat. Eight highly trained male runners completed two runs on a measured outdoor course. The first 35 km of each run was performed at self-selected training pace and the last 5 km at race effort. Under a counterbalanced, double-blind design, subjects consumed 400 ml of either CE or P 30 min prior to exercise, and 250 ml every 5 km thereafter during the run. Rectal temperature, heart rate, rating of perceived exertion, sweat rate, and respiratory exchange ratio were similar during the run for CE and P. Serum Na+, K+, Cl-, total protein, osmolality, blood lactate, urea nitrogen, and % change in plasma volume were also similar for both drink conditions; however, blood glucose was significantly higher (P less than 0.01) with CE. Running performance in the last 5 km was significantly faster (P less than 0.03) during CE (21.9 min) compared with P (24.4 min). Subjects reported no differences in stomach upset, bloating, or nausea between P and CE. Results indicate that CE replacement elicits similar thermoregulatory and physiological responses during prolonged running in the heat but increases run performance and blood glucose when compared with P.     

Factors influencing the restoration of fluid and electrolyte balance after exercise in the heat.

Maintenance of fluid balance is a major concern for all athletes competing in events held in hot climates. This paper reviews recent work relating to optimisation of fluid replacement after sweat loss induced by exercising in the heat. Data are taken from studies undertaken in our laboratory. Issues investigated were drink composition, volume consumed, effects of consuming food with a drink, effects of alcohol in rehydration effectiveness, voluntary intake of fluid, and considerations for women related to the menstrual cycle. The results are presented as a series of summaries of experiments, followed by a discussion of the implications. The focus of this review is urine output after ingestion of a drink; fluid excreted in urine counteracts rehydration. Also included are data on the restoration of plasma volume losses. Ingestion of large volumes of plain water will inhibit thirst and will also promote a diuretic response. If effective rehydration is to be maintained for some hours after fluid ingestion, drinks should contain moderately high levels of sodium (perhaps as much as 50-60 mmol/l) and possibly also some potassium to replace losses in the sweat. To surmount ongoing obligatory urine losses, the volume consumed should be greater than the volume of sweat lost. Palatability of drinks is important in stimulating intake and ensuring adequate volume replacement. Where opportunities allow, the electrolytes required may be ingested as solid food consumed with a drink. There are no special concerns for women related to changes in hormone levels associated with the menstrual cycle. Ingestion of carbohydrate-electrolyte drinks in the post-exercise period restores exercise capacity more effectively than plain water. The effects on performance of an uncorrected fluid deficit should persuade all athletes to attempt to remain fully hydrated at all times, and the aim should be to start each bout of exercise in a fluid replete state. This will only be achieved if a volume of fluid in excess of the sweat loss is ingested together with sufficient electrolytes.     

The work experience of a military infectious disease hospital during repeated overloading

The article analyses the work of the military isolation hospital when it was overloaded 2-2,5 times in the period of 1984-1986 during the war in Afghanistan. Essential conditions were created for the treatment of patients, including up-to-date methods of intensive therapy: hyperbaric oxygenation, hemosorption, quantum autologous hemotherapy, etc. That made it possible to reduce considerably the fatal cases of typhoid, and, in general, the lethality of infectious diseases.     

Effect of acclimation on changes in water and electrolyte homeostasis in soldiers during exertional heat stress

BACKGROUND/AIM: Exertional heat stress is a common problem in military services. The aim of this study was to exemine changes in body water and serum concentrations of some electrolites in soldiers during exertional heat stress (EHST), as well as effects of 10-day passive or active acclimation in a climatic chamber. METHODS: Forty male soldiers with high aerobic capacity, performed EHST either in cool (20 degrees C, 16 degrees C WBGT-wet bulb globe temperature), or hot (40 degrees C, 25 degrees C WBGT) environment, unacclimatized, or after 10 days of passive or active acclimation. The subjects were allowed to drink tap water ad libitum during EHST. Mean skin (Tsk) and tympanic (Tty) temperatures and heart rates (HR) measured physiological strain, while sweat rate (SwR), and serum concentrations of sodium, potassium and osmolality measured changes in water and electrolite status. Blood samples were collected before and immediately after the EHST. RESULTS: Exertional heat stress in hot conditions induced physiological heat stress (increase in Tty, HR, and SwR), with significant decrease in serum sodium concentration (140.6 +/- 1.52 before vs. 138.5 +/- 1.0 mmol/l after EHST, p < 0.01) and osmolality (280.7 +/- 3.8 vs. 277.5 +/- 2.6 mOsm/kg, p < 0.05) in the unacclimatized group. The acclimated soldiers suffered no such effects of exertional heat stress, despite almost the same degree of heat strain, measured by Tty, HR and SwR. CONCLUSION: In the trained soldiers, 10-day passive or active acclimation in a climatic chamber can prevent disturbances in water and electrolitic balance, i.e. decrease in serum sodium concentrations and osmolality induced by exertional heat stress.     

Response of unacclimatized males to repeated weekly bouts of exercise in the heat.

The purpose of the present study was to determine if there is an acclimation effect when unacclimatized males exercise in the heat at weekly intervals. Five subjects performed four exercise bouts, each lasting 1 h at 55% VO2max. The first trial was in moderate conditions (mean(s.d.) temperature (Ta) = 22.0(0.8)degrees C; mean(s.d.) relative humidity (rh) = 67(6)%) and the subsequent three trials were carried out at weekly intervals in the heat (mean(s.d.) Ta = 34.6(0.6)degrees C; mean(s.d.) rh = 60(7)%). There were no significant differences between trials in the heat for heart rate, rectal temperature, skin temperature or VO2 (repeated measures analysis of variance), and total sweat loss (one-way analysis of variance). As changes in these variables are seen with heat acclimation it was concluded that there was no heat acclimation effect and separating exercise bouts by 1 week was a valid method for comparing the effects of different treatments on unacclimatized males during exercise in the heat.