Water versus Carbohydrate-Electrolyte Fluid Replacement during Loaded Marching Under Heat Stress

Transcription

1 MILITARY MEDICINE, 170, 8:715, 2005 Water versus Carbohydrate-Electrolyte Fluid Replacement during Loaded Marching Under Heat Stress Guarantor: Christopher Byrne, PhD Contributors: Christopher Byrne, PhD*; Chin Leong Lim, MSc; Serena Ai Neo Chew, BSc; Elaine Tan Yu Ming, MSc 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 60-minute loaded (14 kg) treadmill walk in 35 C ambient temperature and 55% humidity. CHO-E did not affect urine loss, plasma volume change (WAT % vs. CHO-E %), dehydration (WAT % vs. CHO-E % of body mass), or core body temperature (T C ) and heart rate (HR) responses. Endurance time was greater but not significantly different with CHO-E (WAT vs. CHO-E 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. Introduction rolonged physical activity in the heat often results in dehydration due to sweat loss exceeding fluid intake. 1 Early stud- P ies clearly established the negative impact of dehydration on physiological function and exercise capacity, and the value of fluid ingestion during prolonged exercise in the heat. 2 6 Minimizing dehydration, by matching fluid intake to sweat loss, attenuates the decline in physiological function and is recommended to promote the safety and performance of individuals undertaking physical activity under heat stress. 7,8 In the exercise science literature, fluid replacement with low concentration (4 8%) carbohydrate-electrolyte (CHO-E) fluid or sports drinks, at a rate equivalent to that of sweat rate, is currently recommended to simultaneously meet fluid and energy requirements and offset sweat sodium losses. 8 However, current U.S. Army recommendations for fluid replacement during military physical activity in the heat advocate the ingestion of plain water (WAT). 9 Military physical activity in the heat, such as loaded march- Centre for Human Performance, Defence Medical and Environmental Research Institute, DSO National Laboratories, 27 Medical Drive, Republic of Singapore, *Current address: School of Life Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, United Kingdom. Presented at the European College of Sport Science 8th Annual Congress, July 9 12, 2003, Salzburg, Austria. The views, opinions and findings contained in this research study are those of the authors and should not be taken to represent an official position of the Singapore Armed Forces and the Ministry of Defence, Singapore. This manuscript was received for review in January The revised manuscript was accepted for publication in April Reprint & Copyright by Association of Military Surgeons of U.S., ing, can result in sweat rates of approximately 1 L h 1. 9 Such low-to-moderate intensity activity offers the potential for continual and full replacement of sweat loss during activity. 10 Fluid replacement guidelines emphasizing the prevention of dehydration (e.g., American College of Sports Medicine 8 ) may have contributed to the increased incidence of overdrinking during exercise and the development of WAT intoxication or hyponatremia. 9,10 The hyponatremia of exercise is a rare but potentially life-threatening condition associated with large sweat losses coupled with the ingestion and retention of a large fluid volume Although hyponatremia is strictly defined as a serum sodium concentration below 135 mmol l 1 (normal range, mmol l 1 ), symptomatic hyponatremia of exercise is usually associated with levels below 130 mmol l 1. 11,12 The condition has typically been associated with individuals participating in exercise of more than a 6-hour duration under heat stress, such as competitors in ultraendurance events or military personnel engaged in field training. 11,12 However, symptomatic hyponatremia has recently been observed in a laboratory setting during exercise of only 3-hour duration coupled with sodiumfree fluid (distilled WAT) intake. 13 Studies investigating the merits of CHO-E versus WAT during military physical activity in the heat have generally been field based, have focused on hydration status rather than physiological function, and have provided equivocal findings The present study aimed to examine the potential application of the current fluid replacement recommendations of the American College of Sports Medicine to prolonged loaded marching under heat stress. 8 Specifically, we aimed to establish whether additional benefits occur with regard to fluid balance, physiological function, subjective response, and exercise performance when fluid replacement occurs with CHO-E versus WAT at a rate equal to that of the sweat rate during prolonged intermittent loaded marching under heat stress. An additional aim of this study was to determine whether serum sodium concentration decreases during prolonged marching in the heat when sweat losses are replaced with WAT and to compare this response to that when fluid replacement occurs with CHO-E. Methods Subjects This study had the approval of the Defence Medical and Environmental Research Institute (Singapore) Ethics Committee. Fourteen male soldiers from the Singapore Armed Forces (mean SD: age, years; height, m; body mass, kg; BMI, kg m 2 ) volunteered and gave written informed consent to participate after the nature, benefits, and risks of the study had been explained. Volunteers 715

2 716 had served approximately 1.5 years of conscripted full-time military service at the time of the study and were therefore considered heat acclimatized. Their stature and body mass were representative of young males in Singapore. Volunteers were exposed to the same training regimen and living conditions during their period of service. In general, servicemen in the Singapore Armed Forces remain in camp from Monday to Friday or Saturday of each week and return home over the weekend. Procedures In a randomized crossover design, volunteers ingested WAT or a commercially available (Gatorade, Quaker, Chicago, Illinois) CHO-E fluid immediately before and during exercise. On the evening before each trial, volunteers swallowed an ingestible temperature-sensing capsule (CorTemp, HQI Inc., Palmetto, Florida) for the measurement of body core (gastrointestinal) temperature (T C ) during exercise. Volunteers arrived at the laboratory between 8:00 a.m. and 9:00 a.m. after having consumed 500 ml of WAT upon waking and a light breakfast at approximately 7:00 a.m. Volunteers voided and produced a urine sample for the analysis of urine-specific gravity (USG) using a digital refractometer (UG-1, Atago Company, Tokyo, Japan). If USG exceeded 1.02, volunteers consumed 400 ml of WAT. Nude body mass was measured using a digital scale (KCC 150, Mettler- Toledo GmBH, Giessen, Germany) to the nearest 0.01 kg. A HR monitor (Polar Vantage, Polar Electro Oy, Kempele, Finland) was fitted and volunteers dressed in marching attire (socks, boots, trousers, long-sleeved shirt, and helmet; webbing consisting of ammunition (dummy) cartridges and two filled 1-L WAT canisters; and backpack), resulting in a load of approximately 14 kg. Exercise, consisting of 3 60-minute cycles of walking at 4.4 km h 1 and 5% gradient, separated by 15 minutes of seated rest, was performed in an environmental chamber (35 C ambient temperature, 55% relative humidity, 2 m s 1 wind velocity, and 600-W solar radiation). The walking speed and work:rest ratio were designed to simulate the actual long march regimen performed as part of military training in Singapore. Environmental conditions represented the hottest conditions of the Singapore climate. A fluid bolus of 5 ml kg 1 body mass was ingested immediately before exercise, followed by 3 ml kg 1 every 15 minutes during exercise and rest periods. CHO-E fluid provided 5.8 g of carbohydrate, 46 mg of sodium, and 13 mg of potassium per 100 ml. Criteria for exercise termination during each cycle were as follows: (1) cycle completion (2) T C exceeding 39.5 C, or (3) volunteers unable to maintain predetermined treadmill speed, equilibrium, or volitional exhaustion. Premature termination of a cycle resulted in a 15-minute seated rest before volunteers proceeded to the next cycle. Exercise was terminated completely when volunteers were unable to continue after resting or were in the final 60-minute cycle of marching. HR and T C were recorded continually during exercise and rest. Rating of perceived exertion (RPE) was recorded at 15-minute intervals during exercise using the Borg 6 20 scale. 18 After exercise, nude body mass was measured and the change in mass, corrected for fluid intake, urine losses, and water losses due to respiration and substrate exchange, was used as an estimate of sweat loss. 19 Estimates of metabolic cost and exercise intensity were calculated based on the formula of Pandolf 20 and assuming a VO 2 max of 53 ml kg 1 min 1 for this population (C. Byrne, unpublished observations). Calculated estimates were: metabolic rate (mean SD) Wor kcal hr 1, oxygen uptake 1, ml min 1 or ml kg min 1, and exercise intensity % VO 2 max or a metabolic equivalent of 5.7. Blood Analysis Venous blood (10 ml) was sampled from an antecubital vein into a plain tube (5 ml) and an ethylenediaminetetraacetic acid tube (5 ml) before exercise and after each marching cycle. The plain tube was allowed to clot for 20 minutes, then centrifuged (3,500 rpm for 10 minutes at 25 C), and serum was removed for analysis of sodium and potassium concentrations (9181 Electrolyte Analyser, Roche Diagnostics, Indianapolis, Indiana). The ethylenediaminetetraacetic acid tube was used to perform a full blood count using an automated analyzer (AC.T diff 2, Beckman Coulter, Miami, Florida). Blood glucose was measured from whole blood using a portable analyzer (Accu-Check, Roche Diagnostics, Indianapolis, Indiana). Percentage change in plasma volume from pre-exercise to the end of each marching cycle was calculated using the method described by Dill and Costill. 21 Statistical Analysis Data are presented as means SEM. Data were analyzed with SPSS (SPSS, Chicago, Illinois). Paired sample t test examined the effect of fluid (WAT versus CHO-E) on fluid balance variables. Data analyzed over time (i.e., exercise duration, T C, HR, RPE, and blood variables) were analyzed with a two-factor (fluid time) mixed model analysis of variance with repeated measures on the time factor (e.g., pre-exercise, postmarch 1, postmarch 2, postmarch 3). Statistical significance was accepted as p Post hoc analyses were performed with a paired sample t test using the Bonferonni correction procedure. Results Exercise Performance Total cumulative endurance time was longer with CHO-E ingestion but the difference was not statistically significant (Table I). Endurance time decreased progressively and significantly across marching cycles and was independent of the type of fluid ingested (Table I). Marching cycle 3 endurance time was greater with CHO-E but the difference was not significant (Table I). Although no significant influence of fluid on endurance time was observed, CHO-E resulted in a greater frequency of soldiers completing the 3 60-minute marching task (CHO-E 50% vs. TABLE I MARCHING DURATION FOR THE TOTAL MARCH AND FOR EACH CYCLE WHEN VOLUNTEERS INGESTED WAT OR CHO-E FLUID a WAT CHO-E Total marching duration (minutes) Cycle Cycle 2* Cycle 3** a Values are means SEM. Mean duration of marching cycles 2 and 3 are significantly less than marching cycle 1,, p 0.05;, p

3 717 WAT 21%) and a lower frequency of soldiers terminating exercise completely due to volitional exhaustion (WAT 50% vs. CHO-E 14%). The frequency of volunteers terminating exercise completely because of T C 39.5 C was equivalent (WAT 29% vs. CHO-E 36%). Fluid Balance The type of fluid ingested did not influence total fluid intake, rate of fluid intake, sweat loss, sweat rate, urine loss, postexercise fluid balance, or percent dehydration (Table II). The percentage of sweat loss replaced by fluid intake and the percentage of total fluid loss (sweat urine respiratory) replaced by fluid intake were similar with WAT versus CHO-E (Table II). Changes in plasma volume across marching cycles (cycle 1 WAT % vs. CHO-E %; cycle 2 WAT % vs. CHO-E %; cycle 3 WAT % vs. CHO-E %) and the mean change in plasma volume (WAT % vs. CHO-E %) were greater with WAT versus CHO-E, but no significant differences were observed. CHO-E Ingestion The rate of carbohydrate ingestion (57 3g h 1 ) during the CHO-E trial was within the 30 to 60 g h 1 rate of ingestion recommended by the American College of Sports Medicine and resulted in a total intake of g. Sodium intake from Gatorade (460 mg L 1 ) resulted in a total intake of 1, mg and a rate of intake of mg h 1. T C, HR, and RPE As a result of volunteers terminating marching cycles prematurely, repeated measures analysis of variance were performed on T C, HR, and RPE values at minute 30 of each marching cycle and the final value of each cycle (i.e., at cycle completion or termination). Fluid (WAT versus. CHO-E) did not influence T C responses at minute 30 of each cycle (Fig. 1) or the final T C of each cycle (cycle 1 WAT vs. CHO-E C; cycle 2 WAT vs. CHO-E C; cycle 3 WAT vs. CHO-E C). Similarly, no differences were observed for HR responses at minute 30 of each cycle (Fig. 2) or the final HR of each cycle (cycle 1 WAT vs. CHO-E beats min 1 ; Cycle 2 WAT vs. CHO-E beats min 1 ; cycle 3 WAT vs. CHO-E beats min 1 ). CHO-E ingestion resulted in a significantly lower RPE at minute 30 of marching cycle 1 (Fig. 3). CHO-E also resulted in a significantly lower final RPE of cycle 1 (WAT vs. CHO-E , p 0.05) and cycle 2 (WAT vs. CHO-E , p 0.05), and although this trend continued for cycle 3 (WAT vs. CHO-E ), the difference was nonsignificant. Serum Electrolytes Serum sodium concentration decreased significantly by the end of exercise in both WAT and CHO-E trials (Fig. 4). The extent of sodium decrease was equivalent for WAT and CHO-E ingestion (WAT vs. CHO-E mmol L 1 ). However, 29% of the sample recorded sodium concentrations below 135 mmol L 1 (clinical definition of hyponatremia) with WAT ingestion versus none with CHO-E ingestion. No sodium concentrations below 130 mmol L 1 or symptoms of hyponatremia were observed. Serum potassium concentration did not change significantly over time or as a function of the type of fluid ingested. Blood Glucose CHO-E ingestion resulted in significantly higher blood glucose concentrations after each marching cycle (Fig. 5). Discussion This study suggests that when the rate of fluid intake matches sweat rate, CHO-E provides no benefit over plain WAT in regulating fluid balance (i.e., urine loss, dehydration, and plasma volume change), serum sodium concentration, and attenuating core temperature and HR during 3 hours of intermittent loaded treadmill walking in 35 C ambient temperature and 55% humidity. CHO-E did elevate blood glucose, reduce perceived exertion, and increased the incidence of task completion. CHO-E therefore offers potential for enhancing exercise capacity during prolonged marching under heat stress. The primary goal of fluid replacement during prolonged exercise under heat stress is the prevention of heat illness due to the detrimental effects of dehydration on body temperature regulation and cardiovascular function. 8,9 In this regard, research suggests that low-concentration CHO-E fluid is as effective as plain WAT in attenuating core temperature and HR and maintaining stroke volume during exercise under heat stress. 22 That is, CHO-E offers no advantage (or disadvantage) with regard to TABLE II FLUID BALANCE VARIABLES WHEN VOLUNTEERS INGESTED WAT OR CHO-E FLUID A WAT CHO-E Total fluid intake (ml) 2, , Rate of intake (ml h 1 ) Total fluid loss (ml) 2, , Sweat loss (ml) 2, , Sweat rate (ml h 1 ) Urine loss (ml) (n 4) (n 6) Sweat loss replaced (%) Total fluid loss (sweat, urine, respiratory) replaced (%) 90 6% 89 6% Postexercise fluid balance ( ml) Dehydration (% of body mass) a Values are means SEM.

4 718 Fig. 1. Core temperature at 15-minute intervals during exercise when volunteers ingested WAT or CHO-E fluid. Values are means SEM., minute 30 T C is significantly higher than T C at minute 30 of cycle 1, p No significant differences were observed between WAT and CHO-E. Fig. 2. HR at 15-minute intervals during exercise when volunteers ingested WAT or CHO-E fluid. Values are means SEM., minute 30 HR is significantly higher than HR at minute 30 of cycle 1, p No significant differences were observed between WAT and CHO-E. regulating body temperature and cardiovascular function. The results of the present study support such a contention. Fluid volume rather than CHO-E content appears to be the most important fluid variable when attempting to regulate body temperature and cardiovascular function. 7 Montain and Coyle 7 reported that a large fluid volume (80% of sweat loss) was superior to no fluid, small fluid (20% sweat loss), or moderate fluid (48% sweat loss) volumes in attenuating core temperature and HR and in maintaining cardiac output during 2 hours of moderate-intensity exercise in the heat. 7 In the present study, where equal volumes of WAT and CHO-E ( 100% of sweat loss) were ingested, no differences in core temperature or HR were observed. Thus, our study provides no rationale for the addition of carbohydrate and electrolytes to replacement fluid based on improved thermoregulatory and cardiovascular regulation. This finding supports recent research investigating WAT or placebo versus CHO-E fluid replacement during exercise in the heat Fluid replacement with CHO-E does not consistently result in a greater retention of ingested fluid. 8 This may explain the failure of CHO-E to effect core temperature and cardiovascular responses in the present study and previous research. In the present study, no differences were observed between WAT and CHO-E for plasma volume change, urine loss, and dehydration, suggesting no differences in intra- or extravascular fluid volume between WAT and CHO-E. Field-based studies of marching exercise have also failed to consistently demonstrate additional hydration benefits of CHO-E over WAT or placebo fluid. 14,16 For example, during 30-km of intermittent (30-minute work:10- minute rest) loaded marching (5 km h 1, 15-kg load, C, 53 34% humidity), CHO-E did not affect fluid balance, although a nonsignificant trend for greater preservation of plasma volume was observed. 14 During more extensive exercise, CHO-E and WAT provided similar hydration benefits during a 4-day, 134-km loaded march (5 6 km h 1, 10- to 12-kg load, 50-

5 719 Fig. 3. Rating of perceived exertion at 15-minute intervals during exercise when volunteers ingested WAT or CHO-E fluid. Values are means SEM., significant difference in RPE between WAT and CHO-E at minute 30 of cycle 1, p 0.05., minute 30 RPE is significantly higher than RPE at minute 30 of cycle 1, p Fig. 4. Serum sodium concentration before exercise and after each marching cycle when volunteers ingested WAT or CHO-E fluid., WAT and CHO-E values are significantly lower than pre-exercise values, p No significant differences were observed between WAT and CHO-E. minute work:10-minute rest, 7 8 h day 1, 40% VO 2 max, C, 60 14% humidity). 16 Conversely, significant reductions in urine loss and preservation of plasma volume were observed with CHO-E versus WAT during 3 hours of walking and running exercise covering approximately 16 km (15-minute run:10- minute walk, 39 C, 28% humidity). 17 Serum sodium concentration decreased significantly by the end of exercise independent of the type of fluid ingested (Fig. 4). This finding supports previous research suggesting that the sodium content of commercial sports drinks ( 20 mmol L 1 )is inadequate to prevent a decrease in sodium concentration when attempting to prevent dehydration during prolonged exercise under heat stress. 28 Furthermore, it has been stated that if dehydration is to be prevented, both WAT and sodium sweat losses (40 80 mmol L 1 ) must be replaced during exercise. 10 This can be achieved by increasing the sodium content of replacement fluids to approximately 60 mmol L Failure to replace sodium losses during exercise may explain the failure of CHO-E fluids to positively affect fluid balance and physiological responses in comparison to WAT. 14,16,23 27 The U.S. Army s revised fluid replacement guidelines for training under heat stress recommend a maximum WAT intake of 1 quart or 950 ml h 1, in an attempt to minimize dehydration but also protect against overdrinking or hyponatremia. 9 The

6 720 Fig. 5. Blood glucose concentration before exercise and after each marching cycle when volunteers ingested WAT or CHO-E fluid., CHO-E value is significantly higher than WAT value, p International Marathon Medical Directors Association has also recently recommended an upper limit for fluid intake of 800 ml h 1 during marathon running to protect against hyponatremia. 29 Our study indicates that during moderate work ( 450 W), ingestion of ml h 1 of WAT and ml h 1 of CHO-E significantly reduced serum sodium concentration. Our findings therefore concur with the approaches described above of recommending an upper limit of fluid intake as a safety measure against hyponatremia. CHO-E ingestion (57 3g h 1 ) elevated blood glucose (Fig. 5), suggesting that glucose was absorbed and available for oxidation. 26 The metabolic basis for performance enhancement by carbohydrate ingestion during exercise is the maintenance of blood glucose, thereby preventing hypoglycemia, and maintenance of high rates of carbohydrate oxidation. 30 During prolonged low-intensity exercise under thermoneutral conditions, carbohydrate ingestion was shown to maintain blood glucose, attenuate lipolysis, and enhance time to exhaustion during lowto-moderate intensity (45% VO 2 max) treadmill walking by 11.5% ( minutes). 31 In the present study, CHO-E increased endurance time (although not significantly), increased the incidence of task completion, and reduced perceived exertion. In the absence of a placebo control, it is possible that such effects were indeed placebo effects. However, study design limitations may have masked any true differences in exercise performance. For example, ethical constraints required participants to stop exercise when core temperature reached 39.5 C and participants exercised for a maximum of 180 minutes. Hypoglycemia was not observed and glycogen availability was an unlikely performance-limiting factor during low-to-moderate intensity (30 40% VO 2 max) treadmill walking of 3-hour duration. 32 The possibility of carbohydrate ingestion affecting performance in the heat via a mechanism independent of its role in energy supply, for example, by attenuating central fatigue, warrants further study. 33 The practical implications of this research are that WAT and CHO-E fluid replacement, when consumed at rates equal to sweat loss, are equally effective in regulating thermoregulatory and cardiovascular function. When the goal is safety, fluid replacement with plain WAT is sufficient since CHO-E does not provide additional benefits. When the goal is performance enhancement, fluid replacement with CHO-E should be considered, particularly when exercise is prolonged ( 3 hours) and food intake of carbohydrates is compromised. In summary, CHO-E fluid replacement offered no benefit over plain WAT in regulating fluid balance and serum sodium concentration and attenuating core temperature and HR during 3 hours of intermittent loaded treadmill walking under heat stress. However, CHO-E did elevate blood glucose, reduce perceived exertion, and increase task completion, thereby offering potential for enhancing exercise capacity during prolonged marching under heat stress. Acknowledgments We greatly appreciate the technical assistance of Margaret Yap, Vivian Oh, and Teh Chai Hong and extend gratitude to the volunteers and commanders of the 1st and 4th Singapore Infantry Regiments. References 1. Sawka MN, Montain SJ: Fluid and electrolyte supplementation for exercise heat stress. Am J Clin Nutr 2000; 72: 564S 72S. 2. Bean WB, Eichna LW: Performance in relation to environmental temperature: reactions of normal young men to simulated desert environment. Fed Proc 1943; 2:

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