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Effect of Preexercise Electrolyte Ingestion on Fluid Balance in Men and Women

JOHANNSEN, NEIL M.1; LIND, ERIK2; KING, DOUGLAS S.3; SHARP, RICK L.3

Medicine & Science in Sports & Exercise: November 2009 - Volume 41 - Issue 11 - p 2017-2025
doi: 10.1249/MSS.0b013e3181a82940
BASIC SCIENCES
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Purpose: This article aimed to study the effect of preexercise ingestion of an electrolyte-containing beverage and meal on fluid balance during exercise in men and women.

Methods: Twenty healthy, college-aged people (10 males, 10 females; mean ± SD = 51.2 ± 9.8 mL·kg−1·min−1) exercised at 58 ± 4% V˙O2peak for 90 min, 45 min after ingesting 355 mL of chicken noodle soup (SOUP; 167 mmol·L−1 Na+), carbohydrate-electrolyte beverage (CE; 16 mmol·L−1 Na+), or water (WATER). After 90 min of exercise, participants completed a physical performance task (PPT) consisting of the calculated work that would be completed in 30 min at 60% V˙O2peak (n = 19). Water was allowed ad libitum throughout all trials.

Results: Fluid balance was improved in SOUP compared with WATER (−251 ± 418 vs −657 ± 593 g, respectively; P = 0.002) because of greater water intake and retention throughout the trial. Water intake was also greater in CE compared with WATER mostly because of an increase during the PPT. Plasma osmolality increased after ingestion of SOUP and remained elevated throughout exercise compared with both CE and WATER. Men and women had similar fluid balance results, with women having lower relative water intake and evaporative water losses compared with men. Physical performance was similar in all trials.

Conclusions: SOUP ingested before exercise improves fluid balance because of increased ad libitum water intake and reduced proportional urinary water loss. The increase in water intake and, subsequently, the improved fluid balance may be because of a greater plasma osmolality before and throughout exercise.

1Department of Preventive Medicine, Pennington Biomedical Research Center, Baton Rouge, LA; 2Department of Physical Education, SUNY College at Oneonta, Oneonta, NY; and 3Department of Kinesiology, Iowa State University, Ames, IA

Address for correspondence: Neil M. Johannsen, Ph.D., Department of Preventive Medicine, Pennington Biomedical Research Center, 6400 Perkins Road, Baton Rouge, LA 70810; E-mail: neil.johannsen@pbrc.edu.

Submitted for publication October 2008.

Accepted for publication April 2009.

The American College of Sports Medicine (ACSM) position stand on exercise and fluid replacement sets general guidelines for fluid intake during and after exercise to offset normal body fluid losses in sweat and urine (28). These guidelines are based on extensive research focusing on hydration during and after exercise and, for the most part, neglect preexercise hydration status. Preexercise hyperhydration studies using water alone or in combination with osmotic agents, such as glycerol, have yielded equivocal results and may increase the risk of excess urination during exercise (1,5,11,16,17,19). However, the authors of the position stand recognized that the consumption of sodium-containing beverages, snacks, or small meals may stimulate thirst and reduce urine output (28).

Research performed in our laboratory suggests that ingestion of 350 mL of chicken noodle soup (167 mmol·L−1 Na+) after exercise/thermal-induced dehydration expedites recovery of plasma volume compared with an equal quantity of water or commercially available carbohydrate-electrolyte beverage (25). Beverages and meals containing high concentrations of electrolytes, especially sodium, also promote fluid retention by reducing water losses in urine (10,21,22,30). Sims et al. (31,32) noted that a high-sodium beverage (164 mmol·L−1 Na+) before exercise led to improved fluid balance and reduced urine output compared with a low-sodium beverage (10 mmol·L−1 Na+) in both men and women despite absolute differences in sweat rate. Similarly, Greenleaf et al. (9) reported improved total body water balance after exercise when a beverage containing a high concentration of sodium (∼164 mEq·L−1) was ingested before exercise compared with several beverages containing lower sodium concentrations. In additional studies, Greenleaf et al. (8) also determined that beverages containing higher sodium concentrations reduced urine output and improved endurance performance (7) compared with beverages that were more dilute. However, the aforementioned studies by Sims et al. (31,32) and Greenleaf et al. (8) either restricted or controlled water intake during the exercise phases of the studies, making it impossible to determine the effect of sodium-containing beverages on ad libitum water intake after mild hyperhydration.

This study was conducted to determine whether ingestion of a sodium-containing beverage (commercially available carbohydrate-electrolyte beverage) or meal (chicken noodle soup) before exercise improves fluid balance by increasing ad libitum water intake and by reducing urinary water losses in men and women. Secondarily, we wanted to determine whether men and women respond similarly to preexercise ingestion of sodium-containing beverages. We hypothesized that chicken noodle soup ingested 45 min before exercise would increase ad libitum water intake and reduce urine output during exercise leading to improved fluid balance compared with water intake before exercise in both sexes. We further speculated that ad libitum water intake and urine output after a carbohydrate-electrolyte beverage would be similar to water with no commensurate improvement in fluid balance. Last, we believed fluid balance responses would be similar in both men and women despite potential differences in sweat rates and fluid consumption during exercise (6,12,13,15).

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METHODS

Participant characteristics.

Twenty healthy, college-aged (mean ± SD: 24 ± 3 yr, 174.4 ± 9.3 cm, and 68.8 ± 11.9 kg) men and women (n = 10 each) of varying fitness levels (V˙O2peak = 51.2 ± 9.8 mL·kg−1·min−1) volunteered to participate in this study. One male participant was unable to complete two of the three trials, and all his data were removed from the analyses. The physical characteristics of the 19 remaining participants separated by sex are presented in Table 1. Before any physical testing, participants completed a medical history questionnaire that was reviewed and approved by the primary investigators. The study was approved by the Iowa State University Institutional Review Board, and all participants gave written informed consent.

TABLE 1

TABLE 1

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Preliminary testing.

After initial study consent, body mass and height were measured, and participants completed a graded exercise test to exhaustion on an electronically braked cycle ergometer (Lode BV, Groningen, the Netherlands) to determine peak oxygen uptake (V˙O2peak). The incremental, graded exercise test began at 50 and 40 W for men and women, respectively, and was increased by 50 and 40 W, respectively, every 3 min until participants reached volitional fatigue. Respiratory gases were analyzed with computer-interfaced oxygen and carbon dioxide analyzers calibrated with standard gas mixtures (Physio-dyne Instrument Corp., Quogue, NY).

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Experimental protocol.

Three trials were completed in a randomized, counterbalanced order separated by at least 1 wk. The trials differed by the beverage ingested 45 min before exercise and the amount of water consumed during exercise. The beverages ingested before exercise were chicken noodle soup (SOUP), commercially available carbohydrate-electrolyte beverage (CE), and commercially available bottled, distilled water (WATER). All beverages were served at temperatures in which they are typically consumed (SOUP = 50°C, CE = 4°C, WATER = 20°C). Table 2 provides the nutrient composition of the experimental beverages. Participants were allowed to drink room temperature water (20°C; same water that was ingested before the WATER trial) ad libitum during exercise. Three-day diet and physical activity diaries were kept before the first trial and replicated before subsequent trials. Participants were also required to drink at least one extra liter of water, fast for at least 10 h, and to refrain from exercise the day before each trial. Throughout the study, women recorded the number of days that had passed since the start of their last menstrual cycle and their typical cycle length to use as covariates if necessary.

TABLE 2

TABLE 2

Upon entering the laboratory, participants voided and gave a urine sample. A rectal thermometer (YSI 401, Dayton, OH) was inserted to a depth of 10 cm past the anal sphincter, an HR monitor (Polar Electro Oy, Kempele, Finland) was attached, and a nude body mass was recorded to the nearest 0.05 kg (Befour, Inc., Saukville, WI). A Teflon indwelling catheter was inserted into an antecubital vein, and participants rested quietly for 10 min before a resting blood sample was drawn. Three milliliters of sterile saline was infused after each blood draw to keep the catheter patent. All blood samples were taken without stasis and placed in potassium ethylenediaminetetraacetic acid (K3EDTA; BD Biosciences, San Jose, CA) collection tubes. Hematocrit (Hct) and hemoglobin concentrations were measured immediately after the blood draw. The remaining blood was centrifuged at 400g for 10 min, and the resultant plasma was stored at −20°C until further analyses. Participants also indicated their current rating of perceived thirst (Likert scale 1-9, 9 = very, very thirsty). Participants then ingested 355 mL of the designated experimental beverage and rested quietly in a seated position for 35 min. No additional fluid was allowed during the rest period.

After the 35 min of rest, nude body mass was measured again, and participants rested on the ergometer from 40 to 45 min while all preingestion (Pre-I) measurements were retaken. A preexercise (Pre-Ex) blood sample was drawn exactly 45 min after ingestion of the experimental beverage, and participants began the metabolic phase of the study consisting of 90 min of steady-state exercise at 58 ± 4% V˙O2peak under normal indoor conditions (∼20°C at 30% relative humidity). Every 30 min (EX30, EX60, and EX90) during steady-state exercise, a blood sample was drawn and perceived thirst was recorded. HR and rectal temperature were monitored continuously, and exercise was halted if HR or rectal temperature exceeded 180 bpm or 39.5°C, respectively. Expiratory gases were sampled during the last 5 min of each 30-min interval to ensure consistent metabolic stress between trials. The mass of water ingested during the metabolic phase was recorded to the nearest 0.01 g immediately after each drink. However, participants were blinded to the mass and timing of water ingested with each drink.

After 90 min of steady-state exercise, participants were weighed again after toweling dry, remounted their cycle ergometer, and began the physical performance task (PPT). The amount of time between the metabolic phase and the subsequent PPT was 5 min. The PPT was the time to finish the amount of work equivalent to 30 min at 60% of V˙O2peak. Participants were given no encouragement or indication of time or revolutions per minute during the PPT except verbal acknowledgement when they had completed 25%, 50%, 75%, and 100%. Participants were allowed to drink water ad libitum during the PPT, and the mass of water ingested was recorded to the nearest 0.01 g immediately after the trial. After the PPT, dry, nude body mass was measured and total urine volume was collected.

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Biochemical analyses.

Blood samples were analyzed in triplicate for hemoglobin concentration by cyanmethemoglobin spectrophotometric assay (Stanbio, Boerne, TX) and Hct by microcapillary centrifugation. Hct values were corrected for trapped plasma volume between red blood cells (0.96) and venous-total body Hct ratio (0.91) (2). Percent plasma volume change was calculated using the equations of Dill and Costill (3) with Pre-I as a reference. Plasma and urinary osmolalities were measured by vapor pressure osmometry (Westcor, Inc., Logan, UT), and sodium concentrations were measured by flame photometry (Cole-Parmer, Chicago, IL). Recent research indicates that measurement of plasma osmolality is susceptible to freeze-thaw cycling (29). Although freezing tends to lower the plasma osmolality, we consistently measured osmolality after the first freeze-thaw cycle in every participant. Plasma glucose concentration was measured by enzymatic spectrophotometric assay (Stanbio Laboratory), and urine-specific gravity was determined by handheld spectral refractometry (Leica Microsystems, Inc., New York, NY). All plasma and urine measurements were performed in duplicate.

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Calculations.

Fluid balance was calculated as shown in equation 1, where BMPre-I is the initial body mass (g), BMPost-PPT is the body mass (g) immediately after PPT and before urination, and UTot (g) is the estimated total urine mass (Post-PPT urine volume × specific gravity) after PPT.

The efficacy of preexercise beverages to increase water retention independent of ad libitum water intake during exercise was determined by calculating the percent water retention as shown in equation 2.

Percent dehydration after the metabolic phase of exercise was calculated as the percent change in body mass from Pre-I to Post-PPT after adjusting the Post-PPT body mass for estimated total urine mass (UTot). We used the Post-PPT body mass and total urine mass (UTot) rather than body and urine mass after the metabolic phase (EX90) to calculate changes in fluid balance because we wanted to test the effectiveness of each beverage in a competition setting (equal quantity of work for all three trials within a participant).

Total evaporative losses, including respiratory and sweat losses, were calculated by adding the total amount of fluid ingested to the total body mass lost after accounting for the estimated urine mass (UTot). Because evaporative losses can be influenced by body surface area (BSA) and total trial time, evaporative water losses were adjusted for BSA using the DuBois equation (4) and for total exercise time (90-min steady-state exercise + PPT time) and expressed as grams of water lost per BSA (m2) per hour.

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Statistical analyses.

Statistics were performed with JMP 7.0.1 (SAS, Cary, NC) statistical software. Baseline body mass, plasma osmolality, and urine-specific gravity were compared using one-way repeated-measures (RM) ANOVA to determine the efficacy of pretrial dietary and exercise control. Mean percent V˙O2peak during exercise was analyzed using a one-way RM ANOVA to ensure consistent exercise intensity between trials. Fluid balance, ad libitum water intake, percent water retention, total evaporative losses, urine variables (volume, specific gravity, osmolality, and sodium output), physical performance time, and mean wet bulb globe temperature were also analyzed using one-way RM ANOVA. Plasma variables (plasma volume, glucose, sodium, and osmolality) and perceived thirst were subjected to two-way (treatment × time) RM ANOVA. Sex differences were also investigated using two- and three-way ANOVA, with treatment and/or time as an RM and sex treated as a split plot. Significant treatment (preexercise beverage), time (Pre-I, Pre-Ex, EX30, EX60, and EX90), sex, and interaction effects were evaluated further using Tukey HSD post hoc analyses when appropriate (P < 0.05). Ratings of perceived thirst were collected on 12 of the 19 participants (n = 4 for men). All values are reported as mean ± SD, and statistical differences were declared if P < 0.05.

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RESULTS

Study control.

Pretrial body mass, urine specific gravity, and plasma osmolality were similar for all trials (P = 0.72, 0.76, and 0.27, respectively), indicating adequate pretrial dietary (food and fluid) and physical activity control. Mean percent V˙O2peak during state exercise was also similar for all trials (56.5 ± 4.0, 57.1 ± 4.2, and 56.7 ± 3.8 mL·kg−1·min−1 for WATER, CE, and SOUP, respectively; P = 0.59) and for each sex (P = 0.28). Mean wet bulb globe temperature was 16.0 ± 1.5°C and was similar for all trials (P = 0.71).

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Fluid balance.

Total ad libitum water intake (Table 3) was greater after ingestion of SOUP (1227 ± 602 g) and CE (1062 ± 675 g) compared with WATER (852 ± 554 g). Total water intake during the SOUP trial was greater than WATER because of a persistent increase throughout exercise and during the PPT (Table 3). However, the greater water intake during the CE trial compared with WATER primarily occurred during the PPT (Table 3). Although water intake was clearly greater in SOUP, perception of thirst increased similarly across all trials (treatment × time interaction, P = 0.89). Urine output was similar after each trial (Table 4). However, the percent of ingested water retained was higher after SOUP compared with WATER (79.7 ± 26.0% vs 61.7 ± 37.1%; P < 0.05). Urine specific gravity after SOUP was significantly higher than WATER, and urine osmolality was higher after SOUP compared with both WATER and CE (Table 4). The differences in urine specific gravity and osmolality after SOUP were not related to urinary sodium output (P = 0.24; Table 4), although sodium intake was higher in SOUP compared with both WATER and CE (Table 2). Calculated total evaporative losses (sweat + respiratory losses) were similar across all trials (1.63 ± 0.57, 1.63 ± 0.61, and 1.65 ± 0.56 kg for WATER, CE, and SOUP, respectively; P = 0.92).

TABLE 3

TABLE 3

TABLE 4

TABLE 4

Fluid balance was improved after SOUP compared with WATER (−251 ± 418 vs −657 ± 593 g; trial effect, P = 0.002; Fig. 1). Carbohydrate-electrolyte beverage ingestion before exercise had an intermediate effect on fluid balance and was not different from SOUP or WATER (−429 ± 607 g). Further analysis using 95% confidence intervals (CI) revealed that CE (95% CI = −687 to −172 g) and WATER (95% CI = −914 to −399 g) ingestion before exercise did not prevent a significant reduction in fluid balance (95% CI for fluid balance <0), whereas SOUP permitted adequate fluid balance (95% CI = −508 to +7 g).

FIGURE 1-F

FIGURE 1-F

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Plasma constituents.

Plasma osmolality was consistently greater in SOUP compared with both WATER and CE starting before and persisting throughout exercise becoming statistically different from CE after 60 min and CE and WATER after 90 min of exercise (interaction effect P = 0.003; Fig. 2). However, the effect of beverage composition on plasma osmolality was independent of plasma sodium concentration (treatment effect P = 0.78; Fig. 3). Despite marked differences in plasma osmolality and fluid balance with SOUP, plasma volume responses were similar for all trials with a normal negative plasma volume shift after the start of exercise (time effect, P < 0.001; Fig. 4). Plasma glucose responses were similar between treatment groups during rest except for a small increase after ingestion of CE. Subsequently, after the onset of exercise, plasma glucose significantly dropped after CE compared with both SOUP and WATER (interaction effect, P < 0.001). Plasma glucose concentrations also remained significantly lower throughout the trial after CE ingestion compared with resting levels (P < 0.05).

FIGURE 2

FIGURE 2

FIGURE 3

FIGURE 3

FIGURE 4

FIGURE 4

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Physical performance time.

The total amount of work to complete during the PPT (30 min at ∼60% V˙O2peak) was 271.3 ± 80.7 kJ and differed by sex (214.2 ± 43.2 vs 334.8 ± 62.7 kJ for women and men, respectively; P < 0.001). Time to complete the set amount of work during the PPT was independent of beverage ingested before exercise (27.6 ± 9.6, 25.9 ± 5.7, and 27.8 ± 9.0 min for WATER, CE, and SOUP, respectively; P = 0.36).

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Sex differences.

Men drank approximately twice as much water compared with women throughout the trial (Table 3; P = 0.003) even after adjusting for body mass (mL·kg−1 body mass·h−1; P = 0.03). The greater water intake in men was not related to ratings of perceived thirst (P = 0.44). Men and women had similar urine output (P = 0.42); however, because the rates of water intake were different, percent water retention was significantly greater in men (P < 0.009). Women produced more dilute urine than men after every treatment (sex effect for urine specific gravity and osmolality, P = 0.003 and 0.002, respectively; Table 4), although urine sodium output was similar (P = 0.29; Table 4). Women had lower total evaporative losses compared with men (1.26 ± 0.27 vs 2.06 ± 0.51 kg) even after adjusting for BSA and total exercise time (376 ± 80 vs 500 ± 128 g·m−2·h−1; P < 0.05 for both). Fluid balance improvements observed in SOUP compared with WATER were present in both men and women (sex × treatment interaction, P = 0.82; Fig. 1). Women and men responded similarly for plasma osmolality and sodium and glucose concentrations (sex effect, P = 0.16, 0.44, and 0.96, respectively). However, women tended to have a greater plasma volume for all trials (sex effect, P = 0.09). Physical performance was similar in men and women (P = 0.17).

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DISCUSSION

The main outcome of this study was that ingestion of only 355 mL of chicken noodle soup before exercise improved fluid balance by increasing ad libitum water intake and by reducing the amount of ingested fluids lost in urine during 90 min of exercise and a subsequent performance task in a normal indoor environment. In fact, on average, participants were in fluid balance after exercise in the SOUP trial as indicated by the 95% CI overlapping zero. The effect of SOUP on fluid balance was similar in both men and women, with women having lower rates of evaporative losses, drinking less water ad libitum during exercise, and losing greater quantities of ingested water to urine. The aforementioned results were not observed in either the CE or the WATER trials, indicating a novel approach to preserving fluid balance during exercise. These results provide the first direct support for preexercise fluid intake recommendations set forth by the ACSM position stand on hydration stating that preexercise sodium-containing beverages/meals may improve water intake and urinary water output during exercise (28). Although we did not see differences in physical performance, the improvement in water intake and fluid balance after soup may delay the onset of critical dehydration during long exercise bouts and/or exercise in a heated environment.

Consistent with our first hypothesis, approximately 90% of the improvement in fluid balance in SOUP compared with WATER was due to an increase in ad libitum water intake (∼375 g of the ∼406 g difference in fluid balance). The increase in water intake in SOUP did not simply occur at the onset of exercise but persisted throughout the steady-state exercise and PPT (Table 3) unlike CE, which only seemed to increase water intake during the PPT compared with WATER. Ad libitum water intake during exercise can be influenced by beverage temperature, flavor, and electrolyte (especially sodium) content (14,23,26,34,35). Wemple et al. (34) demonstrated that during recovery from dehydrating exercise (∼3% dehydration), beverages with a sodium concentration of 50 mmol·L−1 resulted in improved fluid balance after 3 h of recovery compared with water ingestion and improved the dipsogenic response compared with lower sodium concentrations (25 mmol·L−1). These data are supported by Nose et al. (23) who found greater net fluid gain with the addition of NaCl to ingested water during recovery. During exercise, fluid intake is greater with beverages containing as low as 18 mmol·L−1 sodium compared with water (14,35). Given this, it is likely that the higher sodium content in SOUP compared with WATER and CE (Table 2) resulted in greater ad libitum water intake during exercise.

Although the aforementioned studies have determined that electrolyte ingestion during and after exercise increases fluid intake, few studies have investigated electrolyte supplementation before exercise. Sims et al. (31,32) examined the effects of preexercise sodium loading (high [Na+] = 164 mmol·L−1; low [Na+] = 10 mmol·L−1) on fluid balance and exercise tolerance in the heat. In these studies, urine output, fluid balance, and exercise performance were improved by additional sodium in the preexercise beverage. Similarly, Greenleaf et al. (7-9) found improved total body water balance, reduced urinary water loss, and improved endurance performance after ingesting beverage with high sodium content compared with no fluid intake or ingesting low-sodium beverages. However, these studies either restricted water or controlled water intake, and the contribution of sodium ingestion to ad libitum water intake during exercise was not studied. In the present study, we demonstrated that ingestion of a beverage containing electrolytes caused a persistent increase in water intake, resulting in greater improvements in fluid balance during exercise compared with those in Sims et al. (31,32). These results are similar to studies investigating preexercise glycerol hyperhydration (17), indicating that, to maximize the benefits associated with preexercise fluid intake, water consumption is required throughout exercise.

Water intake was greatly increased after SOUP compared with WATER, although perception of thirst was similar in all trials. A study by Passe et al. (24) determined that runners could accurately estimate the quantity of fluid ingested during endurance exercise under compensable heat stress but were unable to accurately judge water losses in sweat resulting in voluntary dehydration. This could indicate that drinking behavior is directly tied to perception of thirst and, to a lesser extent, sweat losses during exercise. In our study, it is logical to assume that participants ingested water during exercise to lower thirst to a given set point. If this is true, chicken noodle soup ingestion before exercise may have changed a physiologic set point thus requiring additional water intake to attenuate their perceptions of thirst.

Fluid balance improvements in the SOUP trial are not entirely explained by increased ad libitum water intake during exercise. The quantity of urine produced was independent of the preexercise treatment; however, water retention was greater in SOUP, representing a reduction in the amount of urine produced per volume of water ingested (Table 4). Previous research has demonstrated that ingestion of electrolytes (such as chicken noodle soup) reduces the quantity of urine produced during exercise or rehydration (8,10,22,23,25,27,30-32). Likewise, the specific gravity and osmolality of urine is increased without commensurate increases in total sodium output indicating greater water conservation by the kidney in SOUP. This finding suggests that intake of beverages with high electrolyte concentrations before exercise forces the body to conserve ingested water during exercise.

The increase in water intake and urinary water retention may be directly related to the higher plasma osmolality observed in SOUP (Fig. 2). Thompson et al. (33) demonstrated that when plasma osmolality is greater than 285 mOsm·kg−1, small increases in plasma osmolality significantly affect the release of arginine vasopressin. Maresh et al. (20) also found a close association between plasma osmolality during exercise in the heat and arginine vasopressin, plasma renin activity, aldosterone, and ratings of perceived thirst. The authors of this study (33) demonstrated that greater quantities of water were ingested during exercise if exercise began in a dehydrated (∼4%) compared with a euhydrated state. They also showed that perceived thirst and hormonal responses subsided when participants were allowed to drink during exercise compared with complete water restriction (20). Our study seems to support these data, indicating that higher plasma osmolality before and during exercise may be responsible for increased water intake and retention and that individuals will drink sufficient water to alleviate thirst (no difference in thirst between trials) before completely offsetting the dipsogenic drive that may have been initiated by the increase in plasma osmolality.

Of particular interest is the observation that plasma osmolality increased in SOUP independent of plasma sodium content. Although this seems contradictory because of the high sodium content in SOUP, other studies have similar findings. For example, Ray et al. (25) found that plasma osmolality remained higher during 2 h of recovery from thermal/exercise-induced dehydration after chicken noodle soup ingestion compared with water, although plasma sodium concentrations were similar to water and a carbohydrate-electrolyte beverage during rehydration. However, unlike the current study, Ray et al. (25) reported higher sustained plasma osmolality after carbohydrate-electrolyte beverage intake during recovery. Also, Maughan et al. (21) did not observe an increase in plasma sodium or osmolality compared with a carbohydrate-electrolyte beverage during recovery after eating a high-sodium meal. There are two possible explanations that exist for the sodium-independent increase in plasma osmolality: either (a) the additional carbohydrate, fat, and protein contained in the chicken noodle soup contribute to the increase in osmolality or (b) the chicken noodle soup ingestion causes interstitial water from the gastrointestinal tract to flow into the lumen subsequently reducing available plasma water and increasing plasma osmolality (18). However, substantial differences in plasma volume were not observed between trials, further confounding which of the aforementioned explanations is responsible for changes in plasma osmolality.

Marked differences in responses between men and women were observed in this study, including a lower rate of evaporative water loss, a reduction in ad libitum water intake, and a lower percent water retention compared with men even after controlling for either body weight or BSA. These results were expected and confirm our final hypothesis that, despite normally lower evaporative water losses in women (6,12,13,15), SOUP promotes similar fluid balance in women and men. We believe this is the first study to report this finding and further supports our idea that ingestion of SOUP before exercise changes perceptions of thirst even when evaporative losses are different between men and women.

In light of the important contribution to uncovering possible mechanisms underlying fluid balance by preexercise hyperhydration, there are, nonetheless, some minor limitations to the investigation. First, we did not control for menstrual cycle in the women in this study, and two women were on common birth control therapy. However, in lieu of this, we did record the length of normal menstrual cycle and day since the last onset to categorize by follicular or luteal phase. After examining fluid balance and its components (i.e., water intake and losses) by menstrual category, the main outcomes of the study remained significant. Second, we were unable to distinguish between the effects of the sodium and the other macro- and micronutrients in SOUP. Although this is a methodological drawback, we felt that given the recommendations of the ACSM position stand (28) to ingest high-sodium-containing beverages/meals or snacks before exercise and the abundance of research during and after exercise implicating sodium as the main electrolyte involved in fluid balance, we were justified in using a readily available food that could be used before exercise rather than a salt water drink. Third, we did not have our participants urinate after the steady-state exercise, limiting our ability to determine fluid balance before the PPT. Last, our estimation for fluid balance does not account for metabolic water gain, water liberated from glycogen utilization, or body weight losses from substrate metabolism (CO2 production). Although the sum of these gains and losses could be significant, the within-subject variability would likely be small considering mean V˙O2 and RER during exercise were similar, indicating similar rates of energy expenditure and substrate utilization. Although the aforementioned limitations are important to consider and should be taken into account in future research, we feel they do not detract from our results, indicating improved fluid balance after SOUP ingestion compared with WATER due to an increase ad libitum water intake and retention in both men and women under normal, competition-like exercise.

In conclusion, ingestion of 355 mL of chicken noodle soup 45 min before the onset of exercise improved fluid balance by increasing water intake and retention during 90 min of steady-state exercise at 55% V˙O2peak in both men and women. This study provides a novel mechanism for improving fluid balance during aerobic exercise after mild hyperhydration. Future research should investigate the mechanism by which chicken noodle soup potentiates greater fluid balance, the extent to which plasma sodium and osmolality are involved, and whether the additional body water will delay the onset of significant dehydration during long-duration exercise and/or exercise in a heated environment.

The results of the present study do not constitute endorsement by the ACSM. This research was funded by Campbell® Soup Company. The authors would like to thank the participants and all of the graduate and undergraduate students that contributed to data collection.

Conflict of interest: none.

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Keywords:

HYPERHYDRATION; WATER INTAKE; SODIUM; PERFORMANCE

©2009The American College of Sports Medicine