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Ad Libitum Fluid Replacement in Military Personnel during a 4-h Route March


Medicine & Science in Sports & Exercise: September 2010 - Volume 42 - Issue 9 - p 1675-1680
doi: 10.1249/MSS.0b013e3181d6f9d0

Introduction: Opportunities to determine optimal rates of fluid ingestion could reduce the mass soldiers might need to carry on military missions.

Purpose: The first objective was to evaluate the effects of an ad libitum fluid replacement strategy on total body water (TBW), core temperature, serum sodium concentrations [Na+], and plasma osmolality (POsm). The second objective was to determine if an ad libitum water intake was sufficient to maintain these variables during exercise. A third objective was to determine if changes in body mass are an accurate measure of changes in TBW.

Methods: A field study was conducted with 15 soldiers performing a 16.4-km route march. The average age of 15 subjects was 27 yr (SD = 4.6 yr).

Results: Their mean hourly ad libitum fluid intake was 383 mL (SD = 150 mL). Predicted sweat rate was 626 ± 122 mL·h−1. Despite an average body mass loss of 1.0 kg (SD = 0.50 kg) TBW, POsm and serum [Na+] did not change significantly during exercise. There was a significant (P < 0.05) linear relationship with a negative slope between postexercise serum [Na+] and changes in both body mass and percentage of TBW. Postexercise POsm and serum [Na+] were significantly related (P < 0.05). Higher postexercise percentage of TBW was associated with lower postexercise POsm and serum [Na+] levels. There was no relation between percent body mass loss and postexercise core temperature (38.1°C ± 0.6°C).

Conclusions: A mean ad libitum water intake of 383 mL·h−1, replacing approximately 61% of body mass losses during 4 h of exercise, maintained TBW, core temperature, POsm, and serum [Na+] despite a 1.4% body mass loss. A reduction in body mass of 1.4% (1.0 kg) was not associated with a reduction in TBW.

1ERGOnomics TECHnologies, Pretoria, SOUTH AFRICA; 2Department of Biokinetics, Sport and Leisure Sciences, University of Pretoria, Pretoria, SOUTH AFRICA; and 3UCT/MRC Research Unit for Exercise Science and Sports Medicine, University of Cape Town, Rondebosch, SOUTH AFRICA

Address for correspondence: Heinrich Nolte, M.A. (H.M.S.), Biokinetics, ERGOnomics TECHnologies, PO Box 6264, Pretoria 0001, South Africa; E-mail:

Submitted for publication October 2009.

Accepted for publication January 2010.

Soldiers are expected to carry heavy loads to ensure mission success. Every kilogram added to these loads increases the physiological burden. Concerted efforts are being made to minimize the soldier's load by optimizing every component contributing to this load. The Research and Technology Organisation of the North Atlantic Treaty Organization reports instances in which US soldiers deployed in Afghanistan carried in excess of their body mass in mountainous terrain at altitudes approaching 3000 m (34).

Water carriage contributes significantly to the soldier's load. For example, during deployment in Afghanistan, US soldiers often carried water supplies for missions lasting between 1 and 3 d representing 9-10 kg or in excess of 30% of their fighting load (34). One of the most hotly debated topics in both military medicine (12) and exercise sciences (35) is the volume of water that persons exercising in hot, arid environments need to ingest to optimize their performance and to maintain their health.

In the early 1990s, new drinking guidelines were adopted by the US military that encouraged high rates of fluid ingestion. The goal of these new guidelines was to optimize performance and to reduce the risk of "heat injury." Adoption of these drinking guidelines led to an increased number of cases of exercise-associated hyponatremia (EAH) in the US military (11). The incidence of EAH fell rapidly in the US Army (6) and elsewhere (41) with the adoption of more conservative drinking guidelines (26), which specifically mandated against the overconsumption of fluids, either water or a sports drink, during exercise (29). These guidelines superseded the 1996 American College of Sports Medicine guidelines, which advocated that athletes should drink "as much as tolerable" during exercise (8). The modern emphasis is now on individualized drinking behaviors, the goal of which is to limit body water losses to <2% of body mass during exercise (36).

This altered emphasis has provided the opportunity to determine the optimal rates of fluid ingestion by military personnel. In turn, this could reduce the mass in the form of water soldiers might need to carry on military missions. Accordingly, we posed the following questions: What are the rates of fluid ingestion freely chosen by soldiers during a 4-h route march? Are these freely chosen (ad libitum) rates of fluid ingestion sufficient to protect against major fluid and electrolyte imbalances?

Thus, the objective of the field study was to evaluate the effects of ad libitum fluid replacement on total body water (TBW), core temperature, serum sodium concentration [Na+], and plasma osmolality (POsm) during a simulated 4-h route march in professional soldiers. We wished to determine if ad libitum water intake during a typical military exercise is sufficient to maintain these variables within the homeostatic range. Because ad libitum drinking is usually 30%-50% of the volume ingested when drinking "as much as tolerable" and substantially less than the 1000 to 1800 mL·h−1 originally proposed for US military personnel in the 1990s (25), such a drinking regime could produce significant mass savings for military personnel.

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Subject selection.

Ethical clearance for this study was obtained from one military hospital research ethics committee within the South African Military Health Services of the South African National Defence Force. Twenty Operational Emergency Care Practitioners were identified and invited to volunteer for this study. All were experienced and conditioned to route marches with payloads of up to 35 kg, medically fit to participate in the study and without any musculoskeletal injuries. Subjects were told that they could terminate their participation at any stage without any consequences to their careers. All were required voluntarily to sign an informed consent form before they were accepted for participation in the study. The subjects were asked to provide basic demographic information for record purposes.

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Exercise intervention.

The route march was of 16.4 km, comprising four laps of 4.1 km each. Subjects were asked to pace themselves with a GPS at an average speed of 5 km·h−1. Each participant carried a minimum mass of 17 kg plus 2 L of water. All backpacks were packed in a similar configuration and weighed before the start of the exercise. Water was available for replenishment if required at the start of each lap. Soldiers were instructed to drink ad libitum during the march. Their core body temperatures were measured at 1-min intervals with a CorTemp ™2000 ambulatory remote sensing system (HQ Inc., Palmetto, FL). The wet bulb globe temperature (WBGT) index, relative humidity, and wind speed were monitored for the duration of the exercise (Davis Health Environmental Monitor and Questemp, Quest Technologies, South Africa).

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Hydration markers.

Before the exercise intervention, each subject emptied his or her bladder and provided a saliva sample to be analyzed for background deuterium enrichment. Because of the ease and noninvasive nature of its collection, saliva is the most convenient fluid in which to measure deuterium enrichment. Furthermore, several studies have established that saliva is a valid sampling medium for determining TBW using the diluted isotope technique (14,37,43). It has been established that enrichments of deuterium oxide in saliva and plasma samples were identical and both reached plateau, 2 h after administration of an oral dose of the tracer. Determining TBW with the diluted isotope technique remains the most reliable method currently available, producing lower coefficient of variation values than rival methods such as bioelectrical impedance (39). A preexercise blood sample (5 mL) was collected from the antecubital vein to determine serum [Na+] and POsm. The samples were collected after the subject had been in a seated position for 45 min. Subjects were weighed wearing only their underpants (to the nearest 0.1 kg). Deuterium oxide doses (±0.05 g·kg−1 body mass) were premixed from 99% deuterium oxide. Appropriate weighing of the dose bottle (to the nearest 0.1 g) was performed to determine the exact dose consumed. After a 2-h equilibration period (4,9,17,23,38,40,42), a second saliva sample was collected to determine the preexercise total body water (TBW). At the completion of the exercise each subject was provided with a towel to dry excess perspiration before reweighing. A third saliva sample was collected and used for the determination of postexercise deuterium abundance. The subjects then received their postexercise deuterium dose before commencing a 2-h equilibration period. Urine voided during this period was collected and analyzed to correct for isotope loss. A postexercise blood sample was collected after 45 min of rest after completion of the march. This period ensured that metabolic rates were closer to that of the preexercise level and allowed for the majority of fluid loss through postexercise sweating to have ceased. This period also allowed the exercise-induced plasma volume shifts to return to preexercise levels. No food or fluids were allowed during this 2-h period. Samples were again drawn after subjects had been seated for 45 min as was the procedure for the preexercise sample. A final saliva sample was collected and body mass remeasured to calculate postexercise TBW. Total body water (TBW), which is comprised of extracellular fluid and intracellular fluid, averages approximately 60% of body mass. However, because of the influence of body composition, specifically individual variances in fat free mass, the range has been reported to vary from approximately 45%-75% of TBW (37). TBW (kg) was calculated using the preferred method of Halliday and Miller (14) according to the following equation:

in which, A = amount of dose solution drunk (g), a = amount of dose solution diluted in T (g), T = amount of tap water into which a was diluted, Ea = enrichment of diluted dose, Et = enrichment of tap water used to dilute the dose, Ep = enrichment of baseline sample, Es = enrichment of postdose sample, and 1.04 = correction factor for over estimation due to exchange with nonaqueous hydrogen.

Corrections were made for the ingestion of the isotope dose, metabolic water production, and water added to the TBW pool through exchange with atmospheric moisture according to the methods of Schoeller et al. (37). Because most of the correction factors depended on metabolic rate, additional corrections were made for the postexercise equilibration period during which an increased metabolic rate (excess postexercise oxygen consumption), although marginal (13,3,21), would increase the overestimation through increased metabolic water production.

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Statistical analyzes.

To determine which statistical test would be most suited for the comparisons of preexercise and postexercise values, the differences (paired differences) between the preexercise and the postexercise results were calculated. The distributions (in the form of histograms) of paired differences of all the results were plotted with the number of classes as calculated according to the rule of Sturge. The normality of this distribution was tested using the Shapiro-Wilks' W test. The statistical rule of Sturge states that the number of classes equals N × 1.4 + 1 (where N = sample size). Student's t-tests were used to compare results where the distribution of the paired differences was normal. Where the distribution of the paired differences was not normal, the nonparametric alternative to the Student's t-test, the Wilcoxon rank sum test was used to compare results. A Pearson's product moment correlation coefficient was used to determine relationships between appropriate variables. Statistically significant differences were indicated by a P value of less than 0.05. The statistical analyses were completed using the STATISTICA© software package.

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Fifteen subjects volunteered for the study. Thirteen of these subjects were men and two were women. On average, the subjects carried a pay load mass of 20.7 kg each. The mean WBGT during the route march was 24.5°C (21.8°C-29.3°C). The mean relative humidity was 57.1% (51%-65%) whereas the mean wind speed was 0.99 m·s−1 (0-2.2 m·s−1).

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Body mass loss, fluid intake, and sweat rates.

On average, the group lost 1.0 kg during the exercise with a range of 0.0-1.8 kg (Table 1). The group consumed on average 383 mL·h−1 during the exercise. The subject with the largest intake consumed 665 mL·h−1, whereas the subject with the smallest intake consumed only 153 mL·h−1 (Table 1). The mean sweat rate was 626 mL·h−1 during the march (Table 1).



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POsm and serum [Na+].

Changes in POsm and serum [Na+] values preexercise and postexercise are also listed in Table 1. Neither POsm nor serum [Na+] changed during exercise.

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Core temperature measurements.

The mean core temperature of the subjects during the exercise was 37.6°C; the highest individual core temperature was 39.4°C. There was no relationship between the peak body core temperature reached during exercise and the change in body mass (P > 0.05; r = 0.10).

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TBW measurements.

Table 2 presents the preexercise and the postexercise TBW results of the subjects. TBW fell insignificantly (∼500 mL; 0.6% body mass) during exercise. Figure 1 shows that the change in body mass was unrelated to the change in TBW (r = −0.49). Note that a zero change in body mass was associated with an approximately 400-g increase in TBW. There was a nonsignificant relationship (r = −0.42) between postexercise serum [Na+] and total percent body mass change during exercise (Fig. 2) and a significant relationship (r = −0.59) between postexercise serum [Na+] and postexercise TBW expressed as a percentage of body mass (Fig. 3).









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The first finding of the study was that soldiers participating in a 16-km route march while carrying packs of 20.7 kg and wearing standard issue battle dress maintained safe body temperatures (less than 39.5°C) and regulated their serum [Na+] and osmolality within the normal range while drinking ad libitum at a mean rate of 383 mL·h−1 although this drinking rate replaced only 61% of their measured hourly body mass loss (626 mL·h−1). As a result, these soldiers showed a mean body mass loss of 1.01 kg (1.4% body mass) during the exercise. TBW fell insignificantly by a mean of 526 g during exercise. These findings suggest several important conclusions.

The first conclusion is that changes in body mass did not accurately predict changes in TBW in these soldiers (Fig. 1). Similar findings were reported in soldiers performing a 194-km unsupported desert march during which their mean hourly fluid intakes of 458 mL were adequate to maintain thermoregulation (mean core temperature of 38.1°C) while producing a 300-g increase in TBW despite a body mass loss of 3.3 kg at the end of exercise (28).

In contrast, Baker et al. (1) have recently reported that body mass loss accurately predicts TBW changes during 2 h of exercise. However, their methodologies differed significantly from those reported here, and there are other uncertainties regarding the manner in which their data were analyzed. Thus, the authors designed their experiments to produce four different levels of body mass loss. They then analyzed the data for the four experiments as if all came from a single experiment. The published data set was also incomplete. Our reanalysis (31) of the published data from the four separate experiments revealed that in none was there a significant relationship between the change in body mass and TBW. The differences in results are most probably further confounded by the unusual manner in which they used different biological samples (urine or serum) to measure changes in TBW in the same individuals. We have suggested that this reanalysis of the originally published data does not support the authors' original conclusions.

The debate of whether the change in body mass (kg) during exercise can be used as a 1:1 predictor of the change (L) in TBW is crucially important because it raises the question of whether or not there is a body fluid reserve of perhaps up to 2 L that may not require replacement to insure that whole-body fluid homeostasis is maintained during exercise. This theory was first proposed by Ladell during and after the Second World War (18-20). In particular, Ladell (18-20) observed, as did we, that body mass losses of 2 kg may occur before any of the expected effects of body water loss are detectable in urine. If this fluid volume exists as free water in the gastrointestinal tract or as water complexed to glycogen (32), it would explain why some believe that body mass losses of up to at least 3% may not carry any physiological penalty during prolonged exercise (19,24,27,39). Indeed the new American College of Sports Medicine Position Stand (36) appears to support this interpretation. A greater number of subjects involved in a more controlled experimental design would be necessary further to investigate the possible existence of this hypothetical whole-body fluid reserve.

Our second conclusion was that the serum [Na+] was maintained by ad libitum drinking. This is to be expected because drinking behavior is determined by changes in serum osmolality so that drinking according to the dictates of thirst would be expected to produce minimal changes in serum osmolality and the serum [Na+] (15). In contrast, drinking to stay "ahead of thirst" by "drinking as much as tolerable" (8,2) must cause serum [Na+] to fall (30,34) if renal-free water clearance is insufficient to prevent an increase in TBW. This occurs when arginine vasopressin (ADH) secretion is not appropriately suppressed by a falling serum osmolality (33).

In contrast our data show that an increase in TBW produced a fall in serum [Na+], whereas a fall in TBW produced a rise in serum [Na+]. These data are compatible with our findings that an increase in body mass (and hence TBW) is the major determinant of EAH (30,33,41).

Thus, we conclude that the ad libitum intake of water during a 16-km route march was sufficient to maintain serum [Na+]. This is compatible with the finding that sodium ingestion is not required to maintain serum [Na+] during exercise (16). Rather, it is the inappropriate regulation of the TBW that determines the extent to which the serum [Na+] falls during prolonged exercise (30).

Finally we show that although the core body temperature of all subjects rose steadily from the start of the exercise, none exceeded 39.5°C at the end of exercise. These values are substantially lower than those measured in athletes competing in a 21-km race under comparable environmental conditions (mean WBGT of 26.5°C), some of whom reached values >40.5°C without developing symptoms (5). Similar high values have recently been reported in athletes running 8 km without any limit to their performance in slightly warmer environmental temperatures (mean WBGT of 27°C) (10).

Thus, the relatively low core body temperatures measured in our subjects indicate that none was under extreme physiological stress or suffering from excessive thermal strain. Accordingly, we conclude that their ad libitum fluid intake was adequate to maintain a safe and an appropriate thermoregulatory response in the soldiers during the march in the particular environmental conditions that we studied. Several previous studies indicate that ad libitum fluid replacement strategies in which between 60% and 70% of sweat losses are replaced are effective in maintaining thermoregulation in athletes despite body mass losses of up to 3% (7,22).

In summary, the results of this study indicate that an ad libitum fluid replacement strategy, which replaced approximately 61% of sweat losses (383 mL·h−1), maintained core temperature, POsm, and serum [Na+] values despite a 1.4% body mass loss. This is compatible with the current American College of Sports Medicine Position Stand, which promotes ad libitum drinking provided that the body mass loss during exercise does not exceed 2% (36).

However, it does not exclude the possibility that greater levels of body mass loss may not be detrimental to either health or performance in those who drink to prevent the development of thirst during exercise (35). Data to address that question need to be collected.

Timothy D. Noakes was funded by the University of Cape Town, Medical Research Council, and Discovery Health and Bernard van Vuuren by the University of Pretoria.

The authors herewith state that the results of the present study do not constitute endorsement by the American College of Sports Medicine.

The authors thank the Director of Technology Development, Department of Defence, South Africa.

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