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Hydration

Hydration and Physical Activity

Citation:

Latzka WA, Sawka MN, Montain SJ, Skrinar GS, Fielding RA, Matott RP, Pandolf KB. Hyperhydration: thermoregulatory effects during compensable exercise-heat stress. Journal of Applied Physiology, 83(3):860-866, 1997.

PubMed ID: 9292474
 
Study Design:
Randomized crossover trial
Class:
A - Click here for explanation of classification scheme.
Quality Rating:
Neutral NEUTRAL: See Quality Criteria Checklist below.
Research Purpose:

To determine the efficacy of hyperhydration for improving thermoregulation during compensable exercise-heat stress.

Inclusion Criteria:

Not specified.

Exclusion Criteria:

Not specified.

Description of Study Protocol:

Recruitment:  Not specified.

 

Design:  Randomized crossover

 

Blinding used (if applicable):  Experimental solution administration was double blind: the water and glycerol solutions were of similar sweetness, color, flavor and temperature to mask the taste of glycerol.

 

Intervention (if applicable):

  • Prior to study:  Heat acclimation:
  •         Subjects were heat acclimated by walking at about 45% VO2max on a treadmill for two 50 min bouts spaced by a 10 min rest period for 6-10 days in a hot dry climate( ambient temperature=35oC; relative humidity=45%; air velocity=1 m/s)
  • Heat acclimation established when nonsignificant differences were observed in final exercise cor temperatures and heart rates on 2 consecutive days
  • Subjects were asked to participate in additional heat acclimation session non-test days if they had greater than 2 consecutive days without an exercise-heat exposure.
  • Prior to each HST:   Subject entered climatic chamber 30 min before exercise.
  • Five exercise heat-stress tests (HSTs), administered in random order, with 5 different hydration conditions:
    • euhydration (Eu)
    • glycerol hyperhydration with no exercise rehydration (GD) (glycerol solution:  1.2 g glycerol/kg/LBM)
    • glycerol hyperhydration/exercise rehydration (GR)
    • water hyperydration with no exercise rehydration (WD)
    • water hyperhydration/exercise rehydration (WR) 
    • Administration of fluid:
    •      EU condition required initial plasma osmolality of <286 mosmol/kg H2O
    •     Hyperhydration trials (GD, GR, WD. WR):  Subject first drank 3.9 mL/kg LBM of experimental solution.  After ingestion of experimental solution, subject drank a large volume (25.2 mL/kg LBM) of water (36oC).  Total volume of fluid consumed in a 30 min period was 29.1 mL/kg LBM
    •     Rehydration: Water (36oC) given in equal volumes at 20, 40, 60, 80 and 100 min of exercise to replace fluid lost during HSTs, the volume which was determined during the last day of acclimation. 
  • Heat test - 120 min treadmill exercise (1.56 m/s @ 4.9% grade = 45% of VO2max
  • Rehydration fluid was water given in equal volumes at 20, 40, 60, 80, and 100 min exercise
  • Exercise in heat (35oC, 45 relative humidity)

 

 

 

Statistical Analysis

  • Descriptive statistics:  means, SDs, SEs, and Pearson product-moment correlations
  • Analysis of variance with repeated measures: to determine whether hyperhydration had significant or interactive effects.
  • Student-Newman-Keuls pairwise multiple-comparisons: to identify differences among the means when statistical significance was achieved.
  • Statistical package: Sigma Stat
  • Significance level: P<.05

 

Data Collection Summary:

Timing of Measurements

Measurement

Preliminary

(Baseline)

Pre-exercise

-60 min

-30 min

0 min

40 min

80 min

120 min

Post-exercise

Vo2max submaximal test

 

      X

 

     X

 

 

 

 

 

 

 

Body composition

   

      X

 

      X

 

 

 

 

 

 

 

Body mass (Kg) – clothed

 

 

 

     X

 

 

 

 

 

 

 

 

      X

Body mass (Kg)-nude

  

       X

  

     X

 

 

 

 

 

 

 

 

     X

TBW (L)

      X

     

 

 

 

 

 

 

 

Plasma osmolality

 

    

 

     X

 

   X

 

 

  X

 

   X

 

   X

 

   X

 

Core temperature

 

 

 

 

*

 

 

 

 

Heart rate

 

 

 

 

*

 

 

 

 

Skin Temperature

 

 

 

 

*

 

 

 

 

Esophageal temperature

 

 

 

 

*

 

 

 

 

Venous blood sample

 

 

    

 

 

 

X

 

X

 

X

 

X

 

Urine volumes

 

 

 

X

 

 

X

 

 

 

 

 

Dependent Variables

  • Baseline body weight: During preliminary period, nude body mass measured for 2 weeks in morning, after voiding.
  • Total Body Water (TBW): deuterium-labeled water dilution technique in final week of acclimation; conducted after 8 hours of abstinence from food or drink.  TBW measurement used to calculate change in TBW during the HSTs.
  • Change in TBW:  change in body mass adjusted for fluid volume and urine volume during HSTs
  • Body density:  hydrostatic weighing
  • Lean body mass (LBM) and percent body fat:  Siri equation
  • VO2max:  progressive intensity and continuous effort protocol (Swaka)
  • Core temperature (: esophageal probe swallowed and rectal probe inserted)
  • Skin Temperatures: measured at 5 sites: forearm, upper arm, chest, thigh, and calf using a thermocouple skin thermometer
  • Tsk: mean of temperature at 5 skin sites
  • Esophageal temperature (Tes): tehrmocouple at level of heart; saliva spit, not swallowed to avoid decrease in measurements
  • Rectal temperature (Tre): thermistor inserted 10 cm beyond anal sphinctor
  • Local sweating rate ( msw) of upper arm: dew point sensors enclosed in a ventilated capsule and calculated
  • Sweating sensitivity:  slope of regresion line when msw was plotted as function of Tes during first 20 min of exercise
  • Threshold for actgive thermoregulatory sweating: Tes when msw exceeded 0.06 mg ·cm-2 ·min-1
  • Total body sweating rate: calculated from pre- and post exercise masses and corrected for water intake and urine output
  • Hemoglobin, hematocrit, lactate, sodium, potassium, osmolality, glycerol, and protein: venous blood samples
  • Percent change in plasma volume and blood volume: calculated from appropriate hemoglobin and hematocrit values
  • Urine volumes

Independent Variables

  • Heat-stress test (HST): 12O min of treadmill exercise at 45% VO2max in the heat (ambient temperature=34.9+0.1oC; dew-point temperature=25.9+0.6oC; air velicity=1 m/s
  • Hydration conditions: EU, GD, GR, WD, WR

 

Description of Actual Data Sample:

 

Initial N: 9 males

Attrition (final N):  final N = 8

One subject could not tolerate glycerol solution (nausea)

Age: 23 + 6 years (range: 19-36)

Ethnicity:  Not specified

Other relevant demographics:  None specified

Anthropometrics:

  • Body mass (kg):  76 + 15 (56-100)
  • Lean body mass (LBM) (kg):  63+ 9 (53-73)
  • Total Body Water (TBW)(L):  46.4 + 6.4 (38-54)
  • VO2max (ml · kg-1· m-1):  56 + 8 (42-69)
  • Heat acclimated

Location:  Massachusetts (United States Army Research Institute of Environmental Medicine, Natick; and Sargent College of Allied Health Profession, Boston University, Boston)

 

 

Summary of Results:

Exercise-heat stress

Variable EU GD GR WD WR Significance
Pre-drink body mass (kg) 76.5+4.6 76.4+4.7 76.1+14.6 76.1+14.6 76.1+14.4    n.s.
Pre-drink plasma osmolality (mosmol/kg H2O) 282+3 283+5 284+5 284+5 284+5    n.s.
Volume H2O consumed during exercise (rehydration) (L) 2.36+0.51   2.23+0.72   2.19+0.52    n.s.
Change TBW - afater drinking, 30 min. before exercise (L) no change 1.4+0.39 1.38+0.33 1.5+0.4 1.54+0.31

.05 for HH*

n.s. for EU

Urine output (L) 0.15+0.18 0.52+0.38 0.61+0.19 0.71+0.34 0.7+0.25 .05 **

*  HH:  hyperhydration

** compared to EU

  • No differences in TBW between GR and WR trials or between GD and WD trials during exercise
  • No differences in  plasma volume among EU, GD, GR, WD and WR trials
  • Decreases in serum plasma volumes from pre-exercise to final exercise value in GD and GR trials
  • Total urinary output values were greater in hyperhydration than in the EU trials (P<0.05)
  • No differences in total urinary output between glycerol hyperhydration trials and water hyperhydration trials
  • Pre-exercise serum osmolality values were
    • greater in GD (298+2 mosmol/kg H2O) and GR (288+2) than UE trials (279+1) (P<0.05); mean total osmolar  load from the ingested glycerol=802 mosmol or 16.6 mosmol/L TBW
    • lower during WD (273+2) and WR (272+2) than EU (P<0.05)
  • Serum osmolality values during exercise
    • increased in GD (294+1 @ 120 min) and WD (283+2 @ 120 min (P<0.05)
    • did not change in GR, WR, and EU
  • Serum glycerol levels
    •  were greater pre-exercise in GD (121.6+5.4 mg/dl) and GR (112.5+6.0) than in EU (0.3+0.2), WD (0.0+0.0) and WR (0.1+0.1) trials (P<0.05)
    • decreased in GD (39 mg/dl) and GR (31 mg/dl) at 120 min of exercise (P<0.05)
  • Serum sodium levels
    • were similar in all trials pre-exercise
    • increased during exercise in GD (0 min: 132.8+0.9 meq/L; 120 min: 138.5+1.1) and WD (0 min: 134.4 +1.0 meq/L; 120 min: 137.8+1.6) (P<0.05)
  • Serum potassium and lactate levels
    • were similar pre-exercise and during exercise in all trials.

Metabolic rate

    • increased significantly over time during exercise in all trials ( P<0.05)
    • was not different among trials
    • corresponded to relative intensity oxygen uptake levels 44-45% VO2max.

Heart rate

    • was not different at rest among trials
    • increased over time during exercise (P<0.05)
    • Final exercise heart rate values in GD (158+9 beats per min [bpm]) and WD (161+15 bpm) greater (P<0.05) than in GR (149+13 bpm) and WR (148+17);  GR and WR were not different from EU (150+14 bpm)

Body Temperature

  • Tre:
    • values not different among trials either pre-exercise or during exercise
    • values increased over time during exercise (P<0.05)
    • Final exercise values: EU=38.6+0.4oC; GD=38.8+0.2oC; GR=38.5+0.3oC; WD=38.7+0.4oC; WR=38.6+0.4oC.
  • Tes:
    • Pre-exercise values not different among trials
    • Final exercise Tes values were greater (P<0.05) during GD (38.3+0.2oC) and WD (38.2+0.2oC) than during EU (38.0+0.2oC);  similar final values were found duirng EU, GR (38.1+0.2oC) and WR (38.0+0.2oC).
  • Tsk:
    • increased during exercise (P<0.05) but were similar among trials
  • Tb (mean body temperature) responses
    • were not different among trials either at pre-exercise or during exercise
    • increased during exercise (P<0.05)
  • Final exercise Tb values were: EU=38.2+0.1oC; GD=38.5+0.1oC; GR=38.1+0.1oC; WD=38.4+0.1oC; WR=38.3+0.1oC.

 

Sweating Responses

  EU GD GR WD WR
Whole body sweating rates (g ·m-2 ·h-1) 520+50 497+48 520+67 490+49 524+52
Final msw values (mg ·cm-2 ·min-1) 1.09+0.20 1.08+0.23 1.00+0.28 0.99+0.29 1.05+0.28

Sweating threshold temperatures (oC)

36.7+0.2 36.5+0.1 36.5+0.1 36.5+0.3 36.5+0.4
Sweating sensitivity values (mg ·cm-2 ·min-1 ·oC-1) 1.01+0.30 1.03+0.60 1.03+0.41 0.98+0.42 0.96+0.53

 

Author Conclusion:

Hyperhydration provides no meaningful advantages over the maintenance of euhydration during compensable exercise stress.

Funding Source:
Government: US Army
Reviewer Comments:
  • Small number of subjects.
  • Results not generalizable.
Quality Criteria Checklist: Primary Research
Relevance Questions
  1. Would implementing the studied intervention or procedure (if found successful) result in improved outcomes for the patients/clients/population group? (Not Applicable for some epidemiological studies) Yes
  2. Did the authors study an outcome (dependent variable) or topic that the patients/clients/population group would care about? Yes
  3. Is the focus of the intervention or procedure (independent variable) or topic of study a common issue of concern to dieteticspractice? Yes
  4. Is the intervention or procedure feasible? (NA for some epidemiological studies) Yes
 
Validity Questions
1. Was the research question clearly stated? Yes
  1.1. Was (were) the specific intervention(s) or procedure(s) [independent variable(s)] identified? Yes
  1.2. Was (were) the outcome(s) [dependent variable(s)] clearly indicated? Yes
  1.3. Were the target population and setting specified? Yes
2. Was the selection of study subjects/patients free from bias? Yes
  2.1. Were inclusion/exclusion criteria specified (e.g., risk, point in disease progression, diagnostic or prognosis criteria), and with sufficient detail and without omitting criteria critical to the study? Yes
  2.2. Were criteria applied equally to all study groups? Yes
  2.3. Were health, demographics, and other characteristics of subjects described? Yes
  2.4. Were the subjects/patients a representative sample of the relevant population? ???
3. Were study groups comparable? Yes
  3.1. Was the method of assigning subjects/patients to groups described and unbiased? (Method of randomization identified if RCT) Yes
  3.2. Were distribution of disease status, prognostic factors, and other factors (e.g., demographics) similar across study groups at baseline? Yes
  3.3. Were concurrent controls or comparisons used? (Concurrent preferred over historical control or comparison groups.) Yes
  3.4. If cohort study or cross-sectional study, were groups comparable on important confounding factors and/or were preexisting differences accounted for by using appropriate adjustments in statistical analysis? N/A
  3.5. If case control study, were potential confounding factors comparable for cases and controls? (If case series or trial with subjects serving as own control, this criterion is not applicable.) N/A
  3.6. If diagnostic test, was there an independent blind comparison with an appropriate reference standard (e.g., "gold standard")? N/A
4. Was method of handling withdrawals described? Yes
  4.1. Were follow-up methods described and the same for all groups? N/A
  4.2. Was the number, characteristics of withdrawals (i.e., dropouts, lost to follow up, attrition rate) and/or response rate (cross-sectional studies) described for each group? (Follow up goal for a strong study is 80%.) Yes
  4.3. Were all enrolled subjects/patients (in the original sample) accounted for? Yes
  4.4. Were reasons for withdrawals similar across groups? N/A
  4.5. If diagnostic test, was decision to perform reference test not dependent on results of test under study? N/A
5. Was blinding used to prevent introduction of bias? Yes
  5.1. In intervention study, were subjects, clinicians/practitioners, and investigators blinded to treatment group, as appropriate? Yes
  5.2. Were data collectors blinded for outcomes assessment? (If outcome is measured using an objective test, such as a lab value, this criterion is assumed to be met.) Yes
  5.3. In cohort study or cross-sectional study, were measurements of outcomes and risk factors blinded? N/A
  5.4. In case control study, was case definition explicit and case ascertainment not influenced by exposure status? N/A
  5.5. In diagnostic study, were test results blinded to patient history and other test results? N/A
6. Were intervention/therapeutic regimens/exposure factor or procedure and any comparison(s) described in detail? Were interveningfactors described? Yes
  6.1. In RCT or other intervention trial, were protocols described for all regimens studied? Yes
  6.2. In observational study, were interventions, study settings, and clinicians/provider described? N/A
  6.3. Was the intensity and duration of the intervention or exposure factor sufficient to produce a meaningful effect? Yes
  6.4. Was the amount of exposure and, if relevant, subject/patient compliance measured? Yes
  6.5. Were co-interventions (e.g., ancillary treatments, other therapies) described? N/A
  6.6. Were extra or unplanned treatments described? N/A
  6.7. Was the information for 6.4, 6.5, and 6.6 assessed the same way for all groups? N/A
  6.8. In diagnostic study, were details of test administration and replication sufficient? N/A
7. Were outcomes clearly defined and the measurements valid and reliable? Yes
  7.1. Were primary and secondary endpoints described and relevant to the question? Yes
  7.2. Were nutrition measures appropriate to question and outcomes of concern? Yes
  7.3. Was the period of follow-up long enough for important outcome(s) to occur? Yes
  7.4. Were the observations and measurements based on standard, valid, and reliable data collection instruments/tests/procedures? Yes
  7.5. Was the measurement of effect at an appropriate level of precision? Yes
  7.6. Were other factors accounted for (measured) that could affect outcomes? Yes
  7.7. Were the measurements conducted consistently across groups? Yes
8. Was the statistical analysis appropriate for the study design and type of outcome indicators? Yes
  8.1. Were statistical analyses adequately described and the results reported appropriately? Yes
  8.2. Were correct statistical tests used and assumptions of test not violated? Yes
  8.3. Were statistics reported with levels of significance and/or confidence intervals? Yes
  8.4. Was "intent to treat" analysis of outcomes done (and as appropriate, was there an analysis of outcomes for those maximally exposed or a dose-response analysis)? N/A
  8.5. Were adequate adjustments made for effects of confounding factors that might have affected the outcomes (e.g., multivariate analyses)? N/A
  8.6. Was clinical significance as well as statistical significance reported? Yes
  8.7. If negative findings, was a power calculation reported to address type 2 error? N/A
9. Are conclusions supported by results with biases and limitations taken into consideration? Yes
  9.1. Is there a discussion of findings? Yes
  9.2. Are biases and study limitations identified and discussed? Yes
10. Is bias due to study's funding or sponsorship unlikely? Yes
  10.1. Were sources of funding and investigators' affiliations described? Yes
  10.2. Was the study free from apparent conflict of interest? Yes