Hydration and Physical Activity

Study Design:
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Quality Rating:
Research Purpose:
To determine the effect of intermittent ingestion of two previously tested and two new hypertonic fluid formulations containing various osmotic and carbohydrate concentrations on plasma volume (PV) in euhydrated men at rest followed by upright (sitting) submaximal ergometer exercise.


To test the physiological effect of hyperhydration.

Inclusion Criteria:

Not provided

Passed a comprehensive medical examination

Exclusion Criteria:

Not provided

Description of Study Protocol:

Recruitment: No information provided

Design:  Semi-random, balanced crossover design

Blinding used (if applicable):  Subjects not blinded due to the apparent taste differences between beverages.

Intervention (if applicable):  Intermittent drinking during 90 min of sitting rest, 15 min to move the cycle ergometer and to readjust sensors, intermittent drinking during 70 min of upright (sitting) submaximal leg exercise (70 + 7% VO2peak) followed by 10 min of sitting recovery. Drink volume was 10 ml per kg body weight and was divided into 7 portions and given at 10 minutes intervals during both rest and exercise phases. P1 was consumed during exercise with 5 treatments; there was no drinking during the sixth exercise trial. Drink designations for the 6 treatments in the rest/exercise phases respectively were: P1/P1, P2/P1, P2G/P1, AA/P1, 0/P1, and 0/0.

Statistical Analysis:  Student t-test for dependent variables  P<0.05


Data Collection Summary:

Timing of Measurements

  • Body weight: prior to resting phase and immediately before and after exercise phase
  • VO2: at -40 min time point during the resting phase and at the 35 and 65 min time points during the exercise phase
  • Blood samples at -105, -35, -25 min time points during the resting phase and 0, 10, 30, 70 min time points during the exercise phase and +10 time point during the recovery from exercise.
  • Urine sample at -105, -15 min time points during resting phase and the +10 min time point during the recovery from exercise phase.

Dependent Variables

  • Heart rate
  • Temperatures: rectal (Tre) and skin Tsk
  • Body water balance = [(weight change – (blood + urine loss) + drink volume – (CO2 out – O2in)]
  • Blood measurements: 15 ml each, 20 ml each at 25 and 35 min – hemoglobin, hematocrit, glucose, sodium, potassium, osmolality, red blood cells, glycerol, citrate,
  • Plasma volume
  • Urine volume collected at the end of rest and after exercise, urine exertion rate ml per min
  • Urine osmolality, sodium, potassium

Independent Variables:  Test beverages: Performance 1 (P1); Performance 2 (P2) = 2 x P1 in concentration; Performance 2 + glycerol (P2G) = P2 +4% glycerol; AstroAde (AA) = commercial product designed at Ames Research Center; or No drinking (0).  








Sodium mEq/L





Potassium mEq/L





Chloride mEq/L





Total Carbohydrate %wt/vol





Glycerol (gm)





Osmolality (mOsm/kg water)





 Control Variables: Standardized breakfast on the day of each test.


Description of Actual Data Sample:
Initial N: 6 male subjects
Attrition (final N): 
Age: Mean 30 + 8 years
Ethnicity: Not mentioned
Other relevant demographics: Mean (+ SD) for Plasma volume: 3124 + 505 ml, Vo2peak: 2.99 + 0.45 L/min, non smokers, not on prescription medicine
Anthropometrics Mean (+ SD) for height: 182 + 7 cm, weight: 76.84 + 16.19 kg, surface area: 1.98 + 0.22 m2.

Location: Ames Research Center California

Summary of Results:

Other Findings

Plasma and Mean Corpuscular Volumes:

  • The largest increase in % change in PV after rest phase was in AA/P1 (7.9% p<0.05) and P1/P1 (4.7% p<0.05), 0/0 (1.7%, NS) and 0/P1 (1.0 %, NS).
  • During exercise, % change in PV was +1-+3% AA/P1 (NS), -6%-0% (NS) with P1/P1, P2/P1, P2G/P1, and 0/P1; -8 to –5% (p<0.05) with 0/0.
  • Rate of PV restitution during exercise appeared to be independent of drink composition and whether or not fluid was consumed.
  • Mean corpuscular volumes were not different from each other or over time at rest or during exercise indicating no appreciable net change of vascular fluid into or from red blood cells.

Osmolality, Sodium, Potassium, glucose

  • Plasma osmolality during the rest phase: non-drinking treatments (0/0, 0/P1) remained constant during the first hour, increased with P1/P1 and P2G/P1, and decreased with AA/P2 and 0/P1 by the end of rest.
  • Plasma osmolality increased during exercise with all treatments. Intake of P1 tended to attenuate the increase in PV. 0/0 greatest increase in Osm by end of exercise, while AA/P1 had the least increase. 
  • Sodium concentration followed Osm.
  • Potassium content in the drink did not appear to influence the concentration or content responses at rest or during exercise. 
  • Consumption of glucose during exercise resulted in the increase of both plasma glucose concentration and content.


  • Glycerol was metabolized as evident in P2G by the decrease in glycerol concentration over the 70 min exercise phase.

Urine Data

  • No significant differences between rest and the exercise phase (within beverages) for urine excretion rate, free water clearance, urine osmotic clearance, and urine potassium excretion.
  • Increased HR during exercise was lowest with P1/P1 and highest with AA/P1.  Dehydration (0/0) did not result in the characteristic elevated HR at rest or duirng exerise.  

Physiological Data:  Rectal (Tre) and Skin (Tsk) Temperatures, Water balance, Sweating 

  • P2G/P1 showed the greatest increase in Tre followed by P2/P1, 0/0, AA/P1, 0/P1, and P1/P1.  Thus it appears that glycerol ingestion tends to elevate change in Tre, while P1 tends to attenuate the increase.
  • Absolute average mean Tsk and change in Tsk were not significantly different among the six treatments.
  • At rest treatments P1/P1 and P2/P1 had greater positive body water balance even with increased sweating and AA/P1 had the greatest negative balance; whereas during exercise P2/P1 had the greatest negative balance, and P2G/P1 and AA/P1 the lesser negative balances reflecting reduced sweating. 

Salient Responses During Each Treatment

  • P1/P1: significant increase in % change in PV at rest (%4.7), only positive exercise urinary free water clearance, lowest increase in exercise HR and lowest increase in exercise Tre.
  • P2/P1:  no on % change PV at rest (%2.6) or exercise (-%1.70) , highest positive water balance at rest and greatest negative exercise water balance.
  • P2G/P1:  greatest increase in exercise Tre
  • AA/P1:  greatest increase in % change PV at rest (7.9%), highest level of exercise % change PV, highest level of rest and exercise plasma Na, K, and Osm contents, lowest plasma glucose concentration and increase in content at rest, highest rest and exercise urinary Na excretion, highest exercise urinary K excretion and Osm clearance, greatest negative water balance at rest, and greatest increase in exercise HR.
  • 0/P1:  lowest increase in % change in PV at rest, greatest reduction in exercise plasma glucose content
  • 0/0:  lowest exercise plasma glucose concentration.
Author Conclusion:
  • It appears that drink composition, rather than moderate dehydration or rate of drinking, is the more important factor influencing the magnitude of rest hypervolemia.
  • Hyperhydration seems better than rehydration during short-term exercise for maintenance of PV.
  • It appears glycerol acts to hold body water in part by increasing plasma Na content.
  • Because glycerol distributes into the total body water (all fluid compartments), it may have less of an hypervolemic effect than Na containing compounds which distribute mainly into extracellular fluid space.
  • Fluid formulation ionic contents are more important than its osmotic content for increasing PV at rest and indirectly for maintaining it during exercise.
  • There was no consistent mechanism for variations in water balance with the various fluid formulations; some were caused by change in urinary flow and others by change in sweating.
  • Intake of the fluid formulations does not appear to significantly influence change in Tre or thermoregulatory parameters during 70 min of submaximal exercise.
  • Overall, authors conclude with drinking composition appears to be more important than its Osm for increasing PV at rest and for maintaining it during exercise in previously euhydrated subjects.


Funding Source:
Government: NASA
Food Company:
Reviewer Comments:

Authors do not address limitations of the study, one of which was the statistical analysis.  Using only t-test limits the investigators ability to examine the effects of the test beverages on the outcome measures and to provide more definitive conclusions.  The overall conclusion statement is vague.  Other limitations include:  small sample size, all male, unknown exercise history, ages 22-38 y, unknown how selected for the study, all of which limits the generalizability of the findings. 

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? No
2. Was the selection of study subjects/patients free from bias? No
  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? No
  2.2. Were criteria applied equally to all study groups? ???
  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? No
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.) N/A
  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? No
  4.1. Were follow-up methods described and the same for all groups? Yes
  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%.) No
  4.3. Were all enrolled subjects/patients (in the original sample) accounted for? No
  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? ???
  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? Yes
  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? ???
  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? No
  8.1. Were statistical analyses adequately described and the results reported appropriately? No
  8.2. Were correct statistical tests used and assumptions of test not violated? No
  8.3. Were statistics reported with levels of significance and/or confidence intervals? No
  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)? No
  8.6. Was clinical significance as well as statistical significance reported? No
  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? No
  9.1. Is there a discussion of findings? Yes
  9.2. Are biases and study limitations identified and discussed? No
10. Is bias due to study's funding or sponsorship unlikely? No
  10.1. Were sources of funding and investigators' affiliations described? Yes
  10.2. Was the study free from apparent conflict of interest? No