Hydration and Physical Activity


Maughan RJ, Leiper JB, Shirreffs SM.  Restoration of fluid balance after exercise-induced dehydration: effects of food and fluid intake.  Eur J Appl Physiol 1996; 73: 317-325.

PubMed ID: 8781863
Study Design:
Randomized Crossover Trial
C - Click here for explanation of classification scheme.
Quality Rating:
Neutral NEUTRAL: See Quality Criteria Checklist below.
Research Purpose:

1) To compare the responses to ingestion of plain water together with a standard meal with those to ingestion of a glucose-electrolyte drink. 

2) To examine the reproducibility of these responses by comparing 2 trials in which the same rehydration regimen was administered. 

Inclusion Criteria:
Healthy, physically-active, informed consent
Exclusion Criteria:
None mentioned
Description of Study Protocol:

Recruitment Not mentioned


Design Randomized crossover (3 fluid/food combos, randomly administered for trials 2-3)


Blinding used (if applicable) Not mentioned


Intervention (if applicable) After exercise-induced dehydration followed by 60 min rehydration period during which fluid alone (water or glucose-electrolyte drink) or with food was consumed.  3 trials: Trial A (glu-electrolyte drink), B (glu-electrolyte drink + meal), C (water + meal).  Trials B & C order of administration randomized.  6h follow-up after rehydration.

 Glu-electrolyte drink - 21 mmol/L-1 Na, 3.4 mmol/L-1 K, 12 mmol/l-1 Cl

Meal - 63 kJ/kg BM (53% CHO, 28% fat, 19% pro), 0.118 mmol/kJ-1 Na, 0.061 mmol/kJ-1 K; Drink:  1 mmol/L-1 Na, 0.4 mmol/L-1 K, 1mmol/L-1 Cl

Statistical Analysis Comparisons made between trials B and C after establishing normality of distribution.  Repeated measures ANOVA followed by Student's t-test or Wilcoxon signed rank test between trials or followed by 1-way ANOVA and Tukey's multiple range test or Kruskal-Wallis and Mann-Whitney tests within trials, where appropriate.  Sig level of p<0.05.


Data Collection Summary:

Timing of Measurements

Exercise-induced dehydration: After overnight fast, ingested 500 ml water 2h pre-trial; 1 week between trials; blood sample collected; bladders emptied; weight obtained; Dehydration induced by exercise at 60% VO2max in warm environmental chamber followed by 10 min immersion in 42C water, alternately until 2% body mass lost

Rehydration:   Fluid/fluid+food (60 min): After showering, dressing, 10 min seated rest in cool environment, blood sample taken, urine sample collected; Over following 60 min, 3 trials (A, B, C), order of assignment of fluid/food combos in B & C randomized; drink given in amt equal to 150% estimated sweat loss, in 4 equal volumes to be consumed q.15 min.  Subject feelings measured after consuming 2nd and 4th drinks during 60 min period (hunger, thirst, nausea, tired) on Visual Analog Scale (VAS). 

Follow-up, 6h: Blood and urine samples collected at end of rehydration and at 1, 2, 4, 6 h. 

Dependent Variables

  • Hgb, Hct, packed cell vol (to estimate changes in plasma and red cell vol - Dill & Costill Method)
  • Serum Na, K (Flame photometry)
  • Serum Cl (coulometric titration)
  • Serum Osm (freezing point depression)
  • Plasma angiotensin II (radio immunoassay)

Independent Variables

 fluid/food combo during post-exercise period:  Glu-electrolyte drink alone, water w/meal, glu-electrolyte drink w/meal

Control Variables

 Kept food and fluid intake consistent and abstained from strenuous physical activity 2d prior to trials;  temp during pre-test and recovery (24C)and exercise (34C, 55%RH).  immersion in 42C water pre-trial; body wt loss of 2%

Description of Actual Data Sample:


Initial N: n=8 (5M, 3 F)

Attrition (final N): 8:  5M, 3 F

Age: see subject characteristics in table below

Ethnicity: not mentioned

Other relevant demographics: habitually physically-active, but not highly trained


  n=8 (5M, 3F)
Age, yr 31±3
Ht, cm 171±4
Wt, kg 66.1±4
VO2max, ml/kg/min 46.5±3.1

Location: lab, University Medical School, Aberdeen, Scotland


Summary of Results:



Trial A

(Glu-electrolyte drink)

Trial B

(Glu-electrolyte drink + meal)

Trial C

(Water + meal)

Statistical Significance of Group Difference

Pre-exercise body mass, kg


66.2 ±4.0



Reduction in body mass (2.1% loss), kg





Exercise time, min





Fluid ingested, ml  2042±130 2042±132  2076±131  ns 
Ingested fluid retained at end of study (Median), % 52 51 67* p=0.03, sig diff from B
Blood vol change after rehydration, % 8±1 7±1 8±1 ns
Plasma vol changes after rehydration, % 13±1 12±2 13±2 ns


Other Findings

Urine output & fluid balance:  peak output recorded at 1 or 2h after end of rehydration period.  Cumulative urine output not different at any time between trials A and B.  No difference between B and C at 1st sample point, but thereafter cumulative vol sig higher on trial B than C (p not reported).  No difference at any time between 2 glu-electrolyte trials, and no diff between B and C before or immediately ater rehydration.  

Cumulative urine electrolyte output: no difference in cumulative Na excretion at any time between 2 fluid-only trials (p=0.38), but there was sig retention of Na and K in C vs B (p=0.04, p=0.01).  No diff in Cl excretion (p=0.92). 

Uosm similar between trials A and B (p=0.86), though was sig lower in B than C (p not reported). 

Blood & serum measurements: Sig blood & plasma vol changes w/time during all 3 trials, no treatment effect between conditions.  sOsm:  changed w/time in all 3 trials; small but sig diff between 2 fluid-only trials at end of rehydration (p=0.02).  No diff between B and C (p=0.53). 

No changes across time or between trials in sNa.  sK diff on trial B (vs) A was higher 6 h after rehydration vs immediately after or 1h after rehydration.  On trial C, sK was higher at last sampling point than before or immediately after rehydration.  ns diff sCl between treatments or w/time. 

AngiotensinII(pl):  time-dependent changes in all trials:  increase w/dehydration, and return to pre-exercise levels by end of study; C higher than B 1h after rehydration (p=0.04) due to difference in time-course of change in concentration.

Author Conclusion:

Although restoration of water and electrolyte balance after moderate levels of exercise-induced dehydration requires replacement of electrolytes and water, addition of electrolytes to the ingested fluid is not necessary if solid food w/appropriate amounts of Na and K is consumed together with plain water.

In situations where food is not available or where eating is likely to result in GI problems (i.e. subsequent bout of activity), addition of Na and K to rehydration drinks is essential for maintenance of fluid balance. 

Funding Source:
University/Hospital: University Medical School (Aberdeen Scottland)
Reviewer Comments:
See figures for time-course of urine output, net fluid balance; small sample size
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? ???
  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? N/A
  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? N/A
  3.1. Was the method of assigning subjects/patients to groups described and unbiased? (Method of randomization identified if RCT) N/A
  3.2. Were distribution of disease status, prognostic factors, and other factors (e.g., demographics) similar across study groups at baseline? N/A
  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? Yes
  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? N/A
  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%.) N/A
  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? ???
  5.1. In intervention study, were subjects, clinicians/practitioners, and investigators blinded to treatment group, as appropriate? ???
  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? ???
  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? ???
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? Yes
  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? Yes
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)? Yes
  8.6. Was clinical significance as well as statistical significance reported? ???
  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