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Hydration and Physical Activity


Mitchell JB & Voss KW.  The influence of volume on gastric emptying and fluid balance during prolonged exercise.  Medicine & Science in Sports & Exercise 1991; 23(3): 314-9.

PubMed ID: 2020269
Study Design:
Randomized Crossover Trial
C - Click here for explanation of classification scheme.
Quality Rating:
Positive POSITIVE: See Quality Criteria Checklist below.
Research Purpose:
To determine the optimal rate of ingestion of a CHO solution in order to elicit the highest rates of gastric emptying without causing excessive fluid accumulation and gastric distress. 
Inclusion Criteria:
Informed consent; competitive cyclists; no others mentioned
Exclusion Criteria:
none mentioned
Description of Study Protocol:

Recruitment not mentioned

Design randomized crossover: 3 cycling trials, 1 week between each trial; ingesting either 11.5 (LV), 17.1 (MV), or 23 (HV) ml/kg/h of 7.5% CHO solution

Blinding used (if applicable) not mentioned

Intervention (if applicable) ingesting either 11.5 (LV), 17.1 (MV), or 23 (HV) ml/kg/h of 7.5% CHO solution during 2h cycling trial

Statistical Analysis one-way or two-way ANOVA for repeated measures; when significance detected by ANOVA, differences were further analyzed using Newman-Keuls post hoc test;


Data Collection Summary:

Timing of Measurements

 Pre-test:  determine VO2max on cycle; %fat estimated; dietary intake recorded, and subjects instructed to keep diet consistent during 3 study days.  Blood sample taken at rest prior to trial

3 cycling Trials:  separated by 1 week; riding for 2h on cycle at workload of 70% VO2max; during trial, ingested 1 of 3 CHO solutions: 11.5 (LV), 17.1 (MV), or 23 (HV), in randomized order, administered at the beginning of the ride and every 15 min during the ride.  Body weight obtained before and after each ride; VO2 measured at 15, 45, 75, 105 min; Ratings perceived exertion (RPE) and fullness taken at 15, 45, 75, 105 min; blood sample taken during (60, 90, 12 min) and after trial.  Volume of gastric reside measured and recorded at end of ride.

Dependent Variables

  • VO2 (expired gas analysis) --> rates of fat and CHO oxidation calculated
  • RPE and Fullness (scales)
  • Plasma vol changes (calculated from hgb and hct), electrolytes Na and K (flame photometry)
  • Blood glucose (spectrophotometric method)
  • Gastric residue volume (NG tube to aspirate contents) --> calculate volume of original drink emptied by stomach and vol of gastric secretion

Independent Variables

 Volume of CHO solution ingested during cycling trial: 11.5 (LV), 17.1 (MV), or 23 (HV)

Control Variables

controlled temp during testing (28.3C);

CHO solution feeding rates based on initial reference body weight of 70 kg (designed to produce feeding volumes of 200, 300, 400 ml/15 min, or 14, 21, and 28 ml fluid/min, and 65, 97, 130 g CHO/h for the 3 conditions, repectively);

timing of ingestion of CHO solution (q. 15 min during trial);


Description of Actual Data Sample:


Initial N: 8 males, competitive cyclists (UCSF category 3 & 4)

Attrition (final N): 8 M

Age: 27.9±1.9 years

Ethnicity: not mentioned

Other relevant demographics: no others mentioned

Anthropometrics Wt: 75.6±2.1 kg; Ht: 181.2±0.8 cm; VO2max: 4.64±0.15 L; %fat:9.2±1.3%

Location: lab, Texas Christian University, Fort Worth, TX, USA


Summary of Results:






Statistical Significance of Group Difference

Gastric emptying rates (ml/min)




 p<0.05, n=7

% Drink emptied range ( mean%)

 77-92 (83.7)

64-94 (73.1)

 52-77 (66.8)

 p<0.05, n=7

Kcal emptying rates (ml/min)




 p<0.05, n=7

Change in Plasma vol (%)  0.01±2.20  1.05±1.62  0.97±1.28  ns
Change in body weight (kg) 0.77±0.14* 0.38±0.13 0.14±0.14 *sig, p not reported
Replacement of sweat and respiratory water loss (%) 65.3 82.6 94.2 sig, p not reported


Other Findings

 Volume fluid ingested vs volume emptied produced r=0.89 (linear regression) and r=0.93 (n=21, polynomial regression).

Fullness ratings: sig differences between trials and across time within trials; correlation of r=0.75 between final fullness rating and vol drink in stomach.

RPE: ns differences between trials; in HV trial only, 105 min rating sig higher than 15 min rating (p not reported).

Electrolytes: ns differences between trials or between pre- and post-exercise measurements (n=4). 

Blood glucose: ns differences between trials; level at 60 min sig greater than 0min in all 3 trials (p not reported).

CHO ox rate: ns differences.

HR: ns differences between trials.


Author Conclusion:

During prolonged exercise the higher volumes enhanced by the rate of gastric emptying but had little effect on plasma volume changes, electrolytes, and blood glucose compared to the lower volume. 

In addition, large feedings may be most effective in prevention dehydration but can lead to large gastric residues which cause discomfort that may impair performance.

Funding Source:
University/Hospital: Texas Christian University Research Foundation
Reviewer Comments:
*See Figures for gastric emptying results*
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? Yes
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? 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