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HYD: Effect of Caffeinated Beverages on Fluids (2007)


Armstrong LE, Pumerantz AC, Roti MW, Judelson DA, Watson G, Dias JC, Sokmen B, Casa DJ, Maresh CM, Lieberman H, Kellogg M. Fluid, electrolyte, and renal indices of hydration during 11 days of controlled caffeine consumption. Int J Sport Nutr Exerc Metab. 2005 Jun; 15 (3): 252-265.

PubMed ID: 16131696
Study Design:
Randomized controlled trial
A - Click here for explanation of classification scheme.
Quality Rating:
Positive POSITIVE: See Quality Criteria Checklist below.
Research Purpose:

To examine three levels of moderate, controlled caffeine consumption and to determine how they affected fluid-electrolyte balance, renal function and other indices of hydration in active young men.

Inclusion Criteria:
  • Healthy
  • Not taking medications that affect caffeine metabolism or the physiological variable measured
  • No cardiovascular, metabolic or respiratory disease.
Exclusion Criteria:
  • Tobacco smoking
  • Body mass 60kg to 86kg
  • Varsity athlete status
  • Participation in prolonged or intense exercise
  • Excercise less that twice per week
  • Lactose intolerance
  • Consuming more than eight cola beverages or four cups brewed coffee (total of 600ml) per day.
Description of Study Protocol:
  • Recruitment: Not described. 
  • Design: Three randomized caffeine-dose groups were stratified for age, body mass, and % body fat. 
  • Blinding used: Double-blind procedure that was overseen by a nutrition scientist who was not otherwise involved in the study.


  • Caffeine capsules provided daily group mean (calculated) doses of zero mg, 226mg and 452mg caffeine per day. They were prepared by a pharmacist in seven denominations ranging from zero mg to 200mg per capsule (SD=0.0% to 4.1%)
  • Capsules were ingested in two equal doses
    • Morning dose (7:00 to 9:00 a.m.): Ingestion observed by investigator
    • PM dose (12:00 to 2:00 p.m.): Observed or verified by phone.
  • Subjects consumed no caffeine-containing food or beverages during the intervention.

Statistical Analysis

  • Used commercial software; Base 10, SPSS Inc.
  • Anthropometric characteristics and caffeine consumption prior to the study were analyzed by analysis of variance (ANOVA)
  • Dietary components analyzed using a 3x11 (group x day) repeated-measures ANOVA
  • Significance was set at P<0.05
  • To avoid violating the assumption in independence, means were compared separately across days, using post-hoc paired sample T-tests and Holm’s sequential Bonferroni correction.
Data Collection Summary:

Timing of Measurements

  • Days One to Six (equilibration phase): Subjects consumed three mg caffeine per kg (-1) per day (-1)
  • Days Seven to 11 (treatment phase): The treatment, amount of caffeine per kg (-1) per day (-1) in capsules and number of subjects were as follows. Subjects consumed no other dietary caffeine. 
    • Tx C0 (placebo): Zero mg caffeine per kg; N=20
    • Tx C3: Three mg caffeine per kg; N=20
    • Tx C6: Six mg caffeine per kg; N=19
  • Blood was drawn on Days Six and 11, approximately 1.75 hours after dose ingestion, to assess compliance with the experimental protocol. Serum was analyzed for caffeine concentration using high-performance liquid chromatography.

Attrition (final N)

59 males (initial N not stated).


18 to 34 years (mean=21.6 years; standard deviation, ±3.3 years).


Not described.

Other Relevant Demographics

  • Habitual caffeine consumption
    • Subjects' pre-study intakes were similar (P>0.05)
    • Mean daily intake was 98±17mg caffeine (1.3mg per kg per day)
    • This equated to six ounces brewed coffee per day.
  • Habitual level of exercise
    • Activity questionnaire was administered to evaluate the duration, type and frequency of exercise during the 30 days prior to data collection
    • Subjects reported an average of 4.1±2.41 sessions per week for 1.1±0.7 hours per session at 2.2±0.7 on a three-point intensity scale
    • Subjects were instructed to maintain their usual level of exercise. 


Physical characteristics were similar; P>0.05

  • Body mass: 75.6±7.9kg, measured on a platform scale (±100g; Model DS44L, Ohaus Co.)

  • Height: 178.1±5.7cm, measured to the nearest cm (Model DS44L, Ohaus Co.)

  • Body mass index (BMI): 23.9±2.41.


University of Connecticut Human Performance Lab, Storrs, CT.

Description of Actual Data Sample:

Dependent Variables

24-hour urine collections were completed on Days One, Three, Six, Nine and 12. Urine analysis included:

  • Urine volume
  • Urine color: Used hand-held refractometer (Model A 300CL, Atago Co.)
  • Specific gravity: Used hand-held refractometer (Model A 300CL, Atago Co.)
  • Osmolality (mOsm per kg): Measured with freezing point-depression osmometer (Model 3DII, Advanced Digimatic)
  • Sodium and potassium (total daily electrolyte loss): Analyzed using ion-sensitive electrodes (Model 4003, alectrolyte analyzer, Medica Corp.). When multiplied by the urine volume, the concentration provided total daily loss.
  • Urine creatinine: Used a spectrophotometer calibrated at 500nm (Spectronic 401, Spectronic Instruments).

Blood analysis included:

  • Total Plasma Protein: Measured in duplicate with a hand-held refractometer (Model A300CL, Atago Co.)
  • Microhematocrit: Measured in triplicate following centriguation for 15 minutes
  • Serum Sodium and Potassium: Measured in duplicate using ion-sensitive electrodes (Model 4003, alectrolyte analyzer, Medica Corp.)
  • Serum Osmolality: Measured with osmometer (Model 3DII, Advanced Digimatic)
  • Blood Urea Nitrogen: Plasma samples were analyzed using Ortho Vitros 950 analyzer (Johnson and Johnson).

Fluid and electrolyte balance:

  • Daily Sodium and Potassium Excretion (mEq per 24 hours), calculated as the product of the urine Na+ or K+ concentration x 24-hour urine volume
  • Whole body Na+ or K+ balance; difference between electrolyte intake and loss.

Controlled Variables

Dietary consumption, mean ad libitum values (Days One to 11)
  • Energy intake: 2,782±699 Kcal
  • Carbohydrate: 52±8% of total Kcal
  • Fat: 31±5% of total Kcal
  • Protein: 18±4% of total Kcal
  • Sodium: 205±53 mEq
  • Potassium intake: 76±25 mEq.

Independent Variables

Treatment with caffeine or placebo (described above).

Summary of Results:

The following variables were unaffected (P>0.05) by different caffeine doses on Days One, Three, Six, Nine and 11 and were within normal clinical ranges: Body mass, urine osmolality, urine specific gravity, urine color, 24-hour urine volume, 24-hour Na+ and K+ excretion, 24-hour creatinine, blood urea nitrogen, serum Na+ and K+, serum osmolality, hematocrit and total plasma protein.

Significant within-group changes occurred during the study, but were insignificant by Day 12. These minor changes were viewed as resulting from differences in day-to-day dietary content and spontaneous weekday vs. weekend eating patterns. All means consistently fell within normal clinical ranges for urine, blood and renal function. Examples include:

  • C3 and C6: Experienced 0.3kg to 0.5kg body mass loss from Day Six to Day Nine, (P>0.05), but this was recouped by Day 12
  • C0 and C6: Serum sodium increased on Day Nine (P>0.05), but abated by Day 12.
  • C0 and C3: Experienced significant changes in Na+ and K+ 24-hour urine excretion in on specific days, but C6 did not
  • C3 and C6: Acute urine specific gravity and urine color measurements fluctuated from Day Six to Day 12, but the trends varied.

See paper for extensive tables.

Author Conclusion:
  • Moderate caffeine doses (three mg and six mg caffeine per kg per day, totaling 226mg and 452mg per day, respectively) did not increase urine output above that of a placebo
  • None of the treatments (C0, C3, C6) exhibited evidence of hypohydration
  • This investigation found no evidence that healthy, active males should refrain from consuming moderate levels of caffeine.
Funding Source:
Foundation associated with industry:
Reviewer Comments:
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? N/A
  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? 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.) 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? No
  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%.) No
  4.3. Were all enrolled subjects/patients (in the original sample) accounted for? ???
  4.4. Were reasons for withdrawals similar across groups? ???
  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? 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? 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? 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