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HYD: Effect of Caffeinated Beverages on Fluids (2007)


Neuhauser-Berthold, Beine S, Verwied SC, Luhrmann PM. Coffee consumption and total body water homeostasis as measured by fluid balance and bioelectrical impedance analysis. Ann Nutr Metab. 1997; 41 (1): 29-36.

PubMed ID: 9194998
Study Design:
Randomized Controlled Trial
C - Click here for explanation of classification scheme.
Quality Rating:
Neutral NEUTRAL: See Quality Criteria Checklist below.
Research Purpose:

The study had two purposes:

  • To examine the effect of coffee consumption on total body water homeostasis in humans on a regular diet
  • To study the use of bioelectrical impedance analyses for assessing total body water homeostasis.
Inclusion Criteria:
  • Normal weight
  • In good health
  • Non-smoker.
Exclusion Criteria:
None identified.
Description of Study Protocol:
  • Recruitment: Not discussed
  • Design: This was a two-day intervention trial with Day One as the control and Day Two as the coffee treatment day
  • Blinding: Design did not include blinding.

Statistical Analysis

The analysis software was SPSS PC for Windows 6.0.1.

  • Kolmogorov-Smirnov test was used to assess data
  • Difference in variables between males and females were tested for significance with the unpaired T-test
  • Difference in variables before and after treatment were tested for significance with the paired T-test
  • The Pearson coefficient of correlation was used to test the degree of linear association between parameters
  • Significance was set at P<0.05 (two-tailed).
Data Collection Summary:

Timing of Measurements

  • Subjects abstained from consumption of methylxanthines for five days and during the two days of the study
  • During the first day, subjects were supplied with a standardized diet. Their fluid requirement was met by mineral water.
  • On the following day, subjects were supplied with a standardized diet. The same amounts of fluid were supplied, but the mineral water was in part replaced by six cups of coffee (three cups for breakfast and three cups in the early afternoon), which provided a total of 642mg of caffeine.

Dependent Variables

  • Body weight: Measured with digital scale to the nearest 0.1kg (SECA, Wogel & Halke)
  • Total body water: Analyzed by using BIA and measured at 50kHz and 800µA in supine position with an AKERN-RJL, BIA 101/S Body Composition Analyzer using Kushner formula (Data Input, Frankfurt, GE)
  • 24-hour urine volume: Single urine samples were collected throughout each of the two days from 8:00 to 8:00
  • Fluid output: The sum of 24-hour urine collection and calculated losses from lungs, skin and feces (Robertson and Beri)
  • Fluid balance: Calculated by subtracting fluid output from fluid intake
  • Urine specific gravity and pH: Samples collected before each measurement of BIA and BW; specific gravity determined by an aerometer (Fischer, Frankfurt) and pH measured using a 12pH/ISEmeter (Beckmann Instruments, Munich, GE)
  • Urine sodium and potassium: Acidified aliquots of the 24-hour urine were analyzed using flame photometry (Zeiss PMQ II, Oberkochen, GE).

Independent Variable

Fluid intake: Measured amount of mineral water, coffee, calculated volumes of water in food consumed and of metabolic water.

Control Variable

Caffeine intake: Prepared as filtered coffee in a coffee machine, using 82g ground coffee for 1,400ml tap water. Analysis of the coffee's caffeine content followed methods described in a paper published in 1986 (Citation #12, German language).

Description of Actual Data Sample:
  • Initial N: 12 (six males, six females)
  • Attrition (final N): N=9 for two urine data sets; for all other analyses, N=12
  • Age: 26.7±3.5 years
  • Ethnicity: Not described
  • Other relevant demographics
    • Usual coffee consumption: Two to four cups per day
    • Females were in leuteinization phase of menstrual cycle; two took oral contraceptives.
  • Anthropometrics
    • Height: 1.78±0.07m
    • Weight: 71.2±8.9kg
    • BMI: 22.4±1.6kg per m2.
  • Location: University of Giessen, Giessen, Germany.
Summary of Results:
  • Caffeine intake was the same for all subjects, resulting in a significantly higher caffeine intake of 10.2mg per kg body weight in females vs. 8.2mg per kg in males (P<0.001)
  • Consumption of six cups of coffee (642mg caffeine) resulted in a 41% increase in the 24-hour urine excretion (753±532ml) (P<0.001) and a corresponding negative fluid balance
  • Body weight decreased by 0.7±0.4kg (P<0.001)
  • Total body water, as measured with bioelectrical impedance analysis, decreased by 1.1±1.2kg or 2.7% (P<0.01)
  • Urinary excretion of sodium and potassium was elevated by 80±62mmol or 66% (P<0.01) and 14±12mmol or 28% (P<0.01), respectively. [Note: The N for this analysis was nine (three males, six females), rather than 12. See reviewer comments below.]
Author Conclusion:
  • In the combined group of subjects, total body water measured with bioelectrical impedance analyses was fairly consistent, with losses of total body water measured by urine excretion and fluid balance
  • However, when evaluating males and females separately, BIA overestimated total body water changes in male subjects and underestimated the changes in the female subjects. Further analyses suggest that the overestimation in males was random, as the coefficient of variation for estimation of total body water with BIA was 5% (1.7kg).
Funding Source:
University/Hospital: Justus Leibig University
Reviewer Comments:
  • The description of this study contains some serious omissions or the study had serious flaws
  • Female subjects in the study consumed significantly more caffeine per kg body weight than males (P<0.001), yet for some analyses the data was pooled
  • Table Three: Excretion of Sodium and Potassium in 24-hour Urine and Figure Two: pH of Urine Specimens indicate that analysis was conducted on nine (three males, six females), rather than 12, as with the other urine tables and figures. Although the text included discussions about both sets of findings, there was no mention of the reduced number of males subjects, which may have confounded or skewed the results.
  • The investigator-authors did not randomize the test days. For all subjects, Day One was the control, with no caffeine and Day Two was the caffeine treatment day
  • While subjects received duplicate standardized meals on Days One and Two, dietary intakes (pre-test and two test days) were not analyzed for electrolyte content, yet sodium and potassium excretion were measured
  • It is possible that differences in the sodium content of the usual pre-test diet and the two-day test diets may have affected fluid homeostasis (urine output) by test Day Two.
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) No
  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? No
  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? No
  5.1. In intervention study, were subjects, clinicians/practitioners, and investigators blinded to treatment group, as appropriate? No
  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? No
  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? No
  6.4. Was the amount of exposure and, if relevant, subject/patient compliance measured? No
  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? No
  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? ???
  7.3. Was the period of follow-up long enough for important outcome(s) to occur? ???
  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? No
  7.7. Were the measurements conducted consistently across groups? No
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? No
  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)? No
  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? 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? Yes
  10.1. Were sources of funding and investigators' affiliations described? Yes
  10.2. Was the study free from apparent conflict of interest? Yes