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HYD: Effect of Caffeinated Beverages on Fluids (2007)


Stookey JD. The Diuretic Effects of Alcohol and Caffeine and Total Water Intake Misclassification. Eur J Epidemiol. 1999; Feb 15 (2): 181-188.

PubMed ID: 10204649
Study Design:
Cross-sectional study
D - Click here for explanation of classification scheme.
Quality Rating:
Positive POSITIVE: See Quality Criteria Checklist below.
Research Purpose:
To propose a theoretical method for adjusting total water intake for the diuretic effects of caffeine and alcohol and evaluate the potential for related misclassification bias.
Inclusion Criteria:
  • Males aged 20 to 29 years
  • Non-smokers.
Exclusion Criteria:
None noted.
Description of Study Protocol:


Selected subset from the 1994 CSFII data set.


A CSFII data subset was used to calculate crude water intake (on a per-meal basis) and minimal water requirements. The intake estimates were summed to create a 24-hour adjusted total intake. Subjects were ranked according to total water intake relative to calculated optimal water intake [above or below minimal water (osmotic load) requirements]. Regression coefficients were applied to discern the impact of alcohol- and caffeine-related misclassification upon estimates of effect.

Statistical Analysis

  • Pearson correlation coefficient was used to compare the continuous unadjusted and adjusted water intake estimates (See Tables Two and Three in paper for example of data analysis for one individual by eating episode)
  • Linear regression models were used to examine differences between adjusted and unadjusted BMI.
Data Collection Summary:

Timing of Measurements

Data from the 1994 USDA nationwide survey (CSFII) were processed on a per-meal basis. Four assumptions were made for the purposes of this study:

  • Of the 190 subjects, two subjects reported not knowing how much tap water they consumed. For these two subjects, a volume of zero grams of drinking water was used in the calculations.
  • The 1994 food composition table provided information on the water, energy, protein, carbohydrate, fat, sodium, potassium, phosphorus and alcohol content, but not the caffeine or chloride content of the foods and beverages covered
  • The caffeine content of selected coffees, teas and caffeinated colas were adopted from the USDA Nutrient Database for Standard Reference 11-1 (1997) and mapped to similar beverages in the 1994 USDA data set
  • The water content information in the 1994 USDA food composition table (salt nutrient vector) was compared to that in the 1995 food composition table. Of 7,251 foods, 94 foods differed by more than 10% between the years. The mean percentage difference in water content between the years was 0.9%.
    1. At the start of the day of dietary assessment, all subjects were in water balance
    2. Estimates of Neuhauser-Berthold, et al** and Eggleton are linear and applicable to the levels of caffeine and alcohol intake observed in this study
    3. Alcohol and caffeine effects are additive
    4. If water intake for one meal is less than the minimal urine volume required to clear the associated osmolyte load at 750mosm per L, the effects of alcohol and caffeine consumed at later meals will be reduced by 32% (based on the findings of Taivainen, et al**).

Based on these assumptions, water losses due to caffeine (1.17ml per mg caffeine) and alcohol (10ml per g alcohol) were subtracted from crude intake estimates. **Note: See reviewer comments.

Dependent Variables

Body mass index was manipulated to fit unadjusted and adjusted water variables.

Independent Variables

  • Water intake: Water intake from food, drinking water and metabolic water was calculated and analyzed as unadjusted and adjusted variables
  • Potential osmotic load: Calculated as net water intake = unadjusted water intake - losses due to alcohol losses due to caffeine
  • Energy, macronutrient and micronutrient intakes: Mean and medians were calculated from one-day dietary recall data.
Description of Actual Data Sample:
  • Initial N: The 1994 CSFII N=5589; this study examined a selected subset of non-smoking males (aged 20-29) N=190
  • Ethnicity: Not described
  • Other relevant demographics
    • Exercise frequency ranged from daily (at 32.1%) to rarely or never (at 18.3%)
    • Health status: 90.7% of subjects were in excellent to good, 7.8% fair and 0.7% poor (percentages totaled 99.2%, not 100%)
    • Individual interviews were conducted during all months of the year.
  • Anthropometrics
    • BMI below 18.5: 1.9%
    • BMI 18.5 to 30.0: 85.8%
    • BMI above 30.0: 112.3%.
  • Location: The CSFII surveys were conducted nationwide. The distribution of this sample was as follows: 15.3% from Northwest, 21.3% from Midwest, 43.7% from South, 19.8% from West. None of the subjects were from the Eastern half of the US.
Summary of Results:
  • The mean unadjusted and adjusted water intakes differed by 321.5g, roughly equivalent to one glass of water. The difference in median intakes was 194.5g.
  • Water intake ranged from 788.5 to 11,779.3g for unadjusted intake, and 760.4 to 9,386.3g for adjusted intake
  • Approximately 92% of the water consumed by this sample appeared bioavailable, with 8% theoretically lost to the diuretic effects of alcohol and caffeine
  • The largest component of total water intake was water from food and non-tap water beverages. Although 34 people (17.9% of the sample) reported drinking no water, on the average drinking water contributed 33% to the total adjusted water intake.
  • Metabolic water intake constituted 10% of the total
  • Alcohol and caffeine intakes were non-normally distributed with median intakes of zero grams and 74.4mg, and highs of 345.6g and 1,098.8mg, respectively
  • Of the 190 subjects, six consumed greater than 600mg of caffeine (about five cups of coffee) during the 24-hour dietary recall period.

Mean (SD) and Median 24-Hour Energy, Nutrient, Unadjusted and Adjusted Water Intakes





Energy (kcal)




Fat (g)                        




Protein (g)      




Carbohydrate (g)




Alcohol (g)




Caffeine (mg)




Sodium (mg)




Potassium (mg)




Phosphorus (mg)




Potential osmotic load (mmols)




Water from food (g)




Drinking water (g)




Metabolic water (g)




Unadjusted total water (g)




Adjusted total water (g)




Bioavailable water (%)




  • Linear regression models, each with a water intake variable as an independent variable and body mass index as the outcome, were fit to evaluate the potential for alcohol- and caffeine-related misclassification bias
  • Misclassification appeared worse at higher water intakes (see regression correlation figure in paper).

Change in Linear Regression Coefficients Due to Caffeine- and Alcohol-Related Misclassification Bias

Total Water Intake Variable

Unadjusted Variable

Adjusted Variable

Change in Estimate (%)

RDA (g water/2,900 kcal)




RDA (g water/energy intake)




Above or below osmotic load (250 mosmol/L)




Above or below osmotic load (350 mosmol/L)




Above or below osmotic load (450 mosmol/L)




Above or below osmotic load (550 mosmol/L)




Above or below osmotic load (650 mosmol/L)




Author Conclusion:
  • On average, the diuretic effects of caffeine and alcohol appeared responsible for an 8% decrease in total water intake of more than one cup of water
  • Misclassification of subjects appeared worse at the higher water intakes, with about 10% of subjects in the highest category reclassified
  • Misclassification resulted in large changes (all over 10%) in linear regression estimates of effect. This was greater than the 10% criterion for confounding.
Funding Source:
University/Hospital: University of Chapel Hill
Reviewer Comments:
  • The investigators' second assumption, that caffeine's diuretic effect was linear, was based upon the Neuhauser-Berthold, 1997, study, which was also reviewed for this Hydration question. Given the issues with the caffeine treatment and the data omissions and flaws in the Neuhauser paper, this assumption may not be valid. 
  • The investigators' fourth assumption cites Taivainen et al, 1995, which examined the diuresis associated with acute alcohol intoxication.

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? 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? 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) 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.) 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? 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.) Yes
  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? N/A
  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? N/A
  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.) N/A
  5.3. In cohort study or cross-sectional study, were measurements of outcomes and risk factors blinded? Yes
  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? N/A
  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? 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? ???
  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? ???
  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? N/A
  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