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HYD: Effect of Caffeinated Beverages on Fluids (2007)


Scott D, Rycroft J, Aspen J, Chapman C, Brown B. The Effect of Drinking Tea at High Altitude on Hydration Status and Mood. Eur J Appl Physiol. 2004; Apr. 91 (4): 493-498.


PubMed ID: 14872247
Study Design:
Randomized Crossover Trial
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Quality Rating:
Positive POSITIVE: See Quality Criteria Checklist below.
Research Purpose:

To investigate the effects of drinking tea at high altitude on hydration status and mood. There were two research objectives:

  • To investigate whether there is any evidence to show that tea does lead to body water loss under these conditions and hence, should be avoided by mountaineers and trekkers at high altitude
  • To investigate the effects of tea consumption on mood, to test whether it has any positive psychological effects at high altitude.
Inclusion Criteria:
  • Mountaineer with supporting roles in expeditions attempting to climb either Mt. Everest or Mt. Lhotse
  • Regular tea drinker.
Exclusion Criteria:
  • Past medical history of hepatic, renal, cardiac, pulmonary, digestive, haematological, neurological, thyroidal or psychiatric disease
  • Anyone taking medications that might influence weight or fluid and electrolyte balance (e.g. over-the-counter medications or supplements and especially those containing methylxanthines)
  • Sherpas were excluded.
Description of Study Protocol:


Not specifically addressed; however the authors did state that all participants were volunteers who held supporting roles in expeditions attempting to climb either Mt. Everest or Mt. Lhotse.


Participants were subjected to a crossover experimental design comprising two 24-hour dietary interventions that closely resembled the conditions in which tea is widely drunk in base camp environments.   

  • Prior to the testing day, subjects were supplied with a beverage diary and two graduated bottles, one for measuring fluids consumed and the other urine
  • Participants were assigned to one of the two conditions according to a randomized schedule, beginning the trial with either the tea or no-tea condition to balance for order effects
  • In the tea condition, hot brewed tea formed a major part of fluid intake, whereas in the no-tea condition, tea was excluded from the diet
  • In both conditions,  participants were prohibited during the 12 hours prior to the start of testing from consuming alcohol or caffeinated drinks or food, including tea, coffee and chocolate. This washout period ran from 20:00 hours to the start of the 24-hour test period at 08:00 hours. Since this was the time when most subjects usually slept, the standardized washout period represented a minimal disruption from their normal routine.
  • In the tea condition, participants were required during the course of the day to consume a minimum of two mugs of tea using vessels of known volume (400ml) supplied by the investigators. To ensure that study results reflected the effects of naturalistic tea drinking practices, the tea was prepared and consumed according to subjects’ own taste. To remain further consistent with natural behavior under these conditions, no further constraints were placed on tea consumption and no upper limit was imposed on participants.

 Statistical Analysis 

  • Statistical analysis was conducted using paired Student’s T-tests after appropriate manipulation of the data
  • Differences in urinary measures of hydration status (specific gravity, colour, pH) were computed between pre- and post- time points; then the difference between these measures for the two conditions was analyzed
  • Fluid volumes were analyzed by comparing the ratios of urine volume to total fluid intake for the tea and no-tea conditions. This data did not follow a normal distribution, so the log of the values was taken and subjected to analysis.
  • The difference in mood measures between conditions was compared directly
  • A probability level of 0.05 was set as statistical significance.
Data Collection Summary:

Timing of Measurements

Data was collected over a 10-day period at Mt. Everest base camp, at an altitude of 5,345m.

Dependent Variables

  • Fluid intake and output: Subjects were asked to record in the diary the type and quantity of every beverage ingested over the 24-hour period, as well as the volumes of every bladder void. Fluid intake was measured using the graduated bottles and the 400ml cups. This provided a 24-hour measure of total fluid volume ingested and urine excreted.
  • Urine-specific gravity and pH: First urine void of the morning at both the start and end of the 24-hour test period was tested within an hour of the void for specific gravity and for pH, using reagent strips for urine analysis (Labstix SG; Bayer)
  • Urine color: Estimated from a color chart with eight levels (Armstrong, 2000) 
  • Urine sodium and potassium levels: Aliquots (15ml) were taken from the morning void, preserved in chilled conditions in insulated containers for the duration of the expedition and, on completion, were stored with dry ice and transported by air to the Bedford General Hospital laboratory for analysis (Cobas Integra, Roche Diagnostics) 
  • Mood: Ratings were made at the end of each testing day on 12 adjectives derived from the Profile of Mood States (POMS; McNair et al, 1981). The highest loading items from each of the seven sub-scales of the POMS were selected. Participants rated these adjectives in response to the question, "How have you been feeling generally today?" using a 10-cm non-graduated line as a visual analogue scale, with endpoints "Not at all" and "Extremely." The line was subsequently measured in millimeters to form a 101-point scale. Where the factor was represented by two items, the mean was computed. Subjects also completed a short questionnaire to measure any deleterious effects of altitude experienced while participating in the study, based on the Lake Louise symptoms of acute mountain sickness (Hackett and Oelz, 1992). This includes items on gastrointestinal symptoms (anorexia, nausea or vomiting), fatigue or weakness, dizziness or light-headedness, and difficulty sleeping.

Independent Variables

Tea: Subjects used Taj Mahal tea bags, containing a blend of Indian black teas and principally Assam tea. The tea was analyzed to test for the levels of methylxanthines present using an HPLC system (Dionex) with UV detection at 274nm. Caffeine was present at 67.3mg per gram tea solids and theobromine at 4.8mg per gram. No trace of theophylline was found.
Description of Actual Data Sample:
  • Initial N: 13 subjects; nine males and four females
  • Attrition (Final N): All 13 subjects recruited to the study successfully completed both conditions. None experienced any adverse effects as a result of the experimental intervention or any symptoms of acute mountain sickness during the study.
  • Age: Mean age was 33.5 years, the age range 18 to 51 years
  • Ethnicity: Not stated, but the authors did state that the participants were not Sherpas
  • Other relevant demographics: None described in paper
  • Anthropometrics: None described in paper
  • Location: Mt. Everest base camp in Tibet at an altitude of 5,345m for the study duration.
Summary of Results:
  • Mean fluids ingested [mean (SE); tea=3,193 (259) ml vs. no tea=3,108 (269) ml] and urine volume [tea=2,686 (276) ml vs. no tea=2,625 (342) ml] were similar under both conditions
  • Statistical analysis found no difference in urine stimulated as a result of the tea intervention (P=0.81)
  • None of the markers of hydration status indicated a difference in hydration status as a result of the dietary intervention in either the control or tea condition. A difference was found in mood, with subjects reporting reduced fatigue when tea was included in the diet (P=0.005).
  • No significant differences were found on urinary measures of hydration after 24 hours (P<0.05).
Author Conclusion:

Results suggest that tea consumed under the quasi-naturalistic conditions of the study does not promote fluid loss and dehydration. The total fluid intake of participants was relatively high, with a mean of 3,150ml across the two conditions and this appears to have been sufficient to counter the diuresis typically experienced at altitude. The results, therefore, relate to high-altitude conditions in which hydration status is stressed, but not where participants are already dehydrated.

  • Using two different types of hydration measure, one based on 24-hour urine volume and the other on analysis of early-morning voids, no differences were found as a consequence of drinking tea
  • At the start of the two conditions, participants were found not to be dehydrated. Urine specific gravity was 1.021 and 1.022, which is within the typical range of 1.013 to 1.029 for normal specimens for healthy adults and well below 1.030, which is the level that indicates dehydration or hypohydration. This finding is consistent with the urine color results of 1.88 and 2.04, compared with scores of one to three on the urine color scale that correspond to adequate hydration.
  • An effect was found on mood, with subjects reporting feeling less fatigued during the day in which they drank tea.
Funding Source:
Unilever RDColworth
Food Company:
Reviewer Comments:
Four of the authors were employed by Unilever RD Colworth in the UK, which is affiliated with Hindustan Lever. Hindustan Lever manufactures the Taj Mahal tea that was used in the study.
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? Yes
  2.3. Were health, demographics, and other characteristics of subjects described? No
  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) Yes
  3.2. Were distribution of disease status, prognostic factors, and other factors (e.g., demographics) similar across study groups at baseline? ???
  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? 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? 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? 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? 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? N/A
  6.4. Was the amount of exposure and, if relevant, subject/patient compliance measured? N/A
  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? N/A
  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? ???
  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)? N/A
  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? No
  10.1. Were sources of funding and investigators' affiliations described? No
  10.2. Was the study free from apparent conflict of interest? Yes