EAL

Hydration and Physical Activity

Citation:

Study Design:

Class:

- Click here for explanation of classification scheme.

Quality Rating:

Research Purpose:

No study has systematically examined the contribution of correction factors (fluid intake, urine loss, respiratory water loss, metabolic mass loss and trapped sweat in clothing) to the measurement of total sweat losses (TSL) during prolonged exercise nor has the accuracy of common sweat loss prediction equations been tested with women.

The purpose of this study was:

To evaluate the error produced by four commonly used field estimates and two prediction equations of total body sweat loss.

It was hypothesized that field measurements (uncorrected) would generally overestimate sweat losses while prediction equations would specifically overestimate sweat losses in women. Warm and cool environments were employed to induce a relatively high and low sweat rate, respectively.

Inclusion Criteria:

Inclusion criteria included:

female
eumemenorrheic and heat acclimated
distance runners (competitive and long-distance).

Exclusion Criteria:

not discussed

Description of Study Protocol:

Recruitment -- not discussed

Design --cross-over experimental trials

Blinding used (if applicable) not applicable

Intervention (if applicable)

eight female distance runners were studied during two experimental trials spaced one to two months apart in counterbalanced order. The experimental trials consisted of a 30 kilometer treadmill run (mean relative exercise intensity of 71±7 percent VO_2MAX) in a warm (30⁰ C) and a cool (14⁰C) environmental chamber. All trials were performed at the same time of day and in the follicular phase of the menstrual cycle.

Dietary

Subjects kept a three day diet record before each trial.

Methods used to estimate total sweat losses (TSL)

Control (CON)=Total sweat loss with all corrections determined from changes in body mass and corrected for fluid intake (FI), urine loss (UL) respiratory water loss (RWL), metabolic mass loss (MML or CO_2/O₂ exchange) and trapped sweat in clothing (TS).
Field 1 (F-1)=changes in body mass only + FI
Field 2 (F-2)=changes in body mass + FI + TS
Field 3 (F-3)=changes in body mass +Fl - UL
Field 4 (F-4)=changes in body mass+ FI + TS - UL
Predictve equation 1 (P1)=equation of Shapiro et al (1982)
Predictive equation 2 (P2)=equation of Barr and Costill (1989)

Statistical Analysis

Data were analyzed using a one-way analysis of variance (ANOVA) with repeated measures to compare each sweat loss determination method.

Multiple comparisons against control were made using Duncan's post hoc test.

Paired t-tests were used to compare data between environments.

3 day food records were analyzed with Nutritionist Five software.

P<0.05 was established as statistically significant.

Data Collection Summary:

Timing of Measurements

Pre-trials

1 week before trials, anthropometrics and cardiorepiratory measurements were taken.

VO_2MAX was determined to reflect each subject's personal best marathon race pace averaged over 42 km. The same belt speed was selected for both 30 km trial runs and the treadmill set at one percent grade to reflect an energy requirement similar to outdoor running on a flat surface.

Subjects kept a three day diet and training record before each trial. Pretrial hydration status was monitored using urine specific gravity and serum osmolarity measurements.

Exercise trials

At the beginning of each trial, in the laboratory setting, subjects (and their clothing which was the same for both trials) were weighed and urine and blood samples were collected.

Runners were assigned to perform 30 km trial runs perform under warm (30^oC) and a cool (14^oC) environmenatal chamber conditions against a constant artificial wind resistance (2.1 m per s). Temperature and humidity were monitored at 10 minute intervals.

Environmental chamber temperture and humidity were monitored at 10 minute intervals. Gas exchange measures were taken for a three minute duration every five kilometers.

120 ml fluid was offered after each gas measurement. Subjects were instructed to consume fluids ad libitum as they would under race conditions.

At the end of each trial, subjects were weighed (and their clothing) and urine was collected.

Dependent Variables

total sweat loss- determined by changes in weight changes; comparison of 4 field estimates and 2 prediction equations
urine specific gravity--measured by refractometer
serum osmolarity--measured by vapor pressure osmometer

Independent Variables

treadmill run trials in a warm and a cool environment
fluid intake

Control Variables

Diet
Training
Menstrual cycle phase

Description of Actual Data Sample:

Initial N: 8 women; 5 were competitive runners (Boston Marathon age-class qualifiers) and 3 were recreational long-distance runners

Attrition (final N): eight women

Age: mean±standard deviation (SD) 37±4 years

Ethnicity: not discussed

Other relevant demographics: not discussed

Anthropometrics: mean±SD

height-- 158±9 cm; weight--53.3±7.8 kg; body surface area--1.5±0.2 m²; VO_xMAX--50±5 ml per kg per minute (range 41-58)

Location: Florida State University

Summary of Results:

Pretrials:

No difference were observed between trials for diet, training, or menstrual cycle phase.
Preexercise body mass was lower (P<0.05) before the warm trial (53.8±8kg) than the cool trial (54.5±7kg).
Preexercise urine specific gravity was not different between the two trials (warm trial=1.003±0.002, cool trial=1.004±0.005g per cm)
Preexercise serum osmolarity was not different between the two trials (warm trial=284±5; cool trial=283±6mosmol per kg)
Thus all subjects were considered adequately and equally trained, fed, hydrated, and under similar hormonal influences at the beginning of the exercise trials.

Trials:

Measurement of TSL-- Control versus four field methods and two prediction equations:

Field methods used to determine TSL from changes in body mass varied widely in their capacity to accurately estimate estimate actual TSL (CON).
In the warm environment, F-1 (6.4 percent), F-3 (0.2 percent) and F-4 (6.9%) accurately estimated TSL (P>0.05); whereas F-2 produced a large error (15.3 percent; P<0.05).
In the cool environment, all four estimates overestimated TSL and produced large errors (14-41 percent; P<0.05).
Both prediction equations markedly underestimated TSL for women runners in the warm environment (20-22 percent; P<0.05) and underestimated or overestimated (41 percent and 20 percent, respectively; P<0.05) TSL in the cool environment.
The results show that, in warm conditions, accounting for FI and UL (F-3), although less practical in the field, provided the closest value to TSL (0.2 percent). However, UL measurements are often impractical and F-1 is a simpler and more commonly recommended technique.
Total sweat loss during competitive 30 km running ranged from 1.9 to 2.9 L.
Statistically significant overestimates of sweat loss in this study (15 percent warm, up to 41 percent cool) would result in fluid intakes of 435-779 mL in excess of sweat losses. Excessive fluid intakes may produce gastric discomfort and hinder performance in women runners is a practical concern.

Other Findings

Author Conclusion:

TSL can be accurately estimated from changes in body mass using F-1, F-3 or F-4 in warm environments; however, none of the methods accurately estimate actual TSL values in a cool environment. Neither prediction equation provided accurate estimates of TSL in warm or cool conditions for female distance runners. Because the majority of field studies emply F-1, TSL values reported under cool conditions (<14^oC) may be in error by 31 percent or more but are probably accurate (approximately six percent) for data collected in warmer (>30^oC) environments. Sweat data reported in temperate field environments (14-30^oC) are logically overestimated by a wide range of 6-31 percent.

These results demonstrate the difficulty of accurately estimating and predicting sweat losses in the field. Failure to match fluid intakes to swat losses can result in fluid inbalance disorders.

Funding Source:

University/Hospital:

Florida State University

Not-for-profit

Foundation associated with industry:

Reviewer Comments:

Limitations may include:

limited generalizability due to study sample consists of female long distance runners; age 37±4 years.

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?		No
	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?	No
	2.2.	Were criteria applied equally to all study groups?	N/A
	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?	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)	N/A
	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?		???
	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?	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?		N/A
	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.)	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?	N/A
	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?	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?	Yes
	7.6.	Were other factors accounted for (measured) that could affect outcomes?	Yes
	7.7.	Were the measurements conducted consistently across groups?	N/A
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?	Yes
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