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Hydration

Hydration and Physical Activity

Citation:

Sims ST, vanVliet L, Cotter JD Rehrer NJ.  Sodium Loading Aids Fluid Balance and Reduces Physiological Strain of Trained Men Exercising in the Heat.  Med Sci Sports Exerc, 2007 Jan;39(1):123-30.

PubMed ID: 17218894
 
Study Design:
Randomized Crossover Trial
Class:
A - Click here for explanation of classification scheme.
Quality Rating:
Neutral NEUTRAL: See Quality Criteria Checklist below.
Research Purpose:

To determine whether ingestion of a highly concentrated sodium-citrate beverage would induce hypervolemia in trained individuals and enhance running capacity in the heat.

Inclusion Criteria:

Healthy, nonsmoking, male, moderately trained runners

Exclusion Criteria:

Not specified.

Description of Study Protocol:

Recruitment: Not specified.

 

Design: Double blind, placebo controlled, crossover trial

 

Blinding used (if applicable):

  • Beverage was blind to the researcher conducting the trials and to the participant. 
  • To avoid any perceptive taste variations in the beverages, the participants were told that the study was designed to investigate  varying concentrations of sodium in a pre-exercise beverage.

Intervention (if applicable)

  • Subjects heat acclimated
  • Trials randomized, separated by 1 - 3 weeks
  • Control beverage:  Low Na+, 10 mmol Na+/L-1, 0.58g NaCl, 42 mOsm/kg-1
  • Experimental beverage:  High Na+, 164 mmol Na+/L-1, 7.72g sodium-citrate with 4.5g NaCl, 253 mOsm/kg-1
  • Volume:  10 mL/kg body mass (approximately 757 mL) 
  • Protocol:
    • Participants voided on arrival to lab,
    • Nude body mass recorded (+ 10g), catheter placement in left arm,
    • Participants stood for 20 min before baseline blood samples were taken to ensure steady-state plasma volume and constituents.
    • Beverage ingested in seven equal portions, one every  10 min.;  Participants were required to walk approximately 1 min every 20 min to limit venous pooling;
    • Before blood sampling, they were required to stand in one place for 20 min.
  • Standardization of training effects:
    • Each participant maintained training diary of duration, mode and intensity of activity, replicated for consistency preceding each trial
    • Each participant completed a 40 min treadmill run at 50% V02max 48 h before each testing day and refrained from training until the experimental trial
    • Same meal of participant's choice consumed the evening before testing
    • No smoking, alcohol caffeinated beverages on the day before and day of test
    • Standardized breakfast on day of testing:  1680 kJ, 13 g pro, 10 g fat, 65 g CHO, 2654 mg Na+ consumed 2-2.5 hr before experimentation
    • 500 mL water given between breakfast and start of testing
    • Exercise commenced 45 min after consuming beverage in climatically controlled chamber (32oC, 50% relative humidity, wind speed 1.5 ms).
    • Speed of treadmill set to elicit 70% of temperate environment VO2max
    • Exercise stopped when participant could not maintain exercise at given intensity or when ethically constrained rectal temperature of 39.5oC was reached.

Statistical Analysis

  • Significance of effects of beverage and time on plasma volume, plasma osmolality, respiratory exchange ratio (RER), plasma sodium concentration:  two-way repeated measures ANOVA
  • Differences between means:  Bonferroni-corrected post hoc tests
  • Differences in fluid loss, urinary loss, mass loss, rate of change in urine [Na+], and plasma osmolality, RER, time to exercise termination, and slope of heart rate and rectal temperature:  paired t-tests
  • Relationships between selected dependent measures: Pearson product correlations.
  • Differences considered statistically significant when P<0.05.
Data Collection Summary:

Timing of Measurements:  Trials were separated by 1-3 weeks.

 

Dependent Variables

  • Time on treadmill with speed at 70% of temperate-environment VO2max
  • Rectal temperature:  Thermister 400, Mallinckrodt, St. Louis, MO
  • Skin temperatures:  insulated thermistors (Type EU, Grant Instruments Ltd, Cambridge, UK) at 4 sites - biceps, calf, chest and thigh;  mean temperature calculated
  • Carbon dioxide production, VO2, and ventilation:  measured for 2-min periods at 5 min. intervals using a gas-analysis system (Cortex Metalyser 3B, Borsdorf/Leipzig Germany)
  • Urine [Na+ ], urine specific gravity:  urine collected at baseline (-105 min) and after drinking (-45 min),  immediatly before exercise (0 min), and at exhaustion.
  • Hematocrit (Hct), Hemoglobin (Hb), plasma sodium concentration (P[Na +] and osmolality:  blood samples taken immediately before urine sampling
  • Change in plasma volume (PV) from baseline (%change PV) =100[(Hb0/Hbt) ((1-Hctt)/(1-Hct0))]-100%;  t=time; 0=at baseline(-105 min); Hb=gm/100mL; Hct multiplied by .96, the.91 to correct for trapped plasma and the menous to whole blood Hct excess.
  • Sweat loss:  estimated by change in body mass corrected for urinary and blood losses and fluid intakes.
  • Rates of sweat loss:  approximated by dividing by exercise time.
  • Final temperatures - rectal temperature (Trec) and skin temperature (Tskin):
    mean of five mesurements recorded during the 2.5 min before the end point of the shortest exercise trial;

 

Independent Variables: Low sodium (Low Na+) or High sodium (High Na+) beverage

  • Control beverage:  Low Na+, 10 mmol Na+/L-1, 0.58g NaCl, 42 mOsm/kg-1
  • Experimental beverage:  High Na+, 164 mmol Na+/L-1, 7.72g sodium-citrate with 4.5g NaCl, 253 mOsm/kg-1
  • Volume:  10 mL/kg body mass (approximately 757 mL) 


 

Control Variables:  None specified

 

Description of Actual Data Sample:

 

Initial N: 9 males

Attrition (final N):  Final N=8

Age:  Mean + s.d. = 36 + 11 yr

Ethnicity:  Not specified

Other relevant demographics:  VO2max:  58 mL/kg-1/min (SD 5)

Anthropometrics :

  • Weight (mean + s.d.) = 75.2 + 6.7 kg
  • Height (mean + s.d.) = 179.5 + 5.5 cm

Location:  University of Otago,  Dunedin, New Zealand

 

Summary of Results:

 

Cardiovascular and Thermal Responses

Outcome

Low Na+

High Na+

Significance (P)

Plasma volume after drinking, before exercise (% change)

0.0 +0.5

4.5+3.7

.04

Plasma volume during exercise (% change)

-3.1+3.4

-2.5 + 2.6

n.s.

Heart rate during exercise (bpm)

161+ 16

157 + 11

n.s.

Cardiovascular drift *  (min-2)

0.44

0.22

n.s.

* rate of rise after initial 5 min of exercise

  • Plasma volume decreased immediately before exercise in both trials.
  • Time matched final heart rate was higher in Low Na, but differences were n.s. between conditions
  • Mean Trec and mean Tskin increased over time in both trials (P < .001); no treatment effect was observed.
  • Time matched final Trec was lower in High Na+ than in Low Na+ (P=0.00)
  • Repeated measures ANOVA indicated a significant beverage effect (P=0.038) of changes in plasma volume before exercising to exhaustion.

Plasma osmolality and [Na+]

  • Overall increase in plasma osmolality during exercise between  High Na+ and Low Na= n.s.
  • Rate of change in osmolality with High Na was slower than that of low Na+ (P=0.00)
  • Plasma [Na+] was stable at rest after drinking for both High Na+and Low Na+ (n.s.)
  • Plasma [Na+] increase during exercise:  High Na+ = 0.8 mmol Na+L-1;  Low Na+= 2.8 mmol Na+L-1 ; P=0.06 

Fluid balance

  • After ingestion of 757 mL of fluid in each condition, sweat rates and volumes were equivalent.
  • More urine(mL) was produced throughout the Low Na+ trial (mean +s.d. for 8 subjects: 658.5+168.0) than during the High Na+ trial (492.0+196.5); (P<0.05)
  • Rate of urine production (mL per h-1) throughout the Low Na+ trial (mean+s.d. for 8 subjects: 182+260.1) was greater (P<.05) than the High Na+ trial (130.1+252.1)
  • During exercise, total sweat loss, rate of sweat loss, and rate of change in body mass= n.s.

 Exercise Tolerance:

Core temperature increased during exercise in both conditions, reaching the 39.5oC. limit in 6/8 in Low Na+ and in 5/8 in High Na+trial.   

Time to exercise exertion (mean+s.d.)

  Reason for ending trial Low Na+ High Na+ Significance (P)
  39.5oC. 46.4 + 4 57.9 + 6 .04
  exhaustion 75.3 + 21 96.1 + 22 .03
         

  • Time-matched final rating of perceived exertion (RPE) was higher in the Low Na+ than in High Na+ trial (P=.04).
  • The respiratory exchange ratio remained equivalent between beverage conditions ( Analysis included only 7/8 subjects)

 

Author Conclusion:

Preexercise ingestion of a high-sodium beverage increased plasma volume before exercise and involved less thermoregulatory and perceived strain during exercise and increased exercise capacity in warm conditions.

Funding Source:
University/Hospital: University of Otago, New Zealand
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? 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? 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")? Yes
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? 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? ???
  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? Yes
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? N/A
  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