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Trial registered on ANZCTR

Registration number
Ethics application status
Date submitted
Date registered
Date last updated
Type of registration
Prospectively registered

Titles & IDs
Public title
Can increased sodium intake reduce the risk of exercise associated hyponatraemia ?
Scientific title
Can increased sodium intake reduce the risk of exercise associated hyponatraemia in healthy male athletes?
Secondary ID [1] 288540 0
Universal Trial Number (UTN)
Trial acronym
Linked study record

Health condition
Health condition(s) or problem(s) studied:
exrcise associated hyponatraemia 297642 0
Condition category
Condition code
Diet and Nutrition 297832 297832 0 0
Other diet and nutrition disorders
Metabolic and Endocrine 297842 297842 0 0
Other metabolic disorders

Study type
Description of intervention(s) / exposure
We will utilise a randomised cross-over intervention in which participants will perform two, 3 h exercise trials (cycle ergometry @ workload to elicit 55% VO2max, determined in a graded maximal exercise test on a separate day) in the heat (34 degrees C, controlled climate chamber), periodically ingesting a beverage, with either 60 or 20 mmol Na+/L and the rate of plasma sodium change monitored. The experimental protocol is based upon earlier work (Vrijens & Rehrer, 1999). The beverages will use a base of Gatorade (5.94% glucose 20 mmol Na+/L ) to which NaCl will be added to bring to 60 mmol Na+/L for intervention beverage. (a pre-trial will be done for acceptability and if unpalatable or gastrointestinal distress a mix of NaCl and sodium citrate will be used). The volume given will be determined by net body mass losses in a 1 h exercise pre-trial conducted on a separate day prior to experimental trials under the same conditions (specified amount of water given). The amount of fluid provided in 3 h experimental trials will be at a rate (given every 15 min) to replace mass loss observed in the 1 h trial (i.e. slight excess since some mass loss is due to substrate metabolism). This will be measured out in advance, chilled and provided to participants who will be told to ingest completely as quickly as possible. No other beverage will be ingested. A researcher will be continually supervising participants during exercise trials. During trials heart rate and rectal temperature will be continually monitored. If age predicted heart rate maximum or a core temperature of 39.5 degrees C is reached exercise will be stopped. Experimental trials will be separated by at least 7 days. The evening before trial 1 litre of sports drink (Gatorade [5.94% glucose 20 mmol Na+/L] provided) will be ingested. Breakfast is to be consumed 2 h before the first experimental trial, which will be of their choice, but they will be required to eat this same meal before the second trial. They will also be instructed to drink 1 L of a sports drink (provided) the evening before each trial. On the day of the trial before coming to the laboratory, they may drink water ad libitum, but may ingest no other foods or fluids after breakfast. In addition, the subjects will be instructed to refrain from strenuous exercise 24 h before each trial.
Intervention code [1] 293918 0
Comparator / control treatment
Same exercise and volume of fluid ingestion but with 20 mmol/L sodium in beverage, which is similar to most sports drinks.
Control group
Dose comparison

Primary outcome [1] 297361 0
Change in plasma sodium concentration
Timepoint [1] 297361 0
Baseline prior to exercise and every 30 min thereafter until exercise ceases.
Secondary outcome [1] 320803 0
Change in core temperature, with a rectal thermistor and Squirrel data logger.
Timepoint [1] 320803 0
Measures continually and means per 15 min time period will be calculated across 3 h of exercise or until exhaustion.
Secondary outcome [2] 320804 0
Change in heart rate, with a Polar heart rate monitor which consists of a chest strap and wrist band.
Timepoint [2] 320804 0
Measures continually and means per 15 min time period will be calculated across 3 h of exercise or until exhaustion.
Secondary outcome [3] 320805 0
Ratings of perceived exertion
Timepoint [3] 320805 0
Assessed very 15 min across 3 h of exercise or until exhaustion
Secondary outcome [4] 320806 0
Exercise time (exhaustion) assessed by review of recorded time participant stopped exercising.
Timepoint [4] 320806 0
During 3 h of intended exercise
Secondary outcome [5] 320962 0
Plasma volume change as determined by haemoglobin and haematocrit (Dill & Costill, 1974)
Timepoint [5] 320962 0
At rest, after sitting for 10 min, and every 30 min during exercise for 3 h or until exhaustion.
Secondary outcome [6] 320963 0
Urine sodium output, by volume produced and [Na+] (Flame photomtery).
Timepoint [6] 320963 0
Collected as produced during exercise and immediately following exercise.
Secondary outcome [7] 320964 0
Sweat sodium excretion.
Determined by volume produced (estimated by body mass change corrected for fluid in and urinary output) and sweat [Na+] determined by flame photometry.
Sweat from 4 patches attached on thigh, back, chest and arm..
Timepoint [7] 320964 0
Sweat patches attached at the beginning of exercise and removed after 1 h of exercise.
Body mass measured at the before and at end of exercise.

Key inclusion criteria
Endurance trained males. This does not need to be cycle specific.
Minimum age
18 Years
Maximum age
55 Years
Can healthy volunteers participate?
Key exclusion criteria
On prescription medications (except nutritional supplements) or other drug use (except caffeine),
Have kidney disease or diabetes,
Contraindications to participate in maximal exercise.

Study design
Purpose of the study
Allocation to intervention
Randomised controlled trial
Procedure for enrolling a subject and allocating the treatment (allocation concealment procedures)
Beverage prepared by just one staff member sodium content unknown to others, labelled by code.
Methods used to generate the sequence in which subjects will be randomised (sequence generation)
Random number generator
Masking / blinding
Blinded (masking used)
Who is / are masked / blinded?
The people receiving the treatment/s

The people assessing the outcomes
Intervention assignment
Other design features
Not Applicable
Type of endpoint(s)
Statistical methods / analysis
Based upon earlier work (Vrijens & Rehrer, 1999) and the rate of plasma sodium change observed, with a similar protocol with ingestion of water versus a typical sports drink with 18 mmol/L sodium, we calculated a sample size of 12 to detect a difference of 1.6 mmol/L/h, with standard deviation of 1.5 mmol/L/h, a=.05, power= .90, for paired samples. A target sample size of 14 will be recruited to allow for drop outs or experiments in which technical error occurs.
Data will be analysed by linear mixed effects model analysis of variance, with time as a fixed effect and subject as a random effect, for data collected over time.

Recruitment status
Active, not recruiting
Date of first participant enrolment
Date of last participant enrolment
Date of last data collection
Sample size
Accrual to date
Recruitment outside Australia
Country [1] 7592 0
New Zealand
State/province [1] 7592 0

Funding & Sponsors
Funding source category [1] 292888 0
Name [1] 292888 0
School of Physical Education Sport & Exercise Sciences, University of Otago
Address [1] 292888 0
PO Box 56
Dunedin 9054
Country [1] 292888 0
New Zealand
Primary sponsor type
School of Physical Education Sport & Exercise Sciences, University of Otago
PO Box 56, 46 Union ST West
Dunedin 9054 ,
New Zealand
New Zealand
Secondary sponsor category [1] 291631 0
Name [1] 291631 0
Address [1] 291631 0
Country [1] 291631 0

Ethics approval
Ethics application status
Ethics committee name [1] 294388 0
University of Otago HUman Health Ethical Committee
Ethics committee address [1] 294388 0
c/o Gary Witte, Manager Committees
PO Box 56
University of Otago
Dunedin 9054
Ethics committee country [1] 294388 0
New Zealand
Date submitted for ethics approval [1] 294388 0
Approval date [1] 294388 0
Ethics approval number [1] 294388 0

Brief summary
Exercise associated hyponatraemia is a life threatening condition that can occur in (ultra-) endurance athletes who exercise for long periods of time. The primary risk factor is overhydration with hypotonic fluid, i.e. drinking at a greater rate than any excess can be excreted (Hew-Butler et al 2015). However, other factors can play a role, including sodium losses (Godek, et al 2008) and intakes (Vrijens & Rehrer, 1999). Although ingesting a beverage with 18 mmol/L sodium reduces the rate of decline in plasma sodium when compared with water (during endurance exercise in the heat) it still does decline. Sweat sodium ranges from ~30-80 mmol/L and studies with 60 mmol/L sodium (oral rehydration solutions) in clinical situations (at rest), in which plasma sodium is compromised via losses, result in improved hydration and normalisation of plasma sodium. Therefore, we intend to test the efficacy of a beverage with 60 mmol/L sodium, when consumed in slight excess (i.e. approximately matching body mass loss) throughout longlasting exercise in the heat (34 degrees C) in healthy, endurance trained individuals. The purpose of this study is to establish whether increasing the concentration of sodium in a beverage (~ 3 times that normally in sports drinks), can keep blood sodium concentration from decreasing during longlasting exercise in the heat, when fluid is ingested in amounts to nearly match body weight loss.
The hypothesis is that ingesting a beverage with 60 mmol/L of sodium during 3 h of cycling in the heat will prevent the fall in blood sodium that typically occurs with a beverage with 18-20 mmol/L of sodium. We will utilise a randomised cross-over intervention in which participants will perform exercise in the heat, and will ingest a beverage with one of two concentrations of sodium (20, 60 mmol/L) and then repeat the trial with the alternate concentration.
Trial website
Trial related presentations / publications
Public notes

Principal investigator
Name 63554 0
A/Prof Nancy J Rehrer
Address 63554 0
School of Physical Education Sport & Exercise Sciences
University of Otago
PO Box 56, 46 Union St West
Dunedin 9054
Country 63554 0
New Zealand
Phone 63554 0
Fax 63554 0
Email 63554 0
Contact person for public queries
Name 63555 0
A/Prof Nancy J Rehrer
Address 63555 0
School of Physical Education Sport & Exercise Sciences
University of Otago
PO Box 56, 46 Union St West
Dunedin 9054
Country 63555 0
New Zealand
Phone 63555 0
Fax 63555 0
Email 63555 0
Contact person for scientific queries
Name 63556 0
A/Prof Nancy J Rehrer
Address 63556 0
School of Physical Education Sport & Exercise Sciences
University of Otago
PO Box 56, 46 Union St West
Dunedin 9054
Country 63556 0
New Zealand
Phone 63556 0
Fax 63556 0
Email 63556 0

No information has been provided regarding IPD availability
Summary results
Have study results been published in a peer-reviewed journal?
Other publications
Have study results been made publicly available in another format?
Results – basic reporting
Results – plain English summary