None listed
Conditions
Brief summary
The aim is to evaluate the effects of chronic sodium (salt) intake on acute responses to low sodium intake with fluid overload during prolonged exercise in the heat. It is queried whether a chronic high sodium diet will upregulate sodium excretion, ie less re-absorption in kidney tubules, and result in a greater decrease in plasma sodium with acute fluid overload, during exercise, than a chronic low sodium diet. Participants will follow both a High (normal western diet) and Low sodium diet for 10 days (prescribed with detailed dietary instruction) in a randomised order with at least 10 days washout between diets. Urinary outputs (24 hour) will be collected on days 7-9; from these samples urinary sodium will be measured. Participants will fill out food frequency and activity questionnaires. On day 10, 3 hour exercise sessions (running or cycling) in the heat (55% VO2max, 34 degrees C, 50% rel. humidity). Water will be given every 15 minutes at a rate equalling the rate of fluid loss in a 1 hour pre-trial of the same exercise in the heat. Methods will be as used previously by Vrijens & Rehrer (1999) including plasma (0, 15 minutes and every 30 minutes thereafter) and urinary sodium, Hb, Hct, and body mass changes (before and after exercise), sweat sodium and core temperature and heart rate continuously monitored. Exercise will be curtailed if core temp. is above 39.5 degrees C or if plasma sodium drops to <130 mmol/L or voluntary exhaustion. Vrijens, D. M. V. and Rehrer, N. J. Sodium free fluid ingestion decreases plasma sodium during exercise in the heat. Journal of Applied Physiology 86: 1847 1851 (1999).
Interventions
The intervention will consist of 10 days on either a “normal western “, i.e. High salt diet, vs. 10 days on a Low salt diet. Participants will be given a pamphlet in which foods to reduce intake of will be listed and cooking advice will be given prior to commencing the Low salt diet to reduce the dietary salt intake on their own accord. For the High salt diet participants will simply be asked to maintain their normal dietary habits. Participants will fill in a food frequency questionnaire (FFQ)
The intervention will consist of 10 days on either a “normal western “, i.e. High salt diet, vs. 10 days on a Low salt diet. Participants will be given a pamphlet in which foods to reduce intake of will be listed and cooking advice will be given prior to commencing the Low salt diet to reduce the dietary salt intake on their own accord. For the High salt diet participants will simply be asked to maintain their normal dietary habits. Participants will fill in a food frequency questionnaire (FFQ) on both diets. The FFQ is being validated at present and the low salt pamphlet developed with New Zealand product examples. On days 7-9 total urinary output will be collected to monitor Na+ output as a measure of intake. There will be at least 10 days "wash-out" period between two diets. Participants will be able to do their own exercise training (noted in a diary) during the dietary interventions but no exercise on day 9 and on day 10 of each diet they will conduct a 3 hour exercise (cycling or running at 55% VO2max) trial in the heat (34 degrees C) laboratory (supervised by the researchers) in which blood and urine samples will be taken.
Sponsors
School of Physical Education , University of Otago PBRF funding
Study design
Allocation
Randomised controlled trial
Intervention model
Crossover
Primary purpose
Prevention
Masking
Blinded (masking used)
Eligibility
Sex/Gender
Male
Age
18 Years to 45 Years
Healthy volunteers
Yes
Inclusion criteria
Endurance trained in running or cycling.
Exclusion criteria
The presence of kidney disease, hypertension and on any medication or supplements other than multivitamins or sports drinks.
Outcome results
None listed