Dietary salt and responses to fluid overload during exercise

INTERVENTION: 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. CONDITION: Exercise associated hyponatraemia PRIMARY OUTCOME: Plasma sodium concentration Urinary excretion (volume and Na+) SECONDARY OUTCOME: Resting blood pressure ; This will be measured, in the fasted state, participants having sat for 10 minutes with arm supported, legs uncrossed, with an automated sphygmomanometer. Two measurements will be made and if they are not similar a third measurement will be made. An average of the two (closest) will be taken. INCLUSION CRITERIA: Endurance trained in running or cycling.