The competition season is upon us, and a lot of athletes struggle to get dry on stage. Here are the basics of how your body works when it comes to water balance:
The majority of fluid output occurs from urination. Some fluid is lost through perspiration (part of the body’s temperature control mechanism) and as water vapor in expired air.
The body’s homeostatic control mechanisms ensure that a balance between fluid gain and fluid loss is maintained. The hormones ADH (antidiuretic hormone, also known as vasopressin ) and aldosterone are responsible for this.
What does this mean: your body is smart, it will always try and restore the water balance.
If you drink too little water, it will retain fluid by the kidneys and reduces the urine output.
When you drink too much water, your body will try and push it out by increasing your urine output. Drinking too much water also increases the amount of water in your blood and your sodium and electrolyte levels drop. Sodium helps balance fluids between the inside and outside of cells.
When sodium levels drop due to excess water consumption, fluids shift from the outside to the inside of the cells, causing them to swell. When brain cells swell, pressure inside the skull increases. This pressure causes the first symptoms of water intoxication: headache, nausea, vomiting.
Aldosterone increases water reabsorption through sodium cotransport.
ADH increases water reabsorption by increasing the nephron’s permeability to water, while aldosterone works by increasing the reabsorption of both sodium and water.
Over hydration happens when you drink more water than your kidneys can get rid of via urine.
But the amount of water is not the only factor. How long you take to drink the water also counts.Your kidneys can only get rid of about 0.8 - 1 liter of water per hour. Therefore to avoid water intoxication you should not drink more than 1 l of water per hour on average.
Extra care should be taken when you ‘load’ the water before a competition, and also when you rehydrate following a dehydration after a competition - don’t drink too much water at once.