Saturday, December 3, 2011
Thoughts on sodium retention in exercise
I was recently given the opportunity to read a chapter of the upcoming book "waterlogged" by Dr. Tim Noakes. I've been looking at the research the Dr. Noakes references as well as other research around sodium loss. I agree with Dr. Noakes proposal that the main cause of hyponatremia is over hydration. All Dr. Noakes also proposes that the human body will stay in sodium balance during exercise and that the *only* cause of hyponatremia is over hydration. The research that I've looked at the to me to believe the for athletes exercising from moderate lengths of time in moderate conditions Dr. Noakes is probably correct, and that sodium balance will be naturally maintained. However, all of the research have found indicates that the sodium concentration of sweat is proportional to the sweat rate. This means the sweat rate increase, sodium losses increasing disproportionately quickly. You for instance, if I'm sweating at 0.5 L per hour, my sweat concentration should be around 15 mmol per liter which means I'm losing only about 350 mg per hour of sodium. If my sweat rate doubles to 1 L per hour, my sweat concentration triples to around 50 mmol per liter and I'm losing around 1200 mg per hour of sodium. Doubling the sweat rate again causes sodium losses to jumped to over 4000 mg per hour of sodium which is nearly 3 teaspoons of salt per hour. Dr. Noakes proposes that there are inactive sodium stores in the body that can compensate for sodium losses but the evidence to support this is rather dubious. In conclusion it is my belief that those running in hot humid conditions need to increase their sodium intake. If this running is for protracted periods and then the sodium intake needs to be increased during the run. I have updated my wiki in a number of places based on this information. I have a heat calculator which gives an estimate of the perceived temperature while running in different conditions and there is an option to search sodium losses in this calculator (http://fellrnr.com/wiki/Hydration_101). I've created a calculator specifically to show sodium losses for different sweat rates (http://fellrnr.com/wiki/Sodium_Loss). I've also updated my pages on hydration (http://fellrnr.com/wiki/Hydration_101) and Hyponatremia (http://fellrnr.com/wiki/Hyponatremia). Please let me know if you have any questions or comments.