As you know, when you sweat, you lose sodium, an electrically charged molecule that is a part of salt. Sodium and other electrolytes help maintain proper fluid balance inside and outside of cells. Hormones and kidneys help sodium levels stay within a tight range during the course of normal daily activity.
When athletes sweat, they lose proportionately more water than sodium. This concentrates the sodium that remains in their body. Hypernatremia—high sodium levels—happens when athletes under-hydrate; the concentration of sodium in their blood increases as they become progressively dehydrated. A study with 81 half-marathoners indicates about 7-percent (n=6) ended up with hypernatremia. They tended to drink less often and in lower amounts than the runners who maintained normal sodium levels. Runners who drank less than 0.9 liter during the hot, humid half-marathon ended up with high sodium levels. We need more research to help identify the optimal range for fluid intake.
On the other hand, when athletes drink too much water—or even too much sport drink (a relatively low source of sodium), they can experience hyponatremia (low blood sodium). The high fluid intake dilutes the sodium in their body. This has been documented in up to 13% of Boston Marathon runners and 45% of runners in the Comrades Marathon. This is dangerous and can lead to swelling in the brain, coma and yes, even death.
The bottom line: Learn your sweat rate by weighing yourself naked before and after an hour of exercise with no fluid intake. If you lose 2 pounds, you have lost 32 ounces of sweat. You now know you should target drinking 8-ounces of fluid every 15 minutes during exercise to help maintain proper fluid and sodium balance. If you will be exercising hard and sweating heavily for extended periods in the heat, you might want to eat some salty foods before you exercise to get sodium into your system. This helps retain water in your body and delays dehydration.
Reference; Martinez-Cano JP et al. Dysnatremia among runners in a half-marathon performed under hot and humid conditions. BMJ Open Sports & Exercise Medicine 2018; 4:e000351
For more information on sodium: Nancy Clark’s Sports Nutrition Guidebook
See also: http://blog.nancyclarkrd.com/2017/03/11/the-science-of-fueling-for-performance/