Sodium is the primary electrolyte lost in sweat. Therefore, when exercise exceeds one hour, the American College of Sports Medicine (ACSM) recommends consuming sodium along with fluids to replace both water and sodium lost in sweat. It is not adequate to simply take sodium tabs after a hard training session to replace lost sodium; an athlete must also consume adequate amounts of fluid with this sodium.
Sodium and water are required in appropriate ratios based on an athlete’s sweat rate. It is recommended that an athlete determine their sweat rate in various conditions (link to Sweat Rate Calculator) and consume sodium based on the amount of fluid they require. The American College of Sports Medicine recommends that people who are active for more than one hour consume 500–700 mg of sodium for every 32 oz (~1L) of water they consume. However, there are some who recommend 500–1,000 mg of sodium per 32 oz of water, or per one hour of intense exercise.
Experimental data has demonstrated that sweat rate and sodium loss is highly individual, ranging from 460–1840 mg/L of sweat.
This can be further influenced by numerous other factors including genetics, fitness, acclimatization, and weather conditions. As such, both the sweat rate and the amount of sodium per oz (or liter) of sweat is highly individual. Athletes must experiment in training to find the right balance that works for their body and exercise conditions.