However, there is little direct evidence of a beneficial effect of electrolyte replacement for any but a small proportion of endurance athletes. The fact that electrolyte concentrations in plasma usually rise during exercise without fluid replacement (Costill et al., 1970, 1974, 1976, 1981) indicates that electrolyte supplements are not needed. Furthermore, during repeated exposures to prolonged physical exertion, the kidneys very effectively conserve sodium and potassium so that the electrolyte balance is usually maintained when an athlete consumes a normal diet or a diet low in potassium (Costill et al., 1976), or a diet high or low in sodium (Armstrong et al., 1985). However, recent case studies have been reported in which athletes who participated in very prolonged exercise experienced severe hyponatremia, i.e., low plasma sodium concentrations during exercise (Hiller et al., 1985; Noakes et al., 1985) or up to 7 days after competition (Noakes et al., 1985). These athletes usually consumed large quantities of water or beverages low in electrolytes. -Fluid Replacement and Heat Stress, Institute of Medicine
The effects of consumption of the individual components of a sports beverage (electrolytes, water, or carbohydrate) and of the combination of all components have also been assessed. Compared with the electrolyte-only trial, performance during the water-only and carbohydrate-only trials was approximately 6 percent faster. However, the combination of carbohydrate and water caused a performance enhancement that was approximately 12 percent faster than the electrolyte trial and 5 to 6 percent faster than when water only or carbohydrate only were consumed. -healthline.com
If no fluid is ingested during prolonged exercise, serum Na+ concentration, osmolality and anti diuretic hormone activity all increase (9). The rise in serum Na+ concentration and osmolality correlate with the increase in (body) esophageal temperature and may be a stimulus for the reduction in skin blood flow and sweating that develops at advanced levels of dehydration (10). An important goal of fluid ingestion during exercise may therefore be to prevent rises in serum osmolality and thereby maintain sufficient skin blood flow for maximum evaporative and convective heat losses. -ultrunr.com
MY COMMENT: I still am a bit perplexed by the hype surrounding the need for electrolytes and the perception that supplementation will somehow enhance marathon performance. Americans are overloaded with sodium to begin with, and dehydration increases concentration in the blood. So why would the typical marathoner need to take additional electrolytes? Unless you are sweating heavily and drinking more water than you are losing, marathoners do not need to replace electrolytes until they cross the finish line.