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Distance running has become increasingly popular among US adults. In 2009, more than 397 marathons took place in the United States, resulting in more than 468000 finishing times, a 10% increase from 2008. (1) The majority of these participants probably serve as their own coaches and are unlikely to be supervised by health care professionals. Currently, no consensus for hydration guidelines exists in the scientific community and, consequently, the information runners encounter varies greatly. (2) Past hydration guidelines (3) encouraged athletes to replace sweat losses during competition. These recommendations received much scrutiny from influential people in the running community, (4) some of whom believe that such messages encourage overdrinking and increase the incidence of hyponatremia among marathoners in the United States. More recent position stands for athletes (5-7) have been fairly conservative, emphasizing the need for sufficient fluid intake before, during, and after exercise to minimize loss of body weight between exercise bouts and recommending that consumption during exercise not exceed sweat losses but be sufficient to avoid body weight reduction greater than 2%. All 3 position stands also promoted the addition of carbohydrates and electrolytes to fluids when exercise is to be prolonged (approximately 1 hour or more) and large sweat losses are expected, and they advised incorporating various methods to measure hydration status (eg, urine specific gravity, urine color, and acute and day-to-day changes in body mass). However, the International Marathon Medical Directors Association (8) has presented opposing viewpoints, particularly advocating that thirst be used as the primary gauge for fluid intake and deemphasizing the relevance of sodium ingestion during events. Organized road races of half-marathon distance or longer are typically scheduled during cooler times of the year and in cooler regions, so that sweat losses can be partially mitigated. Most courses also feature numerous aid stations that provide water and sport beverages. Such ready availability of fluids may result in runners actually gaining weight from the beginning to the end of a race: in some cases, they ingest enough fluid to develop hyponatremia. (9) However, during training runners are responsible for providing their own fluids, and consequently hydration opportunities may be more limited. Access to sufficient hydration can be a particular concern for runners who train in warmer weather.