This is one of the biggest areas of research right now in the ultra endurance world. Our current knowledge base is hampered by limited studies, very small sample sizes, no good pre-race measures of the athletes' blood and kidney function, and a focus entirely on sodium levels and no other metabolites or electrolytes.
The problem really developed with the broadening of endurance events to include very slow participants. If it takes you 6 hours to complete a marathon, and you are told to drink 8 oz of water every 20 minutes...goodness gracious that's a lot of water!! Then more people started doing ironmans, and ultra marathons...and were told to drink all this liquid every 10-20 minutes...it's crazy actually.
Anyway, the current best practice guideline is to drink when you are thirsty and not to worry otherwise. This is a highly relevant abstract from a piece in the journal Wilderness Environmental Medicine:
Wilderness Environ Med, 2009 vol. 20(2) pp. 139-43
Exercise-associated hyponatremia: overzealous fluid consumption.
Rogers, IR; Hew-Butler, T
Exercise-associated hyponatremia is hyponatremia occurring during or up to 24 hours after prolonged exertion. In its more severe form, it manifests as cerebral and pulmonary edema. There have now been multiple reports of its occurring in a wilderness setting. It can now be considered the most important medical problem of endurance exercise. The Second International Exercise-Associated Hyponatremia Consensus Conference gives an up-to-date account of the nature and management of this disease. This article reviews key information from this conference and its statement. There is clear evidence that the primary cause of exercise-associated hyponatremia is fluid consumption in excess of that required to replace insensible losses. This is usually further complicated by the presence of inappropriate arginine vasopressin secretion, which decreases the ability to renally excrete the excess fluid consumed. Women, those of low body weight, and those taking nonsteroidal anti-inflammatory drugs are particularly at risk. When able to be biochemically diagnosed, severe exercise-associated hyponatremia is treated with hypertonic saline. In a wilderness setting, the key preventative intervention is moderate fluid consumption based on perceived need ("ad libitum") and not on a rigid rule. (Editor's Note: This paper was written at my request in an effort to increase awareness of this important clinical entity among members of the wilderness community, many of whom are involved in activities that place them at risk of its development. I thank the authors for their diligent efforts.)
URL - http://www.ncbi.nlm.nih.gov/pubmed/19594207?dopt=Citation