Only recently a hot discussion among scientists about how much water one should drink during a longdistance endurance race like for example a triathlon race, was published in the British Journal of SportsMedicine. Before the 1980ies  athletes were advised to avoid drinking during exercise, leading to the development of hypernatremia and dehydration in some athletes. Since that time, it generally has been advised that athletes consume as much fluid as possible during exercise, and rates of fluid intake during running races vary widely from 400 to 1500 ml/h or greater. In fact, most race organizers currently provide copious supplies of water and „sports beverages“ throughout the race course to fend off dehydration. Concomitant with these recommendations, the incidence of hyponatremia in athletes seems to be increasing.

Water Balance: complex intertwined regulatory feedback loops

All in all, we may conclude that keeping the body fluid in balance is
not that simple as one may think. It is not only drinking enough and
taking salt tablets. The studies on the issue of water and salt balance
are not conclusive at all. I think, because here again, we are
confronted with complex regulatory feedback loops including among
others hormones from the pituitary gland, central nervous circuits,
peripheral inactive sodium stores, extracellular and intracellular
water as well as osmotically active ions as there are sodium and
potassium as the most prominent ones. Moreover, your individual body
condition makes a difference too. Athlete is not like athete, only the
fact that some of you take round about 9 hours for a IRONMAN triathlon
race and others may need 13 to 15 hours make a huge difference. And
there are environmental variables, of course that count. What is good
for you, may not be good for somebody else. Therefore, you have to make your own experiences, find your own suitable way to keep in balance. It can always be dangerous to follow a recipe unquestioned.

Below, I cite some lines from a review that may give you an idea about this controversial topic.

Exercise-associated hyponatremia* has been described after sustained
physical exertion during marathons, triathlons, and other endurance
athletic events. As these events have become more popular, the
incidence of serious hyponatremia has increased and associated
fatalities have occurred. The pathogenesis of this condition remains
incompletely understood but largely depends on excessive water intake.
Furthermore, hormonal (especially abnormalities in arginine vasopressin
secretion) and renal abnormalities in water handling that predispose
individuals to the development of severe, life-threatening hyponatremia
may be present.

Until recently, the incidence of hyponatremia during endurance exercise
was unknown and thought to be relatively uncommon. However, recent
studies have shown that endurance athletes not uncommonly develop
hyponatremia at the end of the race, usually in the absence of clear
central nervous system symptoms. For example, in the
2002 Boston Marathon, Almond et al. found that 13% of 488 runners
studied had hyponatremia (defined as a serum sodium concentration of
135 mmol/L or less) and 0.6% had critical hyponatremia (serum sodium
concentration of 120 mmol/L or less).

Speedy et al. investigated
330 athletes who finished an ultramarathon race. In this study, 58
(18%) were hyponatremic (defined as a serum sodium <135 mmol/L) and
11 had severe hyponatremia (serum sodium <130 mmol/L). Studies of
other endurance events have reported the incidence of hyponatremia to
be up to 29% (9,10,12,15–25). These incidence rates may be
overestimations as a result of sampling biases. For example, in the
2002 Boston Marathon study, of 766 runners enrolled in the study, only
488 runners had serum sodium values assayed. Some of these runners
did not finish the race, and others had time constraints that did not
allow them to have blood samples obtained. As is discussed later, the
majority of these athletes are asymptomatic or mildly symptomatic
(nausea, lethargy). However, severe manifestations such as cerebral
edema, noncardiogenic pulmonary edema, and death can occur.

There have been at least 8 reported deaths from exercise-associated hyponatremia (EAH).
Many of these reports relate to a series of fatalities in the military
between 1989 and 1996. During this period, military recruits
were encouraged to ingest 1.8 L of fluid for every hour they were
exposed to temperatures above 30°C (30). At least four other deaths
have been attributed to EAH in the United States. It is
interesting that two of these deaths occurred in doctors. The
exact incidence of mortality related to EAH is not known but is likely
to be low.

*In depth review by Mitchell H. Rosner, and Justin Kirven
Division
of Nephrology, Department of Internal Medicine, University of Virginia
Health System, Charlottesville, Virginia, Published ahead of print on
November 29, 2006
Clin J Am Soc Nephrol 2: 151-161, 2007. © 2007 American Society of Nephrology