Typically, runners who are affected by hyponatremia are not elite, competitive runners but are completing these ultramarathons in 9 to 11 hours. Their slow running speeds allow them ample time to drink fluid from the vast number of feeding stations available during these races. But, more importantly, the slow running speeds and therefore the low metabolic rates of these athletes cause them to sweat at much slower rates than the rates calculated previously by researchers who studied only elite marathoners (Pugh et al, 1967; Wyndham & Strydom, 1969). It is clear that sweat rate calculations based on elite runners cannot be applied to the average runner of the same body mass who runs much slower.
For example, researchers originally believed that if a 50-kg runner loses 5.5 L of sweat during a 5:30:00 ultramarathon, then that runner should obviously drink 1 L every hour to maintain a water balance. (This calculation ignores the water lost from glycogen, which does not have to be replaced.) Thus, the general (but incorrect) rule was devised (American College of Sports Medicine, 1975) that a 50-kg person should drink 1 L of fluid for every hour of running, and those who are heavier should drink a little more.
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But we now know that this advice is safe only if the runner is able to finish the race in 5:30:00. A less competitive 50-kg runner who religiously followed that advice but took 10:00:00 to complete the race would finish the race with a fluid credit of 4 L, enough to cause water intoxication (Noakes et al, 1985a) if the runner is predisposed to the condition (Irving et al, 1991). The finding that the incidence of hyponatremia is on the increase among slower ultramarathon runners (Noakes et al, 1990c, 1991c) suggests that this is happening more frequently.
What factors, then, determine sweat rate? Probably the most important factor is metabolic rate, which is affected by the speed of running and by body weight (Costill, 1977). Exercises 2.2 shows that the rate of oxygen consumption and therefore the rate of metabolic heat production increases linearly with increasing running speed. Because sweating is the most important avenue of heat loss during running, it is likely that the sweat rate also increases linearly with the increasing rate of heat production, that is, with increased running speed (see Exercises 4.1).
Increased body mass increases metabolic heat production at any given running speed in proportion to the increased body mass (see Exercises 4.1). Thus, a 100-kg runner will produce twice as much heat as a 50-kg runner when they both run at