The same speed. The 100-kg runner’s sweat rate will also likely be twice as high (see Exercises 4.1).
What happens, of course, is that during competition, the smaller runners run faster for a shorter time and the heavier runners run slower for a longer time. If their finishing times are in the same ratio as their body weights, their total sweat losses should be very similar.
It follows that if we assume that a 50-kg runner sweats at a rate of about 1 L/ hr during competition and needs to replace only half that loss each hour during exercise lasting up to 5 hours, then a fluid intake of 500 ml each hour is probably adequate for runners of all weights, provided they run proportionately slower than the 50-kg runner.
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Surprisingly, mild environmental temperature seems to have little influence on sweat rate during competition; even in cold running conditions (dry bulb temperature 11 to 12 °C), sweat rates of 1.2 L/hr are recorded in the fastest runners (Maughan, 1985), and these values are not different from values measured at dry bulb temperatures of 14 to 17 °C (Wyndham & Strydom, 1969) and 22 to 23 °C (Noakes et al, 1988a; Pugh et al, 1967).
Thus, we may conclude that running speed and body weight are the most important determinants of sweat rates during running, and that on average, an hourly fluid intake of 500 ml should cover the requirements of most runners under most conditions (Noakes et al, 1988a, 1991a).
These ideas are formalized in Exercises 4.3, which shows postrace rectal temperatures, rates of water loss, and weight losses of 70- to 80-kg runners completing standard marathons at different speeds. Note that the postrace rectal temperatures are not greatly elevated; that the rates of water loss rise with increasing running speeds, as would be expected, and are quite low in the slowest runners; and that only the very fastest runners finish the race mildly dehydrated. As most of the other runners do not lose more than 2 kg, equivalent to the masses of water and glycogen stored before the race, they finish the race moderately overhydrated.