Sweat Rate Study

The mean sweat loss rate among runners in our study (1.4 l/hr) appears to be lesser than what has been previously reported in the literature (1.5-1.7 l/hr) as summarised in Table 2. The possible reasons for this discrepancy include the differences in the experimental methods, especially whether or not subjects ingested fluids during exercise, the abilities of the athletes (which determines the intensity of exercise they can sustain) and the environmental conditions. In our study, the participants were asked to strictly avoid any fluid intake during the experimental run and this may have lowered their sweat rates during exercise. Furthermore, subjects in other studies were highly trained, endurance runners who ran greater distances at higher intensities compared to our study in which the subjects were non-elite marathon runners who ran a mean distance of 9.3 km in 1 hour (Table. 2). This supports the finding of several studies which have reported lower sweat rates among non-elite, slower runners compared to elite runners, presumably because they run slower at a lower exercise intensity.

The principal factors which determine sweat rates during exercise include the metabolic rate which depends on the athlete’s size and running speed (exercise intensity) and the environmental conditions particularly humidity and wind speed. However, several investigators have also reported the effect of the degree of acclimatisation, gender, environmental heat stress, exercise intensity and individual variation on sweat rates among runners. The findings in our study were that atmospheric temperature, the BMI and male gender significantly affected sweat rate. This confirms previously published reports that male runners who ran under hotter conditions tend to have higher sweat rates.

It is still controversial as to how much fluid should be replaced in elite professional marathon runners. The most recent stand of the American College of Sports Medicine on exercise and fluid replacement is that fluid should be consumed in response to thirst (or ad libitum) during exercise and athletes are advised to drink enough so that they do not lose more than 2% of body mass. Beis et alin their study of drinking behaviours of elite male runners during marathons reported a mean body mass loss of 2.3 kg/hr when 10 marathon runners were studied in a laboratory as part of acclimation training at 35° C and 70% humidity. They concluded that although these elite marathon runners drink fluid ad libitum and they do not seem to maintain their body mass loss to within 2%. This strategy seemed to produce optimum or winning performances in these elite runners probably because there exists a tolerance range for dehydration which does not negatively affect performance. Whether this same strategy of consuming fluids ad libitum without affecting performance can be replicated in non-elite marathon runners is not known, but there is presently no reason to suspect that this is not the case.