Sweating during Ironman and Triathlon events

The fitter you are the more you sweat. You learn to start sweating at lower temperatures to cool your body; this can stop you getting too hot. The fitter you are the less salt is sweat contains. If you can effectively cool your skin then you can reduce your core temperature. Overheating will reduce performance and create fatigue.

AT REST -1/16 of a liter is lost at rest in a temperature climate.
MODERATE ACTIVITY -1/2 liter of water is lost
HEAVY EXERCISE – 1 liter of water is lost

The above varies depending on type of exercise air temperature and wind chill factor.
We are all different and cope differently depending on our percentage body fat and recent exposure to heat and exercise. Being partly dehydrated can increase our body temperature more than normal.