A few years ago we did a little study to look at fluid loss and re-hydration, during running. 4 groups of runners raced 25k on 3 consecutive weekends. Groups I and II had sport drinks; III had artificially-flavored sport drink and IV had colored water. Each week I, II and III rotated their drinks and IV thought they also were in the rotation, but always had the same green water. No subjects ever knew what they had. Nude weights were recorded before and after each run, and total fluid intake was carefully measured. Each runner filled out a questionnaire after each run to see how they felt about what they had to drink and how they recovered.
Many said they would have preferred plain water to what they had, including some who had plain water; interestingly those who said they would prefer water, drank less when they had plain water. Race pace was more consistent, and a little faster when subjects consumed energy drinks. The 3 different weekends varied from 73 to 84 degrees F and that 11 degree variation produced greater fluid consumption on the warmer days.
Possibly the most interesting result was the great variation in sweat rate. Two runners, both men, same weight, within 1 minute of the same race time, and both of whom consumed 1 liter of the same fluid during their run, had the following loss of weight (fluid) through sweat and ventilation: Runner A lost 2.5 liters (Kg), but consumed 1 liter so had a net loss of 1.5 liters (Kg). Runner B lost 5 liters, replaced 1, so had a net loss of 4 Kg (8.8 pounds), which was 5.5% of his body weight. This runner was sweating 1Kg every 5Km (every 20 minutes) of running, and at that same rate would have sweat over 8Kg (17.6 pounds) in a marathon. Even consuming 2 liters during a marathon, there would have been a net loss of 13.2 pounds (over 8% of his body weight and extremely dangerous). My main point here is that each person has an individual reaction to exercising in the heat, and it can be misleading to prescribe a particular amount of fluid intake for all runners.
I have the runners I coach keep a log of nude weight pre- and post- runs that are under various weather conditions. The log has run duration and distance, body weight change, fluid intake, temperature and humidity (cloudy or sunny), and after various sessions under different conditions, each runner can build a chart that allows him/her to predict how much fluid will be lost (per hour or per mile of running) under whatever conditions are forecast for any coming race day. Then they are to consume enough to avoid losing more than about 2% of their body weight as recommended by the American College of Sports Medicine. They must drink often, rather than large amounts less often and what they drink should contain electrolytes and energy.