During exercise, sweating is one of the most effective ways for our body to dissipate heat. Heat stress and resulting thermal strain is higher when the environmental temperature is high, so one of the main adaptations to training in hot conditions is a higher sweat rate and earlier onset of sweating.
Typical sweat rates range from 0.5-2 L/h but vary between and within individuals based on the environment (temperature and humidity), intensity, and level of heat acclimation. Sweat is hypotonic, or less concentrated than our plasma. When we sweat more, we lose more fluid, resulting in plasma becoming more concentrated. If this hypertonic hypovolemic dehydration is not compensated for by drinking, osmotic gradients cause a shift from fluid inside cells to restore plasma volume and osmolality. If body mass is reduced by more than 3% from dehydration, performance can be impacted. Thirst may or may not be a reliable indicator of dehydration when sweat losses are high. Therefore, it is advised to consider some type of planned fluid replacement strategy during exercise in the heat, particularly when preparing to race longer events in the heat.
Determine your fluid replacement needs. The simplest way to do this is to measure your starting body weight right before a training session, and then reweighing after the session (subtracting any fluid ingested during the session using 16 oz = 1 lb). Ideally, start hydrated, use the bathroom before, and weigh with no clothes on. Subtract your finishing weight minus fluid ingested from your starting weight to determine your sweat rate in ounces or ml per hour by using the 1 lb = 16 oz conversion.
Once your fluid needs are determined, pay attention to whether your perceived thirst drives your fluid intake such that you are meeting your needs. You do not need to replace 100% of your fluid losses, just enough to maintain performance and avoid dehydration of > 3% body mass over the course of an extended workout or race. Test your thirst response and fluid intake in different environmental conditions, in workouts of different durations and intensities. Ideally, test it under similar conditions as your upcoming race/event.
Finally, pay attention to any GI symptoms that may result with your hourly fueling intake (carbohydrates), fluid intake, and electrolytes. Again, take notes and develop your individual strategy in varying temperatures including in race-like conditions.
You can obtain carbohydrates and sodium from carbohydrate-electrolyte sports drinks but you may need to add additional fluid and sodium to compensate for additional sweat losses in the heat. An example of how you might adjust this is as follows:
Cool conditions: 20 oz/590 ml bottle of Skratch Super High Carb (100 g carbohydrate, 400 mg sodium) per hour
Total in cool conditions: 590ml fluid, 100 g carbohydrate, 400 mg sodium (0.68 mg/ml) per hour
Total in hot conditions: 1065 ml fluid, 98 g carbohydrate, 610 mg sodium (0.57 mg/ml) per hour
Finally, remember your fluid intake strategy can adjust and develop with training and heat acclimation. Dehydration makes GI symptoms and thermal strain worse, so addressing your fluid intake to prevent significant dehydration is a logical first step before then fine tuning your sodium and carbohydrate supplementation in the heat.