If you have been doing triathlons for any amount of time, I suspect you do a good job of sweating. Additionally, I'm sure you know that if you want to successfully complete longer training sessions and races you must avoid or delay dehydration caused by fluid losses from the body. Fluids are lost through sweating, breathing and using the toilet.
Years ago the advice was "drink, drink, drink," with experts assuming there was no downside to consuming as much fluid as possible. Unfortunately, consuming large amounts of water without electrolytes can lead to a condition called hyponatremia. Hyponatremia, also known as low sodium concentration or water intoxication, occurs due to prolonged sweating coupled with the dilution of extracellular sodium caused by consuming large amounts of fluid with low or no sodium.
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Sodium, chloride and potassium are electrolytes, with sources divided on whether to include magnesium in that group as well. Electrolytes remain dissolved in the body's fluids as electrically charged particles called ions.
Electrolytes modulate fluid exchanges between the body's fluid compartments and promote the exchange of nutrients and waste products between cells and the external fluid environment.
There is actually an electrical gradient across cell membranes. The difference in the electrical balance between the cell's interior and exterior facilitates nerve-impulse transmission, stimulation and action of the muscles, and proper gland functioning.
If you consume too much water and not enough electrolytes, your body pulls electrolytes from its cells in order to create the right balance for absorption. If you consume too many electrolytes and not enough fluid, your body pulls fluids from within to create the right balance for absorption.
The bottom line is your body likes balance. Keeping your body in balance, or very close to balanced, is part of your challenge as a sweaty endurance athlete.
More: Electrolytes 101
Average and Champion Sweat Rates
How much do we sweat? An average person sweats between 0.8 to 1.4 liters (roughly 27.4 to 47.3 oz.) per hour during exercise. To help you with a visual, the smaller bike water bottles typically hold 0.6 liters (20 oz.) of fluid and the larger bottles hold 0.7 liters (24 oz.) of fluid.
The highest recorded sweat rate for an athlete in an exercise situation is 3.7 liters (125 oz.) per hour, recorded by Alberto Salazar while preparing for the 1984 Summer Olympics. The highest human sweat rate recorded is 5 liters (169 oz.) per hour measured on a resting body exposed to a hot environment. At rest, the skin blood flow was maximum and not competing with exercising muscles.
How do you know if you are an average sweaty person or a champion sweater? You need to do some testing.
More: Measure Your Sweat Loss for Optimal Hydration
Your Sweat Rate Test
The easiest way to measure your sweat rate is to weigh yourself without clothes on before exercising for one hour. After an hour of exercise, return home, strip down and weigh yourself again. Assuming you did not use the toilet or consume any fluids during exercise, your weight loss is your sweat rate. For each kilogram of lost weight, you lost one liter of fluid. (For each pound lost, you lost 15.4 oz. of fluid.)
If you drink any fluids or use the rest room between the two weight samples, you'll need to include both of these estimated weights in your calculations. Add fluid consumed to the amount of weight lost. Subtract estimated bodily void weight from the total weight lost.
Be sure to record the heat and humidity conditions in your sweat test. Repeat the test in cool and hot conditions. Repeat the test for swimming, running and cycling because sweat rates will vary for each sport and vary with environmental conditions.
Now that you know your sweat rates in each sport, you probably imagine that simply drinking enough fluid will replace what you lose to sweat given the environmental situation. If it were only that easy.
More: Proper Hydration for Summer Training