Hydration & Dehydration

Contributed by Dr. Steven T. Devor – Director of Performance Physiology for MIT and OhioHealth, and Associate Professor of Exercise Physiology, Department of Human Sciences, and Department of Physiology and Cell Biology, The Ohio State University

 When it comes to exercising for long periods of time in the heat, there are three stages of heat stress you need to be aware of.  

Heat Cramps   The first, and least severe, is called heat cramps.  Heat cramps are often the result of substantial electrolyte loss (e.g., sodium, potassium, magnesium, and calcium) through sweating.  Heat cramps highlight the importance of using a carbohydrate and electrolyte beverage (i.e., Gatorade) for your fluid replacement when exercising.  

Heat Exhaustion   The second and more severe heat pathology is called heat exhaustion.  Heat exhaustion is related to a dramatic decrease in total body water and results in sweat rates slowing down dramatically as the body tries to save water for other functions.  Often with heat exhaustion individuals may develop blurry vision and a slight amount of disorientation.  Seek immediate shade, stop exercising, and ingest fluids.  Medical help should be sought for individuals with heat exhaustion.  

Heat Stroke   Finally, and most severe, is a condition called heat stroke.  Heat stroke is extremely dangerous and can be fatal.  Individuals in this state will have fully blurred vision, significant disorientation, a complete lack of sweat, and will require IMMEDIATE medical attention.  

How to Delay Dehydration   If you want to be successful and complete longer training sessions and races you must avoid or delay dehydration caused by fluid losses from the body.  Fluids are primarily lost through sweating, breathing and using the toilet.  Years ago the advice was "drink, drink, drink," and we assumed there was no downside to consuming as much fluid as possible.

Hypotremia   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.

Sodium, chloride and potassium are electrolytes, and these electrolytes remain dissolved in the body fluids as electrically charged particles called ions.  Electrolytes help to modulate fluid exchanges between the different body 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 interior and exterior of cells facilitates nerve-impulse transmission, stimulation and action of skeletal muscles during running and other activities, 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 electrolyte and water balance, or very close to balanced, is part of the challenge as an endurance athlete.

Sweat Rate   The easiest way to measure your sweat rate is to weigh yourself without clothes on before you do a one hour exercise session. After the hour session, return home, strip down, wipe of any excess sweat from your skin, 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.  To convert it to pounds, for each pound lost, you lost 15.4 oz. of fluid.  If you drink any fluids or used the rest room between the two weight samples, you will 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.  I would be sure to record the heat and humidity conditions in your sweat test. Repeat the test in cool and hot conditions. If you are a triathlete, repeat the test for swimming, running and cycling because sweat rates will vary for each sport and vary with environmental conditions.

Through the years I have been able to come up with the following guidelines based on weight and different environmental temperatures.  They are only guidelines, so I would still do the individual tests I have outlined above. None the less, I believe these are good averages.

Fluid Ounces Needed Per Mile at Varied Temperatures (by weight):

Best wishes for your continued training success,

Dr. Devor