Cardiac Drift and Dehydration

It’s hot and humid out and that means more sweating.  Sweat rates vary from person to person so determining your water loss is important.  Weigh yourself before and after you exercise to see how much water weight you have lost.  Studies have shown that a 150 pound athlete can lose 1.5 to 3 pounds of body weight per hour in 70 degree weather (imagine what it could be in 90+). Be sure to drink plenty of fluids throughout the day AND while training.  During training you should be drinking 16-32 ounces per hour, again depending on your body type and sweat loss. For maintaining hydration a general rule of thumb is 1/2 an ounce of water per body weight for an inactive person, and 2/3 ounce of water per body weight for athletes.  That means our 150 pound athlete should be drinking 100 ounces of water a day.

Anyway, we know dehydration is bad but what exactly does it do to performance?  Have you ever been on a long ride or run and noticed that your heart rate continues to rise while your effort/pace stay the same- or even decrease?  This phenomenon is known as cardiac drift.  As you lose fluid through sweat your blood volume decreases.   When you have less blood in your circulation your heart has to beat harder to move the blood you have left to all your tissues. In fact, studies have shown that cardiac drift can start in as little as 20 minutes of moderate-intensity exercise,  AND that your heart rate increases 7 beats per minute for each 1% of body weight loss from dehydration.  Using our same 150 pound athlete, heart rate could increase up to 14 bpms over an hour of training- while not changing the intensity.  In longer efforts that means that you eventually will have to slow down while your heart rate is still sky high, which can be a dangerous (not to mention painful) situation.

It’s ALSO important to realize that over-hydration is also bad.  Since your blood is made up of 50% water if you drink excessive water you will decrease the amount of essential electrolytes leading to a very serious condition known as hyponatremia.

When I train in the summer months and I need electrolytes but not calories I use BANa rehydration water.  It’s full off the electrolytes that I am losing in sweat but doesn’t contain sugar or anything I don’t want.


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