Hydration and Marathon Performance

I live in Fort Worth, Texas.  It’s hot all summer long.  On long runs (>15 miles), I notice my performance drops off.  Of course it gets harder to maintain pace.  Could it be running out of glycogen?  How about the heat here?  Maybe it’s dehydration?  Perhaps it is a combination of the above.

Here’s the physiology of heat and hydration.  You run, you get hot.  You sweat and cool off through evaporative cooling.  Of course this causes your body to pump blood to you skin area.  As you heat up, you have to work harder to pump the blood out to the skin areas.  Your heart rate goes up.  I sweat a lot in the heat.  Experts say you lose about 1 quart per hour.  I would say, at least.  If you are not hydrating enough, the water content of your blood goes down leading to greater viscosity of your blood.  Guess what, your heart has to work even harder and your heart rate goes up to pump that sludge out to your skin so you can cool off.

On my long runs of 2-3 hours, you can really see the hydration problem.  Remember 1Q/hour is lost.  I always drink at least 16oz. of water on my way out the door.  On long runs, I have a fuel belt with 4 7oz. bottles.  These I am taking at about 1 per ever 30-40 minutes.  I have a few places to refill if necessary.   Do you see the issue?  After 1 hour, I might be okay.  After 2 hours, I am at least 16 oz. low.  After 3 hours, I am 1 quart or more low.  Theoretically I was not drinking enough, but can we measure it in terms of my performance?

Let me share with you how my lack of hydration may have affected my physiology.  I use a heart rate monitor on many long runs.  My rate is in the rage of 142-150 when pacing 8:00 to 8:30 per mile or 70-75% of max.  What happens after 2 hours?  My pace slows to about 9:00/mile and my heart rate drifts up to between 152-160.  Did dehydration lead to this slower, yet harder running?  This weekend I will be running a 20 miler with a big change.  I will be carrying my 70oz. Fuel Belt and loading up with about 24oz. of water before I leave.  Come back Sunday to see how it turns out…