November 30th, 2012

Slowing to a trickle: The importance of hydration

I learned several important lessons on my 20 miler this week.  The first, of course, was to review the course and it’s elevation before setting out.  The second was the power of hydration.

I left for the run with my fuel belt and two small bottles of water, intending to loop back past my parent’s house to hydrate and refill my bottles.  When I missed my turn, I knew it meant I would have to last through the whole 20 with minimal water.  Although nothing terrible happened, I learned first hand the effects of dehydration in a way I don’t think I’ve ever experienced before.

Not nearly enough for a three hour run

I was well fueled and my legs felt fine.  However, my pace dropped off those last few miles, and I knew it was related to my poor hydration.  I had recently read the chapter on hydration in Advanced Marathoning and was remembering these words:

Studies have found that dehydration of 2 percent of body weight leads to about a 6 percent reduction in running performance… This is because any reduction in blood volume will reduce the amount of blood returning to your heart.

…It’s not unusual to lose 3 pounds of water per hour when running on a warm day….For a 140 pound runner, this would represent more than a 4 percent loss of body weight and more than a 12 percent decrement in performance.

The effect increases as the run progresses, so this runner … would likely slow by up to a minute per mile by the end of the run.  Staying well hydrated, then, can be the difference between training hard enough to provide a strong stimulus for your body to improve, or just going through the motions in training and never reaching your potential.

I was slowing down by about a minute per mile on those last few miles, partly wondering if I had made the right turn to make it home, and partly because my body was telling me it was done.

As you become dehydrated, your blood volume decreases so your heart pumps less blood with each beat and you produce less energy aerobically, slowing your pace.

Working my way up to one of these per hour

How much should you drink?

Most sources suggest four pints per day, with additional fluid to make up for that lost through exercise.  During a run, you should drink only as much as your stomach can empty, or a max of 24 ounces per hour.  I rarely drink anywhere near that amount, but this week’s run reminded me that I cannot overlook the importance of staying well hydrated.

Advanced Marathoning reports studies that have found racers drink less than 16 ounces of water per hour, and lose an average of 3.2 percent body weight during a marathon.  This can translate to a loss of more than 9 percent in performance.  So the average marathoner may race slower than his/her potential due to progressive dehydration.

Many sources, such as this article, confirm that consuming a drink with carbohydrates can substantially increase your chances of reaching the finish without running out of glycogen.  Using gels, chews or other easily digestible snacks is another alternative.

Although staying hydrated may not make you run faster, being poorly hydrated will definitely slow you down.  Now I have a few more long training runs to get it right before race day.

Do you have a long run hydration plan? Have you ever experienced the effects of slight dehydration?

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