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Wednesday, April 13, 2016

Study Spotlight: How Does Hydration Status Influence Endurance Performance?

By: AJ Jagim

With race season right around the corner 1,000 of runners will be hitting the trails and courses for various 5k's, marathons and even some crazy-distance events that I can't even fathom.  With the start of race season, it's important to do everything you can to enhance performance, particularly if you are a competitive runner. This means everything from proper training, recovery, nutrition and hydration strategies. The focus of today's article highlights the importance of hydration status during exercise and what happens to our bodies as we progressively become dehydrated.

What did they do?
Researchers tested 8 elite cyclists on four different occasions. During each experimental trial they received different amounts of fluid (No fluid, Small fluid, Moderate fluid or Large fluid) and had them complete a 2 hour bout of exercise at 62-67% of their VO2 max in a warm climate.

They were also monitored for changes in body weight, core temperature, sweat rates, heart rate and stroke volume.

What did they find?
As would be expected the increasing amounts of ingested fluid helped to minimize dehydration throughout the 2 hr. ride. The "no fluid" condition resulted in a 4% decrease in body weight (likely due to a mismatch between fluid loss via sweat and withholding fluids during the ride; while the "large fluid" condition only lead to a 1% reduction on body weight. The researchers also found that the "no fluid" and "small fluid" conditions lead to greater increases in core temperature and heart rate while at the same time leading to decreases in stroke volume which is the amount of blood ejected from the heart each beat (essential for endurance performance). 

Take Home Message:
During exercise in warmer climates our bodies increase our sweat rates in an attempt to maintain core temperature. As this continues we lose more and more fluid and if we do not replace these fluids during a long run/ride we progressively become more and more dehydrated. This disrupts our ability to maintain core temperature (not good) as we are more likely to "overheat" on a really hot day if we are not consuming enough fluids.  To make matters worse, if we do not replace fluids, our plasma volume also decreases (major source of fluids within blood) which means less and less blood is getting back to the heart.

As a result, we aren't able to pump as much blood out per beat and per minute which means less blood is getting to the active tissues during exercise. Our bodies try to counteract this by increasing our heart rate (pumps blood faster, hoping to compensate for less blood ejected per beat) which means we are placing more stress on our cardiovascular system just to maintain our current pace.

Essentially this means you are likely to reach fatigue sooner or will have to slow your pace if you are progressively becoming more and more dehydrated during a warm ride/race.  So moral of the story: MAKE SURE YOU DRINK WATER DURING A RACE, ESPECIALLY IF IT IS A LONG RACE ON A HOT DAY!  Aim for 1-2 cups of water every 15-20 min if at all possible. 

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