Hydration Tips for Marathon Training & Race Day Performance


Marathon or endurance race training requires a ton of attention to detail – run plans, cross-training, diet, shoes and other equipment, to name a few. But one often overlooked attribute to your performance (AND safety) is your hydration level. Because some training runs (and the race day) can take hours at a time, dehydration sneaks up on you!


The bottom line is hydration affects performance in a BIG way.


The most well-known mechanism that hydration affects is what’s known as thermoregulation, or your body’s ability to maintain its optimal temperature. Just a 1% decrease in body mass from fluid loss puts you at risk of exertional heat illnesses (heat exhaustion, heat stroke, etc.) Worse yet, at fluid loss levels of 3% or greater the risk of exertional heat illness increases from moderate to severe.

Heat illnesses can certainly put an early end to your race despite all your hard efforts. You’ll head straight to the Med tent for some IV fluids and cold cloths. It often takes days to recover.

In multiple studies, decrements in strength, power, and endurance are consistently apparent in athletes who are JUST 2% dehydrated, that is, a 2% decrease in body mass.  For example, if you are a 150 pound athlete, then we are talking about only 3 pound deficit if you are a 150 pound athlete,

How does this happen? It’s quite the cause and effect. When you become dehydrated, you have fluid loss. Much of that fluid comes from your blood stream and is measured as plasma volume. If your plasma volume decreases, then your blood volume decreases. This loss in volume causes the consistency of your blood to thicken, resulting in the need for your heart to work harder. Remember, with each pump, your heart has to get your blood throughout the body, deliver the oxygen to your muscles, and return the blood back up to the heart.

THIS is where the performance can take a hit. Because your heart is now working harder, it’s going to compensate with a heart rate increase. With just a 1% fluid loss, your heart rate increases 3 to 5 beats per minute. Just like a car, you can’t run on high RPM’s forever. As an endurance athlete, you have to be able to sustain your heart rate. Otherwise, your efforts will ultimately be compromised through reduced performance.

Long story short, make sure you are hydrated!

Keep an eye out for my next post on what to notice or feel to monitor hydration level, as well as hydration strategies to implement for your training and race day.


Michael Jordan, PT, DPT, OCS, CSCS

Doctor of Physical Therapy

Director of Research & Education



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