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How Dehydration Can Affect Your Performance

Runners drink from a water bottle during a summer track workout

I recently gave a sports nutrition talk to a local running club. As an ice breaker, I started by asking a few basic questions to gauge their knowledge about proper hydration while running. Their answers and misconceptions mirrored those that I have heard from hundreds of athletes over the past three decades.

When I ask about their hydration practices, runners most frequently respond by saying things like, "I don’t need to hydrate while running. I’ll drink after I run.” Other athletes have no hydration plan at all.

Some runners will make excuses about why they don’t drink while running: They don’t want to carry fluids, and sport drinks hurt their stomachs or make them nauseous. Trust me, I have heard every excuse for why runners do not hydrate during their runs.

I get it, carrying fluids during a run can be inconvenient, cumbersome and one more detail to consider before heading out the door. However, a significant loss of body fluids, known as dehydration, can be detrimental to your running performance.

Runners have a healthy fear of hitting the wall or bonking, so they will prioritize nutrition, including gels or another sports fuel, over hydration. But dehydration will stop you in your tracks long before your fuel reserves run low.

Remember this rule: The primary cause of early fatigue while running is dehydration. Simply put, drinking during a workout is essential, especially in a warm or humid climate. If you are not hydrating while running, your performance and fitness gains are exponentially reduced.

A man sweats while running during a workout at a gym

Why is Hydration So Important for Runners?

Water is required for the majority of chemical reactions during athletic performance. This water comes from plasma volume, which makes up 50-60 percent of total blood volume. Therefore, when you become dehydrated, the most significant impact is a reduction in your total blood volume.

In the body, blood circulates from the heart, throughout the body and back to the heart through the lungs, and it continuously repeats this circuit. When your blood volume decreases, performance of key bodily functions decrease with it.

Blood has three critical tasks during exercise:

  • Assist with the absorption of calories and fluids consumed. Blood is diverted away from the gut to the working muscles during exercise, which makes it more difficult to absorb nutrients from the fuel you eat. In a dehydrated state, this is the first task to be shut down, which can lead to gastrointestinal distress. In other words, hydration promotes digestion.
  • Deliver oxygen and nutrients to the working muscles for energy. The faster you run, the more oxygen and nutrients the body demands. In a dehydrated state, this is the second task to be shut down.
  • Regulate body temperature through sweat. If your sweat rate decreases as a result of dehydration, your core body temperature will rise. This is the first priority and the last to fail in a dehydrated state. If your body is unable to cool itself, severe hyperthermia (>104 °F) can lead to organ failure and death.

The Effects of Dehydration on Performance

A runner drinks from an Amphipod water bottle during a run

All runners battle fatigue associated with dehydration since it is impossible to maintain 100 percent hydration status without experiencing a health and performance backlash.

Runners can experience a noticeable decline in performance with as little as 2 percent fluid loss. When fluid loss exceeds 2 percent, performance rapidly deteriorates, and the rate of fluid absorption from the intestines decreases even further once reach 4 percent fluid loss, making it nearly impossible to counteract while you’re running.

Signs and symptoms of dehydration include:

• Dizziness, confusion, lightheadedness

• Dry lips, mouth, skin

• Physical and mental fatigue

• Decreased pace and performance

• Darkened urine (one of the first indicators)

• Increased body temperature, heart rate, and rate of perceived exertion (RPE)

The Impact of Humidity on Dehydration

As if heat’s impact on dehydration isn't bad enough, athletes in non-arid environments must also contend with humidity, which further increases core body temperatures and sweat rates.

Your body cools itself through sweating, but it’s actually the evaporation of sweat from your skin that does the heavy lifting. When the air is humid—saturated with water vapor—your sweat doesn’t evaporate as easily. Instead of turning to water vapor and mixing with the air, your sweat stays on your skin, which leads to that uncomfortable sticky feeling. Your body can’t cool down as efficiently when the humidity is high.

So if you’re running in high humidity, expect your body temperature, heart rate, sweat rate and perceived exertion to be significantly higher than if you were running in dry air. The cumulative effects of humidity on your body mean you need to be even more diligent about hydration.


By Susan Kitchen. Susan is a Sports Certified Registered Dietitian, USA Triathlon Level II Endurance Coach, IRONMAN Certified Coach, published author and founder of Race Smart, a sports nutrition and coaching company.

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