Much like running itself, hydrating is one of those things that should be simple. (It’s the most natural thing in the world! You’ve been doing it for years! Your body craves it!) But, unfortunately, it’s not quite that easy.
“By the time you feel thirsty, you’re already dehydrated,” says Craig Horswill, Ph.D., clinical associate professor of kinesiology and nutrition at the University of Illinois at Chicago. “But that doesn’t mean you should drink random, ungodly amounts of water. In fact, it’s probably better to be a little under-hydrated than over-hydrated.”
That’s because the consequences of over-hydration are severe (read: death), and it’s important to note that everyone requires different amounts of water based on their personal physiology, and even the time of year/day that they’re running. (Here, Horswill explains how to determine the right amount of water for your needs.)
Greg Grosicki, Ph.D., an assistant professor and director of the exercise physiology laboratory at Georgia Southern University notes that severe dehydration can have major consequences, too, including death in extreme cases (losing 10 to 15 percent of your body weight) and passing out. Grosicki points out that Sarah True collapsed at the 2019 Ironman European Championship in Germany. “She was winning and fainted just before crossing the finish line,” he says. (It was 100 degrees that day.)
You might even be dehydrated without knowing it: A European Journal of Sport Science study shows that 91 percent of pro basketball, volleyball, handball, and soccer players start practice dehydrated. (Here’s how to tell when you are dangerously dehydrated.)
So what can you do? Don’t go crazy with the H20 (“You don’t need an aggressive hydration strategy if you’re exercising for less than an hour,” Grosicki says), but do look out for these dehydration symptoms (especially when it’s hot out!) to have your best run possible.
You have a headache.
“Dehydration seems to provoke a shrinking of cells in the brain, so a headache is a hallmark sign of dehydration,” Grosicki says. That’s likely why you get a dull ache in your head during long, hard efforts on hot days or after a few too many postrace celebration beers. Try drinking some water before popping an Advil, or adding in your favorite hydration mix to help your body replenish fluids.
You’re not as sharp as usual.
Dehydration compromises your focus, executive function, and motor coordination, according to a Medicine & Science in Sports & Exercise review of 33 studies on the topic. We can only speculate as to why, but Grosicki says this could have something to do with the same shrinking brain cells causing your headache.
Your pee is dark.
You’ve probably heard this one since grade school, but it’s true: “When you’re dehydrated, your kidneys try to reabsorb that water back into your body, resulting in darker, highly concentrated urine with a very low volume of fluid,” Grosicki says.
Your easy run feels hard.
Your usual, steady pace may suddenly feel like a struggle. “A loss in fluid volume alters the concentration of your blood volume, so your heart needs to work harder to provide your muscles with the oxygen and nutrients they need,” Grosicki says.
This means your heart rate will increase in order to keep your usual pace, too, so keep an eye on your fitness tracker if you have one. “You can be running at your usual pace, but your heart rate will be 10 to 15 beats higher,” Horswill says. “Same work, same person, same environment—just lack of fluid in the bloodstream.”
You’re cramping up.
“A loss in fluid changes your body’s concentration of electrolytes, like sodium and potassium, which are responsible for muscle contractions,” Grosicki says. “If you disrupt your body’s electrolyte balance, your muscles may contract and cause cramps.”
You feel tired.
Even mild dehydration can make you want to take a nap. “Core temperature is elevated with dehydration, which can affect the brain and induce fatigue,” Horswill says. “There’s also greater strain on the cardiovascular system—indicated by the elevated heart rate—so if dehydration worsens, the heart will have a reduced ability to deliver oxygen,” which—you guessed it—makes you feel tired.