Dehydration = Poor Performance

As the weather gets hotter, the risk of dehydration gets more serious. And for athletes that train hard all winter, or build for months at a time, hot weather can hurt or even destroy your race day efforts. A lot goes into your training, and as much as nutrition is a key, hydration can play an even greater importance. As a rule, for the rest of this article, everything I am referencing is for endurance exercise over 2 hours. This can apply to shorter exercise, but for the most part, 2+ hours is what I am speaking of.

My definition of dehydration, as well as most in science, is: having lost more than 2% of your body weight.

Take a typical athlete of 150lbs. 2% of 150 is 3 lbs. So, take your weight, multiply it by 2% and you’ll get your magic number. If you lose more during a workout or race then 2% of your body weight, you’re dehydrated and if will impair your performance. In addition to the general “impairing performance,” being dehydrated by just 1% will increase your heart rate 5-8 beats and increase your core temp by .2-.3 degrees C. For each percentage increase of body weight loss, your core body temp will go up and your heart rate will increase.

One interesting fact is, if it’s cool outside (50’s and 60 degrees), and you lose 1-2% water, that amount of dehydration has not been shown to impair performance. But take the same 1-2% loss of water in a hot environment (80’s-90+degree), and it will significantly impair your performance. I wish I had some specific numbers for you like “if you lose 2%, you will lose 10% of your speed.” Unfortunately, the facts just show, if you lose 2% of your body weight, specifically in warmer environment, you will have an impaired performance. The more dehydrated, and the hotter it is, the worse you will do. After your exercise session or race is over, drink 1 20-24oz bottle of electrolyte fluid (sports drink, nuun, etc) to replace each pound of weight lost. This could take several hours to do, not all at once.


The history of recommendation of hydration is a varied one.

  • 1960’s-most athletes drank little to no water during exercise
  • 70’s/80’s-athletes were told  to drink 100-200ml every 203km. Depending on your speed, that could amount to 330-2000ml/hour. Or for those of us in the US, 11-67oz/hour. That is a huge range and likely to cause problems.
  • 90’s-athletes were encouraged to drink “the maximal amount of fluid during exercise that can be tolerated without causing GI distress and up to a rate equal to that lost from sweating.” The problem was people took this to say that you should drink as much as possible, without the last part “equal to the amount lost from sweating.” Without the last part, over hydration could happen.
  • Today-the recommendation for athletes is to drink enough to prevent excessive dehydration (2%) and electrolyte losses. It is a more individualized approach and requires athletes to drink to their OWN needs.

*The best way to say this is, drink to match your fluid loss. The average athlete loses over 1 L of sweat an hour. Some might lose more, some lose less.*

If you don’t know how much you should drink, performing a sweat test is key. If you’d like to do one, an easy way to do it is to go to and follow their sweat test. For most athletes, I recommend at least 24oz (one typical cycling water bottle) per hour and up to 48oz/hour in hot conditions. Many athletes drink 32oz, or a bottle and a half of fluid per hour.

Why Does Dehydration Cause Impaired Performance:

There are many reasons that dehydration causes performance, not just one

  1. Increased cardiovascular strain-there is a greater strain on the cardiovascular system due to the competition of the body trying to dissipate heat while supplying blood to the muscles
  2. Reduced blood volume, reduced stroke volume and reduced blood flow to the muscles-This is due to the water lost, reducing plasma volume levels, making the heart beat “harder” and not as much.
  3. Hyperthermia-this is an increase in your core body temperature. A 1 degree (C) increase in body temp alone isn’t enough to cause a decrease in performance, but take the 1 degree increase, and add in dehydration, this will make things a lot worse.
  4. Others including muscle metabolism, neurological function and increased muscle glycogen use


Sodium is the most important electrolyte lost in sweat. This is also very important to replace while working out and racing. Yes, there are other electrolytes lost in sweat, however they have not proven to be as important to you to replace. I recommend 400-800mg of sodium per hour in hot conditions. Some athletes need more than this, with some athletes taking in 1500mg+ per hour. This is very individual and will require some trial and error on your part.

If you are working out and/or racing in hot conditions for 2+ hours, taking in sodium is even more important. Remember, cooler weather and shorter bouts of exercise don’t always need electrolyte replenishment, however this isn’t always the case.

Take Away:

  • Drink to your own fluid loss. replenish the fluid and sodium lost in your workout or race
  • Keep body weight lost to less than 2% of your body weight
  • The hotter and longer you exercise, the more important fluid and electrolytes/sodium is
  • Perform a sweat rate test to determine your own losses, however a good starting point is 24oz of fluid and 400mg of sodium per hour.
  • Becoming dehydrated will impair your performance, potentially causing you to DNF, impair your performance and possibly putting yourself in serious harm.
  • Drink 1 bottle of fluid post exercise, to replace each pound of weight lost over your 2%






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