Please ignore dehydration issues. Is there any evidence or does anyone know if losing water without replacing it will make you slow down? I was shocked to see that I lost about 5 lbs of water weight after running, so I'm thinking that the body would automatically slow down to counter the loss of water. This is a question about whether or not this phenomenon exists or not. Math to calculate it would be great though.

One other interesting phenomenon I've learned is that running in heat will divert blood from your muscles in order to help regulate body temp which will then slow you down automatically. Also humidity will bottleneck this cooling process and make it harder to keep cool. When winter came I ran pretty fast! Thanks.

  • I've read about a study some time ago that showed that this does not happen. If you hydrate athletes via IV, one half gets a placebo and the other half gets real fluid replacement after one workout session and they go on to do the next, then there is no difference between placebo and the fluid replacement group. But I don't know about the degree of dehydration, I need to dig up that article... Commented Jul 15, 2015 at 16:26
  • @CountIblis - How do you do a placebo IV?
    – JohnP
    Commented Jul 17, 2015 at 21:22
  • 1
    Can you clarify what you mean by "ignore dehydration issues"? Most studies I've read show performance impact starting at 2%(ish) water loss.
    – JohnP
    Commented Jul 17, 2015 at 21:23
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    @JohnP It was published in this article Commented Jul 17, 2015 at 21:50
  • @JohnP I would like to ignore the fact that you will eventually face serious dehydration problems that will slow you down or stop you from running all together. In other words I'd like to avoid the obvious answers to a question that Im not really trying to ask.
    – Jason
    Commented Jul 18, 2015 at 2:55

1 Answer 1


While this question is quite broad and mainly focused on the person there are some broad parameters that can be applied. Most of all does your body "Automatically" slow you down when it gets hot or you are low on water. (Wouldn't this be defined as dehydrated?) The answer would be that you mind will slow you down because you are uncomfortable far sooner than your body will shut down. Of course your heart rate will rise as the blood volume decreases. So there is a point in which you will stop. But your question is what is that point and what is the measurement of dehydration that will cause this.

Lets look at an edge case: Scott Jurek.

Scott was put in a heat chamber

In the end, Ruby said, the runner cracked as temperatures reached 135 degrees. The lab shut the experiment down for safety reasons, just before the fuse tripped. The equipment wasn’t rated to go that high, Ruby said, and neither was Jurek.

“He had to lower his pace and yet his heart rate kept driving up,” Ruby said, adding that Jurek’s core temperature had climbed to 104 degrees. “That’s about the limit we allow people to get in our chamber.”


Most marathons will cancel the race if the heat index is above a certain level, but still the question remains does this exist? The answer is yes. What is the math? The answer is it is dependant on the individual.

I run with large group of runners every Saturday morning. (About 100-200 people) There are runners that will complete 20 miles in a little more than 2 hours and runnings that complete the 20 miles in 4 hours. All at the same temps. All in a heat index above 85-90f. Every person is different and how much they train and how much acclimation they have had makes a difference. I don't think there is any math that will calculate the exact point in which you can't go further.

In the Book "RUN: The Mind-Body Method of Running by Feel" a study was done on a group of Rugby players where they were asked to exert themselves until they had to stop. They measured the wattage they could produce. After they could go no further they were asked to go as hard as they could one more time. They were able to exceed the previous attempt. They cycled to a point of failure at 242 watts, but the second attempt they produced 731 watts for 5 seconds immediately after.


The point is that your mind will tell you to quit far before you are physically not able to go on.

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