morning jog.

“I was at about 9 miles, when all of a sudden I had a very unsettling feeling in my bowels

and I knew it wasn't a question of if I was going to have to go to the bathroom, but

exactly where.

And it was gonna happen in the next 15 seconds.”

That's right: runner's diarrhea, the sudden and uncontrollable urge to poop, right in

the middle of a long run.

But how and why does this happen?

I'm Anna Rothschild, and this is Gross Science.

When we talk about runner's diarrhea we're not referring to people who go for a run with

a stomach bug or GI illness.

Runner's diarrhea is something different that happens to healthy people just going

out for some exercise.

And it's super common.

One study from the 80s found that a third of the 707 marathon runners surveyed experienced

“the urge to defecate, both during and immediately after running.”

And other surveys have found even higher numbers.

Now, there's no one answer for why this happens, but scientists have a few ideas.

First of all, when you go for a run, your internal body temperature rises.

This is especially true on a hot day.

“It was a very hot day—it was about 85 degrees, which is a little bit hotter than

I normally run in.”

Right, and when you heat things up, they tend to get kind of melty.

That's probably part of what happens to the feces in your colon.

On top of that, as one scientist I spoke with put it, the decelerating forces from your

feet landing probably cause an “emulsifying” effect.

It's kind like you're creating a poop milkshake in your gut with every bounce.

“Luckily there were lots of bushes nearby, and I found a discreet place to take care

of the situation.

Let's call it an extreme bowel event.”

So, why the urgency?

Well, again, the bouncing motion probably puts extra pressure on your colon, kinda simulating

the type of pressure it's under when your bowels are totally full—indicating that

you gotta go.

Doctors suspect the jostling motion is involved with this phenomenon because runners experience

these types of extreme bowel events more often than bikers, who are also aerobically taxing

their bodies.

Other things may come into play as well.

Stress—say during a big race—may have an effect on your gut.

Hormone fluctuations during exercise may also be involved.

And, as a side note, dehydration and limited blood flow to your bowels may cause damage

to your intestinal lining, leading to bloody stool—which is another issue that can affect

serious runners.

Sadly, there isn't a sure-fire way to prevent these problems.

Obviously, stay hydrated.

And, if you can, try to go to the bathroom before your run, and maybe eat whatever foods

help that process along for you.

Running at around the same time everyday can help too, because for many of us, our GI tract

tends to be on a pretty reliable schedule.

If the problem gets really bad though, you can talk to your doctor about getting some

pills to slow the action of your colon.

But remember, runner's diarrhea happens to the best of us, and there's nothing wrong

with picking a route with lots of pit stops so you don't end up in an awkward situation.

“I used leaves, in case you're wondering.”

Ew.