The problems with "Drink to thirst"

Since the NC sauna continues unabated, I will stay on the subject of hydration.

In my previous post on dehydration, a commenter posted a link to this credible looking article at competitor.com: Running 101: Hydration During Running.

Here are my reactions to this article:

Hydration during running is not as complicated as you may have been led to believe.

Oh, really? Tell me more.

Runners almost never experience dehydration levels sufficient to cause major health consequences

This is technically true. Runners are hardly ever taken to the hospital. But some of us have a higher standard of success than simply avoiding a ride in an ambulance.What if we want to actually perform well?

The new exercise hydration advice is in fact to drink according to your thirst... you will naturally drink enough to optimize your performance...

...research has shown that drinking to completely offset sweating offers no advantage with respect to performance or body temperature regulation compared to drinking by thirst.

How about a link to the this "research"? Are you saying "drink to thirst" applies to all situations, no matter how hot it is, or how far and fast you are running?

Now you know everything you really need to know about hydration during running

To sum up the advice: "Don't worry about how much water you need, just drink to thirst, and your performance will be optimal."

This seems like fine advice for runs under an hour. But when it comes to marathons and ultras, I am highly skeptical.

So I started researching this, and have discovered that the topic of hydration is way more complicated and controversial than I knew.
Even the "Sports Scientists" are recommending "drink to thirst".  They have PHDs in Exercise Science, and loads of scientific articles going way over my head on these topics. They seem to dismiss dehydration as a concern and think people drink way too much.


I understand that sometimes people drink too much water, and end up with hyponatremia, which is worse than dehydration. This "drink to thirst" advice is meant to prevent this. But someone running for 3 or more hours in the heat can get hyponatremia if they drink to thirst, but don't get extra salt. I'll get back to hyponatremia in a future post.

The problems with "Drink to thirst"

1.  Ready access to fluids
The big problem I have with "drink to thirst" is a practical one. It assumes you have ready access to fluids. 
But most of us don't run laps around a drinking fountain, or carry gallons of water.

So when we head out for our 20 mile run, we have to know how much to bring with us. How do you know how thirsty you will be?

During a 4 hour training run in Uwarrie, I ran out of fluids half way through. I thought 40oz would have been enough, but I ended up mighty thirsty and still 2 hours away from the car. It would have helped if I had a formula to calculate how much I would need in advance.

2. Wisdom of thirst
It seems that the harder, and faster I run, the more fluid I lose and the less thirsty I am.
When I am running a marathon I am usually too focused on maintaining a constant pace to even notice if I am thirsty. I learned through trial and error how much I needed to drink, not from how thirsty I was.
I know this is just my own personal experience, but I would like to see more studies on the this.

3. Delayed Reaction
I ran 2 marathons last year where I drank exactly 1 cup of water at each aid station, one every mile. I finished strong, so this seemed to work for me.

This year when I ran Umstead, I continued with 1 cup per aid station strategy.
However, I had failed to consider that the aid stations at this race were 2.5 miles apart. It wasn't until mile 20 when I got thirsty and realized my mistake. I stopped and guzzled 2 bottles of water. Then I had belly full of water sloshing around. I have to wonder how quickly this water is absorbed.

At mile 23, my poor calf muscle popped off and went seeking water on it's own. Of course, the Sports Scientists don't think cramping is caused by dehydration, so the cause of the cramp was debatable. 

4. Efficiency
It may be a minor point, but when you are trying to qualify for Boston or get a PR, every second counts.
You can run through each aid station and swallow half a cup without slowing down. But if you wait to get thirsty at mile 12, and then have to stop there and drink 6 cups, you might waste a few minutes.

It's complicated
In any case, all this hydration stuff is very complex. Not all of your fluid loss is from sweat, some is exhaled from burning calories. You should lose some weight after a run, there is no point in trying to maintain your original weight. But there is debate about how much is too much, and what should be considered "dehydrated", and at what point your performance suffers.


Anyway, I have spent way too long on this post, and have to pack for our mountain trip.
I will revisit this subject, and do a post on  hyponatremia after I do some more investigating.

Some research I found
I found a paper suitably titled "Effects of Dehydration on Exercise Performance", which cites actual research. It finds that even modest dehydration of 1.8% of body weight will effect performance:

...exercise time to exhaustion at 90% VO, peak was significantly lower (6.8 ± 3.0 min vs. 9.8 ±3.9 min

It cites another study that

...compared subjects' responses when they drank enough fluid to replace about 80% of sweat losses ( 1,330 ml) and when they drank only 200 ml of fluid during 50 min of cycle ergometry at -80% VO, max.
This was followed by a "sprint to the finish" in which they completed a finite amount of work as fast as possible. Performance improved by 6% when fluid intake was high

Here is what is says about "drinking to thirst":

...ad libitum fluid intake frequently leads to the modest levels of dehydration observed in these studies...

And those studies were short, only 1 hour long. There is another good paper, The Importance of Good Hydration, which looks at a number of studies.