Thursday, May 29, 2008

Running: Physiologically Speaking
The Danger of Rapid Rehydration

Now that I’ve recovered from my own running troubles, I’d like to help the running community by discussing a common problem I’ve heard some runners complain about after their long runs. I’m also including this as my contribution for this week’s Take It and Run Thursdays series on "Running In The Heat" because it has to deal with dehydration and rehydration, which as we all know, is all too common in the summertime. So sit back, grab your favorite margarita, tequila, or whatever alcoholic or non-alcoholic beverage strikes your fancy, and prepared to be educated…


THE PROBLEM

Has this ever happened to you?

It's a bright and sunny day and you're out running. It took a while to find your stride, but now that you found it, you feel like you could run forever. You feel relaxed, floating in a sea of endogenous opioids and running a comfortable pace when suddenly you realize the water bottle you're carrying has less than a sip left. You know you should slow down and turn left towards home at the next intersection, but heck, it took you so long to get to this happy place that you'd like to stay just a little while longer so even before you realize it you're turning right for another 5-mile loop around town. Four miles into this second trip, you're out of water, sweating bullets, and starting to feel a little delirious. Your lips are dry and your skin is flushed. You're still feeling pretty good about the 5 "extra" miles you’re putting in, but not so much about the dehydration. Luckily, you're less than a mile away from home, and because you rather get there sooner rather than later, you bust it all the way home. When you finally get there, you proceed to drink up everything in sight. Within 15 minutes, you've downed 16oz of water, 16oz of gatorade, and the quart size container of chocolate milk intended for your kindergartener when he comes home from school. Feeling as if you've somewhat rehydrated, you change out of your stinky clothes, take a shower, and proceed back to the kitchen intending to carbo-reload. Five minutes later, the sandwich is made, you're sitting down to eat, but you're no longer hungry. Instead, you feel nauseous and have a splitting headache. It takes all of your energy to take a few small bites before you head back upstairs to put your head down. Before drifting off, you review the events of the last two hours in an effort to figure out how you ended up this way...

Over the past few weeks, I've read similar accounts from many running friends out in the blogosphere. So, what is the problem here? Why is it so common to be totally wiped and exhausted a few hours after a hard run in the afternoon? Most people would blame dehydration as the root of all evil. Well, I'm here to tell you that although the lack of fluids is a contributing factor, it is actually the replenishment of the lost fluids that is really the culprit. More specifically, it is the rapid rate of fluid intake that we runners take post-run that is most responsible. You heard it right folks. Rehydration can sometimes cause more problems than dehydration.

THE CAUSE

In order to understand why rapid rehydration is such a problem, we need to go over some priniciples of human physiology. If you remember back to high school biology, we learned that chemical particles, just like people, always tend to move from an area of high concentration to an area of low concentration unless prevented from doing so by an outside force. This is referred to as the process of diffusion. Water, in a similar fashion, will always move from an area of high to low pressure. The technical term for the diffusion of water across a selectively permeable surface (such as a cell membrane) is osmosis. Now, because all the molecules and electrolytes found within the human body exists in solution with water at different concentrations, there is constant pressure for both molecules and water to move across cell membranes (in opposite directions) so that the final concentration of solute over water would be equal on both sides of the cell membrane. This pressure for solutes to move across a membrane along a concentration gradient is termed diffusion pressure, while the pressure for water to move along its concentration gradient is similarly termed osmotic pressure.

Now, let’s observe what happens to the brain and the rest of the body during the process of rapid dehydration, as can occur in the middle of a long run on a hot day. We tend to lose a lot of electrolytes and water in the periphery in the form of sweat when we run. Most of these electrolytes and water are drawn from fluids found outside of cells (technically, blood and the interstitial space). Inside the cells, the concentration of water and electrolytes are kept relatively constant by the protection offered by the cell membrane. At some point however, as the dehydration gets worse, the osmotic pressure becomes too great for the cell membrane to handle and water is eventually drawn from inside cells (called the intracellular space) as well. As more and more water is lost from cells through the circulation to the environment as sweat, cells lose their volume and shrink. As they shrink, they also lose the capacity to function normally. In order to prevent this process from happening at the level of the brain, where maintaining a critical water concentration is vitally important for nerve conduction and other high level processes to take place, it has the special ability to generate free osmotically active molecules, called free osmoles, which allows it to negate some of the water pressure exerted on by the periphery. Essentially what happens then, is that as more water is lost in the circulation, the more osmoles are generated in the brain to balance the solute/water concentration. That way, water is drawn away only from the periphery and not from the brain, where centrally processes can continue to function normally.

This unique adaptive ability of the brain to generate free osmoles in the face of dehydration is not without consequence however. The most important side effect, for the purposes of this discussion, is that once these osmoles are generated, they take a very long time to go away. This important principle comes into play in the rehydration phase, after the long run. When we rehydrate, we are refilling our body with a large quantity of water (and electrolytes). And because the brain hasn’t yet had a chance to decompose the free osmoles, there is a high residual osmotic pressure for water to move intracellularly (which is no longer compensated by the osmotic pressure for water to move outside the cells since the periphery is now water replete, and not dehydrated.) As a result, water is drawn into brain cells as fast as it is consumed in the periphery and the cells swell up. Unfortunately, the brain handles water intoxication much worse than it does with water deprivation. As a result, brain function goes haywire and you end up with a splitting headache. By the way, this is actually the same mechanism by which people die from severe hyponatremia (as caused by too much water consumption and too little solute consumption) during marathons.

THE SOLUTION

The recommendation I’m proposing is the same recommendation that is used to treat patients suffering from severe dehydration in the emergency room. The key to rehydration is to replace HALF of the fluid deficit in the first 2-4 hours and the REST over the next 12-24 hours. This means that if you estimated that you lost about 16 ounces of water in your long run, you shouldn’t gulp down two 8-oz bottles of Gatorade before you hit the showers. Instead you should have one bottle and wait for about an hour or so before you have the second one slowly with your post-race meal. The slow rehydration allows the brain to eliminate some of the free osmoles that are no longer needed and the rest of the body to readjust gradually from a dehydrated state back to normal.

This slow rehydration technique is more important for small-framed individuals and people who tend to sweat a lot when they exercise because they will have more fluid shifts during their dehydration and rehydration phase. Also, because these central osmoles are generated only with moderate to severe dehydration, these guidelines are much more applicable for those who participant in long endurance training events than those that exercise for shorter periods of time.


Hope you all find this little dissertation on the physiologic consequences of rapid rehydration useful. Please feel free to ask any questions. Let me know if this was all too confusing to follow. I’m thinking I’d like to post some other physiology lectures on running in the future so if there are any burning questions that you want answered, I’ll take suggestions for those too.

Okay, hope you all learned something today. Class dismissed!

24 comments:

audgepodge said...

Ah yes, you diagnosed me with this from my dehydration experience! Very helpful information - I'll have to bookmark this post for future reference.

running private said...

Nice post. I've definately had this before! Looking at the weather in London it appears I won't be getting it again any time soon!!

Meg said...

Thanks for the explanation, it was really interesting! I'm looking forward to more like this!

Amy@RunnersLounge said...

Thank you for taking the time to write this and share this! This really helped me understand some things about my own running.

I think other runners would really enjoy this - would it be possible to add this as an article in the Know How section of the Lounge (www.runnerslounge.com)?

Amy
amy@runnerslounge.com

Jamie said...

Very informative - thanks for lesson doc! I enjoyed this post.

Non-Runner Nancy said...

Great post, Lam. I thought I would be the only geek that enjoyed it but looks like you have a following. I think you should definitely post this on Runner's Lounge for sharing purposes.

Small framed? HA HA HA HA :D

Nitmos said...

Very interesting. And do you realize this article alone would easily have scored well over 100 points in my word snob game. You are now the Word champion! Sorry, no trophy.

Tree said...

Thank you - super interesting.

Betsy said...

Thanks, Dr. Laminator!

MissAllycat said...

Very interesting! This was an excellent read. Thanks!!

Laura said...

Science was my absolute worst subject, and it always confounded me... but I think I actually understood most of that. Thank you!

P.O.M. said...

Thanks Doc. I love this type of information. Keep it coming! I need to add you to my blog roll. You have such great info all the time.

Thanks again.

heatherdaniel said...

Water + Gatorade + Chocolate Milk sounds like a recipe for a gastro-intestinal nightmare.

I liked the post and have been there myself! I am somewhat small framed and I sweat like a rhino during my long runs. I am always surprised how woozy I feel afterwards.

Andrew is getting fit said...

I like the medical explanation! More posts like this!

D10 said...

Interesting and informative post. This post brought me back to my college days sitting in physiology of exercise class.

The Laminator said...

Thanks you all for your positive comments. I really wasn't sure if I explained the concepts well enough or not, but judging from these reactions, I guess it was okay. I will definitely try and write more posts like this in the future! Thanks again.

sRod said...

This completely happened to me after last weekend's long run. I bought a 16 oz. bottle of gatorade and two sips into it the cap fell on the floor of the subway. Uck. So I started chugging down the whole thing down because I didn't want to be walking around with a capless bottle of Gatorade. An hour later I was feeling sick, as if there was a big water ballon where my stomach was supposed to be.

Although come to think of it, I often drink about 16 oz. of fluids within the first hour after a long run. Maybe I'll be cutting that back on your reco Doc.

bill carter said...

Hi Lam

This is such great stuff. When you take the time to put out a post like this, it just shows how much you care for your fellow runners/bloggers. Thanks for taking the time to explain not only the process, but the solution to the problem. BTW, been there done that on the whole doing the dehydration fixup the WRONG way.

Robert W. said...

Thanks, that was really interesting and informative!

One question: any advice on how to estimate how much water we've lost during a long run? I have to admit that I have no idea, and I imagine it varies from runner to runner, and depending on weather conditions.

Any tips?

Frayed Laces said...

Thanks for the technical explanation for my splitting headaches after a long run. After taking your advice from a few weeks ago, my headaches have magically disappeared! Now I get the detailed explanation--enough to satisfy the scientist in me.

The Laminator said...

sRod - Don't cut back the fluds. Just the rate at which you're taking them.

Bill - Thnaks for the cool comment. I think it takes quite a while to find the "RIGHT" way to hydrate. And even then, you have to make adjustments for weather, temperature, humidity, etc.

Robert - Quick answer to your question: Weigh yourself before and after your run. Every pound difference translate to one liter of fluid lost. I'll try to work on a more scientific explanation for a later post.

FL - Yeah, didn't have time to explain all of this back then. But at least now you know.

The Happy Runner said...

Wow. Very, very interesting. Just heard about this post from Frayed Laces. This is such good information! Thank you.

Lori said...

Thank you for explaining this is detail. I'm going to start doing things differently after long runs and save myself some agony!!

Jessica said...

Just found your blog! Great info and good explanations! Thanks for the tips. Luckily, I am naturally slow at rehydrating! :) Jessica

 
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