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.
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 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!