Dehydration vs. Overhydration

Are you drinking enough fluids? How much is enough? According to doctors, mothers and nutritionists everywhere, 8 to 13 glasses of water per day is absolutely essential for health and optimal function. If you exercise or participate in sports, then your trainer or coach may recommend drinking even more fluids before and after, as well as at regular 15-minute intervals during practice, training or competition. The emphasis is not merely on RE-hydration, but on PRE-hydration, the assumption being that any amount of dehydration is deleterious to performance and potentially dangerous to your health. I, too, shared this belief until I considered compelling reasons to question what is generally accepted as stock and standard protocol. It turns out that popular recommendations for fluid intake not only contradict what takes place during actual athletic performance, but may also introduce unnecessary health risks associated with overhydration.

You might not be as dehydrated as you think

You’ve probably heard that losing 2% of your bodyweight will negatively affect performance. The implication is that the closer you get to this 2% value, the more your performance will drop off. Does this match up with actual observations, though? Obviously, at some point dehydration is going to be a bad thing. However, if you look at marathon runners, those who complete the race in the fastest times also lose the most weight. The average 160 lb. male can lose up to 8 pounds during a marathon, which is 5% of his bodyweight. To reiterate, this is one of the guys at the front of the pack, not a last place finisher. So, what’s going on? Well, for one thing, using the loss of bodyweight alone to determine dehydration is just bad physiology, because some of the water lost in sweat is a byproduct of the metabolism of stored calories.

To illustrate: that same average 160 lb. male will burn about 2800 calories in a marathon. Assuming his average heart rate was about 80-85% maximum, that means he will burn carbs and fats at a ratio of approximately 80/20. (Protein use is negligible, so we will ignore it here.) Converting calories to pounds will give you about 1 1/4 pounds of carbs and about 1/8 of a pound of stored fat utilized as fuel. This means that over 6 1/2 pounds of water will have been lost, but that’s not the whole story.

There is 3 to 4 times the amount of water stored for each unit of carbohydrate. For each unit of fat stored in the body, there is an equal unit of water stored along with it. Do the math and you get anywhere from 4 to 5 1/4 pounds of water loss that was associated with stored calories. What happens to that water once the fuel is metabolized? It’s not needed for calorie storage any longer, so it’s released into the bloodstream and is available for use, conceivably being released as sweat. Taking this into account will result in a net water loss of only about 1 1/4 to 2 1/2 pounds of water (0.6 – 1.2L,) giving you a maximum 1.5% loss of body weight, well within the 2% rule. This makes sense, doesn’t it? As your body utilizes calories, the water that was stored with those calories becomes available for cooling through perspiration. And this mild level of dehydration obviously doesn’t hinder the performance of elite runners.

Water Station

If you’re not thirsty, then you don’t have to stop at the water station

Most if not all long-distance runners do drink during an event, and studies on elite marathon runners have revealed a typical self-reported fluid intake of about 200 milliliters, or 6 ounces, per hour. At that rate, running for 2 hours and 13 minutes would result in an intake of one pound of water, and a total water loss of over 7 1/2 pounds for our hypothetical 160 lb. runner. Of course, this is an estimate based on average performance and physiologic response, and individual response and measurements will vary. The point was merely to show that all water lost during exercise does not have to be immediately replaced in order to avoid critical levels of dehydration, and even mild dehydration is not detrimental to performance.

Mild dehydration does not cause hyperthermia

Maybe winning the race “at any cost” isn’t your primary goal; you just want to finish in one piece. In that case, is it better to just drink as much as possible, for health reasons? After all, you run the risk of overheating even if you’re only slightly dehydrated, right? Very simply, no. Runners and cyclists competing in distance events do not get hotter and hotter as the race progresses and they become more and more dehydrated. Rather, body core temperature rises after beginning sustained activity, then levels off and is regulated at that higher temperature over the duration of the event. This is the normal physiologic response to an elevated metabolism. Body core temperature is a product of metabolic rate and environment, not hydration.

While dehydration may certainly play a role in overheating and heat stroke, it is not the cause, despite what you will hear and read from all the so-called “experts.” You can be sufficiently hydrated and still develop heat stroke! Furthermore, you can become dehydrated to the point of death without overheating. Despite the fact that endurance athletes are virtually guaranteed to experience mild dehydration, as well as an increase in core body temperature, there have been NO documented cases of death due to dehydration in any marathons. However, a few fatalities, as well as many other cases of illness, have been caused by something that may surprise you.

The danger of hyponatremia

Drinking too much water can actually be hazardous to your health, leading to a condition called hyponatremia, which is the term for a drop in sodium concentration in your body fluids and has the potential to be fatal. Some overzealous marathon participants have not only replaced their lost body weight with fluids during the event, but have actually gained weight as the race progressed. How serious is this situation? Here are some unfortunate statistics:

  • During the Chicago marathon in 1998, Kelly Barrett collapsed and later died from hyponatremia.
  • After the Houston marathon in 1999, 4 runners were hospitalized with comas due to hyponatremia.
  • During the Boston marathon in 2002, Cynthia Lucero died from hyponatremia. In that same marathon, 13% of 488 runners tested were hyponatremic.
  • At the 2002 Marine Corps marathon in Washington, DC, Hilary Bellamy died from hyponatremia.
  • At the London marathon in 2007, David Rogers died from hyponatremia.

Each and every case was fully preventable and was solely the result of overhydration. This fact then, bears repeating: there have been NO documented deaths in a marathon due to dehydration, even for out-of-shape participants on a 5 or 6-hour pace. What does that say about how much you need to drink during your one hour of cardio at the gym, or on the tennis court? Now, the purpose of presenting these incidents was not to scare anyone into not drinking anything at all. The take home point is that overhydration is a much more serious risk than dehydration during exercise or competition. Maintaining your starting body weight with fluid intake is a bad idea, and if you’re gaining weight during any period of sustained physical activity, then you’re placing yourself in grave danger.

The myth of electrolyte imbalance

OK, so if you want to avoid this situation, then you can just drink Gatorade, right? Not exactly. No sports drink will ever “rebalance” your sodium levels, and this is why: Normal sodium concentration in your body is about 3.2g of salt per liter of water, but there is only about 1.1g of salt per liter of sweat (that varies, of course, but this is the average.) So, the more you sweat, the higher the concentration of sodium becomes in your body, and the imbalance that sport-drinks claim to correct doesn’t even exist! That’s what makes you thirsty, your body’s need to rebalance your fluids, not your sodium levels. Ever notice how you never want to run for the salt lick after an exercise session? All you need to do is replace some water (but not too much.)

Drinking too much of your favorite sports drink will only CREATE a sodium imbalance, with potentially life-threatening consequences

Still, you do lose sodium in your sweat, so Gatorade can only help by replacing some of it, right? Not really. Since Gatorade only contains around 0.4g of sodium per liter, it can only lower the concentration of sodium in your body, which is way more important for fluid balance than the total amount of sodium. In fact, the more Gatorade you drink, the more predisposed you may be to developing hyponatremia, the same as if you drink too much water. That’s why you will receive “smart advice” to drink plenty of fluids AND eat a salty snack. But, the salty snack wouldn’t be necessary if you weren’t overhydrating to begin with! Using sports drinks as part of an in-race energy replacement strategy is one thing, in which case you should determine your intake based on carbohydrate content, not body weight. But, they will never rebalance your electrolytes. End of story.

Some drinks do claim to be hypertonic, because they contain a higher concentration of carbohydrates. This is erroneous labeling, though. While those drinks may have a higher total osmolality, or number of solute molecules, glucose can freely enter cells so the effective osmolality, or tonicity, is low and the drink is still hypotonic. Of course, those carbohydrates do require water to be stored and metabolized, so you will have to ingest more of any given sports drink to overhydrate than if you drink plain water. But you can still overhydrate with Gatorade, just like you can still get drunk on light beer.

Corporate-sponsored physiology

Where does this advice to drink until you drown come from, anyway? In large part, from the bottled water and sports drink industry. Here are a couple examples: The Academy of Nutrition and Dietetics (formerly the American Dietetics Association, or ADA) which recommends drinking even when you’re not thirsty, is partially sponsored by Coca-Cola and PepsiCo, who collectively own hundreds of bottled water, sports drink and beverage companies and labels. Do you think there might be a conflict of interests in that relationship? This situation is not unique to the U.S., either. French food corporation Danone, which owns Evian, Volvic and Badoit bottled water companies, among others, exclusively sponsors Hydration For Health, a European initiative, to “educate” the public on their hydration requirements.

We, the public, have subsequently been exposed to the oft-repeated mantra, “by the time you’re thirsty, it’s already too late.” This seems to be supported by the fact that exercise does suppress thirst, to an extent. However, the whole idea that you are incapable of becoming thirsty enough to prevent damage to your own organism is nothing more than a corporate fairy tale. Have you ever run around and really exerted yourself for an hour or two and then not been thirsty afterward? Nope, it doesn’t happen. You can purposely ignore your thirst for a little while, but only for so long before the urge to drink becomes irresistible.

Beverage companies would have you believe that any time spent in a state of thirst is a near-death experience

Sustained physical activity also suppresses urine production, which surely complements that initial thirst suppression. If your body senses that it will not be taking in fluids, it also shuts down the process of eliminating fluids. Normally, if you do take in excess fluid, you will simply urinate more. However, manually overriding the thirst mechanism by over-drinking doesn’t necessarily re-activate urine production while engaging in strenuous activity. Even hyponatremic athletes usually have to wait a while after they stop running before they can urinate again. The real danger is when overhydrated athletes believe they are dehydrated because they haven’t needed a bathroom break during training or competition, and take in even more fluids.

Let thirst be your guide

So, how much should you drink, then? I’m not going to provide a recommendation, because if you simply listen to your thirst, you will never have to worry about any issues with dehydration. Our built-in thirst mechanism is just too strong to suppress. If you get thirsty enough, and water is available, you simply cannot ignore the urge to drink. And when you drink enough to satisfy that urge, you stop being thirsty. In other words, you don’t need to be told how much to drink! If you frequently experience headaches, nausea, bloating or fatigue during exercise, well, it could be a lot of things and you might want to talk to your doctor. But, if you’ve also been attempting to maintain your body weight with fluid replacement, then you may just want to try drinking less.

Ultimately, your body doesn’t care how much body weight you lose during a training session or sporting event. Rather, it’s concerned with fluid balance, and it’s pretty good at it without any misguided attempts at fluid micromanagement. Remember, marathon runners who finish first also lose the most weight, which is mostly water weight. On the other hand, runners who gain weight over the course of a race sometimes end up dead. This doesn’t mean that you shouldn’t drink, of course. We NEED water, so listen to your body when you’re thirsty! If you drink according to thirst, then you’ll never have to worry about dehydration or overhydration, or the effects that either condition can have on your health or performance.

Sources

1. Tucker, R. & Dugas, J. (2009). The Runner’s Body: How The Latest Exercise Science Can Help You Run Stronger, Longer, and Faster. Rodale Books.
2. Brandis, K. Fluid Physiology. http://www.anaesthesiamcq.com/FluidBook/
3. Schucany, W.G. (2007). Exercise-associated hyponatremia. Baylor University Medical Center Proceedings. v.20(4); PMCID: PMC2014811
4. McCartney, M. (2011). Waterlogged. British Medical Journal. BMJ 2011; 343:d4280

Image Credits: Dominik GoleniaJoe Clark; Selman PINARLI

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