Dehydration and Athletic Performance

Proper fluid balance is a requirement for not only athletic performance, but proper functioning of the human body. Dehydration is a condition that can and does affect a great many people and may cause many undesirable effects. One study found that about 15,000 episodes of dehydration and/or heat illness occurred during a two year research period in high school athletes, which led the athlete missing from 1 day to 1 week of practice or competition (Hoffman, et al). This article will attempt to talk about some issues related to dehydration and provide some strategies to avoid and correct dehydration before, during and after athletic competition.

Relevant Physiology

A prerequisite for optimal functioning of the human organism is a state of homeostasis. In other words, your body not only prefers, but requires all of its components to fall within a certain range in order for it to function properly. Some components have a narrow range, while others have a relatively broad range. For instance, a slight change in the pH of your body can lead to some pretty nasty effects while your blood pressure can vary a great amount without killing you (immediately that is). When your body senses a component go out of range, the Central Nervous System sends signals to certain organs which then respond with some sort of chemical synthesis which results in an action to pull that component back into its optimal range, this is known as a feedback loop. A very simplified example is when blood pressure drops, pressure sensing receptors in the blood vessels tell your brain of the change, the brain activates the sympathetic nervous system, causes your blood vessels to constrict and voila, your blood pressure is back within a normal range.

The human body loses fluid through several mechanisms. Urine and fecal loss account for about 1700mL/day, respiratory losses account for another 300mL, 100mL is lost via sweat (this is at rest with an ambient temperature of 68 degrees F, when active or in a hot and humid environment this mechanism can go up to 5L/day), and about 400mL/day is lost via insensible water loss (Saladin pg 916-917). Insensible water loss is water that diffuses through the skin and evaporates into the air. This is not a glandular secretion of the body, in other words sweat, in response to a stimulus rather, it is just movement of water out of the body via the skin. Some of the mechanisms are variable, like urine and fecal losses which can be down regulated to conserve fluid however, there is a certain amount of water loss, named obligatory water loss, that will happen no matter how dehydrated a person gets. Obligatory water loss is something the body cannot down regulate, it must happen, and explains why the body cannot just keep cycling the water it has to prevent further dehydration.

The body regulates intake of water through the thirst reflex. Blood osmolarity (we will discuss this term in a moment) is the primary driver of this reflex through a series of signals between receptors, the brain, the salivary glands, the small intestine etc. There are feedback loops which both make us thirsty in response to a decreased fluid content in the blood and inhibit the thirst reflex to avoid over hydration. These mechanisms work constantly to maintain fluid and electrolyte balance in the body.

The next issue we should get a handle on before going any further is osmosis and osmolarity. Osmosis is the tendency of fluid to move across a semi-permeable membrane towards higher solute concentrations, for the purposes of this article, and the main drivers of fluid balance in the body, the solutes are electrolytes. Osmolarity is simply a measure of solute concentration within a solution. In other words, the more ‘stuff’ in the fluid the higher the osmolarity and the greater the tendency to draw in water will be. Membranes separate the extra cellular fluid (ECF), which contains the interstitial and vascular compartments, from the intracellular fluid (ICF), which consists of the cells of the body. Normally, in homeostasis, the osmolarity of these two compartments is balanced and fluid doesn’t shift. If for any reason electrolytes are lost from either compartment, the osmolarity of the other compartment is relatively higher (hypertonic or hyperosmolar) and the fluid will shift until a balance in osmolarity is once again reached, while the opposite occurs if fluid is lost from a compartment. This is a passive mechanism and is governed by the laws of Physics. Once one compartment is hyperosmolar, within seconds, the fluid will start to shift and a balance of osmolarity is once again reached. It should be noted here that when we speak of water moving, we are truly referring to the net movement of water across a membrane. Living systems are highly dynamic and there is always movement of solutes and water back and forth, but when the two systems are in equilibrium the rate of movement into the cell is equal to the rate of movement out of the cell.

The human body is truly a wondrous machine but it’s not perfect and can’t always fix everything on its own. Sometimes these mechanisms land us in trouble, as we will see.

Types of Dehydration

There are three types of dehydration that can occur and they are named for the relative amount of electrolyte loss in relation to water loss. The first, and most common, is isotonic dehydration. Isotonic dehydration means simply that fluid and electrolyte losses are equal (the prefix -Iso- in Greek, means equal). The danger with this type of dehydration is that the ECF becomes depleted of fluid. This depletion then leads to a reduction of blood volume and a drop in blood pressure. The decrease in blood volume leads to poor perfusion of organs, for our purposes the organ system of importance is the musculature. With poor perfusion of the working muscles, metabolic wastes accumulate and oxygen is not being delivered efficiently which may lead to a decreased performance. Aside from the poor perfusion of the musculature, we run into problems because appropriate blood pressure is necessary in the skin to maintain adequate thermoregulation. When there is not enough blood to achieve both goals (muscle and skin perfusion), and we are in the midst of strenuous physical activity, scientific literature seems to show that the body chooses the musculature to supply with blood leading to a breakdown of thermoregulatory mechanisms (González-Alonso et al).

The body has many back up mechanisms to fix this problem, the blood vessels constrict to normalize blood pressure, the kidneys begin to reabsorb water and sodium back into the blood stream, the heart starts to beat harder and faster to get that blood moving better. All of this works well to delay problems however, we only have so much water and without proper intake these defense mechanisms will eventually fail and we will not only have poor athletic performance but illness may ensue. This type of dehydration is typically caused by bleeding (well hemorrhaging, and if you are doing that on the field you shouldn’t be worried about athletic performance), vomiting, diarrhea, profuse sweating, and inadequate intake of fluids/electrolytes (Ignatavicius et al, p. 212-222).

The second type of dehydration is hypertonic dehydration. This type is characterized by a greater loss of fluid than electrolytes. Here is where osmosis comes into play. The ECF loses water but at a greater rate than it losses electrolytes. The osmolarity of the ECF relative to the ICF increases and the osmotic pressure (just a fancy name for the pull of water into the blood vessel) causes fluid to shift from the ICF to the ECF. This shift of fluids normalizes blood volume but, unfortunately, now the cells are dehydrated. A dehydrated cell is not an optimally functioning cell and our body then uses a slew of hormones to get things going and to normalize. The kidneys now reabsorb water back into the blood stream and your body makes you thirsty to get those fluids replaced. This is one reason why it is said that waiting until you are thirsty means you are already dehydrated. Your blood pressure and organ perfusion will be adequate and there is no real threat to life, however your cells have given up precious fluids in order to accomplish this task and are now working sub-optimally. And just as before, electrolytes don’t just appear from thin air and if they are not replaced the cycle will keep going until serious problems arise. This type of dehydration is typically caused by prolonged sweating, hyperventilation, fevers and watery diarrhea (IBID).

The third and least common type is hypotonic dehydration. Obviously this is where electrolyte loss is greater than fluid loss. This type of dehydration is the rarest; it does not apply to athletes, and is typically seen in chronic illnesses. A few words should be said about it anyway to reinforce some of the principles we have already discussed. The problems that we run into with this type of dehydration are from fluid shifts. Losing electrolytes from the ECF will increase the osmolarity of the ICF and the fluid will tend to shift into the cells. The problems are 3 fold. First and foremost our cells need a certain range of fluid to work optimally (remember that homeostasis thing we were talking about?). A swollen cell is not a happy cell and a swollen brain cell is particularly cranky. Brain tissue is very sensitive to swelling and neurological symptoms are very common, and are typically first complaints, like dizziness, headache, confusion etc. The second problem is the loss of fluid from the vasculature causing your blood plasma to be depleted thus making your blood more viscous and decreasing blood volume/pressure. This can affect organ perfusion as thick blood doesn’t flow so well and doesn’t deliver oxygen and nutrients so well, either. The third is a dilution of normal electrolyte levels leading to a whole host of other issues. Each electrolyte has its own problems that it can cause when out of whack and a discussion about each of them is well out of the scope of this article, however we will touch upon sodium balance as this has some implications for athletes.

Implications for the Athlete

“So what does all this mumbo jumbo mean to me?”

Well now that we have a somewhat decent understanding of the causes, mechanisms and complications of dehydration we can talk about what it means to the athlete.

Heat Illness

Dehydration, hot, humid environments (>95 degrees F, >80% humidity) and strenuous exercise (and by extension profuse sweating) can lead to some pretty serious problems, the least of which is decreased athletic performance. But wait, this article should be about dehydration? What’s this heat exhaustion crap? These two topics pretty much go hand in hand as dehydration will potentiate heat stress and hot and humid environments will potentiate dehydration. They are undeniably linked.

Heat exhaustion is a product of dehydration coupled with hot/humid environments. Dehydration is not a pre-requisite for this condition however dehydration will severely decrease one’s ability to dissipate heat properly and in the real world heat stress is typically accompanied by dehydration. In this state the athlete may feel very ill, and may complain of flu like symptoms, which include headache, weakness, fatigue, nausea and vomiting. Although not a true medical emergency, this condition may lead to heat stroke if not adequately addressed (oral rehydration preferably with fluids containing electrolytes, cool environment, and loosening of constrictive clothing).

Heat stroke is a medical emergency and can have a pretty high mortality rate, approaching 80%, if not treated promptly. This is what happens when our defense mechanisms fail and our body can no longer maintain a safe core body temperature. There are two types of heat stroke, exertional and classic. The former is typical of athletes who are exercising or competing in hot and humid environments. The onset is very rapid. Classic heat stroke tends to occur in the elderly with long exposures to hot and humid environments and has a more insidious, or slower, onset. The athlete’s body temperature may be very high and sweating will stop (a defense mechanism to save essential fluid), as well there will be changes in mental status such as anxiety, confusion etc. Hospitalization is a necessary step at this point to reduce further, possibly permanent, organ damage (Ignatavicius et al, p. 212-222).

Environmental factors have a very big impact on athletic performance. Heat stress combined with dehydration will significantly decrease total time to exertion during aerobic work as well as negatively affecting peak power output (Maxwell et al, Goulet et al). Most of the research in this area has been done on aerobic activity with some anaerobic work built in (think long distance cycling with intermittent sprinting) however, very little research has been done on purely high intensity activities, for example weightlifting or short distance running. These types of activities are thought to not be affected as much by dehydration and heat stress as the activities do not last long enough for the cardiovascular system to be strained and the deleterious affects to be noted (Rothenberg et al). It should be noted, as well, that cold environments attenuate the effects of heat stress, meaning that in colder climates these types of performance declines are not as significant. This last bit does not mean that dehydration is not an issue in a cold environment as cold air is drier and absorbs more water from your breath. There is also the issue that exerting yourself and increasing sweat losses, while wearing lots of gear can potentiate this effect. A study done by Palmer and Spriet looked at sweat losses from hockey players during 1 hour of practice. They found that even in a cool environment (57 degrees F and 67% humidity) the players lost between 1-2L of fluid during the hour-long practice (ranging from less than 1L while many lost more than 2L). This shows that sweat losses can be significant even in colder climates and once the athlete begins to approach a 2% loss of bodyweight, the detriments in performance are noted.

Practical Applications

Now that we have established the problems that can arise, let’s tackle the issue of adequate hydration. The unfortunate part of this, most exciting, article is that proper hydration is something that varies from person to person (what doesn’t, right?). Nevertheless we can scan through some literature and perhaps work up certain strategies to avoid dehydration.

In one study, the authors used 2 groups of athletes to compare hydration strategies. The hyperhydration group was given 26mL fluid/kg body mass coupled with 1.2g glycerol/kg body mass over an 80 minute period prior to the test while the control, or euhydrated, group was not given any fluids. Glycerol was chosen because it has been shown to aid the body in retaining water, and thus hydration, without other ergogenic properties (Goulet et al, Wingo et al, Robergs et al). Both groups were given Gatorade to drink during the exercise. Both groups cycled for 2 hours and then were given a test to measure how long they could cycle until exhaustion. The hyperhydration group had a higher peak power output, and was able to cycle for a longer period of time until exhausted. The hyperhydrated group also showed lower rectal temperatures (although the authors were not able to show this as statistically significant, a look at the data shows about a 1 degree difference between the groups at the 120min mark) and lower heart rates during the 2 hours of cycling (Goulet et al). What can we glean from this? Over hydration allows the body to cope with exercise and heat more optimally under conditions where maintaining hydration during exercise is difficult. The peak power output and time to exhaustion differences is self explanatory while the lower rectal temperatures and heart rates in the hyperhydration group show that the body was able to dissipate heat better and the cardiovascular system did not have to work as hard to keep up with the oxygen demand of the musculature. This was probably due to the maintenance of adequate blood volume and other factors having to do with the Central Nervous System (Alonzo et al). The authors concluded that beginning an exercise session hyperhydrated alleviated some of the detriments to performance seen as athletes become dehydrated during competition. For a 170 lb person the above numbers equal about 2 liters of fluid with 100g glycerol over 2 hours before exercise or competition. This method would over hydrate an individual and help to attenuate the performance detriments associated with dehydration during activity. This over hydration method should be the upper limit of fluid intake unless sufficient electrolytes accompany the water if the glycerol is not used, the reason will be discussed shortly

Another group of researchers suggest that to be appropriately hydrated before competition athletes should consume at least 6-8mL/kg of body mass of fluid containing sodium or with food about 2 hours before exercise. For a 170lb man this translates to about 450-620mL of fluid. The authors don’t deny that water needs are individual and differences and experiences are to be kept in mind when planning a hydration protocol (Shirreffs et al). The National Athletic Trainers Association recommends 500-600mL of fluid or sports drink 2-3 hours before exercise with another 200-300mL consumed 20-30 minutes before exercise/competition as a minimum for athletes across the board with individual differences to be accounted for (Casa et al, NATA Position Statement).

What can we take from all this?

Staying hydrated is best accomplished not only by drinking during the activity, but also by ensuring that you are appropriately hydrated before the exercise/competition. This is especially true if the conditions are hot and humid, as these conditions can greatly increase fluid loss through sweating.

During exercise and competition is where things get a bit dicey. Some sports are not exactly conducive to stopping for water breaks, like marathons. Other sports lend themselves to this but many athletes choose to forego or like to just take a sip of water and get back to working. This is all well and good and shows a good work ethic however, when sweat losses are in the liters and the athlete is replacing fluid with sips (we’re talking mL’s here), he/she is shooting themselves in the foot. A good practice to establish is to take a nice big drink of fluid when the opportunity arises and not just a sip or two, especially if the exercise will last over an hour or is taking place in a hot/humid environment. Using containers, which are measurable, is a great way to figure out how much one ingested and whether or not one should drink more or less. Drinking half of a 1L bottle means you ingested 500mL and if hydration was not maintained that amount can be bumped to getting ¾ or the whole bottle in.

There exist a few methods to establish how dehydrated an individual gets during exercise. There are Urine Specific Gravity measuring devices, urine color charts and such but the one I will discuss is very simple and is available to most people. This method is simply weighing oneself. Weigh once pre-exercise and once again post-exercise. If the change is greater than 2%, then it is a fairly safe bet that you have become dehydrated at some point during the session. So if a 170lb person (pre-exercise) weighs in at 166-167lbs after exercise, he/she should take a closer look at what and how much was consumed before the exercise and what and how much was consumed during. A 1-liter loss of fluid amounts to about 1 kilogram (2.2 pounds) loss of weight (Ignatavicius et al, p. 212-222). Conversely, the NATA recommends ingestion of 1-1.25L of fluid for every kilogram lost during exercise as a rehydration strategy (Binkley et al, NATA position statement; Exertional Illness).

Exertional Hyponatremia or Water Intoxication

Natrium is the chemical name for sodium and is the most abundant electrolyte in the ECF. It accounts for 90-95% of the ECF osmolarity (Saladin, pg 922). As we have learned in the beginning of the article the most prevalent form of dehydration tends to be isotonic dehydration, meaning the body has lost about an equal amount of water and electrolytes. If water is replaced without any replacement of electrolytes, the situation created resembles one of hypotonic dehydration. Fluid starts to shift to maintain an osmotic equilibrium and this may potentially lead to death as cells can swell and possibly even rupture. This is a pretty easily rectified situation; don’t just drink pure water if a training session is going to last over an hour, or if the temperature and humidity are high, and especially if you haven’t had any solid food or other sources of electrolytes before the training.


So this long-winded article has finally come to a close. What have we learned? Dehydration can be, at worst, a very serious health concern and, at the least, can decrease performance on the field or in the gym or wherever you may choose to exert yourself. Maintaining adequate hydration is not just a matter of taking a couple of sips of water during the training session but is really more about maintaining a consistent hydration status throughout the day. Pre workout hydration may be more important than intra-workout hydration, as certain activities do not allow for breaks. Continuing with the same theme, assessing one’s own hydration status may be as simple as monitoring the weight changes pre/post workout and qualitatively analyzing urine color. Just as important is rehydrating oneself post training, and this is where the idea of hyponatremia comes into play. I say it is always better to be safe than sorry, so a whole food meal (with water of course) or some type of electrolyte rich sports drink is the preferred method to ensure safe and effective rehydration.


By: Yan Levitsky, RN, CPT

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