Transcription
Hello, and welcome to our second to last online lecture. As we approach the end of the quarter, today's topic is fluid requirements and heat stress. And I think the saying, "It's important to stay hydrated," is probably one of the most repeated pieces of sport and exercise nutrition advice that you're going to hear. And while it's very good advice, generally, when you ask someone to be a bit more specific in terms of hydration recommendations, you're often met with generalizations, advice that is meant to apply to everybody and sort of ignore some of the individual differences in hydration needs. Or you're given some very non-specific follow-up answers that don't really go much beneath the surface of just, "Oh, it's important to drink water," and "It's important to stay hydrated when you're exercising."
And as we will see today, there are a number of unfortunate outcomes that can result from both under and over hydration. So, having a major goal of this lecture being identifying the risks associated with under and over hydration physiologically and what some of the outcomes are, but more importantly, establishing some more specific practical guidelines that athletes and coaches and exercisers can use to try to avoid some of these, some of these dangerous outcomes. And then, of course, because many of these risks are exacerbated by performing exercise in hot environmental conditions, we're also going to discuss some guidelines for monitoring heat stress and for maintaining hydration in extremely challenging environmental conditions.
Now, one of the primary challenges the body faces during exercise is the need to dissipate the heat generated from the mechanical work that's being performed by skeletal muscles in order to maintain a safe core temperature. And again, this is especially true when exercising in hot environmental conditions. And so, we have several mechanisms by which we exchange heat with the external environment in order to attempt to maintain core temperature. And so, the first that I'll highlight is conduction. And in this sort of graphical representation of the methods of heat exchange, we can see here that conduction, this is one of our primary means. And this method of heat exchange is achieved through contact with external objects. So, in this particular picture here, you can see this individual is wearing a backpack. Depending upon the gradient of heat between the backpack and the body, because of the contact with that object, heat could be moving in either direction, and we could either be gaining or losing heat to that backpack. We also see this method of heat exchange very simply depicted when you sit on a seat or a bench of some sort. If you sit there for long enough, when you get up, you'll feel the bench, and you feel that it's warmer, and that's because of the conductive met, the conductive method of heat transfer occurring between your body and the bench as it is in contact. So, conduction is one way in which we can dissipate heat.
The second way that we can, that we exchange heat with the environment is via exposure to the sun's rays or radiation. So, we take radiation on both directly from rays that come from the sun. We also do this in a diffuse fashion because cloud cover will affect the amount of radiation that makes it down to the surface. So, more cloud cover will kind of lessen the effect of solar radiation on core temperature. And then we also absorb some of the heat that is reflected off of the ground and objects in our surrounding environment via radiation as well. So, this is definitely a method that we can gain a lot of heat via when we are exercising outside during the day, especially in hot environmental conditions.
The third method by which heat is transferred with the external environment is via convection. And convection heat transfer involves transferring heat between the body and the air that is moving around the body. So, this can be greatly affected by the wind speed and the wind angle on a given day. And depending upon whether the air that is blowing on us is hotter or colder than our, than our body temperature, will depend on whether heat is transferred to or from the body to the air and vice versa.
And then our final method of transferring heat to the environment is via evaporation. And some evaporation occurs via ventilation. So, water vapor coming out of the air that we are breathing out, blowing out that hotter air and it evaporating into the environment. But a great deal of evaporation occurs primarily via perspiration. And via perspiration, of course, this is what the body moving heat from the core to the skin's surface, the peripheral vasodilation, so increasing the amount of blood flow that's going to the periphery and to the skin so that we can move the heat via fluid, via sweat to the skin's surface. And when it evaporates, that's what gives us a cooling effect and allows us to reduce body temperature.
So, I'll ask the question, out of all of those methods, which is the body's primary means of dissipating heat during exercise? And this probably makes a lot of sense to you, but the primary way that which we do that is via evaporation of sweat or perspiration from the skin surface. And this is primarily what makes hydration during exercise and hydration during exercise in extreme heat stress extremely challenging. The fact that in order to maintain core temperature, we need to move large volumes of heat via fluid to the skin's surface. And the way that we do that is we have to transfer some of that fluid from plasma, so blood volume, to the skin via sweat so that we can achieve that.
So, exactly how much fluid do we lose in these scenarios? Well, on average, depending upon environmental conditions and exercise intensity, normal sweat rates usually range between about a half to two and a half liters per hour. But there are certainly outliers when it comes to these normal rates of sweating. And many highly trained athletes fall into that category, especially when you have them performing in extreme environmental conditions. So, for example, this gentleman depicted here is distance running legend Alberto Salazar. And not only is he a legend in the sport of distance running, but he also has the distinction of having the highest sweat rate ever recorded. And as I said, the fact that he's an elite athlete, but also the environment in which this condition was measured had a great impact on it. And so, this highest sweat rate ever recorded was about 3.7 liters per hour. And Alberto Salazar recorded this while he was training for the 1984 Olympic Marathon, which just so happened to take place in Los Angeles. And being that this was taking place during the Summer Olympics, he knew that he would be having to perform in a hot environment. So, his training leading up to the marathon involved some prolonged running in a 90 to 100 degree environment, which is quite a hot environment to be exercising in. So, during these conditions, he recorded this sweat rate of about 3.7 liters per hour. And if you want to kind of visually think about what that looks like, that means that per hour of exercise, he was losing just under the equivalent of four large Nalgene water bottles per hour. So, a whole lot of sweat that needs to be replaced. You might also be asking yourself at this point, how do you measure somebody's sweat rate? Well, it's actually not as hard as it sounds, and we're going to learn how to do this later in the presentation.
So, what are the potential consequences of losing that much fluid and failing to replace it? Well, in terms of performance, it doesn't really take much fluid loss at all. In fact, functional decrements and associated impaired performance outcomes can occur from just losing one to two percent of your body weight in fluid. Which, as you can imagine, in the case where an individual is losing three, four liters per hour of fluid, isn't really that difficult to do. Now, specifically, some of the physiological and performance-related outcomes that can occur from losing one to two percent of your body weight in fluid are things such as increased thermal strain due to an increased core temperature. Again, as you begin to lose more fluid, you then have less fluid to move to the skin's surface and dissipate the heat that's being generated. As a result, you can see also impaired skin blood flow when losing this much body weight. And of course, one of the things that can happen when you reduce plasma volume is, in order to maintain cardiac output, well, you're going to have a decreased stroke volume because you're losing the plasma volume component. But in order to try to maintain cardiac output at an appropriate rate, you might increase your heart rate in the face of that decreased stroke volume. So, increase the number of times that the heart is beating in the face of each beat having less fluid or less blood being pumped. And this can also create an increase in vascular resistance. So, it can also increase the load that the heart has to pump against to get that cardiac output out into circulation and deliver it to working muscles.
Now, all of these physiological decrements can lead to a decreased time to exhaustion. So, individuals will be able to exercise at a given intensity for significantly less time due to these physiological manifestations of dehydration. And of course, from a perceptual standpoint, they'll perceive that working at any given rate is harder than it would be if they were in a well-hydrated state.
From a metabolic standpoint, because of the compromised delivery of blood and potential reductions in plasma volume and oxygen carrying capacity, we typically see more of a shift towards glycogen utilization and anaerobic metabolism at any given intensity of exercise. So, you may be familiar with this from your Kinesiology 303 Physiology of Exercise labs, but because this forces a shift towards more anaerobic and carbohydrate metabolism, you'll see an increased rate of glycogen utilization, as well as an increased lactate response at any given exercise intensity.
Another challenge that can occur, especially if you're trying to deliver nutrients during a bout of exercise, is because of the decreases in plasma volume, there's going to be less blood flow not just making it to the periphery, but also less blood flow making it to the gastrointestinal system. And this is going to occur as a result of dehydration, but it's also going to be exacerbated considering that in exercise situations, we're already reducing the flow of blood to the gastrointestinal system in order to shunt more blood to the periphery. So, in exercising in hot environments that challenge hydration status even more, you're going to see potentially impaired rates of gastric emptying and fluid and nutrient delivery.
Now, beyond performance outcomes, when fluid losses start to exceed about three percent of body weight, the consequences can become far more drastic. And while all of the performance decrements that can be observed with fluid losses of one to two percent are certainly going to be magnified once fluid losses begin to exceed three percent, we also now begin to see risk of illnesses, exertional heat illnesses, increased dramatically. So, things like heat exhaustion and heat stroke. And these illnesses can be fatal if not treated. So, the stakes get exponentially higher once fluid losses start to exceed about three percent of body weight lost.
So, what I want to do now that we've outlined the issues and challenges from a physiological and practical perspective is begin to take a look at some practical guidelines for hydrating and for assessing an athlete's hydration status. So, let's start by taking a look at the period of time prior to an exercise training session or competition. Now, these recommendations that I'm going to present to you are based on the guidelines for hydration from the National Association of Athletic Trainers. And there are two general recommendations that are given by this organization. The first is that two to three hours prior to exercise, athletes should ingest about a half a liter of water. And if you think about what a half a liter of water looks like, if you think about a squeeze bottle, a typical sport-size squeeze bottle that you'll see athletes use in practice and in training, about one of those squeeze bottles full of water or a sports drink will suffice in the two to three hours prior to an exercise session. And then, as the exercise session draws closer, about 10 to 20 minutes prior to that exercise session, it's recommended that athletes consume about another seven to ten ounces of water or a sports drink. And so, in terms of visually what that looks like, it's about equivalent to one of the little mini water bottles that you can get in like a 24-pack from Walmart or Costco or wherever you like to get your groceries.
So, if we look at these recommendations, you might be saying to yourself, "Wait a second, these seem too general, given that there is differences in individual needs for hydration and in fluid loss. Shouldn't, for example, a larger individual be consuming more fluid than this prior to exercise, or a smaller individual consuming less?" And if you're asking yourself that question, first of all, good for you for critically thinking. However, this is a situation where the general guidelines might actually suffice. And the reason for this is that assuming that adequate hydration status or adequate hydration practices were employed in the days previous to this exercise session, so assuming that the athlete is already at a hydrated state, and we're going to talk about some ways to assess whether or not an athlete is hydrated enough to perform, assuming that that's true, we don't really need more specific individual recommendations in this case because of where fluid absorption takes place. Fluid absorption takes place, as we've discussed, in the intestines. And anatomical studies have revealed that intestinal size doesn't really vary much with body mass. So, larger individuals, you might think would have way larger intestines, and smaller individuals. However, we actually know that's not true, and intestinal size is pretty standard in adult human beings when you look across different body sizes. And so, if they have the same amount of intestinal area, as we'll remember back to our unit on gastric emptying, digestion, and absorption, they therefore have similar absorptive areas and therefore are going to absorb fluid at a similar rate. Therefore, an individual who is larger may still have more fluid requirement needs related to the amount of fluid that they're going to lose during an exercise session. But by ingesting a larger amount of water prior to exercise or fluid prior to exercise, they're not going to then absorb a greater amount of it because the absorption area of the intestines is similar. So, in this particular case, the general recommendation should suffice in most situations.
So, how then do we assess an athlete's hydration status prior to a bout of exercise or competition? Well, by far the simplest method to do this is to measure their pre-exercise body weight, to do so consistently from session to session, and look at the change in body weight relative to the previous exercise session. And this is based on the principle that if we assume proper hydration practices and consistency across two consecutive days or training sessions, that the change in body weight should be due to changes in fluid and hydration status. And so, this is a really simple way to assess hydration status. And if we look at some of the guidelines for looking at this, based on previous session weight and the percent body weight change, an athlete is considered well hydrated if they have a plus one to minus one percent change in their body weight from the previous session. So, that would indicate that not much has changed, minimal dehydration. Enough to impact performance would be considered a negative one to three percent reduction in body weight. And then more significant dehydration occurs when we begin to go above three percent, and serious potential medical attention being needed at a loss of five percent or more. So, this is one method, a very simple method, to assess whether or not someone is ready to go hydration-wise.
But there are a lot of problems with using this method or relying on it as our sole method to assess pre-exercise hydration status. Some of the issues that occur are primarily due to things like the time of day and natural fluctuations in body weight that occur over a time of day. This could occur for a number of reasons, it could just be diurnal rhythms and fluids, but it also has a lot to do with the meals that have been eaten given on a given day. So, if you assess somebody's body weight in the morning before they've eaten anything versus assessing their body weight at night after they've had, you know, three square meals plus snacks, you're going to get different results. This is also going to have a lot to do with whether or not they have gone to the bathroom prior to that session. And is also going to be impacted by the practice that many athletes employ, which is frequent exercise training sessions, many times more than one training session or practice occurring within a given day. So, while this is the simplest method of assessing pre-exercise hydration status, there are a lot of sources of error. And so, therefore, it can be useful, especially when the stakes are high, to have more objective means of assessing hydration status.
And one method that we can use to do this is to take a look at an individual's urine. And while looking at urine may not seem like the most glorious task, a very simple rating of the color of an athlete's urine can give you a lot of useful insight into their hydration status. So, as we can see, the urine chart depicted here has been scientifically validated as an acceptable means for doing so. And the idea is really simple: the darker someone's urine is, the more dehydrated they are. And as a practical recommendation for using this chart, the National Association of Athletic Trainers suggests that in order for an athlete to safely perform, they should be at a urine color of one, two, or three, so three or lower. So, in order to use this effectively, while it is more objective than just looking at changes in body weight and doesn't isn't really prone to the same errors as changes in body weight are, there can still be some error in the discrepancy of rating different urine colors. So, may have a difficult time differentiating whether or not that urine is a one or a two, or a four or a five, and vice versa. So, there could be a little bit of misclassification that occurs due to the fact that this is still going to be subject to human error and rating of color. But if avoiding that error is of great importance, a more objective urinalysis method would be to look at the specific gravity of an athlete's urine sample. And this certainly involves a bit more interaction with the urine itself, but it's quite simple to perform and can be done using a device known as a refractometer. And these devices only cost about $150, so they're pretty low cost, and they give you some good information about the specific gravity of the urine sample.
Now, what exactly is specific gravity? Well, a higher specific gravity is associated with a more dense and more concentrated urine. So, that means that there is less fluid per unit of urine. So, that urine is going to be more concentrated, more dense, and therefore associated with a higher specific gravity. So, higher specific gravity indicates a denser, more concentrated urine, and a lower hydration status. And so, when using this method, you can see that this is a pretty objective physiological method for assessing hydration status. And again, the National Association of Athletic Trainers has some specific guidelines that you can see here in the table to the right for using urine specific gravity to determine an athlete's hydration status.
So, now that we've covered hydration practices prior to exercise, we can begin to think about how much fluid should be taken in during exercise. And this is another place where looking at changes in body weight can be quite useful. While these changes may be limiting to assessing pre-exercise hydration status, they are critical in determining appropriate levels of fluid replacement during exercise. And this is because, as we've already established, in order to avoid performance decrements, we really want to keep fluid loss at less than two percent of body weight at the most. And in order to achieve this, most of the general recommendations that you'll find are to ingest seven to ten ounces of fluid, water or sports drink, every ten to twenty minutes during a bout of exercise. But as you are probably already saying to yourself, there are some problems with relying upon these general recommendations. While the generalized recommendations for pre-exercise hydration status may be sufficient due to the physiological limits of intestinal absorption, there are major individual differences in needs and sweat rates across different individuals, but also across different performance conditions within the same individual. So, using these generalized during exercise hydration requirements doesn't get at these differences. And getting this wrong could result in significant hyper or hypo hydration.
So, I think we tend to think about this just as a dehydration issue, but coming up shortly in the presentation, we'll talk about the risks of overhydrating. And so, following these general recommendations may result in hydration as well as underhydration. And so, because the consequences of this cannot just be detrimental to performance, but also fatal, calculating sweat rates across a variety of conditions in a given individual and calculating an individualized sweat rate is a superior method to relying on these generalized recommendations. And luckily, this is actually quite simple and effective.
So, then how can you calculate an individual's sweat rate in a specific environmental condition? Well, one of the things that you can do is a really simple three-step process that starts with measuring an individual's change in body weight for a given training session or competition. So, first, we need to know how long the given training session was. So, for the example that I'm going to give you, let's say that we have an individual performing an hour and a half long training session. And before that training session, they weigh 61.7 kilograms. And after that training session, they weigh 60.3 kilograms. So, if we look at the difference, we can ascertain that about 1.4 kilograms of body weight were lost during that hour and a half long training session. And 1.4 kilograms is equal to 1400 grams. And this is important, not as a review of the metric system, but because one gram is equal in mass to about one milliliter of fluid. And that's really convenient considering that we are trying to express rate of fluid loss in a fashion that translates to rehydration practices. So, knowing that for every gram of body weight loss during the session, we can assume that one milliliter of fluid was lost, we could then assume that this individual, in the given example, lost about 1400 milliliters of fluid over the course of this hour and a half exercise session. So, we have a decent sense of how much fluid was lost. We also then need to account for any fluid that was taken in during the exercise session. So, let's say that we had the athlete drink using a marked water bottle, because we're trying to calculate their sweat rate and we want to account for any fluid that they intake during the exercise session. Well, we then might see that, okay, let's say the athlete drank about 420 milliliters of fluid over the course of that session. We're going to have to add that back into our calculations so that we can account for that in our sweat rate calculations. So, we've measured the change in body weight to get an idea of the amount of fluid lost. Now we want to take a look at how much fluid the athlete took in during the exercise session. And then, if the athlete urinated during the exercise session, or more likely, after the exercise session before you got a chance to take their post-exercise body weight, you also then need to account for the volume of urine and take that out of your sweat rate calculations. Now, if calculating sweat rate is a goal for that day's training session, and it's possible to do so, you might have the individual refrain from urinating until after you've taken their post-exercise body weight. If you do that, then you don't need to account for this third step. But if that does occur, then you would need to account for the volume of urine that was excreted before taking that post-exercise body weight. So, we have the amount of fluid lost, we have the amount of fluid drank, and then we have the volume of urine that needs to be accounted for if it occurred prior to the post-exercise body weight.
So, what can we do with that? So, let's take an example here, and let's imagine or let's say that this individual did use the bathroom before post-exercise body weight, and they excreted 90 milliliters of urine. So, we can then take those three numbers, and we can use those three numbers to quantify the absolute amount of sweat lost. And this is simply done by taking the change in body weight. So, remember, in this case, the individual lost 1400 grams of body weight. So, we can assume that they lost 1400 milliliters of fluid. We then add back in the amount of fluid that they drank during the session, so we're adding in the 420 milliliters of fluid and taken. And then subtracting any fluid that was lost due to urine before the post-exercise body weight was taken. And so, in the specific example that we've outlined step by step here, this would equal about 1730 milliliters of sweat loss, or 1.73 liters of sweat lost. Now, it's important to remember that that is relative to the time of that exercise session. So, the individual lost 1730 milliliters of sweat over the course of one and a half hours. But if we want to calculate their sweat rate in, say, like liters per hour, we want to take that sweat loss, the absolute number, and we want to divide it by the total amount of time exercising. So, if we take that 1730 milliliter absolute amount of sweat loss and we divide it by the one and a half hours of activity performed, we would then be able to determine that this individual loses about 1153 milliliters, so just over one liter, or 38 ounces of fluid per every hour of activity at that intensity in those environmental conditions. So, it can be relatively simple to calculate sweat rate. You clearly would want to do this in a variety of different environmental conditions, environmental conditions that mimic the events that the individual was training for would be some good guidelines for this, and also at similar exercise intensities. But nonetheless, you're able to do this, you're able to get a much more individualized target for how much fluid needs to be ingested during exercise to maintain hydration at a level where performance decrements can be avoided.
Now, we've discussed the risk of dehydration pretty extensively, but what about the risk of overhydration? Now, in order to take a look at this, I'll turn our attention to a series of events that occurred at the 2002 Boston Marathon. Now, this series of events came into light in the 2002 marathon around the bottom of a portion known as Heartbreak Hill. This is a pretty steep incline on the course that takes place pretty late in the race, somewhere around like the 20th mile, where things are starting to get a little hairy from a metabolic standpoint and fatigue is really starting to set in. So, that's where it gets its name, Heartbreak Hill, as it's been the site of peril for many, many individuals who have run the Boston Marathon. But in this particular day and location, what happened was two runners, two female runners, Cynthia Lucero and Bridget Jones. Both of these runners were not qualifying runners. They didn't run the marathon because they met the criteria in qualifying time. They were more recreational and were running the race through other means, which you can do it, which is to donate or raise a significant amount of money for charity and run for the cause. So, they were both doing this. And in this particular scenario, around the bottom of Heartbreak Hill, both these female runners appeared to be in trouble. And minutes after passing Heartbreak Hill, they both collapsed and were rushed to Massachusetts General Hospital.
Now, upon arrival at Massachusetts General Hospital, both of these individuals were experiencing similar symptoms. They were light-headed, they felt fatigued, they felt nauseous, and they were reporting some incidents of muscle cramping and or muscle weakness. So, by and large, they were presenting with similar conditions. And you might have expected them to be in similar condition, considering they just performed the same physical task. Now, given the similarity in their symptoms, they were also administered similar treatments. They were given fluids via an IV. After they were, a blood sample was taken to the laboratory for subsequent analysis to see what else may have been going on. And so, following this treatment, as they were being given fluids via IV, Bridget's condition began to improve, and within a few hours of the event, she appeared to be making a full recovery. However, unfortunately, Cynthia was not experiencing the same responses to the treatment. Her condition continued to worsen, and it eventually manifested itself into seizures. Her heart rate was extremely rapid, extreme tachycardia, and she was experiencing rapid drops in blood pressure. And this persisted for a couple hours until shortly after Cynthia was unfortunately pronounced dead. So, she didn't make it through this set of circumstances, whereas Bridget did respond to the treatment and was able to leave the hospital on that day.
So, the question becomes, if these individuals were experiencing similar symptoms and were given similar treatments, why did Bridget survive and not Cynthia? So, both patients were treated as if they were dehydrated. However, the results of the blood test indicated that these were two completely different scenarios. Unfortunately, by the time these results came in, it was too late, really, to do anything about it and turn this around. So, let's start by looking at Bridget's blood work. Now, she clearly exhibited signs of dehydration. And this is evident in the blood work, given that if we look at her electrolyte levels, her serum sodium levels, her potassium levels, and chloride levels. Now, these are expressed as concentrations, so millimoles of each one of these electrolytes per liter of blood. And we reference those to what would be a normal range. It appears that her electrolyte concentrations were above normal. And what this indicates is that she lost a significant amount of plasma that wasn't replaced, and therefore her blood was more concentrated. So, the treatment that they administered worked to correct this. Number one, because all they did was give her water, which drove down the concentration of these electrolytes in her blood to more normal levels, and then she started to feel better. But also, we'll see later on in this presentation that the body has some built-in protective mechanisms that are pretty well suited to helping someone make it through this type of situation.
If we, on the other hand, look at Cynthia's electrolyte concentrations, we can see that her sodium and chloride levels were well below the normal limits, and that her potassium levels were also low, albeit within the normal limit. And now, given that she was treated as though she was experiencing dehydration, this treatment probably didn't help and likely made the scenario worse. Because in contrast to Bridget's blood, these low concentrations of sodium, potassium, and chloride indicate that her blood was more diluted. And by giving her more water, you further diluted her blood. So, the treatment that potentially played a role here, but that's not all to blame of the doctors in this scenario, because as again, we'll see, the body doesn't have the same protective mechanisms in place in order to deal with this scenario as it does the scenario that Bridget was undergoing. And we'll take a little bit more of a look at the physiology behind that.
And so, what this really comes down to is again, the electrolyte content of blood, and this is known as osmolarity. So, osmolarity is the number of electrolytes, sodium, potassium, chloride, per unit of fluid, so the concentration. And in order to maintain blood volume and blood pressure and contraction gradients for sodium and potassium, etc., osmolarity is maintained physiologically in plasma in a really, really tight range of about 280 to 300 millimoles per liter when you add together all three of the electrolytes that make up the majority of the electrolyte content in blood. So, we want to keep this in a really, really tight range. And so, how do we do that? Well, we do that via osmoreceptors that sense changes in blood osmolarity. And we also attempt to do this via receptors in arteries that are sensitive to changes in blood pressure and blood volume. So, as blood is making its way to the brain, primarily in the hypothalamus, there are sensors in the hypothalamus and in the arteries leading up to the hypothalamus that sense changes in blood osmolarity and changes in blood pressure and volume, so that these things can be adjusted to keep osmolarity in the physiologically optimal range for functioning.
And in response to changes in osmolarity, as I sort of alluded to, we're actually really well equipped to adjust the changes in osmolarity as a result of losing fluid. And this is what's usually going on during most exercise scenarios, and what certainly seems to be the case in Bridget's case, and why she potentially was able to make it through this. And what happens is, as you might imagine, as you sweat, your blood becomes more concentrated, and therefore your osmolarity goes up. And this is exacerbated even further by the fact that in general, sweat is pretty hypo-osmotic. And what that means is that when we look at the content of sweat, we lose more fluid than we do electrolytes. So, by doing this, this is going to cause a higher concentration of electrolytes or osmotically active particles in blood, and an increase in osmolarity. Now, as a result of this, what ends up happening is plasma volume also goes down, right? Because we're losing fluid, and blood pressure also goes down. So, we lose fluid, osmolarity goes up, plasma volume goes down, and blood pressure goes down. And so, these are all three things that are sensed in the hypothalamus. And when these are sensed in the hypothalamus, what we do is we then signal, the signal from the hypothalamus is sensed in the blood that makes it to the brain. It senses the high osmolarity, the lower blood pressure, the lower blood volume. And when all of this input into the hypothalamus is there, the pituitary gland then releases a hormone known as anti-diuretic hormone. And we can kind of see this axis depicted here in response to things that would cause a decrease in plasma osmolarity. So, we have elevated osmolarity, lower blood pressure, lower blood volume, all being sent, sensed centrally. And then that message is being passed to the pituitary, where the release of antidiuretic hormone will then bind to the kidney. And when that happens, it causes the kidney to reabsorb sodium. And when we reabsorb sodium, because sodium is an osmotically active particle and the primary electrolyte found in extracellular fluid, well, then water will follow that sodium. So, as a result of that water being reabsorbed because of the reabsorption of sodium, that's going to serve to lower the osmolarity, increase our blood volume, and subsequently increase blood pressure. So, we have a pretty solid system and fail-safe that will help us stave off the effects of driving osmolarity up and plasma volume and blood pressure down when we're losing fluid during exercise. So, we have some protective mechanisms, albeit they won't keep you alive for very long if you don't replace fluid, but the body is pretty good at regulating this in this direction.
In contrast to what was going on with Bridget, clearly Cynthia was undergoing a condition that was the result of overhydration. And by overhydrating, as you might imagine, what you are ultimately doing, and what was made worse by the administration of intravenous fluid, was that you ultimately will lower the blood osmo, or the molarity. And this condition of low blood osmolarity is known as hyponatremia. Natremia is the word natremia means the presence of sodium in blood. And so, hyponatremia means there are less than optimal levels of sodium and electrolytes in blood. So, that based on the blood work that we saw from Cynthia's lab tests, seemed to be what was going on. And if we look at studies that have looked at the incidence of hyponatremia during endurance-related events, this actually happens more than you might think. And the increasing number of athletes that are experiencing this are usually experiencing this because they're drinking too much water or too many sports drinks during a longer duration endurance bout. So, by doing that, they're diluting their blood, they're lowering their concentrations of blood electrolytes, and therefore posing the challenge to the body to then lower osmolarity back down to physiological range.
And so, why does this happen? If we have a system in place that is meant to regulate blood osmolarity, how come Bridget was able to overcome this, and although Cynthia was not able to overcome her more diluted blood? And the answer to this is because of how these systems developed from an evolutionary perspective. If you can imagine, in the early days of mankind, we very seldom, if ever, ran into a situation where there was an abundance of fluids to the point where we would have been able to consume enough fluid to drive our osmolarity down to levels where we would need some sort of feedback mechanism in order to re-equilibrate that osmolarity. In contrast, what was an evolutionary issue was that dehydration occurred frequently when fluid availability was limited. Thinking about the fact that fluids were not as available to our ancestors as they are to us, they had to travel in search of fluid for many days at given times. And evolutionarily, this system, the system of sensing elevations and osmolarity and lower blood volumes and blood pressures, evolved in response to the need to maintain and protect against dehydration when fluid availability was limited. So, the system was kind of designed to deal with elevated osmolarity, but it's not sensitive to a rapid decrease of blood sodium or hyponatremia. So, when this happens, it can be very difficult to turn this situation around physiologically.
So, what can we do to sort of avoid hyponatremia or lower the incidence of hyponatremia in these athletes? Well, one thing that we obviously have talked about already at this point is the need to calculate rates of fluid loss so that you can estimate an appropriate level of fluid replacement during an event. Again, this is not just meant to prevent dehydration, but potentially, in this case, had these, and had Cynthia had a good idea of her sweat rate over the course of over the course of this type of exercise for this type of duration in that type of environment, she may have been able to have a better idea of how much fluid she should be drinking over the course of the race to avoid this type of thing from happening. But the other things that we can do and are identify some of the risk factors for hyponatremia. And one of the factors that is consistent across a lot of the cases that have been observed in endurance athletes are those that are performing the event at a more recreational or slower pace. And because this has happened a few times in the marathon, one of the things that has been sort of a red flag is an individual that is going to be on the marathon course for greater than four hours. Is going to have enough opportunity to drink fluid during that time at a rate that may put them at risk for hyponatremia. So, because they're out on the course longer, they have more opportunities to ingest fluid over the course of the race, and this may exacerbate the risk of ingesting too much fluid. The second thing that you might notice are individuals who are drinking a lot of water and stopping at every single aid station in order to drink again. This may be coupled with being on the course for longer periods of time, but in general, most individuals should not need to stop at every single aid station at every single mile marker of an event and drink water or sports drinks. So, this could be a red flag for someone who might be at risk for hyponatremia. You could see individuals who, you know, might be out on the course for a little bit longer, and individuals who are stopping a little too frequently to ingest fluids. And then, in terms of identifying this after the fact, if somebody is exhibiting some symptoms and it's hard to tell whether or not it's dehydration or hyponatremia, one of the first things that you can do to identify this right out of the gate is again, get a post-race body weight. Now, that involves also having a pre-race body weight, but in order to assess this quickly, you could simply look and see if the individual has gained weight. If the individual is experiencing dehydration, as we've established, they should weigh less than they did before the race. However, because the symptoms of hyponatremia might mimic those of dehydration, if an individual has gained weight over the course of this type of event, well, then you might point towards hyponatremia before dehydration. And this might be helpful in helping the medical staff make more informed decisions about how to treat that condition and save that individual's life.
So, we've discussed recommendations for pre-exercise hydration, for during exercise hydration, and now we want to draw our attention to post-exercise rehydration. And it may seem obvious what one of the main goals, or the primary goal, is of post-exercise rehydration, which is to correct any fluid lost during the session or event. So, we can quite simply think about this as, again, a good, useful place for pre and post-exercise body weights. We want to replenish the body weight up to pre-exercise values during this rehydration period. So, that's the primary goal, is replacing any fluid that was lost or any differential from the exercise session. And then there may be a couple of other goals of this rehydration period. One could be to replenish glycogen stores. So, one way that we can kind of potentially kill two birds with one stone there is by drinking sports drink. By drinking a sports drink, we are seeking to replace the fluid that was lost, but also because the, as we know, commercially available sports drinks primarily consist of high glycemic index carbohydrates, they also make a good candidate for glycogen re-synthesis during this critical time period. So, that might be one reason to choose a sports drink over water. If glycogen stores aren't necessarily a concern, then water may be just as effective in that particular scenario.
Now, another thing that we might need to do if we're trying to speed up rehydration, or if dehydration has occurred to a significant level, is additionally intake some electrolytes in order to augment that rehydration process. And so, this can be really simply done by adding some sodium chloride, which is just general table salt, to water or sports drinks if the electrolyte content of the sports drink is not quite enough to restore those electrolyte levels. And in this case, there have been several research studies where they've looked at adding sodium chloride to rehydration formulas. And in general, about 0.3 to 0.7 grams of salt per liter of fluid ingested is enough to speed rehydration without increasing the risk of hypernatremia.
Now, when we think about the situations that we have discussed, specifically relative to those at risk for hyponatremia or overhydration, we could probably say in these situations that sports drinks would be preferable to water. And that's because, via, if we have the situation where an individual is performing for a long period of time, taking frequent opportunities to drink, and potentially drinking too much fluid, by using sports drinks, at least they're getting some electrolytes and diluting their plasma probably less than they would be if they were just drinking water. However, that electrolyte content of sports drinks may not be enough to avoid hyponatremia if you drink too much of it, just because you're still ingesting more fluid than you are electrolytes in that particular case. But if avoiding hyponatremia and rapid rehydration is the goal, then you may choose a sports drink over water in those specific scenarios.
Now, with respect to the timing of rehydration, how quickly you should rehydrate or correct any fluid loss from the event is pretty well established within about two hours. And so, what you should seek to do within that two-hour period is to ingest enough fluid to correct that body weight, that body weight deficit. And then, if that is achieved within that two-hour period, just simply drinking to thirst for the remainder of the day should suffice in most situations. So, within two hours, ideally, you want to replace all that body weight that was lost. Now, if rapid rehydration is necessary, this could be for several reasons, but most commonly from an athletic perspective, this could mean that you need to rehydrate fully before another session or another game that's going to be performed in the same day. You can safely hyperhydrate and avoid hyponatremia if you drink 25 to 50 percent.
More than the sweat losses that were incurred via the previous training session over a four to six hour period. So, if that rapid rehydration is necessary, you can slightly overcorrect. You can do that safely with 25 to 50% more than you lost in sweat, as long as you do so over a relatively extended period of time. And what this also allows you to do is to compensate for some of the urine that you're going to lose during the rehydration period. Um, so up-regulating the amount of fluid to, um, to correct that urine loss will help, um, rapid rehydration if it's needed within the context of the same day.
So now that we've gone over hydration recommendations, I want to briefly talk about heat stress. And so, heat stress is going to be the topic of your research article that you're going to read this week. But one of the things that I want to discuss primarily about heat stress is the way in which you measure heat stress or environmental heat load. And I want to go out ahead and say to you that looking at just the temperature or the ambient temperature, say it's 80 degrees Fahrenheit outside, using just that number to represent the heat load is a poor way to judge the environmental impact. And if we ask ourselves the question, well, why, why is temperature not a good way to, to measure this? Well, if we think about it, it fails to account for all of the primary mechanisms by which heat is transferred or exchanged between the body and the environment. And so we discussed that we exchange heat with the environment via evaporation, radiation, convection, and conduction. Well, simply knowing the temperature outside doesn't really tell us much about all four of those mechanisms of transfer.
If we want to get a better idea, one metric we might look at is potentially the heat index. This is something that you'll see reported on the weather channel quite frequently. The heat index, and this is better than relying on temperature only, but it only accounts for the temperature and the relative humidity. It's a better metric, but it's not an optimal metric if we're trying to account for all of the mechanisms of heat transfer. What is a gold standard way to look at this is by looking at something known as the wet bulb globe temperature. And so this can be done, um, via a fancy wet bulb globe temperature thermometer. Um, they're not terribly expensive, but maybe not in everyone's budget to have a thermometer just for this purpose. Um, but one of the things that I thought would be interesting to provide you with the link to so you can go ahead and drop this down if you want to use this. This is a link to the National Weather Service's WGBT or wet bulb globe temperature map. And what you can do is you can go to this link and you can boil down to where you are on the map, latitude and longitude, and drop a pin to your exact or estimated location. And what it will do is it will look at not just the temperature and the humidity, but also the wind speed, the angle of the sun, and the amount of cloud cover so that it can estimate the wet bulb globe temperature and give you a better idea of what the incurred thermal load will be relative to the environmental conditions that you're going to be performing in. Um, so it accounts for all of these factors and it adjusts the temperature for humidity, wind speed, sun angle, and cloud cover.
And so there are recommendations and suggested actions for impact prevention using the wet bulb globe temp. Uh, and in general, if we take a look at these actions and the effects, there's really not much risk to performing in a wet bulb globe temp under 80 degrees. And so these are given in Fahrenheit again, they're adjusted temperatures. But once we go above 80 degrees, we see that at 80 to 85 degrees wet bulb globe temp, um, working or exercising in direct sunlight will begin to induce significant physiological stress on the body after just 45 minutes. So the recommendation for if you're going to be performing in these temperatures is to take at least 15 minutes of break during each hour of activity.
Now we can see how this sort of scales upward all the way to the upper end of the extreme where if there's a wet bulb globe temp above 90 degrees Fahrenheit, only 15 minutes of working or exercising outside will induce significant physiological stress. And therefore, 45 minutes of break each hour are recommended if you're going to be working or exercising in direct sunlight. So as you can see, this would make performing an endurance event pretty challenging and borderline controversial as to whether you should consider moving the date of the event or moving the time of day to the event to a time when the wet bulb globe temp is not going to be inducing significant amounts of stress in less than an hour's worth of work. Nonetheless, these are recommendations for how to deal with that if you do choose to exercise in these conditions. But the bottom line is, as the wet bulb globe temp starts to go above 80 degrees Fahrenheit, you need to start making some considerations to how and if you're going to exercise or compete in those environmental conditions.
So one of the things that is potentially life-saving, um, when supervising or exercising yourself in the heat is being able to differentiate, um, between two of the primary types of heat-related illnesses: heat exhaustion versus heat heat stroke. Now, as we can kind of see, heat exhaustion is the less serious and less life-threatening condition, but it is critical to be able to differentiate between these two in order to save someone's life that may potentially have heat stroke. And one of the things that can be challenging here is that they may appear similar. For example, nausea and vomiting is a symptom of both heat exhaustion and heat stroke. There are some key differences in the symptomology that can help determine whether or not it is heat exhaustion versus heat stroke. And the easiest ones to discern, um, I'm going to give you three that I think are the easiest and potentially most objective ways that you can quickly get an idea of what's going on.
And so the first one I want to draw your attention to, uh, is the presence or lack of sweat. So during heat exhaustion, the individual will still be sweating, and they'll still be sweating as you might expect in an effort to dissipate heat and lower their core temperature. As where during heat stroke, the individual will have ceased sweating altogether. And the reason the individual will cease sweating altogether is at this point, plasma volume levels have reached such critically low, um, levels that in order to prevent any further losses in plasma volume, the body just stops, um, sweating and trying to dissipate heat via that mechanism altogether. So the presence of sweat may actually be a good thing and indicate a less severe condition. But if the individual is presenting with symptoms and isn't sweating at all, you, um, can be safely sure that they may be at risk for heat stroke and you might want to treat them accordingly.
Now, the second thing that we can look at that is kind of a good dividing line is the differences in their pulse. And so what you'll notice here is that again, via the loss of plasma volume in both cases, the response to the decrease in plasma volume and associated decrease in stroke volume is going to be a rapid heart rate. So they're going to be tachycardic in either one of these situations. However, during heat exhaustion, the pulse may actually feel weak. So the actual, um, the actual magnitude of each heartbeat when you're taking their pulse may feel kind of faint and weak. As opposed to during heat stroke, the individual's pulse will actually feel pretty strong and pretty distinct. Now, what causes this is because in heat stroke, while there is still a significant amount of plasma volume lost, plasma volume levels have gotten to such critically low levels that in order to maintain cardiac output, one of the things that has to happen is the sympathetic nervous system sends signals into the myocardium, into the heart itself, to have the heart not only beat more rapidly, but also beat more forcefully. And this is to increase the amount of blood that is coming out of the ventricles with each contraction because plasma volumes are at such critically low levels. The heart is trying to get out every single drop of blood that it can in order to try and maintain cardiac output. As where this hasn't occurred to critically low levels yet in the case of heat exhaustion. So because of the decrease in plasma volume, you may still observe their pulse to be rapid, yet weak.
The third way, um, to look at this is to simply look at body temperature. So while these individuals who are experiencing heat exhaustion may, may be presenting with similar symptoms, you can take core temperature. And because in the case of heat stroke, the body is being, is completely inefficient in dissipating the heat that's being generated, um, they will begin to see some significant increases in core temperature or body temperature. And so the sort of dividing line for this, um, is a body temperature above 103 degrees Fahrenheit is typically indicative of heat stroke. And so being able to differentiate between these conditions is important, um, in determining what the methods of treatment are. In the case of heat stroke, you may simply be able to get the individual to a cooler place, get them to drink some fluids if they're conscious and able to do so, and do things like a cold shower or cold compresses. That should help them over time decrease their body temperature to a safe level. But if it's heat stroke, they have begun to reach the point of no return, and the only measure that you can take is to call 9-1-1. And while you are waiting for 9-1-1 to get there, you want to take an immediate action to try to cool the person as much as possible until help arrives. So moving them to a cooler place, compresses, doing anything you can to help them last until emergency medical services gets there.
Now, I want to finish this by going over some recommendations for performing exercise safely in the heat if you indeed do need to perform in challenging environmental conditions. And the first thing that we'll see and a lot of what I'm going to show you now is going to show up in this week's research article is acclimating to exercising in the heat can help reduce the rise in core temperature that occurs when performing in these challenging environmental conditions. And so what can you do in terms of acclimating to exercising in the heat? Well, it might go without saying, but one of the things that you're going to want to do is you want to get some practice exercising in environmental conditions that are similar to those that are going to be experienced during training and competition. And as far as the time course for this goes, typically you need about five to ten days of training in the heat with proper hydration if you want to reap the benefits of acclimation. And of course, depending upon how extreme the environmental conditions and the athlete's prior level of acclimation, you want to begin these acclimating processes as close to the event as possible. And the reason for this is because if you perform acclimation too soon, the benefits of the acclimation actually begin to decline. So after one to three weeks of performing in exercise in the heat, if you do say five to ten days of heat training prior to an event, if you wait a week to three weeks, you start to see significantly decreased benefits of acclimation. So that's key is to make sure that the acclimation is occurring as close to the event as possible. And of course, regardless of whether you're acclimated or not, the only way you're going to get a benefit of acclimation is if proper hydration practices are performed in the days leading up to and during the performance. So if you come in dehydrated or you don't drink enough fluid during the actual performance to make up for some of that fluid loss, it's not going to matter whether or not you are acclimated, you're still going to have the same challenges as a result of dehydration. So hydration and timing are key if acclimation is going to be effective.
Beyond acclimating, there are some other things that we can do to help avoid some of the potentially nasty outcomes that come from exercising in the heat. One of the things that you can do is check your hydration status daily. If you're a coach or a trainer, or even if you're employing these practices on yourself, check the hydration status daily before allowing them to continue practicing or playing in the heat. If you have lost too much fluid to indicate that you're well hydrated, don't perform until you have corrected those fluid balances and continue to monitor hydration status accordingly. Um, something else that can be hugely problematic or unhealthy weight cutting practices that seek to induce rapid dehydration in weight cutting sports. So we see this a lot in wrestling, in figure skating, in sports where either there are weight classes or there are aesthetic components to the performance. We see that one of the practices that individuals use are things like exercising in steam rooms and saunas, wearing plastic suits so that none of their sweat evaporates and they continue to sweat, doing unhealthy things in an effort to rapidly dehydrate and and make weight or achieve an aesthetic. This is going to exacerbate these issues and it's definitely something that should be avoided altogether if possible, but should absolutely be avoided if performance is going to occur in hot environmental conditions.
Something else that might be of use, especially in a game or performance scenario where you're not going to be able to avoid, um, these types of environmental conditions, is to check hydration status between halves and quarters, um, if that's possible. It's going to be a little bit more difficult in a continuous event for time, uh, but if there is the opportunity to check hydration status, um, via body weight changes or urinalysis, um, during the actual intermissions or during a game itself, then that might help avoid any dehydration problems before returning to play. Of course, you want to make sure that you're not just relying on the environmental temperature and that you're checking the wet bulb globe temp. And if the wet bulb globe temp warrants, you might consider moving your practice or training to cooler times of day or if it's too extreme, you may, you may consider canceling an event or a practice session or a training session altogether.
Monitoring fluid intake, not just knowing exactly how much fluid should be intaken, but engaging in practices that will allow athletes to monitor, easily monitor how much fluid they've taken in over the course of a training session or a performance is key. So this is why you'll see a lot of athletic trainers marking water bottles or using water bottles that have volume markers on them. This helps the athlete track how much they're in taking during a bout of exercise or performance and then establishing a plan for how much they should have drank at given intervals within a bout of exercise or performance are absolutely critical to maintaining hydration and making sure that they're able to track and employ the practices that you're helping them to establish. And then of course, we always want to make sure that if we are using sweat rate calculations to estimate fluid intake during exercise, that the sweat rates are assessed in similar environmental conditions, um, in which the performance or training would occur. So if you're going to be performing, um, at a wet bulb globe temp of 85 degrees, you might want to know what your sweat rate is from exercising at a similar intensity in that similar environment before performing so you'll know how much fluid to intake during that actual bout. And then of course, this can be very difficult to do, but if possible, being able to assess core temperature in athletes during performance to make sure that they are not inducing or incurring too much thermal strain is going to be a really effective means for preventing heat related illnesses and unfortunate outcomes that can result from improper hydration, especially during exercise in the heat.