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Fluid intake, dehydration and exercise: Part IV
Why waiting until you are thirsty is NOT too late
We really hope everyone is enjoying this series so far. It is proving fun and challenging to write, and we hope that is coming across in the posts. So far we investigated the history of fluid ingestion in Part I, demonstrated why it is the metabolic rate that predicts temperature in Part II, and weighed up the strengths and weaknesses of the lab-based and field studies in Part III. For Part IV we will look at the thirst mechanism and why waiting until you are thirsty is not "too late."Myth busting: If you wait until you are thirsty, it is too lateHow often have you heard this? This is an oft stated mantra of athletes, coaches, and arm-chair quarterbacks everywhere. But where did this concept originate? In 1965 John Greenleaf did a study on four well-trained men to examine how much water they would ingest during exercise in the heat. The title was "Voluntary dehydration in man," and is the first reference to the finding that when given ad libitum access to fluids---that is, when we drink to thirst---humans do not replace 100% of their weight losses. For those of you who have read Part II and Part III, this should be no surprise, since in those posts we introduced the concept that weight is not the regulated variable, and therefore your body does not care how much weight you lose during exercise. This "thirst is bad" guide stuck, however, and some time later you were introduced to the mantra above: "If you wait until you are thirsty, it is too late."
What is it too late for?The argument is that by waiting until you are thirsty, you are already dehydrated. This argument has been perpetuated because you have been led to believe that weight losses equal body water losses. However, even in a class lab we performed recently, our volunteer cycled for just over two hours. During that time he burned nearly 300 g of carbohydrate and fat while ingesting water ad libitum. His weight losses, or "dehydration," were 1 kg. Yet a whole 30% of that "dehydration" was not water at all and instead represented fuel that he burned. Let us say that again---the weight loss method overestimated his "dehydration" by 30%. So the take home message here is that the body weight losses grossly overestimate the fluid losses, and when someone is said to have lost 4% of his or her body weight, at least 10% of that or more will be fuel that has been burned during the exercise.
The thirst mechanism - a well-oiled physiological machine
The reality of the situation is that humans (and mammals) have very well-developed and successful mechanisms in place to help conserve and maintain their fluid balance, although the sports drinks companies have informed you otherwise. As we have said, the body is not concerned about body weight, but rather the concentration of the body fluids---otherwise known as the osmolality, and here is how it works.
Incredibly small increases (1%) above the resting value (280-300) first will trigger the release of anti-diuretic hormone, or ADH. Its job is to keep you from losing any more water in the urine. It has a profound effect so that even small amounts of ADH produce a maximal effect---that is, it is not possible for you to produce any less urine. Next, if ADH does not do the trick, as is the case when you are exercising and sweating, your thirst kicks in. Again, this occurs at a very marginal (4% or less) elevation of the osmolality. The effect is that we seek fluid, drink, and some time later the fluid gets in to the blood and dilutes it back down below the thirst threshold. This cycle continues indefinitely until you stop excreting fluid (i.e., sweating) and restore your osmolality once and for all.
So in fact humans have a very acute sense of when it is important to drink fluid, and it does not take much to stimulate us to seek water. Thirst is a very deep-seated, physiological desire for water, and it has been shown again and again in lab studies to effectively defend the osmolality.Why is the osmolality so important?The reason the body does not care about weight losses and instead "defends" the osmolality is that this concentration of the body fluids is what keeps the fluid balance between the cells. We have fluid both inside and outside the cells, and under normal conditions, the osmolality maintains this balance. The following two changes are possible:
- The osmolality can increase outside the cells. This will cause the fluid to leave the cells. Because this is undesirable, the ADH and thirst mechanisms explained above kick in and we correct the change to restore balance (homeostasis, in physiology-speak!)
- The osmolality can decrease outside the cells. If this happens, then fluid will move into the cells. Similarly, the body will initiate a sequence of responses, including the release of other hormones (aldosterone, for example) that we won't go into here.
"My sweat tastes salty"
Yes, it certainly does, and that is because it does contain some sodium. However it contains profoundly less than the fluids in your body, and is still mostly water---body fluids have a sodium concentration of 140mM while sweat has a value of 20-60mM. Therefore when you remove a liter of sweat from your blood, it has much more of an effect on the volume compared to the solutes (sodium), and what happens is that the osmolality rises in response to sweat losses. This is absolutely crucial to realise - you cannot lose sodium, even if you are a "salty sweater", as Gatorade are now claiming. If the sodium content of the blood is dropping, it's because you're drinking too much water, not because you're sweating sodium!
In fact, a very interesting study was published in 1992 by Robert Cade, the man who invented Gatorade. His experiment took place during a marathon, and the groups of runners were given Gatorade, 1/2 Gatorade (half water, half Gatorade), or water. The really interesting finding was that the water group maintained their sodium concentration (a surrogate for the total osmolality) just fine, while the Gatorade group actually increaesed its concentration. In fact this explains why people drink more of a sports drink compared to water---the sports drinks keep your osmolality higher and therefore makes you thirstier. So instead of lowering osmolality, which is what your body wants you to do, the sports drinks raise it. Seems kind of counter-intuitive, doesn't it?The final word - Drinking to thirst optimizes your fluid intake
We hope it has become clear that, for a number of reasons, it is not necessary to drink so much during exercise, and in furthermore no one needs to tell you how much to drink. As we have shown you here, the thirst mechanism is highly sensitive and very successful at what it is meant to do: maintain your osmolality, not your weight. But the final message here is that when you drink to thirst, you optimize your fluid intake, and by that we mean your thirst will always keep you from drinking too much or too little. There is such a thing as both of those, but drinking to thirst will always prevent you from straying too far in one direction or the other.
In addition, who wants to carry around three Liters of fluid in a backpack when half that volume will be just plenty? And when there is no scientific evidence to support the claims that dehydration increases your core temperature or elevates your risk for heat stroke, it seems quite unnecessary. In fact, the concept that people are "dehydrated" while losing a few kg's is now debatable.
One last thing, is that as humans, we are regarded (by most, anyway) as the smartest animals, right? Yet for some reason, companies making fluids deem it necessary to inform you how much you should drink. Have you ever had to force your pet cat or dog to the water bowl? Have you ever seens signs in the wild pointing animals to the watering hole with instructions to drink before they're thirsty? Yet somehow, the Gatorades of the world have "discovered" the NEED to educate us all about fluid. It does strike one as patently ridiculous - thirst is good enough for every animal in the world, it's good enough for us...!
Looking ahead to next week
We really hope you have enjoyed this series! Next week we will focus more on running again as we preview the USA Men's Olympic Marathon trials and the NYC Marathon. It will be a week of running-related posts, so be sure to join us for the discussion and analysis!
See also:Part I: History of fluid intake and a conflict of interest Part II: Fluid intake, dehydration, and exercise
Part II: Fluid intake, dehydration, and exercise
Part III: Comparison of laboratory and field studies, and implications for fluid intake
39 Comments:
I ran 117 miles over a 24 hour period this past weekend. Temps were in the 50s F and there was a 11mph head wind as well as sun. I drink about 11 liters of fluid and took in at least 6 grams of sodium. I had swelling in my ankles. I weighed in Tuesday when I got home and was up 1 lb from before the run. I expected to lose about 5 lbs from glycogen depletion and fat burn. By Thursday, I lost 6 lbs of water weight.
I deliberately chose to drink no more than 16 ounces per hour, believing that's the max my body could absorb. In addition to waiting till I am thirsty, should I wait till I crave salty foods before taking electroyte capsules or salted potatoes? When do I replace water with sports drinks, particularly the higher sodium Endurance Gatorade?
After the race I did not feel thirsty but I was hungry for sweets.
Thanks.
Hi again Hung-kwong, and thanks for your continued support of The Science of Sport.
Well done on your ultra-running feat. One-hundred seven-teen miles is a long way to run in any amount of time, but in 24h that is quite an accomplishment.
I would say your first mistake was drinking a fixed volume of fluid. As you can see it clearly resulted in a state of fluid overload for you. The environmental conditions were mild, created the following situation:
1. Cool outside temps
2. 11 mph (17.6 kmph) wind
3. Low intensity exercise
These three things predict that you would have a pretty low sweat rate, even though we do not how much you weigh. Therefore ingesting 16 oz (about 450 mL) per hour as it turns out was very close or above your sweat rate, which is why you gained weight.
One question is how much were you urinating both during the run and then from Monday to Thursday?
We will have to do the metabolic calculations to work out how much fuel you burned. If you send us your weight, how fast you were running, and if the terrain was flat or hilly, we can make some estimates.
Yes, you should wait until you crave salty foods before you begin ingesting sodium. There is no evidence to show that sodium ingestion during prolonged exercise is of any benefit.
The one problem is that you need carbohydrates (energy) during a run like this, and sports drinks are a good way to get some of that in to your system. However, if you are running at a sufficiently slow running speed, which I assume you are if you are going for 24 h, then it might be much easier to eat solid food and ingest water or some beverage that you know you can tolerate well.
However, drinking to thirst is the real key here.
Kind Regards,
Jonathan
This last article conflicts with this web site, which states that an athlete is dehydrated by 1.5 to 2 liters by the time thirst sets in. He says thirst is a poor indicator while you say it is very good. Who to believe?
http://www.healthline.com/hlbook/nut-factors-affecting-fluid-intake
Hello Clyde
Thank you for your question and the link. I had a look through it, and I just have to say what a shame. I guess we can only invite everyone to review the evidence, to give it some thought and then arrive at their own conclusions, based on what we've tried to write over the last 4 posts in this series
But this kind of website, set up as credible, is providing, in our honest, scientific opinion, completely incorrect information. There is NO basis for the claims that are made, other than that fact that for years, everyone has been told that "dehydration is a killer" and you must drink, drink and drink before you get thirsty.
Just as an aside, the author of that article, Dan Benardot, is a Fellow of the American Collegr of Sports Medicine (ACSM). ACSM is an organization that has received SUBSTANTIAL FUNDING FROM GATORADE - read Post I of this series where we spoke about the conflict of interests this creates.
What is more, if you look up Dan Benardot, you'll find that a lot of his research was in fact funded DIRECTLY BY GATORADE, through the Gatorade Sports Science Institute. Therefore, when he makes these statements, what you need to be asking is whether he is in any way benefitting from his opinion - financially, research wise, etc.? Of course, just because he's been funded by them does not mean his opinion is immediately discredited. But the trail back will almost always be very very revealing...
But to tackle the specific article, this attitude has unfortunately become so ingrained, that even "experts" are saying it. now, what we've tried to do is present the evidence in such a way that readers can make up their own mind. So we are not going to do the same, and tell you that you MUST follow our approach, because we'd prefer if you arrived at that point through careful consideration of the facts.
But the facts around thirst are pretty clear - it's a good guide. The problem with that article you've read is they begin with the assumption that dehydration is bad. In otherwords, their starting point is that dehydration, a loss of body weight of 1-2kg, is BAD. Once you begin at that point, then you set up a circular argument where thirst will be too late, because you find that if allowed to drink to thirst, people will lose some weight.
But the whole point of this article (number IV in the series) was to emphasize that the BODY DOES NOT CARE ABOUT WEIGHT, it cares about osmolality. And studies in rats and humans have all shown that thirst responds very rapidly to changes in osmolality. Therefore, the correct scientific argument is that thirst IS the best guide.
Finally, I just have to emphasize the point. I don't know if you have pets, a dog or a cat maybe. But honestly, have you ever had to force your dog to drink? Ever had to remind a cat that it better drink because if it gets thirsty, it's too late? Of course not! So why treat humans differently. The answer, of course, is that we have money, and so companies want that money and thus instruct us that we're too stupid to know when to drink. I'd be offended at that approach, frankly.
Thanks again for bringing up that article - I can assure you there are thousands like it on the net, and hopefully, we've presented the counter-argument that eventually gets people to objectively consider their position.
Best wishes!
Ross
Thanks. Just to clarify, that is an excerpt from his book, Advanced Sports Nutrition, which was published in 2006 by Human Kinetics. It appears that the entire book is online at that site. Haven't read the whole thing, only a few pages in chapter 3.
Sorry, a couple more questions. You've focused on body weight and core temperature and all of that makes sense. But at what point does a decrease in blood volume affect performance? If you sweat a liter, does all of that come from blood plasma? If so, does the thicker blood have a harder time reaching and fueling muscles?
If I feel recovered, I will be running the Marine Corps Marathon on Sunday. I will wait till I am thirsty before drinking. I won't take any electrolyte capsules unless I am craving salt.
Should I top off with fluids right before the race?
When I do get thirsty, do I just take one 4oz cup or grab 3 or 4 cups? I assume that there's some time lag before the body tells me I am no longer thirsty.
From what I can recall of the 24 hour run, I was only thirsty in the afternoon sun. I think the headwind on 50% of the course had a great chilling effect that substantially reduced the sweat rate. I urinated twice in small quantities in the daylight hours and more 3 or 4 times at night. For 24 hours after the race, the urination amount was small and on the dark side. Tues through Thursday were clear and in large amounts. I think having some alcohol as a diuretic helped.
Pre-race, my weight was 151 lbs. I averaged a 12:10 pace on flat ground. Friday morning I was 145 lbs.
Even while I was taking 350mg sodium per hour, I did crave potato chips in the late afternoon. I craved fat later on -- I ate grilled cheese sandwiches and pepperoni pizza while running at night.
It's interesting that for the past couple of years, I had worked on a goal of a 100% replacement of water, electrolytes and calories during a race. Now I am comfortable that I will be fine with only a partial replacement during a race. The goal now is to absorb about 200 calories per hour to maintain blood sugar and the rest will take care of itself.
Is there a point when the thirst mechanism or the hunger triggers for salty foods and fat fails, where I should be drinking and eating on a timetable/cnecklist? Can I be in a state of confusion where I will not know that I am thirsty? Could this happen in a 48 hour event or in the Badwater 135 mile ultra through the desert?
Thank you for answering my questions.
You guys rock! I recently discovered your blog and love your candid, fact based approach in presenting information.
Okay, you've convinced me that the high sodium in most of the commercial products is not necessary and may in fact be counter productive.
What do you recommend for getting calories when you're running fast enough that your body won't tolerate solid foods?
Hi RunEmoryU
THank you for the kind words and positive feedback, much appreciated
The question you ask is interesting, and it's something I was going to tackle in a post in the coming week. You see, the companies making fluids have missed a trick when it comes to marketing. that is, they have focused on the fluid requirements, leading to the increase in hyponatremia related deaths, when in fact they might have gone with the energy angle a little more strongly.
From a scientific validity point of view, making the claim that "your body needs glucose energy during exercise" is quite appropriate. But as we've seen, the claim that your body needs X ml of water per hour is NOT. So the marketing angle adopted very early on has led us to this point now, where the sports beverage companies are now suddenly throwing in extra salt, claiming that people are salty sweaters etc. As we've discussed, this is also a dead-end street as far as science goes.
But what these companies might do, and would, in my opinion be well-advised to consider, is to push the energy angle, because a sports drink containing say 7g glucose per 100ml is more than able to provide the energy needed during exercise, even if it is drunk according to thirst - most people will drink about 400 to 700ml per hour, which means they get 30 to 50g per hour, and that's enough for most.
But this is something we'll cover in a post, because I really think the marketing angle is an interesting one.
Thanks again!
Ross
You've really outdone yourselves with this series. Excellent work.
I have a question that I hope you will address in future posts. I've tried before to find out about cramps. But there are three problems:
1) Nobody can tell me what a cramp actually is.
2) Nobody can tell me what causes cramps.
3) Nobody can tell me how to prevent cramps.
Do you guys have any insights? The only things I have been able to find are specious bits of advice like "take salt pills" or "eat potassium" or "drink more". But I can't find any scientific basis for any of this advice.
Stan
Hello Stan
Thank you very much for the kind words!!! I hope we haven't outdone ourselves, because then we might as well 'retire' - I hope there's a lot more to write and many more series to come!
Speaking of which, you have provided us with the inspiration for our next series, thank you! Muscle cramps it is! Another controversial one, with conflicting arguments and that's just what we like! Muscle cramps are interesting and we'll definitely tackle them.
So the plan will be next week, we'll be looking ahead to the NYC Marathon and the US Olympic Marathon trials, the week after will be post-race analysis and a few topical posts, but the week after that, muscle cramps it is!
So thanks for the hot tip! Hopefully you can be patient and then we'll get cracking on that muscle cramps issue. It's actually a complicated one that, but we'll do our best...in due course.
Thanks ahead for your readership, and keep those ideas coming!
Ross
In response to Hung-kwong,
I just now realized that the Marine Corps Marathon was yesterday, 28 Oct.
Apologies for not getting back to you before the run, but we are curious how it went. Perhaps you can share your experiences here in this Comments section?
"Drinking to thirst" means drinking to thirst. . .that is, I cannot tell you if that means one cup or ten cups, but if you listen to your body you will know when you are thirsty.
There is no evidence to suggest that the hunger or thirst mechanisms ever fail. It is possible, however, for you to become fatigued so much that you generally feel terrible and want to stop exercising. I suspect at that point people might feel as if the physiology has failed, but in reality it is working fine since it has stopped you from continuing to exercise.
The fact that your urine was clear and in large quantities definitely confirms that you were overhydrated.
I hope the marathon this weekend went well, and good luck with the continued training and running.
Kind Regards,
Jonathan
I ran Marine Corps dragging my right leg when I should have stayed home. It took 2 1/2 hours longer than I normally would have to complete the 26.2 miles.
I drank mostly Powerade and urinated 3 or 4 times in the first half. I was drinking about 10 ounces per hour in 56 degree weather and 21mph winds. Clearly I was over-hydrated. Did I confuse thirst with a craving for something sweet? I am thinking that hydration, calories and electrolytes should be taken in separately. If I drank plain water only, supplemented with gels, would I have felt the craving to drink more?
I was hungry and sought out food - I found sugar wafers, mini-bagels and cheese crackers. Running at the slow jog, I had no trouble digesting solid food.
Thank you for the excellent series on fluid replacement during endurance exercise. As you correctly pointed out, maintenance of blood volume and plasma osmolarity (a physiologic necessity monitored and controlled by our hypothalamus through neural and hormonal mechanisms) during a long exercise is a complex process. The sweat rate depends so much on factors like ambient temperature and wind speed that it varies tremendously from race to race. To advise a volume-based quantitative regimen of fluid replacement during race, therefore, is simply wrong. As a recreational runner, I have always used thirst as my guide of fluid replacement during exercise. In my opinion, this has not been emphasized enough in the media (until now). I often wonder, however, if perceived thirst can be ignored by individuals motivated to push their limits (I would not be surprised to hear this claim from Gatorade or other sports drinks companies). What do you guys think?
Hi Joe, and thanks so much for your support of The Science of Sport.
As with all things biological and physiological, we will see a normal (Gaussian) distribution. This means that most of the population (68%) will fall within less than one SD of the mean. A further 27% falls within two SD of the mean.
That is a lot of stats mumbo-jumbo, but the gist of it is that an overwhelming majority of a population are pretty close to the mean.
Therefore when we talk about thirst, for 95% or more of the people this mechanism works fine and is an entirely accurate way to assess fluid needs. Having said that, there are people who for whatever reason fall at the extremes of the curve, and for them their thirst might not be the most sensitive mechanism. It might be that they are more or less sensitive to changes in their osmolality and therefore ingest more or less than they actually need to.
Again, though, for the vast majority of the normal population thirst is 100% ok. One exception is the elderly population. the effect of age on the thirst sensitivity is well described and shows that as you age, you become less sensitive to increases in onsmolalioty. Therefore in this population drinking to thirst will mean that they do not ingest sufficient fluids.
With regards to elite or high-level athletes, I suspect it is not about pushing beyond thirst for them. Instead, it is likely that they tolerate different (higher) osmolalities, or even that they have a higher thirst threshold---that is, the osmolality at which they sense thirst is much higher compared to you and me. This allows them to lose more fluid before getting thirsty, this permitting them to do more work before requiring fluid.
Currently the sports drinks companies are focus on two points:
1) Thirst is wrong (that is, your own human physiology is incorrect!)
2a) Sodium losses are large during exercise, especially if you are a "salty sweater," i.e. if you are in the <5% of the population;
2b) Sodium replacement is necessary for both normal physiological function and performance
Currently there is no evidence to suggest that humans incur any meaningful sodium deficits during exercise, even when the humidity is high and the individual is sweating profusely. Again, the vast majority of people (think 95% of the population) maintain their sodium concentrations just fine even when exercising in the heat and sweating, and even while ingesting water to thirst.
Sorry, that is post almost by itself! But I hope that answered the question you asked. In short, one can ignore thirst, but the evidence does show that when performance is a desirable outcome, it will be hampered if you ignore this sensation to drink.
Thanks again for the comments and we hope to see more from you.
Kind Regards,
Jonathan
Dear Ross and Jonathan,
Really an excellent job. I especially like the common sense approach that you invoke. (Another area where I think this is overlooked is diet. When people want to know what they should eat, perhaps they should consider their teeth. Mostly borad, flat surfaces for grinding plants and a few sharp ones for tearing a little meat . . . but I digress).
So many different issues come to mind upon reading this series, and I'd like to share some of them with you and ask some questions.
First, I have recently seen some other literature speaking about the fact that drinking is not necessarily the cure all that it's been held out to be, so maybe we are once again due for a shift in thinking. What you say insofar as core temperature is concerned seems to be supported by the evidence you provide.
I'm a bit unclear on some of this however. If I've got this right, on one hand it seems as if you're saying core temperature depends mostly on work load and even stabilizes at a certain point. Yet some of the other science seems to suggest that core temp stabilizes not because it reaches a theoretical limit but because athletes slow down as part of an adaptive behavior to keep it in check.
If the latter is true, it seems the body encourages less work in the face of increased temperature. Since the goal in a race is to keep the work load up, wouldn't it be to one's benefit to use whatever means possible to bring down temp as much as possible, which it seems the ingestion of extra water can help to do?
Perhaps more importantly, it seems to me that heat disappation is just one of the concerns and perhaps a less thorny one at that. My understanding of the real problem with dehydration is that it leads to a loss of blood volume and that as blood volume decreases, cardiac output increases to keep up with oxygen demand. With this increased heart rate necessarily comes an increase in VO2 uptake, and I believe the problem is that there is an exponentially inverse relationship between VO2 consmption and the amount of time one can exercise at the associated pace.
This leads to another controversial topic: muscle fatigue and its cause. (For intance, I know that some current science has very much called into question the model whereby fatigue follows from lactic acid buildup.)
Regardless, there's no denying that one can exercise for much less time at 98% VO2 Max than at 70%.
So isn't the other crucial question when it comes to dehydration the plain and simple issue of its effect on cardiac output?
That said, since both hydration and the ability to know your hydration status are clearly useful on SOME (perhaps yet to be determined) level, I'd like to share with you what, for me, has been a fairly reasonable strategy for calculating true sweat loss. (Not a perfect approach, but a good first approximation.)
It begins with the fact that, over the period of a number of years, I've weighed myself both before and after countless runs. In each case, I take into account fluids added through drinking. I also have tried either not to urinate between weighings or have measured urine output so as to be able to account for it in my calculations.
Along with this data, I've recorded temperature and dress.
Adding to this survey are similar data points from what I will call training races, races leading up to a target race.
And what I've found, besides the obvious wide range of sweat rates, are some max and min rates that correlate to race dress as well as hotter and colder temperatures and varying speeds. (Humidity is obviously a factor as well).
Ultimately, when calculating a fluid plan for a race, I've been able to take this data, combine it with an estimate of the 2% loss that so many seem to suggest is acceptable, account for the loss through calories, and come up with a target rate for drinking.
In practice, the numbers have borne out pretty well. I admit that the approach is part science, part art, but again, the results seemed to have provided a very good first approximation.
In addition, there are a number of additional points about hydration that I'd like to share with you along with some questions. Please feel free to comment as you see fit.
First off, a lot has been made about the fact that every gram of carb comes complexed with three grams of water and that burning that fuel releases this water. Some scientists claim that such water counts towards hydration. At least one article I've read, however, (admittedly from Gatorade's Bob Murray) makes the excellent point that since only 8% of the body's total water is contained in the blood, only 8% of the complexed water––an insignificant amount––can be considered to go towards maintaining blood volume.
Second, I've read that at about 70% of VO2 Max––the point where most trained endurance athletes start from in an event––the kidneys all but shut down, making urine output/production a poor indicator of hydration.
In fact, I've used this phenomenon to my advantage at the marathon distance. Through experimentation, I've found that it takes me about 4 hours to stop uriniating after I drink a liter of water, with peak output coming at about 2.5 hours. By drinking a liter 4 hours prior to a start, then quitting, I'm able to toe the line fully hydrated but with an empty bladder. Neither the half liter I take immediately before the start nor any of the fluid taken on course produces the need to urinate since at 70% plus of VO2 Max, my kidney's aren't making urine. Great for avoiding bathroom breaks.
Thirdly, it's worth mentioning at least one good thing about weight loss through dehydration: a lighter "chassis" means that you go faster. In running, I think you get about 3 seconds per mile for every pound lost. At some point, this must clearly be eclipsed by the negative effects of dehydration, but it seems to me that this benefit should be considered in the equation.
Finally, another critical component of the hydration equation is gastric emptying rate (GER). My understanding is that normal emptying rate is about a liter per hour and that at 75% VO2 Max, this can as much as halve. In additon, the introduction of carbohydrate slows GER while increasing the volume of stomach contents increases it. Also, cold fluid supposedly emptys faster than warm.
Now in college bio, we always learned that inside the stomach was outside the body. That is, just getting water into the stomach isn't enough, it's got to be able to empty across the membrane and down into the intestine. So GER becomes the rate limiting step. Pumping in a large volume of cold fluid can up things a bit, but basically, your stomach will only transmit what it will transmit at a given exertion level and so to drink beyond a bit of sloshing is fruitless.
Sorry this is so long; I look forward to hearing your thoughts on all this.
Keep up the GREAT work!
Hi Ross and Jon,
First of all, I've just chanced upon your blog and absolutely love it. Great job you guys are doing. From the looks of it, don't expect the questions to stop anytime soon!
You've discussed that drinking sodium laden sports drinks will increase the body's osmolality relative to its cells, making one more thirsty to return the body to homeostasis.
How about sport drinks claiming to be isotonic? Plus, these drinks have essential electrolytes that help the refueling process. I'm guessing that compared to plain old water, the body does not need to do additional work to infuse isotonic drinks with the relevant electrolytes/sodium for ingestion. Therefore, isn't an isotonic sport drink useful in this sense? I'm using a powdered version of Gatorade for my Ironman training and I usually dilute it so that it tastes less "salty" than the bottled ones...comments?
Thanks! I'm officially a fan of your blog now..
Regards,
Shannon
Hi Shannon
Thanks so much for your positive words and feedback! Great to hear from regular readers...
To deal with your question, "isotonic" as you are aware means the osmolality of the ingested fluid is about 280-300mOsm (same as your plasma), and to get this from sodium only, you would have to ingest about 140mM of Na. The current Gatorade is at 18mM, so you can see how this poses an enormous problem for taste, if nothing else!In reality a sports drink can become "isotonic" through the use of carbohydrates (essential requirements, by the way), but this doesn't really deal with the sodium problem.
I do know that Gatorade brought out a new formula called Endurance, which contains substantially more sodium than the regular version - this might be what you are referring to? We should actually take a look at this formula in a future post and dissect whether it has any real chance of working...
The idea here was quite clearly to attack the hyponatremia problem by telling athletes that they needed to replace salt. So they attributed the problem to A LOSS OF SALT. We tried really hard in posts 3 and 4 to make the point that you don't become hyponatremic as a result of salt loss - it's a problem of water balance, not salt!
So what you have here is a case of two wrongs trying to make a right. Firstly, they advocated overdrinking, and then followed this up with advice to take in excessive salt. I was chatting with a colleague yesterday who has just done a PhD on this issue, and she reckons the next big problem we'll see is an athlete with NORMAL SODIUM LEVELS, but elevated plasma volume (and body weight) because they are drinking so much water.
The simple answer is to just drink less - you'll lose some sodium in the sweat, you can never replace this from a sports drink (unless you bottle your own sweat - interesting anecdote coming up later!), but it doesn't matter because you also deplete your fluid volume and the result is that your sodium level is pretty acutely regulated.
There are a couple of arguments for this use of salt to promote water absorption in the gut. They say it promotes the uptake of water through what is called co-transport - think hunting in pairs. And this is true, but again, you don't need it, because your body is telling you just what you need to do via the thirst mechanism.
Finally, one thing that I have noticed from being in the medical tent at the Comrades marathon in the last three years, is that one of the biggest complaints is that athletes feel nauseus as a result of drinking sports drinks for upward of 8 hours! They get to the tent and we offer them some Coke or Powerade, and the word is just about enough to make them ill again! So your problem is not unique. Whether it's the sweetness, the flavourants, the salt, I don't know. Probably a combination of all of them. But most of the athletes just want water, to wash away the taste
So for sure, if you can dilute it, go for it! Just check that you are not diluting the carbohydrates so much that you lose out on valuable energy - remember, you need the glucose - about 30 to 40g per hour. That is why a safe amount to consume is about 500 to 700ml per hour of regular Powerade, for example - it provides between 30 and 50g of glucose per hour.
If you dilute your drink, make sure you are making up for the carbs somewhere else, and you'll be fine!
Oh yes, I said I had an anecdote - speaking of the salt content of sweat being so much higher than that of Gatorade/sports drinks. The biggest company doing sports drinks in Asia is called Pocari Sweat - they were a sponsor of the Asian Games last year! I thought that was a great name - bottled sweat, I wonder what the formula was!
Anyway, hope that helps a little! And keep reading!
Ross
Hello Richard
Thank you so much for teh kind words, we're delighted to count you as a fan.
Thanks for the very detailed and thoughtful question. I must apologize for the delay in replying to you, but we've both had incredibly busy times of late, and so we squeeze in a few minutes here and there to respond. But your question would be poorly answered in a few minutes, so we thought it better to wait for a gap and do it justice!
It's no problem, because we value the contributions made by readers. But I'm going to say that we'll get to it in a little while - when there is more time to give it the time it deserves!
So perhaps check in about a week from now, or maybe sooner, and we'll hopefully have gotten a reply out!
Thanks for understanding! Keep reading!
Ross
Thanks for checking in Ross. I understand completely; no rush. I look forward to hearing your thoughts.
Great series of posts. Not sure how I wound up at them though :)
Been into sport at different levels of intensity and competition - although never elite (national rep or above, for want of a definition) - for around 40 years now.
Also quite interested in sports physiology and medicine generally.
Lately I've been into road cycling. About 75 km for my standard ride, takes about 2.5-3 hours, with traffic lights providing stops along the way.
Also been running for around 30 years, variety of distances - but standard run around 10-12 km and occasionly double that.
I find in warmer weather (say 30 C) my sweat rate is around 1 l/h with my natural drink/thirst rate about half that. So my weight loss is around 0.5 kg/h and I weigh 75-80 kg. All broadly in line with the 2%-5% weight loss.
Some specific points and questions:
1/ I find if I try and drink more whilst actively exercising I seem to slosh too much. However, my thirst response when stopping or finishing seems to recover quite quickly and I usually nearly drink the weight loss back quite quickly - say within 30 min. Been a consistent pattern for years, maybe I've just learnt it, or grativated to it.
Is this a commonly observed pattern for athletes rehydrating to 'normal' weight?
2/ What is a typical figure for a body's fluid absortion rate (is that the GER Richard mentions?)
3/ I find I 'feel' a core temperature drop when I have a drink whilst exercising. And the drop 'feels' bigger the colder the fluid, all the way to O C ice/water mix. I dont go out of my way to try and drink more to drop temperature, but it does seem to have something of an immediate relieving - and boosting - effect, although not that long lasting (say 5-10 mins, but can have another drink of course). I seem to be core/body temperature limited in warmer weather (say 30+ C, and higher humidity in particular).
So my question here is that if the fluid being ingested is (significantly) colder than core body temp, doesnt some physics say heat will be conducted away from the body to heat it. Sure if it is just a balance between the say 75 kg of body mass vs say the 0.5 kg of cold drink, then it aint going to go far, and it wouldnt explain the 0.5 C difference in one of the studies you comment on, but maybe it hits the 'hot spots' or something.
I know there have been various experiments with actively cooled jackets for athletes where the extra weight isnt so much an issue (rowing?). Not sure but I had a vague idea they may have been banned.
4/ Whilst I sweat around 1 l/h I have a friend also into cycling who seems to only sweat minimally - certainly no visible signs whereas I will be drenched with rivulets running down my face. Is there some measurements of athletes sweat rate and variation along with other variables - temperature, humidity etc?
5/ How does one best go about self measuring core temp? Some euphemism will do if it simply amounts to thermometers and orifices :) What about little swallowable instruments? Or imaging (seen those at different times).
Already too long a post, but I was recently reading about bicycle helmets and research into their design and cooling versus protective tradeoffs. Mainly because on hot days I seem prone to a sort of head boilover. In the mountains when going uphill (10 km/h) I will often need to take the helmet off ...
Geoff
Great site !
I find it very interesting that you mention a colleague that suspects that the next problem in electrolyte management in endurance sports will be eunatremic hypervolemia (normal sodium levels with increased plasma volume) from the ingestion of sodium supplements with too much water.
I am running in the Marathon des Sables this spring, and a common problem there is extreme foot swelling. This is variously attributed to 1) heat, 2) too much running, and 3 )not enough salt.
It is my suspicion that the opposite infact is the case - the footswelling is in part due to too much water and salt.
I am very interested in hearing what you have to say about the issue of decreased plasma volume and cardiac output - to me, this might possibly be a reason to stay somewhere near euvolemia.
William
You say that the body encourages you to drink to maintain osmolality, and that sweating out large volumes of water doesn't require intake of large volumes because the osmolality doesn't change, or the sodium concentration even increases. If this is the case, why would you ever feel like drinking enough to replace the lost fluids? Does your body return to a set-point of fluid content after a race, and if so, how does that work if osmolality dictates thirst?
Hi Steve
Thanks for the questions. I'll get to the other one a little later...
As for this one, the thirst response is primarily dictated by the increase in osmolality that results from the loss of hypotonic sweat. In otherwords, your osmolality rises as a result of sweating, stimulating the thirst mechanism. From our data at the finish of major marathons and ultra marathons, we know that most (about 80% or more) people finish the race with HIGHER sodium contents, but within about a hour of the finish line, they have, driven by their thirst, replaced most of the fluid that was lost.
In otherwords, you do replace the fluids, because the thirst mechanism is driven by the increase in osmolality that occurs when the sweat is lost. But it does so in its own time, because that's what the body is monitoring. We can't stress enough that the body is not monitoring weight or fluid content - it's osmolality all the way.
And then after you finish, the raised osmolality, which is the normal response after exercise, continues to stimulate the thirst response until fluid homeostasis is again achieved!
Thanks!
Ross
I found these posts an excellent read and a different perspective from what I've read so far. From what I understand, the summary is that the body attempts to regulate osmolality and core temperature. Natural physiological responses like thirst are the best way to react to variations of these variables during exercise.
A couple of points which this article does not touch upon - relation of water loss to cardiovascular drift and strategies for ensuring maximal performance during exercises lasting over 12 hours.
The first point about cardiovascular drift. The following page (belonging to another exercise physiologist who also has a Ph.D)
http://home.hia.no/~stephens/hrchngs.htm
discusses about cardiovascular drift. Since one of the strategies during long distance events is to attempt to maintain HR within a particular zone, increases in HR require a corresponding decrease in intensity. Would this not suggest that strategies to maintain plasma volume would ensure the best performance ? Since the rate at which water can be absorbed by the body is slower than the rate at which water is lost, drinking frequently would counter such losses in plasma volume or at least minimize it.
Secondly, can you go over exercises which last several hours in duration (ironman triathlon events which typically take 13-14 hours for slow to medium paced athletes). 13-14 hours is a long period of time and needless to say, it is necessary to take in water during such a long period even if you do not exercise. How should one approach such an event. My approach has been to frequently drink a mix of water and calories specially during the bike segment as that is when the body can absorb the most water without sloshing around in the stomach.
Would love to hear what you folks have read/learned/observed about these two points.
Cheers.
Hi guys
Absolutely loving your sports science myth busting blog!
I have a similar query re the hierarchy of physiological control.
My understanding is that maintenance of blood pressure is the king of all, with plasma volume protected at all costs even if it means disrupting sodium concentration when required.
We see this clinically in situations such as gastroenteritis where a child who is dehydrated may present with either hyper, hypo or normonatraemia. In this case the child is holding onto all the salt and water available in an attempt to maintain plasma volume.
How do you see this fitting into the schematic for exercise?
If body fluids levels normally decrease during exercide without triggering thirst and salt hunger via stretch receptors in the great veins and arteries, then either central plasma volume is being maintained via fluid shifts within the body (eg constriction of vessels in non-essential tissues and movement of inter-stitial fluid into the vascular space)or the body responds differently to intravascular volume depletion during exercise.
Given the universality of our physiological responses to different stressors I would propose that exercise is not a unique circumstance and that the first explanation is more likely.
In this scenario intravascular volume depletion will re-emerge, as will thirst and salt hunger, once the athlete stops exercising, blood flow returns to non-essential tissues and intra-capillary pressures normalise.
What are your thoughts?
Hi Craig
Thanks for the comments and your questions. I'll do my best to respond, but it might take a bit of an iterative process to fully answer.
Firstly, the regulation of blood pressure is very important, it's been referred to as "the defended variable", even (or perhaps especially) during exercise. So the regulation of blood pressure is a key, key homeostatic process, and must be central to any discussion. In that regard, a discussion of fluid regulation is not complete without some mention of blood pressure. However, i do believe that the two are reconcilable, which I'll hopefully be able to explain...
First of all, plasma volume varies by relatively large amounts during exercise compared to at rest. So it's by no means true that plasma volume is defended - there is a plasma volume contraction at the onset of exercise, with fluid shifts within the body (ECF to ICF) being responsible. The plasma volume then continues to reduce as exercise continues, caused partly by dehydration, but it is relatively well defended through shifts of fluid between the various compartments.
But it's not the case that plasma volume is protected at the expense of sodium levels. In fact, it's just the opposite - the body will sooner defend the osmolality by shifting fluids between compartments. That is, plasma volume is one of the many effectors to regulate osmolality.
Now, the shifts in fluid, which impact on plasma volume, are also responsible for regulation of blood pressure. I can't comment on the similarities or differences between exercise and gastroenteritis, which is no doubt an area I'd be shown up in! But I will say that during exercise, there are some unique circumstances.
For one, if you lose body fluids, you will trigger thirst. I'm not 100% sure how you would lose fluids and NOT trigger thirst - perhaps you could clarify this? Because for all intents and purposes, it doesn't happen during exercise. The thirst mechanism is triggered by a rise in osmolality, so provided your sweat is hypotonic, you'll see a rise in osmolality. The thirst mechanism would then be initiated. This is an incredibly sensitive mechanism - a change of about 1% in osmolality is enough to stimulate thirst.
As for blood pressure, the sympathetic nervous system is responsible for the maintenance of central blood volume, stroke volume (Frank-Starling Law) and blood pressure. So the body takes care of that 'end of the deal', so to speak, through nervous system responses. In the longer term, te thirst mechanism, which is highly sensitive, helps to defend fluid balance and hence BP, and the hormonal responses (vasopressin and aldosterone) also help.
I hope that clarifies it a little - basically, plasma volume is one of the responses that exists specifically to regulate blood pressure AND osmolality. But with regards to fluid balance and thirst, osmolality is king. In disease, at rest, the situation is different. But exercise is a rather different situation.
Regards
Ross
Very cool, I think I am nearly there.
Your model suggests that during exercise that thirst is controlled by osmolarity alone, whilst I am suggesting that exercise is not an example of unique physiology and that as such thirst is controlled by both osmolarity and volume depletion.
The two perspectives are reconcilable if during normal exercise the blood volume depletion occuring is reserve capacity (primarily stored in the great veins). In this scenario ciculating blood volume remains unaffected, the decrease in total blood volume is within normal limits and thirst is controlled by osmolarity alone. This water intake slows the rate of volume depletion and corrects hypertonicity.
When blood volume drops to the point where it begins to affect ciculatory capacity, thirst is then additionally driven by decreased input from stretch receptors in the great veins.
As such, the athlete will begin to drink to restore blood volume at the expense of osmolarity. As such, we would expect severely volume depleted patients to present with serum sodium levels reflective of the fluid they have been consuming.
Athletes drinking water will be hyponatraemic, those consuming no fluids will be hypernatraemic and those consuming salt and water may be anywhere inbetween.
If we rely on osmoreceptors alone, then athletes presenting to the medical tent affected by volume depletion would be expected to have either high serum sodium if they have consumed no fluids or normal serum osmolarities if they have had access to water or hypotonic electrolyte drinks.
Do you have any data eg from Comrades to help us differentiate between these possibilities? Do athletes with severe volume depletion ever present with hyponatraemia?
cheers
Craig
Hi Craig
That's pretty much the long and short of it! The key to bear in mind is that osmolarity influences volume, so one must recognize that a drop in volume increases osmolarity and this stimulates thirst to correct osmolarity and hence volume. So the interaction is key and it's not a simple case of a switch from osmolarity to volume as the regulator...this is especially true in exercise, because fluid losses are not so great that you reach the point of volume-depletion being so vital.
Then the other is that during exercise, IF the athlete drinks voluntarily, they'll never volume deplete to such an extent that it becomes a factor. I'm not sure what you see in clinical medicine, or at what degree of fluid losses the stretch receptors become important signallers for the thirst mechanism? But I don't believe that during exercise, you ever get the situation where "blood volume drops to the point where it begins to affect ciculatory capacity". Therefore, you would not ever find a situation in exercise where the athlete would need to correct volume at the expense of osmolarity. In clinical medicine,perhaps, but not during exercise, to my knowledge.
So volume depletion can happen, but not in an organized athletic event, in my opinion - maybe if someone finds themselves stranded in the desert with a full day of walking and no access to fluids...
But other than this, the osmolality mechanism seems to be adequate to regulate volume sufficiently so that an athlete doesn't ever compromise circulatory capacity.
As for the final range of possibilities, that's where it gets really interesting. If you drink TOO MUCH water, you become hyponatremic - too much, in this case, is anything that exceeds thirst. The evidence strongly suggests that athletes who do not lose weight during exercise (weight loss is an indirect, though imperfect method to estimate fluid loss) are at risk of hyponatremia.
That's why we drum home the message - Drink to thirst, and you'll be safe. Drink more, and you're at risk.
If you consume water to thirst, your sodium level usually rises very slightly, but not to the extent that it's "abnormal" - the evidence from the Comrades is that most finishers are in the range of 140 to 145mM, which is on the higher side of normal. These people have NO SYMPTOMS and recover soon after. In some people, fluid intake to thirst sees sodium levels rise to reach hypernatremic levels. Some of these people develop symptoms, some do not. At this stage, there is no explanation for why symptoms develop.
One thing that is interesting from the Comrades is that the restoration of normonatremia is not a prerequisite for discharge from the medical tent, or even for the relief of symptoms. That is, people who come to the tent with hypernatremia and symptoms often leave with hypernatremia and no symptoms!
And then finally, drinking sports drinks does very very very little to change the sodium content of the blood. We tried to make this point in a post (not sure if it was this one), but the sports drinks contain so little sodium compard to body fluid that you're still diluting the plasma when you drink them.
Bottom line - the thirst mechanism, for exercise, is sufficient, so we recommend listening to it!
I'd be interested to hear from your clinical medicine background, just when volume depletion becomes a factor. Because I suspect it's far more than the 4% body weight loss that is seen in exercise?
Regards
Ross
Thanks Ross- its great to have learnt something new!
You are right on the money with percentage body weight loss- clinically we are looking at the 5-10% zone.
I am trying to do some myth busting of my own in my blog "Barefoot versus the shoe". Would be interested to see what you guys think?
cheers
Craig
1. Please list all the funding for all the research you have done. Since you seem to pick on GSSI, I think you should disclose any potential conflicts as well.
2. I can tell you guys have been around Noakes.
3. In the post on thirst you state that in a loss of 4% body mass about 10% would be fuel. Well, 10% of 4% is not much at all. A person weighing 70 kg losing 4% means a loss of 2.8 kg. If your numbers are correct, then a whopping 0.28 kg is from substrate use. That leaves 2.52 kg that is fluid and something else.
BTW, I have never received funding from Gatorade (although I have asked for it!); however, they do serve as a sponsor for athletes/teams with which I do work.
Hi Anonymous
Sure, though I can't list ALL the sources of funding, because then we'd be into the small grants and donations from individuals who left money to the University when they died - there are about 40 different funds, as far as I recall.
The main funding source is Discovery Health, which is a medical aid provider, who have worked with us to develop some innovative tests and tools for monitoring health and exercise - obesity, cholesterol, and so forth.
They were preceded by another company who was similarly involved in medical insurance.
The rest of the funding is provided through grants (Wellcome, Medical Research Council etc), and then funding sources from individuals who have left money to the university.
Until about 4 years ago, we worked with Energade, a local sports drink company, to look at protein and its effects on performance. So we did have sponsorship from a sports drink company, we concluded that drinking too much sports drink was dangerous to the athlete's health and that they should drink to thirst. I dare say the source of the funding did not influence the conclusion. So those are the main sources - if there is a potential conflict of interest, it would have been with the Energade research. I think our conclusion eliminates that, unless of course, the company wanted to communicate that drinking their product could be harmful.
As for "picking" on Gatorade, that certainly is a new one - the multi-billion dollar company, the victim of being "picked on"? To me, the fact that people are dying as a result of the "information" being fed to them as a result of the research justifies that.
And yes, we've been around Noakes, no apologies there. I'm not interested in personalities and egos, which unfortunately is the fallback plan when all else fails - how many times we've submitted research papers and been rejected for using too many "Noakesisms" is beyond me - I've never understood that. So if being around Noakes qualifies me to be marginalized by the scientific community funded by Gatorade, then no complaints.
It's worth noting that South Africa, thanks to Noakes, has the lowest incidence of hyponatremia in marathons around. In New Zealand, an Iron Man triathlon produced upward of 10 cases one year, and none the following year, the only difference being the advice on fluid and the available fluid during the race. The USA, unfortunately, is on the other end of the list.
This is a fabulous blog. I wish I had read it before my first IM during which I urinated at least 30 times. According to Richard, I guess I wasn't reaching my 70% of VO2max exertion level. Although if I had shut down my urinary function, I would have probably exploded.
I sure wish you guys would get a break, and answer Richard's post.
Keep up the great work.
HI Weekley Photo
Thank you for the feedback. Do you know that your email reminded me of Richard's question - I'd actually forgotten about that, I must confess!
I'll get a reply to him as soon as possible! Just incidentally, though, we did cover many of his questions in subsequent posts - most notably, his perception that cardiac output is affected by hydration is likely not true until you lose maybe 7% or more of your body water. In otherwords, you don't drop cardiac output as you dehydrate at NORMAL levels.
Incidentally, I'm not a believer that urinary function shuts down only at 70% - very few things in the body function as "on-off" switches, and this is no different. The fact that you had to stop 30 times is more likely a sign that you were overhydrating, though I'm speculating. Would be interested to hear how much you might have had to drink?
in any event, we'll get more of a reply, but have a look at other posts in this series, they might be helpful until then.
Ross
Thanks for you're reply. I was trying to drink about 6 oz. of fluid every 10 minutes. I think that works out to about 175 ml, or 1050 ml per hour. I had been religiously training myself to take in fluids on training rides, but had never experienced this level of urination. I'm sure the cold in combination with much too much fluid caused the problem.
I read the entire series, and am working my way through you're other posts. I'm very impressed. I even looked up Tim Noakes on Widipedia. Interestingly, they mention that he has worked on the brain's ability to determine performance, "This work lead eventually to the construction of a complex central governor model of exercise in which the brain is the primary organ that dictates how fast, how long, and how hard humans can exercise." No doubt this is why I felt better and better on training rides when I was nearly drowning myself with sports drink.
There is a company, Infinit Nutrition that allows you to mix your own custom formulas. I mention it because they discuss osmolality balance in their formulas. I gather from your posts and answers that balanced osmolaity in ingested fluids has nothing to do with balancing one's internal system.
Hi Guys
How will the supplementation with glycerol influence fluid intake for a marathon type stage race like the Cape Epic, especially in hot weather? I read in a Joe Friel's - The Cyclists Training Bible that "in a study" body temperatures increased 40% less using glycerol, compared with water only as well as 5% less increase in heart rate, resulting in a 32% improvement in endurance. Do these claims hold any water, so to speak.
Thanks, Russell
Hi Russel, and thanks for joining us here on The Science of Sport.
The type of research you read about is grounded in the idea that any weight or fluid losses are 1) bad, 2) very detrimental to performance, and 3) negatively impact temperature regulation in a big way. Therefore, if we can "hyper-hydrate" then we will perform better on a number of levels.
I am not sure of exactly which study was mentioned, but if "endurance time" was improved, it was probably an open-ended test to fatigue where the guys rode at a fixed power output.
These tests are highly variable and poor indicators of true performance. On the other hand, they are a great way to control for lots of things and are not as "messy" as a time trial where athletes pace themselves, and that is why we see these open-ended/fixed workload trials so often.
I suspect that if those same subjects did a time trial then we would see only marginal, if any, changes in performance, although admittedly that is a trial we have not yet performed.
The bottom line is that there is not a graded relationship between fluid ingestion and performance. Instead, drinking to thirst optimizes how much you drink and keeps you from drinking too little or too much.
In our research, drinking too little (less than to thirst) causes about a 3% decrease in performance in 80 km time trials in the heat. We are getting ready to do a post on this study for our current series on fatigue, so check back for that one soon.
In the heat your fluid requirements are higher, but your thirst mechanism still is optimal in spite of this.
Thanks again for the question!
Kind Regards,
Jonathan
An interesting topic. Drinking to thirst really makes sense as you have shown here. This is probably the best way to replace water. However, I would like to hear your opinion about replacing sodium in prolonged exercise in heat, such as Ironman racing. For me it seems obvious that at some point the lost sodium (and the following decrease in total amount of water contained in the body, even if you drink to thirst) must lead to a decrease in performance. It would be interesting to know how long it will take to see this kind of decrease if we drink/eat just plain water to thirst. My own experience tells me that ingesting sodium pills towards the end of long bike rides lasting more than five hours in summer will make me feel much better at the end of the ride. One could think that optimum amount of ingested sodium during very long efforts would be the same as the absolute amount of sodium that is lost in sweat. If you only ingest the amount of sodium (or slightly less) that you have lost in sweat and then drink water/carb mixture to thirst only, there should be no increased risk of hyponatremia? This way you could also retain the total amount of water in the body longer than by ingesting just water/carbs mixture to thirst. Do you agree with this or do you have some evidence suggesting that people should not replace sodium att all because it just leads to increased thirst and total body volume..??
This series on hydration has been eye opening for me. Thank you for posting it!
I've been on the 'drink as much as you can' plan because that's what I always thought was right. But it has NOT been working for me.
In 2007 during the Hawaii Ironman I drank as much as I could (exclusively gatorade and accelerade) and collapsed at the finish line, looking like a marshmallow woman, having gained 13 lbs since the beginning of the race. All that fluid had been stored in my tissues. In the med tent they wanted to give me an IV for more fluids but I assured them I had more than enough to drink already. No IV. I peed like crazy for 2 days afterward until my body returned to normal. Reading your posts, I now feel lucky that I didn't die.
My question is about the puffiness I sometimes see in my fingers during long training. Too much fluid intake?
Thank you for your thoughts!
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