Dear Sports Scientists: Will drinking fluids keep me cool?

Another look at fluid ingestion and temperature regulation

First, if you did not catch the NY Velocity interview with Ross, be sure to---Andy Shen and co do a great job over there and produce some excellent interviews.  Their site is a must read for any serious or enthusiastic cyclist, whether or not they reside in NYC.

Back in June I was very fortunate to present two sessions at the National Athletic Trainers Association annual meeting in Philadelphia, PA.  Both talks were about fluid ingestion, temperature regulation, and dehydration, and last week I received the audience feedback from the two sessions.  As usual the sessions produced polarized views on the subjects.  So I thought it might be a good time revisit this topic, one we have written about quite a bit on the site and in The Runner's Body.  After all, it is the end of summer, it is hot and humid, and plenty of people are training and racing in the heat.

The title of this post was not inspired by an email we received, but one of the core junctures where the two sides of this argument split is how much fluid is the right amount and why athletes should ingest it.  Nearly everyone will agree that ingesting fluid does have an effect on one's ability to regulate core temperature.  However one side of the argument is that athletes should try to maintain weight losses or lose less than 2% of their starting mass, while the other side feels ingesting fluid to thirst (which normally results in weight loss and some "dehydration") is the best practice.

Why ingest the fluid to prevent or minimize weight losses?  Well, some might argue that if we do not, we get too hot and this predisposes athletes to "heat illness."  The exact meaning and relevance of "heat-illness" is debatable and probably deserves its own post altogether, but the rationale for warding off dehydration by minimizing weight losses is that dehydration causes a rise in core temperature, and that causes heat illness, and that it might even cause heat stroke according to some.

The lit review (in brief!)


The evidence used to support the practice of replacing all or most of your weight losses comes from studies that control for the workload while asking subjects to run or cycle in hot and humid conditions.  The smaller scale studies measure weight losses (and sweat rates) and core temperature, the larger scale ones look at cardiac output and skin blood flow, among other variables.  This is good science, because if we permit our experimental subjects to speed up and slow down then suddenly we cannot determine what is affecting core temperature because now we have two independent variables (intensity and fluid volume) instead of one (fluid volume).  Therefore I am not slating those studies and authors and accusing them of bad science.  


The conclusions are that dehydration, as measured by weight losses, increases cardiovascular strain and results in an elevated core temperature at the same workload.  Fair enough, and as I mentioned earlier I do not think anyone, us included, will try to say that fluid ingestion has absolutely no effect on core temperature---it does, and these studies all demonstrate this effect.  And in fact their science is good, but it is the application of the conclusions that are bad.  In writing a scientific paper it is quite easy to wander off and begin to speculate about why you found what you found.  And it is at that point in your discussion that the reviewers let rip and often times sharply remind you to remain within the confines of your data and draw conclusions based only on what evidence you have available to you (i.e. your data)!


The issue with this topic of fluid and temperature is that the data are all collected within a strict set of conditions---as dictated by the scientific process--but then applied to every athlete (slow, fast, recreational, competitive, elite) in any situation (practice, race, fun) and any condition (cold, warm, hot).


Size counts


The size and magnitude of this effect is terribly small, however.  I try to teach my students in my stats class the difference between statistical and practical significance, and this is a classical exercise for this.  Take the absolute difference between the core temperatures at the end of a typical study, where the subjects exercise for up to two hours.  It is typically between 0.5-0.8 C, or maybe 38.x C in the fluid trial and <39.5 C in the no fluid trial.  Statistically significant?  Yes, most likely.  Practically significant or meaningful?  You are allowed to disagree, but I say "no."


And to follow up with that conclusion, the advice to replace fluids and prevent dehydration is dished out from this evidence even though none of the subjects in these trials ever report signs or symptoms of "heat illness."  So perhaps the real story is that even when we exercise in hot and humid conditions, our core temperatures rarely reach critical levels, and we cope with the additional stress just fine as evidenced by the lack of symptoms.  To me it begs the question, "Why are we telling people to follow this practice," because although there is a difference in temperature, it is small and not otherwise meaningful.


Ingesting fluid keeps does not keep you cool


Long ago, in an exercise lab far, far, away (ok, in Fort Worth, TX), some bored or motivated (or both) students were testing an athlete preparing for the Honolulu Marathon.  At the end of the heat acclimatization period, the runner did a performance run (80 min) at marathon pace (14-15 km/h, or  8.75-9.4 mph) and ingested quite a large volume of fluid (1.75 L) while we measured the rectal temperature response.  He did not ingest quite enough water to prevent weight losses, but came pretty close, losing only 1.6% of his body mass pre to post.
  

And by the way, the conditions in the heat chamber were 27.3 C and 60-65% relative humidity---the expected conditions on race day in December in Hawaii.  

So if the model is that you must prevent or minimize weight losses, and that you must do that to prevent an excessive rise in core temperature, and this model is based on the evidence I mentioned earlier, how does one explain the graph above?  According to that model, this athlete should be no where near 40 C since he lost only 1.6% of his body mass and was only minimally "dehydrated," yet after 80 min he is nearly to 41 C.  And herein lies a problem, because if some data do not support a particular conclusion, said conclusion must be scrapped and we must formulate a new one that incorporates all the available evidence.

Therefore it is not the fluid you ingest that keeps you cool, but as we have written here before it is your metabolic rate, or how hard you are exercising, that really predicts your core temperature during exercise.  Do not mistake what I am saying here, though---fluid plays a role, but only a very small one, and more importantly when we permit athletes to pace themselves they just slow down in the trials where they do not drink or receive very small volumes of fluid.  The result is that they reach the same core temperature at the end of the time trial, but take a little longer to finish.

For me the bigger message is that if performance is a desired outcome, if the runner wants to go as fast as they can, then they should drink to thirst.  Ingesting volumes that are larger than that have not been shown to produce faster performance times.  If performance is not important, the evidence from where I am sitting tells me that ignoring thirst and/or ingesting very small volumes of fluid will result in a miserable day out, but will not cause you to get heat stroke or collapse---two conditions that result from mechanisms other than changes in fluid balance.

If using a "hydration system" and lugging an additional 1-2 kg of mass during training runs makes you feel better, then please continue, but just know that you are not lowering your risk for anything by doing so.  The normal response is to replace less than what we lose, and it is perfectly normal and healthy to drink to thirst.

A look ahead:  Running economy and the marathon WR

Meanwhile, the Fall marathon season is upon us!   Berlin is around the corner and Chicago is boasting a super hot field that, under optimal conditions, is capable of a record.  Earlier we looked at the Joyner paper but left it before discussing what kind of running economy it would take to break two hours, so watch out for that analysis!

Jonathan

NFL, Gatorade and bananas

It must have been the bananas

Superbowl Sunday has come and gone, and New York Giants played the part of giant slayers by upsetting the New England Patriots, a team that steam-rolled the competition during the regular season, and even when against the ropes always seemed to be able to pull out a win from somewhere. Not in Superbowl XLII however, and Manning (the younger one, amazingly) led the Giants to two fourth quarter scores to win the title.

There are plenty of talking heads and pundits on the web, television, and in print to provide you all the detailed analysis of the game you need. Neither of us have tremendous insight into the game of football, and so we will leave the breakdown to the other guys. However, there is one story that emerged from the Superbowl that does fall within our "playbook", so we thought we'd spend some time on that instead!

New York coach Tom Coughlin does not read The Science of Sport

Back in October of 2007 we did a series on muscle cramps. In it we looked at the different theories of cramps, looked at the prevailing and perhaps dogmatic theory, presented a novel theory to explain cramps, and finally used the debate around cramps to demonstrate how science and knowledge evolve as new evidence comes to light.

The gist of this debate is that for years cramps have been attributed to dehydration and electrolyte imbalances and deficiencies. We suspect many of you who played youth sports were told, when playing in hot weather, to eat lots of bananas. The hypothesis there is that potassium depletion causes muscle cramps, and it is commonly accepted that bananas are a food stuff that is rich in potassium. So, quite simply, to stave off cramps one must just eat plenty of bananas - elementary school knowledge (or so we thought), and it turns out that even in the Superbowl, they adhere to that same dogma!

So in the big game, late in the first half, the crack Fox TV broadcast team crossed to their onfield reporter, who informed the watching nation that as a result of the high humidity in the stadium (the roof was closed), the Giants players were having problems with cramps, and that the coaches, sharp as they are, immediately had boxes of bananas brought to the sidelines. Sure enough, a couple of minutes later the cameras spotted it---a pile of bananas on the Giants sideline!

The first important (though tongue-in-cheek) point here is that Tom Coughlin and his coaching staff clearly do not subscribe to The Science of Sport. . .or perhaps they do, but they missed our series on muscle cramps? The second interesting point is in spite of all of the technology the NFL teams and coaches have at their disposal, all the high-tech strategies they employ, their wealth of human resources---19 coaches for the Giants and 14 for the Patriots---they rely on techniques that are entirely unproven and which no scientific evidence supports.

And then thirdly, and perhaps most thought provoking, is that Gatorade are the Official sports drink of the NFL, and copious amounts of it are available on the side of the field. Yet for some reason, the Giants were not told this - they chose the banana instead of the Gatorade! So calling for the banana backup is an indication that...the Gatorade wasn't working...? That wasn't an ad you saw in the Superbowl! Imagine the tagline..."Gatorade appears NOT to prevent cramps. Try bananas instead..."

No, science does not always have the answer

Admittedly, science does not always have the answers. Human performance even in individual events is incredibly complex. One only has to look at our previous post for some insight into will power and motivation to understand that many factors, perhaps too many and too complicated to measure, predict performance.

But it is still fascinating that at what many consider the pinnacle of professional sports---the NFL---the coaching staff turns to bananas during a game to alleviate muscle cramps. This is a sport in which assistant coaches, perched high above, take moving and still pictures, analyze them, and relay information about their opponents down to the coach on the sidelines. It is a sport that makes exstensive use of video analysis as players watch hours of game film of opposing teams to "get to know" them and their offensive and defensivee formations. They appear to be on the edge of technology. . . or are they? The bananas suggest otherwise, and give hope that maybe there is room for basic science.

In any case, it was a cracker of a game, and in our honest opinion the better team on the day won the match. Somehow the Patriots never really looked like the team that dismantled their opponents 18 games in a row. The Giants found a way to get to them, and came out ahead as a result of their efforts.

Be sure to come back later this week as we move on to Part III of our series on exercise in the cold.

Sports drinks, sweat and electrolytes

The actual data from the lab and the field

In the last post we introduced you to Randy, our imaginary 70 kg average male runner, and we created some potential scenarios regarding his fluid and sodium losses and replacement. The biggest take home message was to listen to your body and to drink to thirst, as this has been shown again and again in the field and the lab to keep people from drinking either too little or too much. We have received tons of feedback and discussion, and as we stated in the comments to that post we are pleased that so many of you are participating in the discussion, sharing your stories, and asking relevant and insightful questions.

Just as we will admit that field studies are not the perfect experiment but play an important role, it is the same with prediction equations. We can predict all we want from imaginary scenarios, and some times the equations are pretty accurate, but there is no substitute for the real thing, and many readers wanted to see more actual data and references that demonstrated why sodium ingestion is not necessary, and how both sports drinks and water will can cause a fall in sodium concentration. So while it is important to go through the exercise in the prior post, the next logical step is to look at the actual data.

Baker LB, Munce TA, Kenney WL. "Sex differences in voluntary fluid intake by older adults during exercise." Med Sci Sports Exerc. 2005 May;37(5):789-96

This study examined ad libitum fluid ingestion in older adults during intermittent exercise in the heat. Basically, they had continual access to water in one trial and Gatorade in another, and they had to cycle for 15 min and then rest for 15 min. The total time of each trial was two hours (four work/rest bouts) followed by an additional 30 min recovery period. A number of variables were measured, but we will focus on the sodium concentrations. One limitation of this study for our purposes here is that it was performed in older adults, and there is a well-documented effect of age on the thirst mechanism so that as you become older you become less sensitive to thirst. That is, your plasma osmolality rises higher before thirst kicks in.

According to the authors, their main findings were (and we quote):

1. When cool palatable fluids were readily available, active adults aged 54–70 yr drank enough to match sweating rates and maintain their body mass;
2. Their fluid intake behavior was repeatable;
   
3. CES [note: Gatorade] promoted greater voluntary fluid intake and restored PV losses faster than water;
   
4. There were sex differences in the fluid intake behavior of older active adults, with women drinking more water per kilogram of BM than men

As we tried to explain in many of our prior posts, the ingestion of any hypotonic fluid in excess of thirst will cause a fall in the sodium concentration. In this case "in excess" means drinking more than to your thirst. This occurs even though sports drinks contain some sodium because they still have much less when compared to the body fluids. Therefore the end result is a fall in sodium concentration. The data from this study show that these older adults, even when drinking to thirst, experience a fall in sodium concentration when ingesting water or Gatorade:


What we see is the time on the x-axis and the sodium concentration on the y-axis. The black dots represent the Gatorade trial, and the white (open) dots represent the water trial. All the subjects started with a sodium concentration of 142 mM per Liter, and in both trials the average concentration fell over time to approximately 139-140. There were no differences between the groups, and the symbols you see on the graph means that those values are significantly (statistically) different from the baseline value. So water and Gatorade ingestion produced a similar effect, and so Gatorade did not prevent a fall in sodium concentration in these subjects.

However one female subject ingested 2.8 L of water and 2.7 L of Gatorade in the respective trials. She gained weight in both instances, indicating an excessive fluid ingestion, and here are the data that support the conclusion that ingesting Gatorade will not prevent hyponatremia:


The problem is that the authors herald this as proof that ingesting Gatorade is much better than water:

"Furthermore, this woman’s data support the notion that a CES [Gatorade] is superior to water in limiting reductions in serum sodium during exercise-heat stress. During the CES trial, this female subject consumed 2.7 L and had a final serum sodium of 131 mmol per L. Therefore, although she consumed similar amounts of CES and water, serum sodium was maintained above that of symptomatic hyponatremia during the CES trial."

While the authors are entitled to their interpretation of the data, we disagree and conclude that both fluids are producing a steady fall in sodium concentration, and that the 131 value in the Gatorade trial is just marginally outside the symptomatic range (< style="font-weight: bold;">ingesting either water or Gatorade produced a nearly identical fall (~2-3 mmol) in the sodium concentration.

The take-home part: Sports drinks do not prevent hyponatremia

In fact Jonathan tried to apply this finding to a more "real world" situation in a letter to the British Journal of Sports Medicine. In that letter he argued that since the mean finishing time of women marathoners in America is five hours, and if the ingestion of Gatorade at rates similar to those found in the study is advocated by races, coaches, scientist, etc., then there would likely be many women (and probably men, too) presenting with hyponatremia. These data demonstrate that sports drinks do not prevent this condition as their ingestion in these subjects and at these rates causes a fall in sodium concentration.

Again one limitation to this study is it was done on older subjects, who are less sensitive to thirst, and what we might see in younger subjects would be a slight rise in sodium concentration when ingesting Gatorade and a maintenance of sodium concentration when ingesting water. The evidence for that statement comes from a 1992 study by Robert Cade, the inventor of Gatorade who incidentally died this week at the age of 80. In that study three groups of runners completed a marathon. One ingested Gatorade, another "half-Gatorade" (50% water, 50% Gatorade), and the third group water:

So in fact ingesting Gatorade to thirst in younger subjects results in a rise in sodium concentration, which is why you drink more---you never lower your osmolality below the thirst threshold and therefore are thirstier when ingesting a sports drink, whereas with water you maintain the osmolality right around the thirst threshold and drink and abstain as your thirst comes and goes. With sports drinks you instead just get thirstier, which seems kind of ironic since their slogan is "The thirst quencher!"

So those are some of the data that support our conclusions, and we hope that helps to clarify some of our interpretation(s) and conclusions. We will still post a "wrap-up" for the series on cramping in which we will try to briefly summarize the main points but more importantly leave you with some practical advice on this complex topic!

Muscle Cramps: Part I

Theories and Fallacies of muscle cramps

As promised in yesterday's post, today we kick off our latest series - Muscle Cramps. We hope that none of you did cramp in the middle of the night, as we mentioned yesterday! Though if you did, we're sure you stretched your calf and avoided the temptation to point your toe!

This is a follow-on from our series on Fluid Intake and Dehydration, and as we were preparing to write this series, we realised that there may actually be even more nonsense and blatant lies in the media than there were for dehydration!

Conflicts of interest revisited

In the dehydration series, we dealt with the very obvious conflict of interests that arise when a company which manufactures and sells sports drinks become the company who are funding and then performing much of the research on fluid and exercise. This is what happened when Gatorade created the Gatorade Sports Science Institute, and began funding research studies all over the USA, that rather unsurprisingly told the world that thirst was not enough, and you just had to drink as much as you could.

Can you imagine Gatorade issuing the results from those first studies saying to people "Folks, we've tested the sports drinks, and we don't have much evidence that you really NEED them. You'd most likely be fine without them, but hand over your money and buy your Gatorade at the counter anyway". An unlikely scenario. Of course, it was never as simple as that, and as we tried to explain previously, some of the early lab-based science was actually sound, but its application became the problem. More than this, the manner in which the research was compromised, becoming a form of shameless endorsement for the sake of sales in subsequent years was the ultimate problem. But that was all covered in our previous series, for those who are interested...

Muscle cramps - even more pervasive mis-marketing, but a complex issue

The same marketing vs scientific integrity debate exists for muscle cramps. The industry that has sprung up around the muscle cramp issue has spread far and wide. It includes Gatorade, who advocate the use of their drinks to replace the loss of salt which is, according to their research, responsible for the cramp in the first place! But more than this, there are dozens of products that claim to prevent cramp - next time you are in a pharmacy, take a look at the range - everything from gels, to creams, to pills, to effervescent tablets.

The two broad theories for muscle cramps

All these products work off the same premise - the put back the electrolytes that exercise will take out. And it's the loss of those serum electrolytes, the theory goes, that are responsible for the cramps during exercise. This theory, over 100 years old, is one broad category of theories for muscle cramps.

The second theory is that muscle cramps are caused by a 'malfunction' in the control of the muscle by the nerves - an abnormality of neuromuscular control which is caused by fatigue.

Our objective in this series is to look at these two theories, beginning with a bit of groundwork and history...

Defining cramp

Perhaps one of the first things to do is provide a definition for cramp, as well the usual disclaimer that we cannot possibly cover all the possibilities and scenarios in this series. Firstly, cramp has been defined as a "spasmodic, painful, involuntary contraction of the skeletal muscle that occurs during or immediately after exercise".

Note that this definition applies to exercise-related cramps only, and therefore, it excludes a whole host of other possible cramps. We must point out that if you do suffer from very regular cramping, there are some conditions that can cause this - endocrinologic, neurologic, and vascular disorders, treatment with certain drugs, and occupational factors. Then of course, some cramps are what the experts call "idiopathic", which means they have no cause (but actually means we don't know what causes them, but it sounds better to say "idiopathic"!). If you are a regular cramper, it's probably worth seeing a doctor and just having an exam to determine whether any of these broad factors might be responsible.

But returning to muscle cramps, the lifetime prevalence of cramping is reported to be as high as 50%, which is remarkably high. Some people are also quite clearly more susceptible, and you can actually predict with a fair degree of accuracy who will cramp during a marathon based on their history and their racing strategies (more on this later).

The history of cramping - the electrolyte depletion theory

The earliest reports of muscle cramps come from 100 years ago, when labourers in hot and humid conditions of the mines and shipyards suffered from cramps. Even that far back, the sweat could be analysed, and it was noticed that the builders had a high chloride level in their sweat (chloride, incidentally, is one half of the salt in your sweat). The conclusion that was made was that the labourers were sweating out valuable electrolytes, causing their muscles (and nerves) to malfunction. The heat and humidity were key factors that caused this situation. It must be pointed out that no one prospectively measured the sweat of the labourers who DID NOT CRAMP, something that we'll look at in our next post.

Later, the builders of the Hoover Dam famously recovered from cramp when they were made to drink salty milk, entrenching the theory that salt loss was the cause of cramp.

And perhaps rather surprisingly, that was it - based on those anecdotal observations, the theory which you probably hold true today, was born. That is, cramp is caused by a loss of sodium, chloride, and later calcium and magnesium were added to the mix. Heat and high humidity were implicated as "accessories", and the term "Heat-Cramps" was even conceived. According to this theory (as seen by this article and the "expert" testimony) , cramps happen because athletes exercise in the heat, lose electrolytes in their sweat, and the depletion combined with high body temperatures cause muscle cramp.

For example, take these testimonies:

"When a young athlete experiences heat cramps, pull him or her off the field into a cool area and gently stretch the affected muscle. "Have them drink, drink, drink, and then drink more," says Albert C. Hergenroeder, professor of pediatrics at Baylor College of Medicine and chief of the sports medicine clinic at Texas Children's Hospital.

"High-sodium drinks will prevent children from getting heat cramps," says Jackie Berning, PhD, with the National Alliance for Youth Sports. "Gatorade has just enough sodium to prevent those cramps. But if you're a heavy sweater, and you're still getting cramps after drinking Gatorade, eat some salted pretzels or salted nuts. Those work fine.""

There is of course more to it than this, but the essence is that the serum electrolyte depletion theory was created without any controlled, clinical studies to establish whether the depletion of salt through excessive sweating was to blame. Rather, the theory was picked up on and used to spawn the numerous products you can purchase today. But, as I'm sure you've guessed, there are some holes in it.

The problems with the serum electrolyte depletion theory

First of all, there is a key conceptual problem here, and that is that when you sweat, you don't actually reduce electrolyte concentration. That is, there are certainly electrolytes in the sweat, but the concentration of these electrolytes is so low, that sweating is likely to make you HYPERTONIC, not hypotonic. We looked at this in our posts on fluid - when you sweat, you lose more water than electrolytes, because the sweat is HYPOTONIC. Therefore, sweating cannot lead to a fall in electrolyte concentration.

What transpired was that Gatorade (and the rest of the 'industry', it must be said) developed the theory of "salty sweaters", which is the term they gave to people who they said have abnormally high salt levels in their sweat. Small problem - no one actually knows what a salty sweater is. How much salt does there need to be in the sweat before you are placed in this group? No one knows. Recently, Professor Martin Schwellnus, widely published in this area, posed this question to scientists at the Gatorade Sports Science Institute at a conference on cramping - he received no answer.

The truth is, even the saltiest sweaters around still have hypotonic sweat, and so the more they sweat, the more they will cause their electrolyte levels to rise, not to decrease. This is a very obvious problem that is overlooked by the electrolyte replacement advocates.

Of course, those of you who read our fluid series might be thinking that if you then drink a lot of sports drink, you can reduce the electrolyte content, but that's yet another reason why drinking too much is not a good idea...

The cramping paradox - why specific muscles?

The second problem is something we asked you in yesterday's post. We asked whether the depletion of serum electrolytes would be expected to cause cramps in specific muscles, or all over? Hopefully it is evident that if a cramp was caused by a loss of serum electrolytes, there is no reason for the cramp to be limited to one muscle only. Rather, you would cramp everywhere. In fact, in people who have lost a great deal of salt and have become hyponatremic (not during exercise, but clinically), we know that they cramp in ALL their muscles.

But somewhat surprisingly, exercise-associated muscle cramps ONLY happen in the muscles that have been used extensively for exercise. The afore-mentioned Prof Schwellnus found in 2004 that the quadriceps, hamstrings and calves made up 95% of cramps in the 56km Two Oceans race in Cape Town.

Leading onto the next post - further evaluation of the electrolyte depletion theory

In the interest of time, we'll call it on this post for today, and say that in our next post, we'll tackle the electrolyte theory in more detail and look at some of the studies that have looked at people who cramp and those who don't and compare their values.

Join us then!
Ross

Further reading:

Schwellnus, M. (2007) Sports Medicine, vol 37, 2007

Schwellnus et al., British Journal of Sports Medicine, vol 37, 2004

Muscle cramp 'teasers'. . .

Apologies for the absence...and some muscle cramp 'teasers'

Ah, the joys of the University calendar! We must apologize for our somewhat lengthy absence - it has been 5 days since we last did a post, which I think is the longest break we've had since we began The Science of Sport in April earlier this year!

But we have good reason, for both Jonathan and I are both deep into marking and examination of undergraduate and post-graduate exams and theses at our respective universities. Though Jonathan and I both enjoy lecturing and find it very rewarding, when we reach mid to late-November, our disposition towards teaching changes somewhat, as hundreds of exam scripts and thesis work suddenly land on our desks for us to plough through! And funnily enough, the other work doesn't seem to realise it and let up!

So we do apologize for the break, but we're back now, hopefully with a bang, as we get stuck into a new series, this one on Muscle Cramps - Science and Fiction.

A follow on from fluids and dehydration

The series on muscle cramps is really an extension of our last series - Fluid Intake and Dehydration. In that series, we tried to explode some of the myths around drinking during exercise, describing how the prevalent "scientific" advice was in fact flawed (sadly, it's often fatally flawed). We looked at the claims and counter-claims and ultimately encouraged everyone to do the wise thing - listen to your body, and when it says you're thirsty, drink! But only then...don't let the fluid companies tell you that you're the only mammal too stupid to know when to drink by itself!

Introducing muscle cramps - some points to ponder

The muscle cramp story is similar, sadly. In South Africa, we have a host of companies who produce cures, preventative tablets, creams and liquids to help you avoid cramp. The premise, as was the case with fluids and dehydration, is that a cramp is caused by a depletion of some electrolyte - sodium, potassium, calcium or magnesium. All have been mentioned, and most often, it's sodium and magnesium that take the brunt of the blame.

We'll take a step by step journey through muscle cramps in the series. We'll tackle it in three parts:

1. What is a cramp? Very brief history and overview of what we know, and how we know it.
2. The electrolyte-dehydration-heat theory for muscle cramps
3. An alternative view - evaluating the gaps in the theory

So that is what you can look forward to (or with dread, as the case may be!) over the next week.

Until then, here are some issues to ponder:

• Despite the theory that muscle cramps are caused by electrolyte and fluid depletion, it has yet to be shown that people who cramp have lower electrolyte levels or are more dehydrated than those who do not. In fact, the studies have found that "Crampers" and "Non-crampers" have similar electrolyte and dehydration levels. Something wrong with that picture...

• If cramps are caused by electrolyte and fluid depletion, which muscles would be most likely to cramp? Would it not be ALL the muscles, because you're losing electrolytes and fluid through sweat, so then all the muscle groups should be vulnerable...yet for some reason, we cramp in the muscles we actually USE. Again, something out of place there.

(Some of you may already have a counter to these points, saying that what is happening in the muscle is not necessarily what is happening in the blood - we'll tackle that one for sure)

• And then finally, if you are reading this before heading off to bed, we hope that you don't experience the dreaded "Night cramp", which we're sure most of you have had at some stage. You wake up in the night, and feel a slight twinge, usually in the calf. Your first impulse is to point your toe, and when you do that, what happens? You may know that if you do this, you'll be writhing in agony instantly! Instead, what should you have done? The answer is you should stretch the muscle - stretching is the quickest way to get rid of cramp. Now, how do we explain that one according to a heat-electrolyte theory?

The answers to follow, so join us then!

Ross

Fluid intake Debate: Comments from a doctor

Last week saw our series on Fluid intake during exercise, where we described the development of our perceptions around drinking during exercise. We looked at how there has been a radical shift in perceptions since the 1970's and how the current scientific evidence is beginning to swing that perception around again. Where it was once recommended that you drink, drink, and drink because thirst was not good enough, there are now studies showing that excessive drinking can be deadly, and that when drinking to thirst the body loses some weight without any risk or detriment to performance.

And when we started The Science of Sport, our intention was always to stimulate debate, to encourage discerning readers to comment, submit questions and discuss the topics we present. So on that note, today we thought we would share the comments of a doctor, which were kindly sent via one of our readers.

So below, highlighted in blue, are the comments of the doctor, word for word as he submitted them, without any corrections from our side. We then have attempted to address his points in a logical manner and using evidence from the scientific literature.

The cause of collapse - the 1% in the medical tent are not different from the 99% who are not admitted

"why do you feel bad when your [sic] in the heat for a long period of time. it is not because your osmolality is off (that is the balance of electrolytes, etc) it is because of "volume loss."

Having worked in the medical tents of the Two Oceans and Comrades Marathons for the past three years, studying this exact question, it is now evident that 99% of the runners finish without any undue symptoms and have lost the same amount of fluid as those who enter the tent. In 2005 at the Comrades Marathon, we found that "Control" subjects who did not report to the medical tent had lost the same amount of weight as those who did report to the medical tent. So clearly, their collapse was not a volume issue, or surely all the athletes who lost similar amounts of weight (i.e. volume) should then collapse. This study is currently in press in the Clinical Journal of Sports Medicine. In addition, no study has ever demonstrated that volume loss is responsible for any raised perception or medical condition during exercise---and again, we cannot stress how critical this is - you cannot study humans outside of exercise and apply the same findings.

The body is more than capable of meeting the circulatory demands during exercise

"what happens when you reduce the volume of a closed system such as your blood stream. your blood pressure drops. perfusion to muscles, brain, gut and other vital organs begin to shut down. you become dizzy, faint, pass out and seize."

In 1979, Ethan Nadel published a study (Journal of Applied Physiology) where he compared exercise in the heat to exercise in the cold, specifically to look at the circulatory system. In that paper, he showed that the challenge to the circulation as a result of plasma volume contraction was more than adequately met by a redistribution of blood from the splanchnic, renal and gastro-intestinal circulatory systems.

Is there a challenge to the circulation whenever plasma volume is reduced (be it high temperatures or fluid loss)? Yes, but the body is more than capable of adjusting to this 'stress'. A number of other studies by scientists in Denmark particularly (Savard, Nielsen, Nybo) confirmed this for exercise in the heat.

There is no evidence that perfusion to the vital organs of the brain, muscle or skin is compromised during exercise, unless you become significantly volume depleted. However, the point we are making, a point borne out by the evidence, is that drinking to thirst is well capable of preventing that kind of fluid loss. If you drink to thirst, you'll never lose enough body water to reach this scenario. Instead, what the doctor refers to is likely to happen only in patients with severe medical conditions, including haemorrhaging or being lost in the desert for a week.

Collapse happens after stopping - it's a venous return issue, not fluid loss

"blood pressure drops. perfusion to muscles, brain, gut and other vital organs begin to shut down. you become dizzy, faint, pass out and seize...BP drops. he starts getting dizzy, nauseated. he is trying to keep standing. BP to brain continues to drop because the heart has no volume to pump to the brain "

We need to be very clear about the point that people collapse after finishing, not while they are still running. This is critical, for it suggests that it is the act of stopping running that causes the drop in blood pressure. This point, which we made in a post about the Chicago marathon, indicates that the blood pressure is more than adequately defended during activity, but once some athletes stop, the removal of the muscle pump means the blood pressure suddenly drops as they are not able to mount a sufficient compensatory response to this fall in venous return. Note that this has nothing at all to do with the fluid loss, as the doctor purports. Instead, it's entirely the cause of a reduction in venous return by what is often called "the second heart," the muscles pumping blood as they contract. The Frank-Starling law of the heart, of which the doctor is no doubt aware, then says that as venous return falls, the cardiac output is reduced, and in the presence of vasodilation (as occurs during exercise) the result is a fall in the blood pressure.

This phenomenon was described in the mid-90's in a series of papers by Holtzhausen and others, which you can find here and here. The point is that it's not the volume reduction, but the decrease in venous return in the presence of sympathetically-driven vasodilation which then fails to reverse quickly enough. For this reason, the best method of treating the collapsed runner is to allow him to lie with his feet elevated for a short time. Of course, there are more serious collapses, but you'll find that these happen on the course, during running, and not when the athlete stops running. It should be noted also that the presence of seizures must indicate some degree of encephalopathy which has not been shown to be associated with any amount of weight losses in otherwise healthy adults.

Finally, we'd also like to point out to this doctor that the athletes who lose the most fluid during marathons tend to be the elite athletes and race winners. An elite athlete drinks probably 200 to 400 ml/hour on average (a generalisation, but one backed up by evidence and our direct knowledge of elite athletes' drinking patterns). Yet in order to run at 3 min/km for 2 hours, the athlete would have a sweat rate of anything between 700 and 2000ml/hour, depending on environmental conditions. This drinking pattern will always result in body weight losses, often as large as 4%. A 60kg athlete, for example, who drinks 400ml/hour, with a sweat rate of 700ml/hour is expected to lose about 1.5% of body weight. On a slightly warmer day, this increases. Yet these athletes do not slow down, and only very rarely do they collapse. That is a paradox of the model that the doctor proposes---that is, if weight and volume losses are so detrimental then it must be the athletes who lose the most weight and volume that suffer the worst symptoms. We are interested to know the explanation for this observation, as well as the earlier mentioned fact that 99% of the field who do not need medical attention have lost as much weight as the 1% who do. To us, it suggests something else is the cause.

The importance of engaging in scientific debate

"these guys are idiots and definitely have an "issue" with the sports drink companies."

"somebody needs to write these clowns and challenge their thinking"

As we mentioned in the beginning of this post, we encourage further discussion around these and any of the points we make here. Knowledge and scientific "truths" are an evolving entity, and expected to change as new evidence becomes available. Therefore we are disappointed that this doctor did not feel he could post his questions and observations here on The Science of Sport, for we welcome debate and challenges to our interpretation(s) of the scientific data. We are also curious why this doctor used the terms "clowns" and "idiots" to describe us, as we have tried to present the scientific evidence and our interpretation of it in a way in which many people can access it.

Regards
Ross and Jonathan

References
Holtzhausen and Noakes, Med Sci Sports Exercise, 1995
Holtzhausen and Noakes, Clinical J Sports Med, 1997
Nadel et al., J Applied Physiology, 1979

(Please email us for the full references)

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.

As our bodies are mostly water, you can imagine why keeping these fluid volumes balanced is so important, and that is precisely why the body defends the osmolality and not the body weight.

"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

Chicago Marathon death - no evidence of dehydration

In the last few weeks, we have run a series of posts investigating the events of the 30th Chicago Marathon, where record high temperatures caused the early cancellation of the race, amid record numbers of medical cases and emergencies. We discussed the possible cause of these collapses, suggesting that blood pressure and unfamiliarity with the heat were more likely the cause of the problems than dehydration and heat stroke, which were being blamed for all the problems.

We also did a post looking at the sad death of Chad Schieber, a 35-year old policeman who collapsed at the 18 mile mark and was later pronounced dead. Initially, the death was blamed on the heat and dehydration, but the initial autopsy found that Schieber suffered from a relatively common condition known as mitral valve prolapse. This condition, which is reported to affect 2% o the US population, does not by itself cause death in athletes, but has been implicated as a contributing factor to a potentially fatal arrhythmia.

New reports from the Medical Examiner

Now, just over two weeks later, the medical examiner's office has released further reports that "tests show no evidence of dehydration." According to Nancy Jones, the Cook County Chief Medical Examiner, dehydration can now be ruled out.

In response to our initial post on this death, where we suggested that it would be prudent and wise to hold the verdict until the autopsy result was announced, a few medical doctors wrote in saying that dehydration may contribute to the death. A few things in response - firstly, there's no evidence for that, it's pure supposition, because until one has actually studied the physiological response of people with mitral valve prolapse to graded dehydration DURING exercise (very important - it must be measured during exercise, as we've tried to emphasize in other posts recently), it's supposition to say that "dehydration contributed to the death".

But in this latest news, what are referred to as "dehydration tests" (presumably measures of body water) have confirmed this. Again, there's an issue around what constitutes "dehydration" - is it 1% body weight loss, is it a change in total body water of "X" %? That's unclear. But it does seem more and more that it was not a dehydration issue.

The new blame-game - not dehydration but lack of maps

Instead, it seems that attention is now being turned to the ambulance and the length of time it took to get Schieber from the course to the nearest hospital. According to one news report,

"You can actually see the University of Illinois at Chicago Hospital from where Schieber collapsed. It takes a minute and a half to get there. But the ambulance took between 8 1/2 and 14 minutes."

Unfortunately, this type of thing does tend to happen after such tragic events. But hindsight is always 20/20, and it's a shame that blame gets assigned so easily. Not that one should gloss over potentially critical details, lest we forget to learn from (possible) mistakes

But this does make me think back to the Comrades Marathon this year, where a runner collapsed perhaps 300m from the finish line. His fellow runners, seeing him lying there without medical support, picked him up and carried him across the line. Sadly, he was pronounced dead at the scene. But then even worse for all those involved, the runners who had attempted to help him were accused of contributing to his death! There was some physiological basis for this (you can read this in a post we did on it here), but it was a shame to have to play that game at such a time, in such a public forum. The death of Chad Schieber seems the same.

Our future posts

To wrap up on a related theme, we're very much into the thick of our series on Fluid Intake and Dehydration: Exploding the myths. We've recently looked at the comparison between laboratory and field studies, having previously explained how it was the advent of "science" into marketing that drove a good deal of research in lab studies. In Post IV of that series, we'll look at the physiology of thirst and what the body is actually defending, as we ask the question: "Is thirst enough and how does it work?"

So do join us for the completion of that series.

In other news, the New York City Marathon takes place on the first weekend in November, and many runners are no doubt hoping for much cooler temperatures. Regardless of what happens, you'll be able to read all the race insights and stories right here!

Join us then!
Ross and Jonathan

Fluid intake, dehydration, and exercise: Part III

Welcome back for Part III in this series on fluid intake and dehydration during exercise! Thus far we have examined a brief history of fluid replacement during endurance exercise in Post I, and in Post II we tried to explain how some of the lab research has perhaps been over-interpreted, and how that has lead to a false belief that ingesting fluid during exercise will keep you cool. In that post we reported the findings of earlier researchers who concluded the following:

• The core temperature is maintained at a higher level during exercise
• It is the metabolic rate (or in other words, how hard you are exercising) that predicts the core temperature

Now, in Part III, we will compare the research that has been done in the field versus that from the lab, and show you the evidence of what really happens when people exercise in a variety of conditions.

The field study - Is it really science?

Many scientists will downplay the importance of field studies as they are largely uncontrolled studies from which we cannot assign causal relationships. So in other words, from a field study alone, we cannot say that dehydration causes one's temperature to rise, or that ingesting fluid will keep one cool. However, field studies do play a very important role as they represent what is actually happening when people exercise in real conditions. Also, they represent a portion of the scientific evidence that we accumulate and therefore these studies contribute to the available data from which we form models to explain physiology.

The very first field studies in Exercise Physiology were published by E.F. Adolph in the 1940's. Adolph and his team performed numerous experiments on American soldiers marching in the desert, and wrote a book on all of this work: "The Physiology of Man in the Desert." We won't try to explain all of his work here, but two of the take home messages from Adolph are the following:

1. Ad libitum access to fluid is sufficient to enhance performance (as measured by hiking in the desert)
   
2. Fluid restriction will affect performance---11 soldiers in a fluid restricted group could not finish the hike, whereas only one in the fluid ingestion group could not finish

It is normal to lose fluid and decrease body weight during exercise

Since then a number of studies have been performed at races and other endurance events, and the one main finding of all of these studies is that athletes replace only between 40-60% of their weight losses, and complete the race 2-5% "dehydrated."

Despite this fact, from real athletes competing in real events, many scientific articles and lay magazines continue to emphasize that "dehydration" of this magnitude (2-5% of the pre-race weight) is detrimental to health and or performance. This is the basis for the many advertisements proclaiming the importance of drinking to runners, as we discussed in Post I of this series.

The evidence from all the field studies, however, shows rather that changes in body weight of this magnitude are not associated with collapse and high core temperatures. One reason for this is likely because, as we stated in a comment to another post, the body weight is not the regulated variable, and so even if you lose some weight the body is fine, and is in fact responding normally to that exercise. We will examine that concept in the next post in this series.

So what does happen during exercise? Data from running and cycling studies

Two studies from Jonathan's doctoral work measured the rectal temperatures of runners during a 56 km road race and cyclists during a 109 km race. The main finding of these two studies was that the rectal temperature rises for approximately one hour and then levels off, after which time it remains within a very narrow range (less than 0.5 C). The data from the runners are below:

And the data from the cyclists are here:
What both these graphs show is that the core temperature is maintained within a very narrow range during the event. Secondly, most of the changes in the core temperature occur at the beginning of exercise, and not at the end. Thirdly, although the two groups were different since the runners were going simply for race completion and the cyclists were highly-trained and racing, the temperature responses are similar. In addition, the environmental conditions were quite different between the two races: the marathon was cool and wet, and the cycle race was warm and dry. Yet, again the temperature response was similar.

The body temperature response - higher is normal

Another important aspect of all of the field studies that have measured the post-race rectal temperature is that 39-41 C is quite a regular (and normal) finding. In 13 different field studies we have reviewed, the range of post-race core temperatures was 37-41.7 C. The one study that included untrained or lesser trained runners was the one that reported the lowest temperature---37 C in a group of 63 marathon finishers. Nearly all the other studies measured highly-trained runners finishing marathons in 2:30 - 2:45.

Their finding is that in a variety of environmental conditions, it is the metabolic rate that determines the core temperature, and not the body weight losses and fluid replaement! Therefore it is those athletes who exercise at higher intensities that have higher core temperatures. Furthermore, although these athletes reach "high" temperatures, they do not exhibit signs or symptoms of "heat illness," and recover quickly. So in fact we would say that a post-race core between 39-41 C is quite normal and well within the limits of the body's coping mechanisms.

The lab vs. the field - assigning importance

We've described that when we exercise out doors, we lose weight without any apparent impairment of performance or risk of hyperthermia. Yet the lab studies, conflict of interests aside, show that when you ingest more fluid you stay cooler, which is a complete contradiction of what is actually observed during your exercise. So which is right?

One very important difference between lab and field studies, in addition to providing insufficient air velocities, is that the subjects in the lab studies are not allowed to pace themselves, and therefore they cannot change their metabolic rate (their running or cycling speed) as they wish. Note that people will slow down based on how they are feeling, and one such drive for slowing down is the feeling of getting too hot. But in the lab, this cannot happen and so an artificial situation is created where the subject simply has to keep going without slowing down, and a vital part of the regulation of physiology is removed.

In addition, it does not mean that there is no effect of fluid ingestion on one's ability to regulate core temperature. When the exercise intensity is fixed, there is most certainly an effect. However, as we have emphasized in Post II, that effect is 1) very small, and 2) is likely amplified by the lack of air velocity in those studies, meaning that it is probabbly even smaller than it has been measured as.

However what this aspect does represent is a limitation to how these studies can be applied. In other words, we cannot take those findings and apply them to the normal exercising population because running at exactly the same running speed for 2-3 hours is simply not how people complete marathons. Instead, they alter their running speed---that is, they slow down--- as they become fatigued or as their brain senses that they might get too hot. And this has a profound effect on their core temperature. Namely, it ceases to rise or even falls if they slow down enough.

So the importance of the lab studies is that they add to our knowledge by controlling for specific variables and measuring how one thing affects another. Without these studies we can never know the exact relationship between different physiological systems and characteristics. But at the same time, we must be extremely careful how we apply the data from these studies, because it does not represent what people are actually doing. Therefore we must rely on both field studies and lab studies to draw our final conclusions. Both types of experiment play an important, but different, role in helping us understand how the body regulates its temperature during endurance exercise.

In the end the message is that yes, fluid ingestion can indeed affect your ability to regulate core temperature. However, this effect is overstated and very small, and your body will always protect you by making you slow down before you suffer and major physiological consequences from this. The evidence to support that is not apparent in this post, but is something we will present here in the future. What we can say now is that when performance is a desirable outcome, you must drink to thirst, for it will optimize the amount of fluid ingestion.

Should you choose to ignore your thirst, you will not collapse from "heat illness," and nor will you die from heatstroke. However, you will be miserable and you will run slower than you would like. So listen to your body and join us later in the week for Part IV in this series!


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