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 II

The electrolyte depletion model of muscle cramps

In part one of this new series we tried to set the scene by providing some history in this area of muscle cramps. At times it might seem like we are a bit heavy on the historical side, but as we mentioned in one of our comments to Part I, understanding the historical record is crucial as often it helps us understand why we think what we do---and this affects one's interpretation of the science. In this post we will focus on the prevailing premise that dehydration and electrolyte disturbances cause muscle cramps.

The first important thing about this area of research is that Professor Martin Schwellnus is hands down the one researcher who has consistently moved this area forward. As a sports physician he has treated many a runner with cramps, and his curiosity and what he was seeing in the medical tents lead him to challenge this paradigm that dehydration and electrolyte problems cause cramps. What he found was that this model was based on not one shred of scientific data, and instead relied heavily on anecdotal evidence. Since 1997 he has published some of the only evidence available that has even attempted to determine what actually is causing the cramps and who is prone to this condition. The first paper he published in 1997 proposed a novel hypothesis for muscle cramps, but we will address that in Part III of this series.

The lab vs. the field

In our series on dehydration we discussed how the lab is not always translatable to the field, and vice versa, but that each has its own important role. Field studies are often cross sectional in nature, and although important we cannot assign direct cause and effect from them. However it is observations and findings from field studies that often lead to the very precise and mechanistic lab studies that are important in advancing our knowledge.

However one major obstacle in this area (cramping), is that no one has yet created a laboratory protocol in which we can reproduce muscle cramps in a controlled manner. Being able to do this is a crucial step in eventually identifying what causes them because it will allow us to make specific interventions to test what the effect is on cramps. So although we are still in the infancy of this area of research, the field studies are a very important starting point and have so far yielded important findings.

One study published in 1990 showed that there was no association between potassium levels and cramps. In that study cyclists rode for up to five hours. Some of the subjects did cramp, but their potassium levels were not uniformly high or low, thus showing no association between that variable and the cramps. However beyond that study (and one more that was presented at a conference but apparently not published) there is little real data out there to support or refute this hypothesis that dehydration or electrolyte disturbances cause cramps.

Study 1: Two Oceans Ultra Marathon

In a 2004 study published in the British Journal of Sports Medicine, Professor Schwellnus and his colleagues examined runners before and after the Two Oceans 56 km marathon in Cape Town. They measured quite a few variables, but since we are discussing changes in electrolytes and hydration, we will talk about those results. Remember that many people, both scientist and personal trainer alike, will profess that cramps are caused by dehydration and/or some disturbance in the electrolytes (sodium, potassium, magnesium, etc.) So the important finding from this 2004 study was that when the crampers were compared to the controls---who were matched for body mass and finishing time---the only differences were that the crampers had lower sodiums and higher magnesiums. The problem with this is that a lower sodium concentration suggests overhydration and not dehydration, and also if magnesium deficiency is meant to cause cramps then surely the crampers should have been lower here?

	Crampers (N = 21)	Controls (N = 22)
Sodium	139.8 ± 2.1	142.3 ± 2.1
Potassium	4.9 ± 0.6	4.7 ± 0.5
Magnesium	0.73 ± 0.1	0.67 ± 0.1
Osmolality	280 ± 6	284 ± 10

The relevance of this study is that if dehydration and electrolyte disturbances really play such a large role in cramps (as they are proposed to), then the crampers should have much higher electrolyte concentrations since they would be losing fluid and causing the concentrations to rise. Yet instead we see something entirely different, first that the crampers had lower sodium concentrations, and second that the crampers were not really different compared to the controls.

What is also noteworthy from this study was that the crampers had an average loss of body weight of 2.9%, compared to 3.6% for the non-cramping controls. In otherwords, the people who DID NOT cramp lost more weight than the people who did. It goes further than this, because Schwellnus et al were able to measure the change in plasma volume as well - a more direct measure for what is happening to fluids. Here, they found that the crampers actually gained a small amount of 0.2% during the race. The non-cramping control subjects LOST 0.7%. So the sum effect of this data is that it suggests very strongly that cramping is not associated with dehydration, or with lower serum electrolyte levels, which is what we have had drilled into us for many years!

The follow-up study from Iron Man - further evidence against serum electrolytes

The next year they published a study in Medicine and Science in Sports and Exercise, and instead of runners it was Ironman triathletes. According to what most of us hear day in and day out, it is these ultra-distance athletes who are exercising for 10+ hours at a time that must be most susceptible to dehydration and electrolyte deficiencies. After all, they are sweating for hours on end, and the numbers tell us that with so many liters of sweat lost then they must also be losing grams and grams of "essential electrolytes" such as sodium. Below you will see the basic data on these athletes, and the important finding here is that we see the crampers and controls were the same age and were similar in mass, had similar pre to post cahnges in mass, and also finished the Ironman in similar times:

	Crampers (N = 11)	Controls (N = 9)
Age (years)	33.5 ± 8.8	35.4 ± 8.1
Pre-race mass (kg)	79.1 ± 5.9	77.7 ± 6.4
Post race mass (kg)	76.3 ± 5.6	74.6 ± 6.5
Body mass loss (%)	3.4 ± 1.3	3.9 ± 2.0
Total race time (min)	660.8 ± 77.9	685.7 ± 48.5

So the two groups were essentially the same in that the crampers did not spend longer in the course or lose more weight (a crude measure of dehydration). Yet again the crampers and the controls looked remarkably similar on paper---except as in the 2004 study the crampers again had a statistically significant lower sodium concentration, and, we will repeat this, that suggests they were more hydrated compared to the controls. . .yet they were cramping. Here are the data from the electrolytes in the two groups:

	Crampers (N = 11)	Controls (N = 9)
Sodium	140 ± 2	143 ± 3
Potassium	4.4 ± 0.06	4.2 ± 0.5
Magnesium	0.9 ± 0.2	0.8 ± 0.1

Recall that what is most often put forward as the cause of cramps is either dehydration or some electrolyte disturbance, but the data from these two studies do not support that hypothesis. Although these are field studies and we cannot assign a cause and effect relationship, this available evidence suggests that these (normal) levels of dehydration do not appear to cause cramps. If these levels of dehydration did cause cramps and were largely responsible for cramps, then what we should see is a very high incidence of cramps in all of the race finishers with the same physiological characteristics as these subjects----or in other words, the vast majority of the race finishers.

Rejecting the old models

In science when the available evidence does not support the hypothesis, we must change the model. Based on this available evidence we see clearly that dehydration and electrolyte levels are not associated with muscle cramping during or after exercise, and therefore we must adopt a different model to explain what is causing them. We cannot just ignore the data we have shown here and keep on telling people that it is dehydration and electrolytes when new evidence suggests otherwise.

So in Part III of this short series we will lay out the newest hypothesis that tries to explain the "why" and the "how" of muscle cramps. It is novel and, as you might have guessed already, has nothing to do with electrolytes and dehydration! So come back and join us for Part III of this series, and then join us for the comments and debate!

See also:
Part I: Theories and fallacies of muscle cramps

References:
Brouns F et al., "Ammonia accumulation during highly intensive long-lasting cycling: individual observations." International Journal of Sports Medicine. 1990 May;11 Suppl 2:S78-84.

Schwellnus MP et al., "Aetiology of skeletal muscle 'cramps' during exercise: a novel hypothesis." Journal of Sports Sciences. 1997 Jun;15(3):277-85.

Schwellnus MP et al., "Serum electrolyte concentrations and hydration status are not associated with exercise associated muscle cramping (EAMC) in distance runners." British Journal of Sports Medicine. 2004 Aug;38(4):488-92.

Schwellnus MP. "Muscle cramping in the marathon : aetiology and risk factors." Sports Medicine. 2007;37(4-5):364-7

Sulzer NU et al., "Serum electrolytes in Ironman triathletes with exercise-associated muscle cramping." Medicine and Science in Sports and Exercise. 2005 Jul;37(7):1081-5.

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 late

How 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