Fluid intake Debate: Comments from a doctor

31 Oct 2007 Posted by

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 [cite source=pubmed]438008[/cite] 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 [cite source=pubmed]9397327[/cite] and here [cite source=pubmed]8614313[/cite]. 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.

Ross and Jonathan

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