The Science of Dehydration

This summer I have begun working on my master's thesis for my degree in exercise physiology.  It has brought back fond and less than fond memories of the comps project I completed no more than a year and a half ago.  Grad school sure flies by!  Here is a hilariously accurate portrayal of my experience with science. http://getrealscience.com/katelyns/wp-content/uploads/2011/06/sciencerage.png

Not surprisingly to those who know me the project is progressing at a modest pace.  In order to motivate my reading of the literature and to restrain myself from becoming that little face on the left side of the cartoon, I plan on blogging my findings, thoughts, questions.  That said, warning to those of you who are looking for traditional scientific style writing, free of gramatical errors.  This is not meant to be a review of literature or to agrue a standpoint, or to be used for any other purpose besides brainstorming questions and ideas.  Please, let me know if you know of any good articles or if you have a concern or different perspective on the topics.  Don't be shy.  Even if it may sound like it, at this point, I have no hard stance on the issues I just want as many perspectives as possible.

My research this fall is looking at temperature regulation and running performance across a cross country season.  So I want to start with a post on hydration.

A lot of the popular assumptions about temperature regulation relate to hydration.  The reason for this is that in many laboratory studies conducted in warm and humid conditions, athletes who consumed fluids during the exercise had lower core temperatures and lower heart rates compared to dehydrated subjects.  As a result, a general consensus confirmed by the ACSM's position stand is that proper hydration (often thought to be maintaining weight loss under at least 2% of pre-exercise weight) can prevent heat-related illness, and improve performance.

Still, there has been no solid evidence to suggest the hydration status is linked to heat-related illness.  More likely, these conditions are related to a failure in the bodies ability to sense injurious temperatures and react accordingly.  This failure of homeostasis is often a result of previous infections, genetic predisposition, or consciously overruling the bodies defenses against excessive heat production (pacing) and storage (ie. wearing a helmet and pads in football, or a sweat suit in the sauna to make weight in wrestling).

Like I stated earlier, many of the early studies that looked at distance races and fluid consumption revealed that the more water weight an athlete lost, the higher their core temperature and heart rate.  But an aspect of these studies that has been over looked is that the top athletes were the ones with the highest core temperature following the race.  There is little evidence to suggest maintaining weight throughout a competition will improve performance.  In fact, in competition durations of over an hour glycogen levels noticeably deplete.  Glycogen in a sense stores water in the muscles, so if you were to maintain weight throughout a marathon you would be considerably over-hydrated due to the lowered water storage capacity of depleted muscle glycogen.  Realistically, maintaining weight throughout a marathon would be considerably uncomfortable and near impossible for anyone running under four hours in a moderate to warm climate.  On the other hand, now that marathon running has begun to appeal to a much larger range of abilities, runners who stop at each aid station and are not "working up a sweat" could easily get themselves into trouble with hyponatremia, a condition in which excessive water intake dilutes the fluids in the body and can have disastrous health consequences.

Until relatively recently marathoners held very different attitudes about hydration.  In fact, fluid consumption in the first half of a marathon would have been considered weak and a sign of poor fitness.  Fluids were not recommended until the final miles because it would pull blood from the muscles to the intestines to absorb the water, hence slowing the runner's pace.

Finally, when we talk about dehydration we often confuse the real issue which is water balance.  The reason heart rate is affected by fluid loss is because when we sweat the volume of our blood (more accurately plasma) decreases.  This means our heart has to pump faster to maintain pressure in our blood vessels.  Our core temperature rises because the loss of plasma volume and blood pressure does not allow for additional blood to flow to the skin to dissipate into the environment.  (Side note: a criticism to a lot of the literature on core temperatures during different intensities of exercise in different climates are that studies are conducted in a laboratory setting in which participants perform without the usual air/wind resistance and convective heat loss.)  A common counter-attack to this reduction in plasma volume includes sodium loading.  A sodium gradient resulting from increased intake allows for two things, increased absorption of fluids in the intestines and increased ability to retain more water within the plasma. Carbohydrates in fluid (ie. sports drinks) can also increase fluid absorption in the intestines and at the right concentrations have definite performance enhancing ability for long duration activity, but that’s a topic for another time.

Most of these thoughts reflect my reading of Tim Noakes and his former student and Johnathan Dugas, whose ideas I have considerable respect for.  I believe they make strong cases against some unquestioned conventional wisdom of our understanding of exercise physiology.  Alternatively, I believe their arguments can lead to misunderstanding in translation and unqualified rash changes in athletic applications.  As a coach I would never encourage athletes to enter races dehydrated, however, I may question them if they consistently carry around gallon jugs of water because someone on tv said that we need at least 64 ounces of water a day (show me the research on the 64 ounces rule and then I might believe you).  Excessive water intake during exercise is not going to keep you from slowing down in the heat and excessive water intake throughout the day is only going to over work your kidneys.  That said yes we need water.  Yes, stopping at the water fountain on a hot day is only going to make your run feel easier, but if you really want to cool your core temperature in the heat dumping water on your head will be more effective than dumping it in your stomach.

For those of you who are still with me, here are the simple practical applications I would recommend from my current understanding of hydration, thermoregulation, and fluid balance.

1.      On really hot sweaty days, either consume sports drinks in place of water since they are specially formulated to be optimally absorbed and mimic the composition of sweat, or add sodium to your diet if you don’t already.

2.      Don’t force yourself to drink water because it’s supposed to be healthy, remember to consider your thirst and drink when thirsty.  Alternatively, don’t ignore your thirst, or get to busy to drink.  Carrying a water bottle is good because it reminds you when you are thirsty, but it is not necessary to get in a habit of drinking just because it is there.

3.      From the weight control stand point pay attention to the caloric value of the fluids you drink throughout the day, you can drink a lot of calories without ever feeling full.  This is good if you're training really hard and cannot keep up with your calories, on the other hand of course if it becomes a habit in the offseason it’s probably going to get harder to balance your caloric intake with energy expenditure.

4.      If you are not working up a sweat, you shouldn’t be overly concerned about hydration.

5.      Hydration during competition should be a balance of maintaining plasma volume, limiting intestinal discomfort, not carrying excessive weight, and improving your perception of exertion.

6.      Following competition it may take awhile to return to pre-competition weight due to the lag in glycogen resynthesis, however, if you, like me, are a urine color detective you can attest to the importance of rehydrating in the hours following a hard effort.

7.      Not a hard and fast rule but I still love it: urine should look like lemonaid not apple juice

If you are still loking for more here are some great recent blogs on the topic:

http://peakperformance.runnersworld.com/2011/07/enjoying-the-heat-neither-is-any-other-runner-but-a-little-pinch-of-salt-might-help.html

http://sweatscience.com/pre-drinking-to-hyperhydrate-and-other-heat-related-research/

Chasing a degree : )
Laura