When its more than just data: More on hydration

I have to come clean. I’ve got a drinking problem. I really cannot decide how much I need to drink before I leave on a training ride. Sometimes I drink more, sometimes I drink less, but I always drink to thirst. When the world’s most interesting man advises you to stay thirsty, he knows what he’s talking about!

So here we are again, once again discussing hydration. I simply cannot escape the bad articles posted on Competitor and the endless questions (not that the latter is a problem). The latest article discussed “new evidence” for 2010 on the importance of hydration, and how losing 2% of your body weight is the beginning of the end. As I’ve recently discussed (as in a few weeks, not years) on Tipcast 37, weight loss can be deceiving. However, my objective with this brief article is to point out some of the issues with hydration research and what are important things to consider when evaluating a research paper by discussing a 2013 Review by Dr. Eric Goulet.

Combining data carries more weight

Unlike individual research papers that can be limited by the subject pool, study design or chance, summarizing data from many studies give a broader idea of the overall effect of a “treatment”, like hydration or carb feeding; this is what a review paper does. More robust recommendations, however, can be made by pooling data meeting specific criteria; this is what a meta-analysis does. In his 2013 meta-analysis titled Effect of exercise-induced dehydration on endurance performance: evaluating the impact of exercise protocols on outcomes using a meta-analytic procedure, Dr. Goulet notes

“The recommendation to limit dehydration to 2% bodyweight during exercise is based from results of studies that used exercise protocols where athletes were forced to exercise at fixed-work rates until exhaustion or at least during part of the exercise protocols. These research designs have a poor reliability or possess a very low ecological validity, thereby suggesting that they should not be used in the establishment of fluid intake guidelines, especially those designed for athletes.”

The best studies rely on a known end point either energy expenditure or distance, thereby allowing athletes to pace accordingly. By allowing athletes to “run to exhaustion” you create a vary unreliable test. In order to be used for his analysis studies must have met the following criteria:

1) laboratory-controlled

2) Exercise induced dehydration (EID) induced during, not before exercise*

3) fluid replacement during exercise given orally

4) data needed to calculate % change in power outputs, effect estimates, variances and EID levels reported

5) if perfect euhydration maintained during exercise (ie, 0% bodyweight loss): minimum dehydration level set at ≥ 1% bodyweight loss

6) if perfect euhydration not achieved during exercise (ie, different from 0% bodyweight loss):

  • A) euhydration considered when end-of-exercise bodyweight loss within ± 1% of start- ing exercise bodyweight
  • B) minimum EID level set at > 1% bodyweight loss and
  • C) difference in EID level between the dehydrated and euhydrated group ≥ 0.45% bodyweight

7) same quantity of carbohydrate provided between exercise trials and

8) Performance assessed in compensable exercise-heat stress.

Most of these are straight forward, but number 2 is important, because the Competitor article cited a paper that pre-dehydrated their runners and then showed that the dehydrated runner ran worse. Despite being a well-designed and well-implemented study, I sometimes wonder about an agenda to confirm a bias. Yes, starting dehydration is a bad idea, no, I would never suggest to any athlete, “Nah, don’t worry about drinking before the your race, just drink as little as possible 24 hrs before.” Starting a race dehydrated is just not smart.

Far be it from me to ruin all your fun, though. I’ve included the paper here to read. However, if you just want the major points, here they are:

▶Results of the present investigation clearly show that only under exercise conditions comprising fixed-power output work does EID impairs EP

▶Because under real-life exercise conditions athletes can constantly adjust their speed according to body cues, drinking to thirst-associated EID of up to 4% is very unlikely to hinder EP

Stay thirsty my friends!

Goulet BJSM meta-analysis 2013

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