Hydration Physiology: Application
My guests today are veterans of Science of Ultra; they joined me in Episode 4. LISTEN TO THAT EPISODE (#4) FIRST IF YOU HAVEN’T ALREADY. Today we continue our series with them on all things sweating, hydration, electrolytes, and fluid balance.
Up first is Team Leader of the Thermal and Mountain Medicine Division at the US. Army Research Institute of Environmental Medicine (aka USARIEM). In addition to his doctorate in exercise physiology, he is also a registered dietician. My first guest is Dr. Sam Cheuvront. My second guest is Principal Investigator in the Thermal and Mountain Medicine Division at USARIEM. He served as the president of the New England Chapter of the American College of Sports Medicine. And, he is an ultra marathon runner himself. So, he knows first hand what it takes to achieve in our sport. My second guest is Dr. Robert Kenefick. Collectively, my guests have published over 200 peer-reviewed papers, book chapters, and reviews. They are two of the world’s leading scientists in hydration and fluid homeostasis, especially during exercise.
They work for the U.S. Army. So, we must provide the disclaimer that "The views and/or opinions of Dr.'s Kenefick and Cheuvront are theirs personally and do not reflect the views or opinions of the U.S. Army or DoD."
In the first part of this series, episode 4, we focused on the physiology of fluid and electrolyte balance. That episode is packed with fundamental physiology and what we talk about in this episode builds on what we covered in episode 4. So, you’ll benefit most from this episode if you’ve listened to that episode.
In this episode, we’re focusing on: FLUID BALANCE AND THERMOREGULATION WHILE PLANNING FOR PERFORMANCE
Quick background: We sweat to put water on the surface of our skin, which evaporates to the environment. The transition from liquid to gas requires a large amount of energy; sweating cools us because that energy comes in the form of heat, which is drawn from our skin. Sweat that drips off of us, does not provide that cooling benefit. Either way, that fluid loss eventually impacts all three body fluid compartments, which are 1) blood plasma, 2) intracellular (inside cells), and 3) interstitial (outside cells but not including blood).
Listen and learn the answers to these questions:
We start with a scenario: I go for a long run and during the run my urine is dark; after the run I try to replace fluids by drinking plenty the rest of the day and by bedtime, my urine is a much lighter color. But, when I wake up in the morning, it’s dark again…what’s going on?
What is the time-frame for fluid/electrolyte shifts among body compartments?
As we sweat, the fluid and electrolytes initially come from the interstitial compartment, specifically around the glands near the surface of our skin. As we run and sweat…what do we know about fluid shifts and electrolyte shifts across the three body compartments during prolonged exercise.
Another example, I run and take water = regular urination and clear; drink electrolyte solution = less urination and darker…we talked about the physiology of this in episode 4 but now, putting a real world example to the physiology, what’s happening to me in those cases?
Then we move into specific preparation for performance
Dr. Kenefick is an ultra runner and a leading expert on this topic, plus he has access to all resources for measurement and testing. He must never have any problem with fluid and hydration...right?
Once in a while, we hear advocates of ‘bonk’ runs where one would purposefully dehydrate or go out without water. Clearly, this can be very very dangerous and we recommend against doing bonk runs. Out of curiosity, thought, is there any evidence that we can train in a way that will help us to perform better in a dehydrated or low volume state?
Keeping ALL ELSE EQUAL, what are the practical, relative effects of each of the following on sweating: long clothing vs short vs nothing (same material - just different coverage), tightness of clothing, type of material, color of material?
What are the definitions of adaptation, acclimation, acclimatization?
What does it mean to be acclimatized to a hot environment with respect to body fluids, hydration, and sweating?
What are best practices for preparing to race in warmer environments? Exercise, sauna,…?
What is the recommended protocol for acclimation to heat in preparation for an event?
What is the time-course of gain and loss of heat acclimation?
When we plan for thermal stress from the environment, we must consider not only temperature but other factors such as wind, sun exposure, and humidity. Let’s say that we have gone through the acclimation protocol. Is there a cut off temperature/thermal stress range, below which, there is no benefit to performance. How can we gauge whether going through the protocol will be of benefit?
Specifically thinking about what’s going on during running: at what body temperature do we begin to sweat and where on the body do we sweat first, most, etc.?
Should we be concerned about gear placement (e.g., hydration pack vs waste belt) with regard to efficient sweating and cooling? E.g., would we expect any appreciable difference in fluid loss or cooling over time for someone wearing a hydration pack vs waist belt vs none or handheld bottles.
To what extent does carrying extra weight affect sweat loss due to the extra work of carrying it; e.g., as much as 5 lbs for some full hydration packs vs 1 lb or so for a full handheld.
Many people have the idea that, while running, ‘if they are continuing to urinate and it isn’t very dark, then they are probably OK’. We’ve established that watching urine color - DURING exercise - is not a reliable method for monitoring hydration status. So, “How can I monitor myself for appropriate fluid replacement and maintenance during an ultra marathon (or during a long training)?”
Our wrap up, big money question today...WHAT’S THE ANSWER?