Oct 20, 2015
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
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
In this episode, we’re focusing on: FLUID BALANCE
AND THERMOREGULATION WHILE PLANNING FOR
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
- 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
- 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
Then we move into specific preparation for
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
- 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,
- 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
- 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
- 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
Our wrap up, big money question today...WHAT’S THE