Dehydration no big deal when running?

Dan Stumpus said...

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"PJ Wilson" <[email hidden]> wrote

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Dan Stumpus said...

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Provided you pace it well, and drink more than 4.2 oz (no spitting!)
There's plenty of research on line about the effects of dehydration on
performance.

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You mean like this?

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http://www-rohan.sdsu.edu/dept/coachsci/csa/vol72/whittles.htm

Interesting cite. Not a good test, though. The subjects could do 10
minutes at half-marathon pace (if I remember my %'s) without using any more
oxygen. Their core temps were higher, though. This doesn't bode well for a
longer test -- my prediction is that core temp would rise to the point where
performance would suffer.

A better test would have runs to exhaustion and time trials, like this one:

http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list

_uids=4033401&dopt=Citation

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It showed significant performance decreases at 1500m, 5k and 10k with much
less dehydration (~2% body weight). And the run to volitional exhaustion on
the treadmill was noticably shorter at those mild dehydration levels.

The two sets of results jibe with my experiences on easy-ish long runs with
very little drinking -- it's not that big a deal if it's cool. Sometimes if
I'm lazy and it's cool I'll run 3+ hours without drinking much and lose 5
lbs, but I finish in one piece. But it becomes a big deal in an all-out
race or it's hot -- I cramp up after awhile (several hours) if I'm down 5%.

OK then... here is a run-to-exhaustion study showing no effects at 4% body
mass loss:

http://www-rohan.sdsu.edu/dept/coachsci/csa/vol72/kenefick.htm

Don't you just love these studies? You get to pick the one that best supports
your point of view. :-)

There is a saying that goes something like, "Statistics are like
prostitutes. Once you have them you can do anything you want to with
them."

An interesting exmaple of dehydration was the
first episode of the current TV series survivor Guatemala.
The first competition was a 11 mile hike in the
jungle. Many people didnt drink enough and had
severe camping and retching. More of the gung-ho
young guys rather than the more paced women
had this problem.

Dan Stumpus said...

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Yeah, but: we can quibble over the effect of dehydration at mild temps.
But dehydration when it's warm can lead to serious overheating...I've run
ultra races in 100 degree temps (had cramps but not heat exhaustion), but
drank like a fish. If you stay in salt/water balance you can sweat quite a
lot (>60 oz/hour), keep your core temp below critical, and keep running.

I won't quibble with your personal experiences - what works for you, works.

But if I remember correctly, Noakes makes a case that if you are headed for
heat exhaustion due to the level of exertion plus enviromental conditions, no
amount of additional fluid will help prevent the onset of hyperthermia. The only
way to prevent it is to either slow down or get to a cooler place.

And lest you think the ingestion of additional salt will keep you safe, he has this
to say:

"Thus, the possibility remains that even if athletes were to ingest very large
amounts of sodium during exercise, they might still develop cerebral edema if
they over-drank, because it is the cerebral edema and not the serum sodium
concentration that causes the symptoms in this condition. There is evidence
that high rates of sodium ingestion during exercise reduce the rate of urine
production [55], thereby perhaps increasing the possibility that fluid overload
will develop when voluntary over-drinking is combined with high rates of
sodium ingestion."

So go figure.

"PJ Wilson" <[email hidden]> wrote

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But if I remember correctly, Noakes makes a case that if you are headed
for
heat exhaustion due to the level of exertion plus enviromental conditions,
no
amount of additional fluid will help prevent the onset of hyperthermia.

Yes, such as heat + very high dew points -- sweating doesn't work, you gotta
stop moving and get in the shade.

Such as from Tony's post: July 8th 2003, in Dhahran: Dew point 95", temp
108", for an apparent temp of 172".

Heat (100 is the hottest I've raced in) and low dew points (in the 40's) is
ok if you stay hydrated and sodium'd. The evaporative cooling system is
pretty amazing when conditions let it operate. Failing to take salt when
you're running many quarts through your system will get you hyponatremic.

Quoted message said:

"Thus, the possibility remains that even if athletes were to ingest very
large
amounts of sodium during exercise, they might still develop cerebral edema
if
they over-drank, because it is the cerebral edema and not the serum sodium
concentration that causes the symptoms in this condition. There is
evidence
that high rates of sodium ingestion during exercise reduce the rate of
urine
production [55], thereby perhaps increasing the possibility that fluid
overload
will develop when voluntary over-drinking is combined with high rates of
sodium ingestion."

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So go figure.

That all makes perfect sense. But developing salt-o-phobia if you're out
there for hours in stressful conditions is not a good plan.

Salt, like water, has an optimal consumption rate, depending on the athlete
and the conditions.