In article said:
When I moved thru the starting chute on the way to the seeded area at Chicago last year (running
like hell because I was late!) I noticed something interesting. Back around 3:30-4:00 pace
starting area, people had shorts with pockets and like 15 Gu/Powergels stuffed in them, belts
with canteens of GU, multiple water bottles, and all sorts of paraphenalia strapped to them like
bullets on Rambo. The more I moved forward, the less of that I saw. There are multiple reasons
for this. First, you might say, someone running sub-3 is going to have less time on-course so
they don't need as much as someone running 3:30-4l to an extent this might be true. However, I
think people are way too swayed by slick salesmanship. For instance, if you think chugging 1000
calories of Gu is going to help you, I doubt it. I take about three during the marathon, so I
can see a few more than this but there are limits. Also, think of the extra weight/pounding on
your legs carring all that stuff is going to cause! None of it is a substitute for good, old-
fashioned training.
From Noakes, iirc, the upper limit of uptake is about 240 calories per hour. That requires that
you be running slowly enough to have full digestive processes going. While that is typically true
at ultramarathon paces, it is probably not true at marathon paces. Note that the relevant pace is
relative to the person's effort level, not absolute pace. A figure I've seen for marathon pace
uptake is more like 150 cal/hour.
For a 4 hour marathoner, that's about 600 cals during the race. Then again, there's time between
consumption and availability, which reduces the amount of during-race calorie conversion. One can
game this a bit by downing some of the fuel before the race starts. But then you're not carrying
the fuel with you in packages and it's the packages Andy is observing. So, how many packets of Gu
are 600 cal?
A different question is: How many calories of carbs do you _need_ to consume during the race? Rule
of thumb-ish, people have enough glycogen stores on hand to complete 20 miles relying pretty much
solely on that. With due LSD training, I think similar rule of thumb says that one can race a
marathon, including 4 hours (note that the 20 miles is time-independant), with on-hand glycogen
and fat metabolism. Taking on 600 cal of carbs during the race is to say that one is running the
whole race on glycogen. On the one hand, this might be advantageous, in that straight glycogen
burning is a more efficient and faster energy source. On the other hand, that 150 cal/hour uptake
is rather idealized and is assuming that you are getting the precisely correct concentration of Gu
to water for maximum uptake. Being off slows down the uptake, and being off in the wrong direction
contributes to dehydration (as fluid leaves the body to dilute the over-concentrated sugar in your
digestive system).
Of course that also says you're not taking any gatorade/ultima/whatever at the stands. Gatorade is
about 50 cals per 8 oz, iirc.
On to wondering about water. Noakes again mentions that typical maximum uptake of fluid is about
0.7 liters/hour (about 23 oz/hour). For a 4 hour marathoner, that's a total of about 2.8 liters,
92 oz. Let's round a bit and call it 96 oz, 6 lbs. Carrying all that with you from the start is a
pretty substantial addition. For the canonical 70 kg runner, it's equivalent to giving up almost
4% of his VO2max (2 of 50). To do it from water stops at the race says 8 oz every (about) 2 miles,
which sounds familiar to me. Note that faster runners would need less frequent stops (to keep up
to maximum absorption rate) and slower runners would need even more.
To the extent that pace is linear in VO2max, and averaging the penalty at 2%, the 'carry it
yourself' runner at 4 hours is giving up about 5 minutes. (Also assumes it remains 2% in spite
of the fact that you expect a much lower VO2max for 4 hour marathoning than the 50.) On the
other hand, those 5 minutes might be made up for in the fact that the runner doesn't have to
battle water stops, walk for the drinking (folks seem to have an easier time drinking from their
own devices than from cups), and have confidence that they've got all the water they want, when
they want it.
Then again, how much water does one need to consume during the race? In a warm, poorly ventilated
area (fitness center treadmill), I measured about 1.5 liters/hour of sweating. Apparently (Noakes
again) this is a fairly typical value. So, sweating off 1.5 liters, absorbing 0.7 liters -> net
loss of 0.8 liters per hour. In 4 hours, 3.2 liters, which is also 3.2 kg. Performance starts to
be noticeably affected at 2-3% weight loss (right?), and is seriously hazardous at 7% ('serious' =
even ultramarathoners pull each other off the course at that point). Let's take 2.5% as the
maximum dehydration for racing. That says that even with maximum uptake, the 70 kg runner is 4.6%
down by the end of 4 hours. (In warm, poorly ventilated, conditions.) For 2 hours, it's only 2.3%
and he's 'ok'.
Two points, one quantifiable here, one I'll have to ask about. One is that the 1.5 liters/hour
occurs in large part because sweating and evaporative cooling is the only means of losing heat in
fitness center conditions. In more reasonoble conditions, you can dissipate a substantial fraction
of your heat through sensible heat loss (you're hotter than the air you're moving through).
Roundly and roughly speaking, the sensible heat loss rate can be about 125-375 Watts, as compared
to 760 Watts production rate for a 4 hour marathoner at 100 kcal/mile. So, from 1/6th to 1/2 of
the dissipation. Taking 1/4th as a working value knocks back the required sweat rate to 1.2
L/hour, and fluid deficit to 0.5 L/hour. In 4 hours, 2 kg, which is still an unpleasant
2.9%. If half the heat is lost sensibly (evaporation is latent heat loss), then the fluid loss is
0.75 L/hour and net of 0.05 L/hour, which is quite manageable. Hence the big concern in
marathoning about the weather (a cool, breezy day is best for sensible heat loss).
The one I have to ask about is how much water is released per kcal of glycogen burned. I only
remember that Noakes mentioned that this is indeed a source, and that for ultramarathoners it is
'significant'.
In any case, the matter of limited uptake of water does point up some of the hazards and concerns
for the people taking a long time for marathons. Even for fairly good weather conditions and a
sensible hydration approach, a 4 hour marathoner can be near the edge of significant (negative!)
effects from dehydration. A 6 hour marathoner is at even greater risk as smaller errors of dress
and hydration (not to mention small variations in ability to take up water) can add up to serious
dehydration debts -- or, as we're hearing about, hyponatremia as they err in the other direction.
--
Robert Grumbine radix.net~bobgOpen ↗ Science faqs and amateur activities notes and links.
Sagredo (Galileo Galilei) "You present these recondite matters with too much evidence and ease; this
great facility makes them less appreciated than they would be had they been presented in a more
abstruse manner." Two New Sciences
