I’ve seen a lot of FB posts recently where runners are asking
how they should fuel for longer runs and races. The answer is… there’s a
formula for that! I’m going to have to do some math to explain this. But don’t
be afraid, I promise I will make it as simple as possible and I will explain
everything!
SCIENCE FACT #1: I AM NOT A NUTRITIONIST! I am a running
coach. I just read some articles online and tried a few things out on myself,
and I’m going to tell you about it. Now that that’s out of the way…
SCIENCE FACT #2: A 2010 paper by Dr. Benjamin Rapoport M.D.,
Ph.D. tells us that a typical human body burns about 0.73 calories per pound of
bodyweight per mile. That’s right – your speed doesn’t matter! (https://news.harvard.edu/gazette/story/2010/10/fuel-efficiency-for-marathoners/)
Using my 150-pound self as an example, I burn about 110
calories per mile. (150 x 0.73 = 109.5) Remember – speed doesn’t matter. So for
a marathon (26.2188 miles to be exact, but I’ll use 26.3 since I can’t seem to
run those tangents closely enough), I would burn about 2893 calories (26.3 x
110 = 2893). Since there are about 4 calories in each gram of carbs, that comes
out to about 723 grams of carbs (2893/4=723.25).
SCIENCE FACT #3: Research has shown that the human body can
absorb about 1 gram of carbs per minute.
Still using myself as an example, my body would need 723
minutes (1 gram per minute) to be able absorb the amount of carbs it needs to
completely replenish the calories I use during a marathon. That’s over 12 hours!
On pace for my BQ time (which I cannot currently sustain for 26 miles, but that’s
what I’m training for) I only have 3:40 – or 3.666 hours. So, during a BQ
marathon effort, my body will only be able to absorb about a quarter of the
carbs needed to replenish all the calories burned. So I can’t possibly keep up.
Now what?
Well, it turns out that your body only uses carbs for about
75% of energy production (the rest comes from fat, yay!) That reduces my
marathon carb needs to about 542 grams of carbs (723 x 0.75 = 542.25). But that
would still take 542 minutes to absorb, which is over 9 hours.
SCIENCE FACT #4: A 2008 paper by John R. Bennett, MS, CSCS
and Michael P. Kehoe, Ph.D. at the University of Central Florida (my alma mater)
cites research showing that performance is impeded when carbohydrate reserves dip below 30-50%.
(https://www.nsca.com/uploadedFiles/NSCA/Resources/PDF/Education/Articles/NSCA_Classics_PDFs/marathon_fueling_techniques.pdf)
Good news! We don’t have to replace 100% of the carbs burned
during a marathon, only a third to a half of it. Even better news! In my
example of myself, 30% -50% of the carbs I would need is 163 to 271 grams (542
x 0.30 = 162.6; 542 x .50 = 271). This is totally doable since 163 minutes (1
gram per minute, remember?) is about 2:43, and 271 minutes is about 4:31. Now I’m
in range! For my fictional 3:40 BQ marathon, I should be able to absorb 220
grams of carbs since I have 3 hours 40 minutes, which is 220 minutes (again, 1
gram per minute.) in which to do so.
So, how does this work in real life? Well, you see that you
can’t really replace all that you are using. But you also don’t want to fall
too far behind. The truth is that you need to practice your fueling strategy
just like you practice running. Common running lore says that you don’t really
need nutrition for runs shorter than an hour. When your marathon-training long
runs go beyond an hour, it’s time to start practicing your fueling. That way
you can find out what works and what doesn’t during training instead of during
your race.
Here’s what I do. I take my first gel at about mile 4. Then
I immediately take a couple sips of water. After that, I take another gel and a few sips
of water every three miles (miles 7, 10, 13, 16, 19, 22, 25). In a marathon,
with only a mile to go, I don’t think it’s necessary for that last dose at mile
25; you should be concentrating on cranking out that last 1.2 miles and looking
good for the finish line photos!
Let me tell you that in my own personal experience, the
first time I followed this plan was during my most recent marathon (April 2017)
and this also happened to be the first of my three marathons where I did not “bonk”,
or “hit the wall” around the 20-mile point. Nope, I was running strong and
happy and evenly-paced throughout. It was also my fastest time. I used Boom! Gels,
and if I remember correctly, I don’t think I could manage to cram in any more
after mile 19. If you look at race pictures, I’m holding a gel for the last
three miles!
Note that many of the leading sports gels (Gu, Boom!,
Hammer, Clif) each have about 25 –ish grams of carbs per dose. Make sure you
take them with water, since the body requires water to digest carbs. Some people
find they can’t handle the concentrated carbs; it’s ok to go natural, but real
food is harder to carry then gels. Some people use pretzels, fig newtons,
dates, etc. You could even combine your carbs & water and just sip a sports
drink, but make sure you are getting enough calories per sip. Another reason to
practice to find out what works best for you.
I hope that helps! Fuel up for your next half or full
marathon, and be sure to let me know how it goes!
