How Boston Marathon runners can avoid hitting the wall

Peter Wehrwein

Contributor, Harvard Health

A Harvard/MIT student has created an online calculator for prerace carbo loading.

Come tomorrow morning, about 27,000 runners will begin the annual 26-mile, 385-yard (42.195 kilometers) mass run from suburban Hopkinton to Boston.

But if past marathons in Boston and elsewhere are any indication, perhaps up to 40% of these optimistic and determined souls will slam into a sudden sensation of overwhelming, can’t-do-this fatigue several miles (typically about five) before they get a chance to experience the glory of crossing the finish line.

It’s called “hitting the wall.”

Getting through, around, or over hitting the wall is part of the mystique of marathon running, although there’s a physiological explanation that’s not all that mysterious: when runners hit the wall, their bodies have run out of the carbohydrates needed to sustain intense physical activities like long-distance running.

Benjamin I. Rapoport believes many runners could avoid hitting the wall if they put a few key facts about themselves and their target marathon time into the online calculator he created, which can be found at The calculator will tell them how many extra calories they should get from pasta, rice, or other high-carbohydrate food or drink before (and in some cases, during) running a marathon.

More precision

No, Rapoport doesn’t have a concession on pre-marathon carbohydrates; he’s a 29-year-old student in the M.D.-Ph.D. program jointly run by Harvard Medical School and the Massachusetts Institute of Technology.

And the anti–Atkins Diet advice to go ahead and gorge on carbohydrates before endurance events is old news; “carbo loading” with heaps of pasta before marathons is a time-honored tradition.

But after Rapoport experienced hitting the wall firsthand several years ago while running the New York City Marathon, he set out to put some precision into the general advice to carbo load. The result: a dense, equation-filled 12-page paper on metabolic factors limiting marathon performance in the October 2010 issue of PLoS Computational Biology. The online calculator is a condensed, user-friendly version of the important conclusions he drew about the mathematical relationships between running pace, aerobic capacity, and carbohydrate use and storages.

To Rapoport’s pleasant surprise, his paper and the calculator have attracted worldwide media attention. (My favorite headline: “Marathoning made easy.” If Rapoport’s calculator figures out carbo loading and does that, he really is on to something big!)

The attention prompted him to start a consulting business, and he has advised the U.S. national cycling team, as well as some private individuals.

All of this is sideline: Rapoport is planning on a career in neurosurgery and his just-completed Ph.D. thesis is on electronic implants in the brain.

Playing with the calculator

So let’s run some numbers through Rapoport’s calculator.

Robert Kiprono Cheruiyot set a course record when he won the Boston Marathon last year in time of 2 hours, 5 minutes, and 52 seconds (which works out to 4:46 miles, a simply astounding pace).

Cheruiyot’s age (22) was easy to find, but I had to guess about his weight (135 pounds) and his resting heart rate (40 beats per minute), which in Rapoport’s calculator is a proxy for the standard measurement of aerobic capacity, VO2max.

According to Rapoport’s calculator, if Cheruiyot wants to match his 2010 time tomorrow, he should have stoked himself with 1,944 calories of carbohydrates, which is about the number of calories in seven, 1-cup servings of pasta.

Rapoport notes that runners need to count just the calories from the carbohydrates, not the calories from the sauce that typically comes with it, and that the carbo-loading carbs are on top of normal calorie intake. In addition, carbo loading should be completed 12 hours before race time to allow time for digestion and storage.

A more typical runner

Now let’s consider the more typical runner in tomorrow’s marathon: a 45-year-old, 120-pound woman with a resting heart of 60 and a goal of finishing the marathon in 4 hours (9:09 miles). Rapoport’s calculator says her prerace carbo load should be 1,347 calories (five servings of pasta).

But say her resting heart rate was 70 instead of 60. She’d need to increase that to 1,715 calories of carbohydrates, according to the calculator. Why? Because a higher resting heart beat means a lower aerobic capacity, and a lower aerobic capacity means a runner is less fuel efficient: he or she will burn more carbohydrates running at the same pace as a runner with a higher aerobic capacity.

Rapoport’s calculator also figures out how much slower a person would need to run if he or she were to skip carbo loading (and yet still wanted to avoid hitting the wall). Without carbo loading, Cheruiyot would need to drop his pace to 6-minute miles (or slower) and a 2:37:37 finish (or slower). The hypothetical 45-year-old woman runner would need to throttle down to 9:50-mile pace and finish at 4:18:04 or slower.

A rice-fueled Rapaport

I hope to catch a little bit of the marathon tomorrow in Boston at about a mile before the finish. I’ll be keeping an eye out for number 1223, the number Rapoport is wearing.

I plugged his numbers into his calculator (age 29, 145 pounds, an impressive resting of heart rate of 40, a target time of 2 hours and 50 minutes). His carbo loading requirement: 1,349 calories.

When I spoke to him earlier today, he was working on finishing up multiple bowls of white rice, plus some fruit juice, before getting to bed at 6.

(Photo courtesy of, 2010)


  1. Sean Gunner

    Thanks for the interesting article – plus the link to Mr Rapoport’s online calculator. As much as diet plays a part in hitting the wall, so I suspect does the mental stress of months of training and anticipation, together with the physical fatigue of overtraining.

    Something I have found is particularly useful in my pre-marathon and post-marathon training (and is particularly good should running injuries occur), is training hard on an exercise bike.

    I do a 10-minute warm-up on the stationary exercise bike, pedal at close to maximum intensity for one minute, recover by spinning for 30 seconds at 90 rpm against very little resistance and then continue with 60 seconds of easy pedalling (adding up to 90 seconds of recovery in all). The pedal at very close to max for another minute.

    I continue this workout routine of 60 seconds of maximal effort followed by 30 seconds of spinning and 60 seconds of light pedalling – until I have completed eight nearly all-out one-minute intervals. I then follow this with a 10-minute cool-down.

    Over time, this workout can be made even more challenging by increasing the number of maximal 60-second repetitions to 15, increasing the resistance on the bike, and cutting down the recovery intervals to 60 seconds (30 seconds of spinning, 30 seconds of light strokes).

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  2. Stephanie Perez

    This year I actually made it to the 14 mile mark. I felt beaten down, flustered, and I had the worst shin splints (the following day) that I have ever had in my life.

    The interesting part on your post is the usage of carbs in for cardio – in your example the 5 servings of pasta. The thing is I try to avoid carbs because I lost a lot of weight (prior to training for CC running) on the Scarsdale Diet. I’m worried if I took the suggestion that I would regain some of my weight. Of course, I can’t imagine it if I keep training like I am, but at what point can I re-implement the carbs as suggested?

    I’m actually blaming the high protein within my diet plan for my failure (well if we can call it a failure at 14 miles seeing that I could not have imagined running 1/2 a mile 2 years ago).

    I’m going to continue training, and I’m going to bookmark your page and ponder on it for a while. Perhaps its just raw fear on so many carbs. (rice & pasta).

    Thanks for the endurance calculator btw. I think it will help with my training. I’ll try to report back next year how far I was able to make it. I’m hoping for total completion!

  3. Anonymous

    I just amazes me that people continue to exert themselves beyond expectations. For your body to be pounding the pavement not only for the race, but just to practice and continue to excel is rather daunting.
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  4. Chad Waterbury Fan

    Well for me it’s the same thing. The desire to run is what really matters, most of us may think there’s some kind hitting the wall experience, but for me there’s no such thing.

  5. Mike

    The reason the marathon is such a challenging event is that the runner has dig into energy reserves beyond the available glycogen stores (from carbohydrates). There are many factors that determine how well a person will perform – not just the amount of carbs consumed! Here are a few:

    – amount & quality of marathon training done
    – physical condition on race day
    – weather
    – course terrain
    – running pace

    To suggest that eating the right amount of carbohydrates can help avoid hitting the wall maybe be partially true but is somewhat misleading.

    i have personally run several marathons successfully without an overload of carbohydrates.

    See my ‘26.2’ website for more detail on this interesting topic.

    • Ruth

      I agree with Mike – there are lots of things that determine if you will ‘hit the wall’ or not. I checked his 26.2 website and really liked the article on Marathon Diet.

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  6. Steve Chapal

    I read your article with interest, and was just as interested to see how Mr. Rapoport fared in the 2011 Boston Marathon, sionce I was a participant as well. I notice that he completed his first half (13.1) in 1:29:33, a rate per mile of 6:50, but deteriorated considerably in the second half, with a time of 2:01:20 and rate per mile of 9:16 per mile. Have you had any feedback from Mr. Rapoport regarding his experience? His overall finish time of 3 hours, 30 minutes and 53 seconds was far behind his target finish time of 2 hours and 50 minutes. Thank you.

  7. health

    The story is interesting, thank you, but makes no mention of how hydration and race day temperature affects the runner as far as hitting the wall. I would be interested in the influence on running by these factor on different individuals. In my case the warmer the tempeture the quicker I hit the wall granted I did not count calories intake before the marathons. Also the effect on the amount of training miles i.e. how much is enough.
    p.s. how can individuals access the formula ?

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  8. Jose Viveiros

    The story is interesting, thank you, but makes no mention of how hydration and race day temperature affects the runner as far as hitting the wall. I would be interested in the influence on running by these factor on different individuals. In my case the warmer the tempeture the quicker I hit the wall granted I did not count calories intake before the marathons. Also the effect on the amount of training miles i.e. how much is enough.
    p.s. how can individuals access the formula ?

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