Last October, two reporters interviewed me about an article published by Benjamin Rapapport on marathon runners hitting the wall. Rapaport list Harvard Medical School and MIT as his affiliations so he's obviously crazy smart plus he's a marathon runner. Combining these two Rapaport came up with this wild looking chart that predicts how far a marathon runner can run before she hits the wall.
you can predict it here. (Warning; you'll need to know you weight in kilograms and height in meters. Good luck with that!) Mine is 49. A little high but close enough.
With the colored lines a smaller percentage of the total energy comes from glycogen as you go down from red down to purple. Yes, the more fit, faster runners burn less glycogen than slower runners. Next, look across the top and find your marathon finishing time. The slower you run a marathon the smaller the percentage your energy comes from glycogen. I'll go with 4:28. Follow your marathon time downward until you intersect with your fitness level. Are you below the gray zone? I am. If so, no carbohydrate loading for you. You don't need it. If you land in the gray zone, break out pasta and load up. Now, if I want to keep up with Bill and finish a marathon in 3:42 I would need to carb up to avoiding hitting the wall. Pretty cool, huh?
Does it work. Rapaport validated it with data from a study that looked at running 30K and glycogen depletion. True, 30K is not 42K so that might be an issue. Still, his theory does illustrate important concepts about running; 1. Not all marathon runners hit the wall. And, 2. A constant running pace is more energy efficient. For example, a runner slows down for 5 minutes and burns less glycogen. He makes up for the time by running faster for 5 minutes but in doing so he burns even more glycogen than the amount saved during the 5 slower minutes of running. In other words, as running speed increases, glycogen use increase exponentially not linearly. (See Figure 1. from the article posted below.)
article from MIT News with an unimpressive quote by me. I can't access the article in Science News but its titled Up Against the Wall.