Harvard Med Student, Benjamin Rapoport, has proposed a Marathon Formula to predict calories required for your race pace. Having hit the wall himself during the 2005 New York Marathon, Rapoport had some personal motivation for investigating the phenomena and notes recent literature showing “more than 40% of runners hit the wall during a typical marathon“. The wall being the point when your body has used up all its glycogen reserves and started burning fat.
Rapoport suggests carbohydrate requirements are influenced more by individual differences and the usual method of carb loading isn’t that efficient. He claims the study demonstrates that energy constraints on endurance runners “depend on several physiologic variables including the muscle mass distribution, liver and muscle glycogen densities, and running speed (exercise intensity as a fraction of aerobic capacity) of individual runners, in personalized but nevertheless quantifiable and predictable ways.”
The formula is
Mc =dmfc (i)
Carbs needed (Mc) = distance in kilometers (d) X weight in kilograms (m) X intensity factor (fc(i))
Rather than work it out yourself, Rapoport has put the calculator online at endurancecalculator.com. This shows the fastest pace at which a given distance, such as the marathon, can be run without exhausting glycogen stores. It also calculates the amount of carbohydrate required to avoid glycogen depletion during the race. The idea is that this will give a more accurate figure for mid-race refueling.
The study also suggests another common sense idea that sticking to a target pace means your more likely to get to get to the finish line. Although on hilly courses he points out “maintaining a constant level of exertion, rather than a constant pace, is most metabolically efficient.” Having run 18 marathons with a PB of 2:55 in this years Boston Marathon the formula looks like it works for him.