Posted by Dave on June 29, 2011 | 11 Comments
In 1967, a landmark study led by Bjorn Ahlborg found that military recruits could improve their ability to perform endurance tasks after a carbohydrate depletion and loading regimen over the preceding week.
The dietary restrictions were quite extreme, and were coupled with exhausting exercise: A hard workout followed by three days of a low-carb diet, another hard workout, and another three days of a high-carb diet. Each day the researchers measured the level of glycogen in the recruits’ muscles. This graph gives a rough summary of the results:
As you can see, after the depletion/loading regimen, the recruits had significantly more glycogen in their muscles than when the study began. Since glycogen is the muscles’ most readily available source of energy, runners of the day reasoned, completing this regimen should allow them to run faster in races. Later research suggested that runners’ baseline level of glycogen was actually sufficient to carry them through most races; it’s only during marathons that this extra energy supply is useful.
But even as carbo loading and depletion became standard practice among marathoners, some questioned whether it was really helping much. After all, two hard workouts the week before a race, in addition to radical diet shifts, might outweigh any advantage gained by a little extra glycogen. Even if muscles don’t have glycogen, they can compensate (albeit less efficiently) by using fat.
In 1981, a team led by W.M. Sherman found that the carbo-depletion phase of carbo loading may not have been necessary after all. All that was needed was to increase carbohydrate consumption a few days before competition, while easing off on workouts (tapering) and the same benefits could be reaped.
Based on this research, I tried to carbo-load for both marathons I’ve run, and I still find it quite difficult. I needed to consume 3,500 calories (about 800 grams, 10 grams per kilogram of my body weight) in carbs for each of three days before the race, and frankly, by the end of Day 2, I was sick of pasta and bananas. Carbo-loading doesn’t require that you limit yourself to just carbs, but any other food you eat simply means you’ll have to consume more total calories. If, like me, you’ve just proudly shed a ton of weight while training for the race, that’s not something you’re particularly inclined to do. Add to that the difficulty of eating well while travelling, and I felt like I never really fully carbed up.
Indeed, a 1991 study led by G.M. Fogelholm suggests that I’m not alone. These researchers instructed athletes to eat 9 grams of carbs per kilogram of body weight for 5 days before a hard run. But when they measured glycogen levels in the runners prior to their run, they weren’t significantly different from the levels at the start of the program, suggesting either that the runners didn’t actually consume as many carbs as prescribed, or that this regimen wasn’t actually helping (rather than resting as in other studies, these athletes did 45 minutes to an hour of easy running each day during carbo loading).
While the research on carbo-loading continues to be done, the key elements appear to be rigorous adherence to the proscribed diet, and rest during the final carbo-loading phase. Does it have to be so hard to carbo-load? A review article by Louise Burke argues that most benefits of carbo-loading occur in the 36 to 48 hours before a race. That’s just two days, not three, and nowhere near five days in the Fogelholm study. Sure, effective carbo-loading requires more than just a plate of spaghetti the night before the race, but I think I might be able to manage a two-day regimen better than a three-day regimen.
What this means for you
If you’re going to attempt a carbo-loading regimen, you need to be truly committed to it. This means eating 10 to 12 grams of carbs per kilogram of body weight (4.5 to 5 grams per pound), for at least two days before the event you’re preparing for. There’s no point in carbo loading for events shorter than roughly 30K (A marathon is 42.2K). Carbo-loading doesn’t substitute for proper in-race nutrition, including hydration and consuming more carbs during the race.
Burke, L. (2007). Nutrition Strategies for the Marathon Sports Medicine, 37 (4), 344-347 DOI: 10.2165/00007256-200737040-00018
Fogelholm, G., et al. (1991). Carbohydrate loading in practice: high muscle glycogen concentration is not certain. British Journal of Sports Medicine, 25 (1), 41-44 DOI: 10.1136/bjsm.25.1.41