Carbo-loading: An idea whose time has passed?

Posted by Dave on June 29, 2011 | 9 Comments

ResearchBlogging.orgIn 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.

Ahlborg, B., Bergström, J., Ekelund, L.G., Hultman, E., & Maschio, G. (1967). Human muscle glycogen content and capacity for prolonged exercise after different diets Forvarsmedicin, 3, 85-99

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


9 Responses to “Carbo-loading: An idea whose time has passed?”

  1. zoon politikon
    June 30th, 2011 @ 12:07 pm

    Blogroll Frühjahrsputz…

    Schon seit Monaten habe ich mir vorgenommen meine Blogroll zu überarbeiten, damit sie meine jetzigen Lesegewohnheiten besser reflektiert. Nun habe ich diesen Frühjahrsputz endlich an die Hand genommen und möchte die Gelegenheit nutzen, gleich ein paar …

  2. Travis
    June 30th, 2011 @ 3:03 pm

    I don’t have any refs to back this up (I think they might touch on it in Lore of Running, but it’s been a while since I’ve dusted it off), but my take is that so long as you’re consuming a relatively high-carb diet in the days before a marathon and have good nutrition during the race itself, carbo-loading isn’t going to give much of an additional performance benefit. Especially considering the relative unpleasantness and/or difficulty of carbo-loading, which may itself have a negative impact on race performance. I’d much rather feel relaxed and happy leading up to a race with slightly sub-optimal glycogen levels, rather than feeling stressed about my diet.

  3. Michael Caton
    July 1st, 2011 @ 3:20 am

    First of all, great blog. Unfortunately I often find exercise science articles focus on proxy indicators. These all seem to use muscle glycogen as the endpoint. I’d be very interested to see other studies looking at carb intake and performance indicators, rather than muscle glycogen. The only reason we care about muscle glycogen is because we assume it correlates with a better time, and/or feeling better at the end of the race. Maybe this is accepted as a strong proxy endpoint in exercise science but if that’s the case it would help readers (including myself to know this).

  4. Dave
    July 1st, 2011 @ 2:47 pm

    Good point Michael, I’ll see what I can dig up. I’d imagine, though, that it’s difficult to find an effect of actual performance because there are so many other variables involved.

  5. Alex Hutchinson
    July 5th, 2011 @ 3:24 am

    Interesting topic, Dave. A couple of quick points:

    (1) As you pointed out, the evolution of carbo-loading has been to shorter and less invasive protocols. I’d take it a step further: researchers (in at least one study) have found that just one day of taking in 10 g/kg is enough to max out glycogen stores, and you don’t get any further benefit from days 2 and 3. Of course, 10 g/kg is a very large amount of carbohydrate: the elite marathoners I know don’t rely on food alone to reach that level — they’re sucking down high-carbohydrate sports drinks on the day before the race. Here’s the reference I was referring to:

    (2) There have been studies that measure performance rather than just muscle glycogen (you’ll find the references in the Burke review). The problem is that, for the sake of scientific clarity, these studies have tended not to allow the subjects to take in fuel DURING the event. Of course, this isn’t consistent with what happens in real life. So does “fully maxing out” your carb stores make a crucial difference if you’re also sucking down gels and sports drinks during a marathon? That’s less clear — especially when you consider that each gram of glycogen is stored with ~3.7 grams of water, meaning that carbo-loading can add several pounds…

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  8. Ernesto Burden
    February 8th, 2014 @ 8:53 am

    I used carb depletion and subsequent carb loading for my last race (Boston 2013) and ran a 5 min PR with really great sustained energy all the way to the end. However, I’m not convinced that the carb loading phase was what did the trick. In retrospect, I think the depletion phase (I went a bit extreme and ate less than 50g carbs a day for about two weeks before carb loading) helped make me much more efficient at burning fat for fuel and running in a glycogen depleted state. So my premise is that depletion phase helps not because it’s allowing you to superload glycogen, but because it’s adapting you to low-glycogen running.

  9. Even for a Carb-Fueled Runner, Fat Burning Can Help Your Marathon Times | Ernesto Burden
    February 8th, 2014 @ 9:07 am

    […] 3. Try a short-term carb depletion right before your marathon. This last one means essentially eating the way I do now – but with an end in sight. It’s an old-school idea for marathoning that has fallen out of favor, but I think still has some validity. The idea used to be, if you could totally starve your body of carbohydrate, when you started taking carbohydrate again, your muscles would super-load themselves with glycogen and you’d be able to skirt the bonk. But studies have  suggested your muscles get just as loaded with glycogen without the carb depletio…? […]