How many carbs do you need to max out your muscle stores?


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- Alex Hutchinson (@sweatscience)


My column in today’s Globe and Mail takes a look at some recent field research on carbo-loading the day before a marathon:

[…] To find out whether this revised advice works in practice, researchers in Britain followed 257 London Marathon participants for five weeks prior to the race, collecting data about their training and eating patterns. The runners had an average age of 39 and an average finishing time of 4 1/2 hours. The results were published in the International Journal of Sports Medicine.

Sure enough, day-before carbohydrate consumption mattered. Runners who ate more than seven grams of carbohydrate for every kilogram of body weight (g/kg) ran 13.4 per cent faster than a comparable group of runners who ate fewer carbohydrates but were otherwise identical in terms of age, body mass index, training and marathon experience. Surprisingly, the amount of carbohydrate consumed during the marathon didn’t matter as much. [READ THE WHOLE ARTICLE HERE]

Most people don’t realize what an enormous amount of carbohydrate you have to take in to maximize your glycogen stores — which is why only 12% of the runners in the study hit the 7 g/kg threshold. Trish McAlaster did a nice job with an accompanying graphic showing just how much you’d need to eat and drink to hit 5 g/kg (the average in the study), 7 g/kg, or 10 g/kg (which is the amount suggested for elite athletes). Note that I’m not suggesting you should actually eat four plates of plain pasta for dinner — this is just to put the amounts in context!

[CORRECTION: Reader Mike LaChapelle just pointed out that the math doesn’t add up in the graphic. The “threshold” lunch should include 500 ml of sports drink. That being said, I should clarify that I’m not recommending these menus as exactly what you should eat; it’s aimed at giving a sense of the quantities involved. In real life, I’d go for more variety, and include things like fruit and vegetables!]

14 Replies to “How many carbs do you need to max out your muscle stores?”

  1. Hi there, great article…i have a marathon coming up in two weeks and i shall try this out. Is the british study available to link to? I am a numbers junkie and like reading studies like that. Thanks

  2. Great article as usual, Alex. I am wondering if this is just too many calories for regular people. Do you know of any research on substituting carb calories for fat/protein calories in the days leading up to a race? Lee

  3. Great article. But there is no mention of what these people were doing in the week before the race. Were some depleting themselves of carbs….some eating normal meals… some increasing carbs 2-3 days before?? Surely that information in addition to whether or not they carb loaded the day before would be significant. The study doesn’t seem to elaborate on this??

  4. @Lee: Depends what you mean by “regular people…” 🙂 Carbo-loading in this way is most relevant for people trying to run a marathon absolutely as fast as possible. If your goals are focused on fitness rather than competition, then you’re probably better off not stuffing yourself with carbs the day before. If you’re not pushing all-out during the race, you’ll burn fewer carbs and more fat, so you’re less likely to run out of carbs.

  5. @DC: It’s always possible that there’s some hidden confounding factor like that, but the assumption is that any behaviours like carb depletion (does anyone really still do that?) would be evenly distributed between the two groups.

    Ultimately, real-world observational studies like this have to sacrifice some control in exchange for greater ecological validity. Ideally, we look at a collection of different kinds of study to draw our conclusions.

  6. Alex: When I tweeted the abstract of this study, a smart friend quickly emailed me to say: clear example of correlation vs causation problem. In other words, did high carb loading produce better marathon times, or do better marathoners run faster because they understand the importance of carb loading. This is the kind of chicken or egg thing you often comment on. This time you didn’t. So, did the study convince you of causation?

  7. @Amby: Good question, of course — and an important point that I should clarify.

    This study, in isolation, doesn’t convince me of causation; no observational trial like this can show causation. So I look at controlled lab studies (like this one) to find out whether raising carb intake before a multi-hour exercise task can improve performance.

    On the flip side, no lab study can really tell us what’s relevant in the real world. After all, lab studies force people to do silly things like perform their “marathon” with no breakfast, or not letting them ingest carbs during the race, to avoid confusing the results of the previous night’s carb loading. So real-world studies like this one are a very important reality check, but not sufficient on their own to draw any conclusions from.

    Of course, you know all this. If you’re asking whether I REALLY believe that carbo-loading to the extremes described in this paper is useful/important… well, it’s more subtle. I definitely don’t think it’s important for distances shorter than a marathon. And there are some lab studies that fail to find a benefit even in long events (e.g. 100km cycling time trial) when the carbo-loading is placebo-controlled.

    Moreover, there are potential problems with carbo-loading, like the fact that you store something like 2.4 grams of water with every gram of carbohydrate, meaning a successful carbo-loading bout might easily add a few kilograms to your weight — bad news in a marathon. And effective in-race nutrition might reduce the penalty for not having your muscle glycogen stores at 100%.

    So what’s the verdict? I think that reasonable people could certainly reach opposing conclusions at this point. But my feeling, which is definitely bolstered by this London Marathon study, is that there’s a good chance that many people are losing a bit of potential performance by starting marathons with their fuel tanks only partly full.

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