Maximizing carbohydrate absorption during exercise


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Great first day at the International Sport Nutrition Conference in Canberra. Several interesting sessions that I’ll post about in days to come. For now, since I have just a few minutes before dinner, a few notes about Asker Jeukendrup‘s session, which he called “Carbohydrate: boring old or exciting new?”

In the “boring old” category, he took aim at the ACSM’s position stand, which states:

For longer events, consuming 0.7 g carbohydrates/kg body weight/h (approximately 30-60 g/h) has been shown unequivocally to extend endurance performance.

Now, it’s well known that if you take a drink with glucose in it, the fastest you can possibly make use of it is at a rate of about 0.8-1.0 g/min, no matter how much carb you have. The rate-limiting step is absorption from the intestine, which doesn’t depend at all on how big or small you are, so the guidelines shouldn’t be expressed “per kg body weight” — it should just be 30-60 g/hr (with the high end corresponding to the 1.0 g/min which is the fastest we can make use of glucose).

However, there are ways around this rate-limiting step. Over the last five or six years, Jeukendrup’s group has done a long series of studies on combining glucose and fructose. On its own, fructose is absorbed slower that glucose. But it’s transported across the intestinal wall by a different mechanism, so you can have both glucose and fructose being absorbed at the same time. Add them together, and you can get about 1.7 g/min that the body actually makes use of.  (And this is, indeed, the formulation used in Power Bar’s “C2 Max carb mix” bars and gels.) So Jeukendrup suggests that ultra-endurance athletes should aim for closer to 90 g/hr of carbs, and he showed some data from Ironman triathlete Chrissie Wellington, who does precisely that (and who just set a new Ironman world record in Arizona).

Does this apply to everybody? Well, it’s not such a big deal for shorter events. He offered these loose guidelines (which I copied down quickly — if I made any mistakes, I’ll correct them when I get the official proceedings):

  • less than 45 min: no carbs needed (but he later noted that some new studies are now showing that “mouth rinsing” with carbs can have an effect with exercise bouts as short as 30 min)
  • 45-75 min: mouth rinsing, with any type of carb
  • 1-2 hr: up to 30 g/hr, any type of carb
  • 2-3 hr: up to 60g/hr, carbs that are oxidized rapidly like glucose or maltodextrin
  • more than 2.5 hrs: up to 90g/hr, MUST be a combination of carbs that are absorbed via different mechanisms (e.g. glucose or maltodextrine combined with fructose in a 2:1 ratio)

Anyway, that’s a quick overview of some of the take-home messages he offered. There was lots of discussion afterwards, so if you have any questions about any this stuff, feel free to post ’em in the comments section and I’ll answer if I can.

30 Replies to “Maximizing carbohydrate absorption during exercise”

  1. Hope you’re enjoying a warm sunny Canberra.

    What I don’t get is how you consume that much. Whilst Adventure Racing I always feel so sick after the first 4-5 hours. But perhaps that’s a sucrose issue, but I can’t see that quite being the case.

    Did they go on to any any issues in the discussion about protein, electrolytes and relations with absorbance or oxidation rates (or mouth ulcers)?

    Anyway, enjoy the free Powerade sponsored sports nutrition conference in which nutritionists get to learn that they should advise athletes take more of the sponsors product 😉

  2. Good points on all counts, Ben. The conference is indeed sponsored by PowerBar, as is Jeukendrup’s research (which is why PowerBar was the first to adopt his glucose/fructose advice, though several other companies now also offer it). The research, in this case, still looks solid to me, but it’s definitely important to be aware of where the funding comes from!

    In terms of tolerating high carb loads, that was definitely a topic of discussion. It’s an individual thing, but you do get better at tolerating high loads with practice, he said. In the example of Chrissie Wellington, the data he showed was from 2007, at which point she was taking 86 g/hr; she’s now apparently taking quite a bit more than that, because she’s managed to train her system to tolerate it. Another change Wellington has made is that she now takes just carbs, no protein. Asker’s opinion is that the early studies that found improved performance from taking something like a 4:1 ratio of carbs to protein during exercise were flawed in several ways. His lab has tried to replicate those results and found no benefit from adding protein — so eliminating the protein might make it easier on the gut. Finally, he noted that his recent studies have found no difference in absorption rate for bars, gels and fluids, so you can pick (or mix and match) to find what your stomach tolerates best.

    He also showed some interesting data from Ironman Hawaii 2009 — a scatter plot of finishing time versus carb intake for a big group of subjects whose times ranged from 8:xx to 16:xx (if I recall corrently). There was a definite and pronounced trend that faster times were correlated with higher carb intake per hour. Of course, that’s correlation not causation, and there are a lot of other factors tied up in comparing 8-hour finishers with 16-hour finishers — but it is food for thought.

    There was also some discussion of tooth decay, which can be a problem — especially since the mouth-rinsing boost is likely tied to time of contact in the mouth, just like cavities. Jeukendrup said that this problem is actually caused mainly by the acidity not the sugar. So it can be eliminated by reducing the acidity of the drink and adding some calcium. THe problem is that reducing the acidity hurts the shelf life, so no manufacturers are currently doing this.

  3. thanks Alex, Canberra has some great running trials, I hope you can get out there and explore. And remember if there are any “need to know” gems shoot them my way would ya!!! especially interested to know how an older athlete best maintains lean muscle mass and avoids the bulk muscle mass.

    Simon Whitfield

  4. Great info on carbohydrate consumption during exercise.
    I would like to know if the athlete needs to be eating every hour of training? I’m assuming they don’t need to start consumption until after the first hour. If they consume 90 grams after the 2.5 to 3 hour mark, is that beneficial? Or should the athlete literally try to consume 30 – 60 grams during EACH hour of training to obtain maximal benefit?

  5. Alex,
    What’s the rate limiting step for fructose? Comparing glucose/fructose mixtures at the gut-level could be misleading, since fructose must pass through the liver before heading on to the muscles, and glucose does not.
    Thanks and keep up the great posts,

  6. Thanks, Simon — great trails in Canberra indeed, as I found out when I got lost in them! Eventually found the highway and jogged back along the shoulder. Also nearly got run over by a kangaroo, quite literally — it ran across the trail about two feet in front of me. Anyway, Stu Phillips from McMaster did give a presentation on muscle, so I’ll go through my notes and post some thoughts soon.

    Cathy: my understanding is that it takes a while (an hour?) for the rate of carb uptake to reach maximum. So if you only start taking carbs after an hour, there will be a delay before those carbs are being fully utilized. I’d think you should consider the total duration of the event, work out how many carbs you want to take, then divide it as evenly as possible throughout the event. (That also makes it easier on the gut.) The time when you can stop taking carbs is towards the end, since you’ll be finished before those carbs are processed — but you might still do some “mouth rinsing,” since those effects are more or less instant.

  7. What is “mouth rinsing” and how is that effective? Is that basically what it says: gargling some gatorade and spitting it out? Why not just swallow it?

  8. John: Yes, the mouth rinsing is exactly what it sounds like. It appears there are some carb sensors in the mouth that detect incoming carbs. And they’re not taste buds, because you get a performance boost from glucose (sweet) and maltodextrin (tasteless) but not from artifically sweetened maltodextrin. Basically, it appears you can fool your brain into thinking more fuel is coming, so it lets you go a bit faster. This seems to work even for relatively short bouts of an hour or less (though possibly only if you’re in a fasting state for the short bouts).

    So why would you bother? Well, depends how well your stomach can tolerate carbs. If you’re in the final 15 minutes of a marathon, you’re not going to get any further fuelling benefit from ingesting carbs (because you’ll be finished by the time their utilized), and your stomach might be in a pretty dicey state, but you can still get a boost from rinsing and spitting.

  9. @Chris

    Hi Chris,

    We’re getting a bit out of my depth in terms of the internal kinetics of carbohydrate absorption, but my understanding (from Asker Jeukendrup’s talk and notes) is that the rate-limiting step is indeed passing from the intestine to the blood stream, not processing in the liver. (Glucose also has to pass through the liver on its way from the gut to the muscles, but it can just pass straight through, while fructose has to be processed.)

    Fortunately, the experimental design takes all that into account. They feed the subjects carbs labelled with carbon-13, so they can determine exactly how much of the exogenous carbohydrate is then being oxidized in the body. With glucose, you get that 1 g/min ceiling (and even lower with fructose), but with glucose AND fructose, you get up to 1.75 g/min — so that’s an experimental measure that incorporates all the steps in the process.

  10. @alex
    In a table in the Conference Proceedings, Asker refers to 7 mouth rinse studies published since 2004. Six of seven studies found significant positive effect attributed to mouth rinsing. The activities studied included a 30 minute treadmill tests, a 60 minute running time trial, and one hour cycling time trials.

    Interestingly the one study that showed no effect Beelen et al (2009) (see involved 14 male endurance trained athletes in a post prandial state – “in the morning after consuming a standardized breakfast”. They undertook a one hour time trial on a cycle ergometer. The treatment group rinsed every 7.5 minutes with a 6.4% maltodextrin solution and the placebo group just rinsed with plain water.

    It’s just one study, but seems to suggest that you might actually be better off training yourself to stomach a decent breakfast, brushing your teeth and avoiding the risk of cavities. Perhaps we’ll hear more about this approach at a Sports Nutrition Conference sponsored by Colgate.

  11. Thanks for the comment, John. I blogged a bit about this question last spring. You get the same difference if you actually drink the carbs rather than just rinsing: it improves 60-minute exercise bouts if you’re in a fasting state, but doesn’t improve them if you’ve been eaten beforehand. The way I’d interpret this is that your brain is monitoring your whole-body carbohydrate stores — if you’re running low, then ingesting some (or tricking your brain into thinking you’re ingesting some) convinces the brain to let you go faster. If you’re not running low, then your brain doesn’t care whether you take more carbs.

    So I’d agree entirely with you that just making sure you’re well-fuelled beforehand is all you need to do before exercise lasting an hour or less. (Though, for the record, when Jeukendrup was asked about this, he said that in more recent data they’ve found that mouth-rinsing can help even in short exercise bouts when you’re well-fed, contradicting the study you linked to — on which he is an author.)

    For that matter, even during longer bouts of exercise (when you do need to take in supplemental carbs), it’s always to your advantage to start as fully loaded as possible. Those studies where they start fasted are really only useful to understand how the body works, not to prepare for optimal performance.

  12. Thanks for the article. Some interesting information you are reporting on.

    Do you happen to have a reference with regard to this statement, “Now, it’s well known that if you take a drink with glucose in it, the fastest you can possibly make use of it is at a rate of about 0.8-1.0 g/min, no matter how much carb you have.”


  13. Hi Calvin,

    Asker Jeukendrup wrote a review of this topic that was published over the summer in Current Opinion in Clinical Nutrition and Metabolic Care. Here’s a line from the abstract: “Exogenous carbohydrate oxidation rates [with the addition of fructose] reach values of 1.75 g/min whereas previously it was thought that 1 g/min was the absolute maximum.” If you’d like a copy of the paper, let me know.

  14. Hi Alex,

    Very nice summary of my presentation in Canberra and of the work of my team! Keep up the great work! I just wanted to comment on one thing. I agree that it is important to keep an eye on where funding is coming from. But for that reason it is perhaps even more important to point out that the initial 10 studies or so that led to the development of the carbohydrate mixture described was NOT funded by PowerBar. At the time they adopted this formula, they had not funded any of our studies. Since then they have funded a PhD student who has looked into several of the practical aspects of the recommendations I suggested. For example we studied whether the high intakes are realistic during running, whether the oxidation is similar in running and cycling, whether delivery of carbohydrate from a gel or a drink is different, whether a bar is different from a carbohydrate drink in terms of carbohydrate delivery and what athletes actually take on board during races.
    I never talk about products, I talk about mixtures of glucose and fructose or maltodextrins and fructose. Of course you can buy products that have these mixtures but you can also make your own, just buy the ingredients, mix them together in recommended amounts and you have a drink that gives you the same benefits.


  15. Thanks for the clarification, Asker — that’s a very interesting and important point that the research preceded the products (and the funding). I shouldn’t have leapt to the conclusion in my comment above that PowerBar was always funding your research! (And I can confirm, for anyone else who’s interested, that Asker indeed didn’t mention PowerBar products at all during his presentation.)

    Anyway, thanks once again for all your work in organizing the conference — I learned a huge amount about the latest research, which I’ll be trying to pass on to others through the blog and in other articles. I also learned to never, ever try pedalling all-out on an exercise bike with no resistance for nine minutes. I’ve STILL got DOMS!

  16. Hi,

    Really interesting topic!

    Does it matter how you prepare the ingredients?

    ‘Of course you can buy products that have these mixtures but you can also make your own, just buy the ingredients, mix them together in recommended amounts and you have a drink that gives you the same benefits.’

    Eg, for a app 8 hour race, would I like to combine drinks and ‘hard’ food. Lets say I can drink 1 liter/ hour, should I use then: 1L water + 60 gram glucose/ maltodxtrine + 30 gram fructose?
    And the same kind of counting for making your own bars? Or do you ‘lose’ some of the effectivity if you use water or bake?

    Sorry for the question, but I try to get the figures living in practice.

    Kind regards,

  17. Hey Kes – sorry for the long-delayed reply. Asker Jeukendrup and his colleagues did, in fact, do a series of experiments comparing the absorption rates from drinks, gels and energy bars. They found no difference — all three forms were equivalent, and produced the “maximum” absorption rate. You can definitely mix your own drinks (i.e. water, glucose and fructose), though it may take some practice to teach your gut to accept such high rates of carbohydrate. Definitely something to practice in training. I must admit, I’m not sure whether baking might change things, or make some of the carbohydrate less accessible — I don’t see why it would in theory, but I don’t know for sure either way.

  18. and even a later ‘thank you’. Have to say, the bars were tasty (although I needed some testing in baking.
    But thanks for the comment.

  19. Thanks for your many thoughtful articles. The main study you mention in this article seems to contradict findings from a study published in the January 2007 edition of Medicine & Science in Sports & Exercise, that state a moderate intake of carbs (1.0 g/min) is preferred over a high rate of intake (1.5 g/hr). I am completely ignorant of any of the science involved so I don’t know if I’m correct or not in seeing any condtradictions. Any thoughts or clarifications here or in a follow-up article would be appreciated!

  20. @Slowpoke: The key difference is that the 2007 study used only glucose. As I wrote in the blog entry above, “it’s well known that if you take a drink with glucose in it, the fastest you can possibly make use of it is at a rate of about 0.8-1.0 g/min, no matter how much carb you have.” What’s new is the idea of combining different kinds of carbohydrate, which are absorbed through difference mechanisms, allowing you to oxidize more than 1 g/min.

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