How many carbs can a super-carb-absorber absorb during a triathlon?


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Following up on my post on maximizing carbohydrate absorption during exercise a few months ago, I got an interesting e-mail from a triathlete named Josh (yeah, I’m still way behind in catching up on e-mail since the trip to Nepal!). His question was basically: Forget about averages, how high can an individual outlier push his or her rate of carb absorption, with training and good genetics?

I’m a tall and lean guy and at Ironman this past year I ate 600 calories [150 g] per hour for 5 hours on the bike and ate at around 450 cal [~112 g]/hour on the run. That’s well in excess of ANY average rate that anyone has ever suggested is “possible on average”…

It’s an interesting question. After all, as Josh pointed out, the average marathon time is around four hours, but we don’t focus our training discussions on how to be average. So I dug up Asker Jeukendrup’s recent review of multiple transportable carbs to see if it would shed any light.

The first key point, of course, is the difference between ingestion and absorption. While Josh was ingesting 150 g an hour, that doesn’t mean all those carbs were reaching his muscles — they could be hanging around in his stomach, or passing through his intestine without being absorbed into the bloodstream, destined for an eventual rear exit. A very nice table in Jeukendrup’s paper sums of the results of 13 studies:


The ingestion rate in some of the studies was as high as 2.4 g/min, which works out to 144 g/hr — pretty much the same as what Josh was doing, and far higher than the 90-100 g/hr thought to be the max. But how much of this intake were they actually burning? The exogenous carb oxidation rate tops out at 1.70 g/min (102 g/hr), in line with expectations. And if you’re just using plain old glucose while stuffing in all those carbs, fully half of them go to waste. So the moral: if you can pack in 150g /hr of carbs while doing an Ironman, you’re blessed with a very strong stomach — but it doesn’t mean you’re using all of it.

Of course, we’re still talking about averages, so that doesn’t answer Josh’s original question about distribution of absorption rates in different people. Here’s another figure from the same paper:

jeuk2The point of this figure is to show that absorption rate doesn’t depend on body mass — it’s limited by transporters in the intestinal wall, not by how big you are. But because data points from individual subjects are plotted, we can get a sense of the scatter (as long as we’re careful to compare only dots from the same study). In Jentjens 2006 (the medium-filled circles), the average is 0.77, but the values range from about 0.38 to 1.09 — 50% lower and 42% higher than the average!

So where does this leave us? Well, an outlier in a study where the average carb absorption rate is 100 g/hr could indeed conceivably be absorbing 150 g/hr. Again, just because you’re able to down that much without puking doesn’t mean you’re making use of it, though it does ensure that you’re definitely not underfuelled.

The follow-up question, of course, is how do boost your ability to absorb (not just ingest) carbs during exercise? The only answer I’m aware of is practice, practice, practice — but I’m all ears if there are other ideas out there!

6 Replies to “How many carbs can a super-carb-absorber absorb during a triathlon?”

  1. Interesting discussion and nice to see the review with results. I have it the other way around. Last year i did 4 ironmans. all with average result. but hte bigest thing is that i could not eat any gels during the run. In my first race in lanzarote i was astonished but as i went to the aother races i accepted the fact that if i took gels i would only make my race worse. The funny thing is that i recovered even faster than before.
    I dont think it is really necessary to take all those carbs in. As you progressing more in training years the better the switch is to lipolysis. to me this is the major factor also in training and racing. Curious what you guys think

  2. That’s impressive that you can do an Ironman with no carbs, Chris! There’s definitely been a lot of interest over the past few years in trying to optimize fat burning during ultra-long endurance exercise. My understanding of the research is that carbs still play a very important role, though I’m more familiar with research on marathon-length exercise rather than Ironman-length, which could be quite different. Still, everyone’s stomach is different, so you have to find what works best for you.

  3. It was not too many years ago really that fueling with all these carbs that are commonly used today wasn’t common. Sure, using gels and carbs when your body is crying out to get more carbs to your muscles can lead eventually to adaptations in your digestive tract to enable faster update of carbs that are eaten during exercise, but when you increase carbs through one route, you necessarily require less adaptation for carb delivery through other routes. I’ve often wondered if by taking in all the exogenous carbs, if athlete’s aren’t also decrease rates of carb liberation from endogenous fats stores… OR… Maybe it doesn’t matter, because you can train your body to uptake carbs at a greater rate than they could ever be provided by endogenous mechanism, or perhaps some athletes perform at levels that require such high rates of carbohydrate delivery to muscle tissues that their intense training stress and therefore develops all endogenous and exogenous uptake mechanisms. Interesting questions, at any rate.

    I don’t find it surprising at all that Chris could complete Ironman triathlons without taking in a lot of concentrated exogenous carbs. Many endurance and ultraendurance events existed before modern ‘fuels’ were in common use, and lots of athletes successfully completed them. There is always the option of simply eating foods that are high in complex carbs at intervals during a race.

  4. Interesting points, Mark. That brings up the difference between training and racing (and, indirectly, the “train low, compete high” theories). My understanding of recent research is that it’s pretty clear that you’re correct: if you do all your training with plenty of exogenous carbs, you body adapts to precisely those conditions, at the expense of, say, learning to use endogenous fat stores more efficiently. People are currently trying to get the best of both worlds by training (at times) in a low-carb state, then racing with plenty of carbs available.

    So far, the hardcore train-low regimens (where you actually deplete your endogenous carb stores before training) don’t seem to be very practical — you lose more in your ability to process carbs than you gain from improved fat oxidation. My gut feeling, if I were advising an athlete, is that you get a useful stimulus from doing some of your training without “full” fueling protocols, but that it’s not worth doing the depletion stages.

    As for the point that people were able to complete ultraendurance events before the advent of modern “fuels” — sure, of course. But the question isn’t just whether we can complete them, it’s how fast we can go. Of course, real foods are a perfectly good option: the carbs don’t have to be delivered in fancy packages!

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