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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:
The 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!