Holmer says glycogen does not explain the carb benefit
Three studies say the standard explanation for why high carb fueling works keeps coming up short.
Brady Holmer says the story every runner has been told about carb fueling is incomplete. The usual version goes like this. You eat more carbs, you spare your glycogen (the carbohydrate stored in your muscles), and you hold your pace longer. Holmer points to three recent studies on carbs and durability that do not line up with that.
carbs aren't a performance-enhancing drug. They let you use the fitness you've built.
I suspect the brain is part of it.
In the first, cyclists took in 60 or 100 grams of carbs an hour over a four hour ride, then did a 15 minute time trial. Power dropped 9.3% versus fresh on 60 grams and 6.4% on 100 grams, a gap of roughly 9 watts. Glycogen fell a lot in both groups, with no detectable difference between them. So the performance benefit was there and the mechanism everyone cites for it was not.
More is not automatically more
The second study pushed the intake higher, comparing 90, 120 and 150 grams an hour. Athletes burned more of what they ate, but whole body carb oxidation was identical at 120 and 150, and exercise capacity did not differ significantly. That matches what Holmer has said earlier about stacking grams past a point.
The third went the other direction. Artificially raising the body's own carbohydrate use almost doubled the drop in power at the first ventilatory threshold (a marker of the effort level where breathing starts to climb) after two hours of riding.
What it actually means
The benefit of fueling well is real and has been measured more than once. The explanation is the shaky part. Holmer's own read is that carbs are not a drug that adds fitness, they let you reach the fitness you already have, and he suspects the brain is involved. When the body senses fuel is coming in, he says, it lets you push longer.
For a marathoner, nothing here changes the practical advice to train your gut and fuel high. It does change the reason you give for it. If you are hitting 100 grams an hour and telling yourself you are banking muscle glycogen, the studies Holmer cites do not back that up.
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