Carbo-loading with a “hyperglycemic-hyperinsulinemic glucose clamp”

THANK YOU FOR VISITING SWEATSCIENCE.COM!

My new Sweat Science columns are being published at www.outsideonline.com/sweatscience. Also check out my new book, THE EXPLORER'S GENE: Why We Seek Big Challenges, New Flavors, and the Blank Spots on the Map, published in March 2025.

- Alex Hutchinson (@sweatscience)

***

In search of the ultimate carbo-loading protocol, I stumbled across a paper in the European Journal of Applied Physiology that was posted online a few weeks ago. Researchers at Liverpool John Moores University in Britain investigated the use of a “hyperglycemic-hyperinsulinemic glucose clamp” as a sort of super-accelerated way of stuffing your muscles full of glycogen (the form in which carbs are stored in the body).

What the heck does that mean, you ask? Basically, the subjects spent two hours the day before the big exercise test, lying around with needles stuck in their arm delivering elevated levels of glucose and insulin. (The “clamp” means they picked a specific level of blood sugar and infused exactly enough glucose to maintain the subjects at that level for the two hours.) Glucose is carbohydrate, and insulin is one of the signals that initiates the storage of carbohydrate in your muscles, so the subjects were able to pack about 198 grams of carbohydrate into storage during the two-hour protocol.

The next day, the subjects did a 90-minute steady-state bike ride following by a 16K time trial — and sure enough, the clamp improved time-trial performance by 3.3% (49 seconds over ~24 minutes) compared to when the subjects received a placebo saline infusion. So yes, you can carbo-load in two hours the day before a race without any special dietary or training intervention (though it doesn’t show whether this method is better or worse than standard carbo-loading protocols). Of course, this really doesn’t have much practical significance. I suppose you could imagine Tour de France riders having the resources and incentive to undertake a protocol like this, but insulin is banned by WADA. (Oh wait…)

The most interesting part of the paper is buried near the very end, and it’s written very confusingly — it sounds very much as if the paper’s peer reviewers have insisted on a bunch of caveats and rephrasings, which wouldn’t be surprising as the topic is Tim Noakes’s highly controversial “central governor” theory. It relates to a somewhat confusing quirk in the data:

[D]espite the evidence of alterations in substrate availability (higher glucose and insulin), the patterns of substrate oxidation were no different.

In other words, carbo-loading makes more carbohydrate available, but it doesn’t seem to change how much carbohydrate (versus fat) is actually burned. A number of other studies have found similar anomalies, which has made some researchers question whether we really understand why carbo-loading works to improve performance:

The essence of this theory, supported by appropriate findings, is that muscle glycogen may have a signalling function that influences pacing strategy. Subjects who start exercise with elevated levels of muscle glycogen would be able to exercise at a higher pace due to signalling between muscle and the brain than when in a glycogen depleted state.

In this picture, carbo-loading is just another version of the carbohydrate mouth wash, whose function is to convince your brain that you have enough fuel. It’s hard to imagine that this is always the limiting factor, but it’s an interesting area of research.