In 2010, a newspaper article in Computational Biology PLOS caused a sensation in the world of running. It offered a comprehensive model of how you store carbohydrates in your body and how quickly you burn them during a marathon, in order to calculate how much more you need to consume. “Marathon runners will never need to ‘hit the wall’ again,” enthuses the media.
The basic assumption behind this model was that you can run happily when you have carbs in the tank, but will stop – or at least hit the wall and slow down dramatically – when you run out. Of course, there are plenty of other factors to consider, like the microscopic muscle damage that accumulates during long runs. But some scientists have argued that there is a more fundamental problem with this assumption, which is that you start to tire long before your carb reservoir is empty – a counter-intuitive idea, equivalent to your car slowing down. automatically when the tank is still half full. .
This is the idea tackled by a new study in Medicine and science in sport and exercise, from a team of researchers in Denmark led by Jeppe Vigh-Larsen of the University of Aarhus. In the world of muscle physiology, Scandinavian researchers are famous for their rigorous experiments, and this one was no exception: Study volunteers underwent a series of protocols that included four muscle biopsies, i.e. say take a piece of thigh muscle for detailed analysis. -in one day. The result is a remarkable look at what happens inside your muscles when you exhaust them and then recharge them.
After a bunch of initial testing, the volunteers did a grueling interval training session on exercise bikes to deplete glycogen (aka stored carbohydrates) in their leg muscles. Then they rested for five hours while consuming high-carb recovery bars and drinks, or low-carb placebo versions. Then they did more exercise tests: six sets of five-second full sprints, plus a two-minute fixed-intensity test to measure perceived exertion.
Here’s what muscle biopsies revealed about the amount of carbohydrates stored in the subjects’ thigh muscles before interval training (Pre), after interval training (Post), and after five hours of recovery, just before the total sprints (Rec ). Open circles are the placebo group, filled circles are the high-carb group:
As expected, interval training drained the leg muscles of carbs. High-carb drinks and bars partially restored those lost carbs; the low carb group has also restocked some of these stores, but to a lesser extent.
Here’s how full sprints fared for the high-carb (CHO) and placebo (PLA) groups, expressed as a percentage of their repeated sprint capacity (RSA):
Their sprint suffered after the grueling split training, which is no surprise. Five hours later, he had partially recovered, but not as much in the placebo group as in the high-carb group.
Likewise, here’s the Perceived Exertion (RPE) rating, on a scale of 1 to 10, for the two-minute ride at a pre-determined power. It got harder when fatigued, then easier after they recovered, but again with greater recovery for the high carb group.
Overall, the results agree with previous data suggesting that performance suffers when your muscles have less than around 250-300 mmol/kg carbs, which is about half full. Why does this happen? The document goes quite deep into the intricacies of muscle physiology, but there are a few points that stood out to me.
The first point is that there is a difference between the overall amount of carbohydrate available in a muscle and that available to an individual muscle fiber. If you have 250 mmol/kg out of a maximum capacity of 500 mmol/kg, your muscles are half full. But it could mean that some fibers are mostly solid while others are mostly empty. And indeed, that’s what the researchers found: In the low-carb group, 19% of individual slow-twitch fibers and 4% of fast-twitch fibers were depleted to less than 20% of their levels. initial carbohydrates. By comparison, no individual fiber was as depleted in the high-carb group. When doing exercise, such as six-second sprints or a sprint at the end of a long run or a football game, you need everything your muscle fibers on fire. If some are empty, your performance will be compromised even if there are carbs in the fiber alongside.
And you can even zoom in a bit further, to consider how glycogen is stored in muscle fiber. There are three main places where you will find glycogen in a muscle fiber, called (since you asked) subsarcolemmal, intermyofibrillar, and intramyofibrillar. The last, known as intra glycogen, has been linked in previous research to how muscle fibers contract. Indeed, in the low-carb group, about half of the individual fibers were depleted to less than 20% of their baseline intraglycogen levels – and each individual’s level of intraglycogen depletion was correlated to the severity of the situation during the repeated sprint. test. This suggests that a muscle fiber can be compromised even though there is plenty of glycogen left, if that glycogen is not in the correct place in the fiber.
All of this biochemistry is in service of a simple point, which is that there is a big gray area between the extremes of being fully fueled and bonking. Most of the time I don’t really care because I train once a day and glycogen levels usually rebound in about 20-24 hours as long as you follow a sensible diet and don’t have to worry post-drive refueling windows and details like that. But there are times when it matters. For example, I usually play basketball on Friday night, then meet up with friends for a tempo run on Saturday morning. The data suggests my legs will still be partially carb depleted the next morning, so I’m very aggressively fueling up when I get home from my basketball game. I probably can’t fill the global tank, but believe me, I need as many individual fibers as possible to shoot.
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