Low Carbohydrate and Metabolic Flexibility

Metabolic flexibility and low-carbohydrate endurance exercise is a topic close to my heart because it's what I researched during my PhD.

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When it comes to low-carb and endurance exercise I believe a lot of people are missing the fundamentals of how a low-carb nutritional approach can benefit endurance athletes. Here, I want to go over metabolic flexibility, which I believe is the main area for improvement. In order to understand metabolic flexibility, we can use these two simple graphs;


The graphs represent the course profile of a race. The lows are easy intensity or the bottom of a hill, and the highs are high intensity or the top of a hill.

Metabolic INflexibility

The left graph displays metabolically INflexibility, referring to an athlete who eats predominantly carbohydrate, avoids fat, and potentially has a weak aerobic base. Once this athlete gets halfway up the hill, they start entering into the red which is carbohydrate. Carbohydrate is finite, meaning there's only a certain amount, around 300-500 grams, we can store within our muscles. Once an athlete starts using this carbohydrate, they are in a position of diminishing returns.

Running out of Carbohydrate

For the remainder of the race, the metabolically INflexible athlete spends a large amount of time in the red. Even on the downside of the hill, the metabolically INflexible athlete is still in the red using carbohydrate. Using carbohydrate when we should be recovering is particularly important for endurance and ultra-endurance events because we need to preserve that carbohydrate for the other hills later in the race.

Metabolic Flexibility

Now look at the right graph, this graph represents the metabolically flexible athlete. These athletes are able to get through the first hill climb and only enter into the red ( using carbohydrate) at the very top. This is doubly important because, on the downside of the hill when you're recovering, the metabolically flexible athlete quickly relies back upon fat (green) as their main fuel source. By the time this athlete begins the next hill climb you start to see the true difference as they have a lot more carbohydrate left in their tank.

The Big Benefit

If we fast forward to the very last peak, imagining it's the decisive climb in a race. The left graph shows how much red, carbohydrate, the metabolically INflexible athlete has already burnt in order to get to the top of that hill. Depending on the total duration of the event and how many carbs this athlete has ingested throughout the race they've potentially run out. That puts the right graph (the individual with metabolic flexibility) in the best position to capitalise on their high fat-burning rates because they have plenty of carbohydrate to get them to the top of the last climb. Not only that, but they also needn't rely so heavily on eating and drinking carbohydrate throughout the race.

Therefore the principle behind removing carbohydrate from your diet and promoting fat oxidation is to reduce the reliance on carbohydrate. It’s actually your metabolic flexibility to be able to burn both carbohydrate and fat, as opposed to one or the other. Metabolic flexibility describes how efficiently you can use both to produce the best possible outcome. Relying on fat during your everyday training can also help with fat-loss.


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Dr Will O'Connor

Educating and inspiring athletes to train smarter

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