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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.

Don’t like to read? watch the video at the bottom!

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 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, where there is no real intensity, the metabolically INflexible athlete is still in the red using carbohydrate. This is particularly important for endurance and ultra-endurance events because we don’t want to be travelling down a hill still using carbohydrate. We need to preserve that carbohydrate for the other hills in the race.

Metabolic Flexibility

Now look at the right graph, the metabolically flexible athlete, they are able to get through the first hill climb and only enter into the red or the carbohydrate at the very top. This is doubly important because, on the downside of the effort, they quickly rely 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.

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 top graph (the individual with metabolic flexibility) in the best position to capitalise on their higher fat-burning rates because they have plenty a 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, and that is the basics of how it works. 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.

Watch the video below.

Will O’Connor – PhD

Sports Scientist