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Metabolic Flexibility

The ONE big idea to understand

Metabolic flexibility really is the holy grail of nutrition for performance (and life). Understand this and you will give yourself a powerful new weapon in your competitive lunchbox. After all, knowledge is power! 

Humans are designed to be metabolically flexible. That is to say, if you want to get the best out of your brain and body then you should be able to rely on fuel from both carbohydrates and fat as and when you need them. Someone who is metabolically flexible can use fat as the primary (and almost exclusive) fuel when they are resting, sleeping and moving around at a fairly slow pace. As they start to move around at a quicker pace – like fast running – they will be able to take advantage of extra fuel supplied by carbohydrate, and when they are going nearly flat out they will rely almost exclusively on carbs for fuel.

We measure metabolic flexibility in our lab using online gas analysis. We measure proportions of inhaled and exhaled oxygen and carbon dioxide to understand just how much fat and how much carbohydrate someone is using from rest to flat out exercise. What you want to see is represented in the figure below; that is, this athlete mostly uses fat for fuel at low running speeds and mostly carbs at faster speeds. This athlete is a male triathlete who has been eating Low-Carb, Healthy-fat for over two years. He is highly metabolically flexible.

Calories per hour derived from carbs (red) and fat (blue) [vertical axis] for a metabolic efficiency test; treadmill-running speed (min/km) [horizontal axis] using respiratory exchange. The athlete is a 39-year-old male elite triathlete who has been LCHF for at least two years.


Other athletes we test aren’t as good at using fat as this athlete. Here’s another test where the athlete is metabolically inefficient (Figure below). This woman is a pretty good age-group triathlete, but she is really not able to access her body fat stores as a fuel source at any exercise intensity.

A metabolically inflexible athlete. This is a high-carb eating, high-level age-group female triathlete.

These two athletes are chalk and cheese. One can easily access his body fat stores as primary fuel at low exercise intensity. He can provide energy from fat right up to very high exercise intensity. He is a fat-burning machine who can access the tens of thousands of calories of fat he has stored around his body. He can maintain a healthy lean body weight easily and doesn’t have to eat sugar and carbs every time he goes training.

The other has to rely on the very limited carbohydrate (around 2000–2500 kcal) she has stored in her muscles and liver. She has to eat extra sugar every time she trains and fuel up again afterwards. She’s tired and has trouble getting her weight down to race.

Being a fat burner has obvious advantages in some sports, like endurance where having enough fuel to make the distance is an issue. Endurance includes long distance running, triathlons, cycling and anything else where you are training, racing or competing for a few hours or more. The fat burner has access to a big fuel tank (fat) and can spare the small tank that provides extra power when you need it (carbs). You can go faster for longer.

But it doesn’t stop there. The fat burner has the potential for significant health and performance advantages in any arena where weight, cognitive performance, fuel, and high training and competition loads are a factor. This includes weight class sports, all day sports like sailing, and team sports, and the workplace where steady thinking is needed.

What makes you metabolically flexible?  Low Carb Healthy Fat eating of course.

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