Fasted Running

Michael Donlevy argues that running on empty could improve performance / bernardbodo

Sometimes, nothing beats an early-morning run. There’s a chill in the air, the sun is rising and the birds are singing. There’s just you, the sound of your breath and your shadow for company. It can also be extremely good for you.

The chances are you won’t have had time to eat, let alone digest breakfast. So you’re running on an empty stomach. You are using up the glycogen in your muscles that have been stored from the previous day.

This is known to have benefits, because it increases your body’s ability to burn fat. You’ve probably heard of this – it’s called ‘fasted training.’

The science

“It’s all about reducing your body’s available glycogen – the stored sugars that are converted into energy to fuel our muscles,” says coach Tom Craggs ( “Our body only has a limited ability to store glycogen. For most of us it wouldn’t cover a half marathon, and certainly not a marathon or beyond.

We do, though, have access to a much bigger energy reserve: free fatty acids in our blood and liver which can be converted to useable energy through a process called gluconeogenesis.

By training in a fasted state and by sometimes actually slowing down some of our easy runs – we can improve our ability to burn stored fats as a primary energy source.”

Fat-adapted training

Fasted training is something of a hot topic in the world of sports science, because you can take it one big step further into the realm of ‘fat-adapted training’.

This is where you burn stored fat during the day and rely on fat, more than carbohydrate, to fuel exercise. It involves eating a low-carb, high-fat (LCHF) diet and training in a fasted state. Over time your body adapts, so you don’t need to stock up on carbs before, during and after exercise.

“This is a topic that inspires polarised views and strong beliefs,” says Mayur Ranchordas, senior lecturer in sport and exercise nutrition and physiology at Sheffield Hallam University.  “When you eat a high-fat, high-protein diet, you break down carbs less efficiently and you’re fat-adapted.”

Fasted or faster?

Running fasted and burning fat is all well and good, “but what happens when you need to increase the intensity? asks Ranchordas. “To produce power – for example in a 5K race or interval session – the aim isn’t to utilise fat. It’s to produce as much power as possible. And for that you need to oxidate carbs.”

One recent piece of research into fat-adapted training put two groups of ultra-endurance runners on different diets – one LCHF and one high-carb – for 20 months.

The results showed that the LCHF runners burned more fat during exercise than the high-carb group.

“This isn’t rocket science,” says Ranchordas. “What this study shows is that fat-adapted athletes oxidise more fat at sub-maximal pace. But fat adaption doesn’t improve performance at high intensity.”

When to run fasted?

You can try it for yourself by following some simple rules below. Be warned though: you’ll feel horrible for at least four weeks while your body gets used to it!

“I’d say it’s ideal for winter training, when you maybe want to lose some weight, or steady-state training,” says Ranchordas. “But if you want to do an interval session at threshold pace, you won’t reach the power output necessary.”

“I’d suggest eating a balanced diet, consuming carbs before a hard run or race. Only train fasted when you’re doing an easy or steady-state run,” adds Ranchordas.

“Being fat-adapted probably works best for amateurs racing very long distances – because the intensity is lower,” says Ranchordas. “For the general, recreational athlete, (fasted training) helps control your weight and blood sugar and reduces the risk of diseases such as diabetes.”

Three-time Tour de France winner Chris Froome is said to eat LCHF, and 2017 runner-up Romain Bardet is known to have taken this approach.

Low carb, high fat

Following a low-carb, high-fat diet is simple, in theory. If you want to give it a go, just follow these rules.

Don’t count calories

They matter less than with other eating plans, because it’s very hard to consume too many on a low-carb diet.

Restrict carb intake

It has to be less than 50g per day.

Eat moderate protein e.g. around 1g-1.2g per kg of lean body mass per day .

Eat high-quality, high-fat foods. E.g. eggs, avocadoes, olive oil. But stay away from processed oils like sunflower, as well as nuts like almonds and macadamias.

Ease off

Scale back your training for the first four weeks while you’re body adapts, then gradually increase the volume.

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