Traditional research states that runners need carbohydrate to train effectively, but new studies show the benefits of fasted training. A nutritionist explains the ‘train low, compete high’ concept

You need energy to fuel you on a run, right? Not always. Fasted training, in certain circumstances, can help boost your performance. It has been a highly researched topic by many top sports science institutions around the world over the past few years, with my colleague, fellow nutritionist Dr James Morton, leading the way at Liverpool John Moores University.

For many, fasted training has proven to be an effective technique. It is not a fad diet or a method to reduce calorie intake by skipping meals – it is simply a strategic training method where carbohydrate fuelling is completed after training rather than before, to enhance adaptation and performance.

This new strategy is in contrast to traditional research that states carbohydrate availability and muscle glycogen stores should be high for every single training session and race. The importance of carbohydrate for performance is well documented, so this new training technique has been described as the “train low, compete high” model. This means choosing a number of running sessions to complete in a fasted state to optimise training adaptations, while actually racing with high carbohydrate stores and availability to maximise performance.

The most practical way to complete fasted training is to train in the morning, before having breakfast. Hydration is still important, so water or a low-calorie electrolyte drink should be consumed. Fasted training is best completed at an intensity and duration that does not require a great input from metabolism: typically training for around 60 minutes at a moderate intensity is recommended.

The reasoning behind fasted training is that it further enhances the mitochondrial adaptations that occur as a result of aerobic training. This improves the body’s ability to use fat as a fuel source during exercise, sparing muscle glycogen for when it is most needed – during the tough parts of a race. However, on race day the body must also be able to use carbohydrate as a fuel source. This is why it is best to select certain shorter training sessions to complete fasted, while others should be completed in a carbohydrate-loaded state. This will ensure the body is well adapted to using both fat and carbohydrate for fuel during exercise.

Although this train low, compete high approach produces great adaptations to training, there are limitations. The main drawback is that during these sessions, exercise intensity may be compromised. When training in a fasted state in the morning, liver glycogen is low after a period of sleep. This can lead to reduced blood glucose, making exercise seem more difficult. Muscle glycogen may also be low depending on your activities and nutrition the evening before, so this can make high-intensity sessions particularly hard to complete. Training with low muscle glycogen levels can also lead to a hormonal and metabolic environment that increases muscle protein breakdown and can impair immune function.

So repetitive fasted training can actually have negative effects, if performed over a long-term period. This again emphasises the importance of splitting your training programme, so certain sessions are completed fasted to promote adaptations, while others are completed with carbohydrate intake.

Often people struggle with the concept of training fasted just because of their habit of consuming breakfast beforehand. So another option is to consume a protein-only breakfast (no carbohydrate) and moderate doses of caffeine (about 100-150mg), as this helps prevent protein breakdown and reduces the risk of fasted training affecting exercise intensity. For example, this could be three scrambled eggs and a cup of strong coffee. As previously mentioned, hydration is also important, so a low-calorie electrolyte drink consumed before and during fasted training can optimise hydration without compromising adaptation.

• Jill Leckey is Science in Sport’s senior nutritionist. For more information on nutrition for running, visit Science in Sport.

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