Will Fasting Diets Make You Faster?

Are you curious about fasting diets? A registered dietitian and sport nutritionist reviews the evidence on how the different types of fasting diets impact energy, running performance and overall health

Fasting diets have become hugely popular among runners. Registered dietitian and sport nutritionist Melissa Kazan explains the theory behind them, what the research says, how they may impact your health and whether fasting makes you faster.

Runners are often told to eat six or more times per day and to ensure that they maximize their fuel stores before, during and after their training or race. We also know that higherintensity bursts of movements – even in a long run – such as the sprint to the finish line, are carbohydratedependent. So how can fasting, which leads to low carbohydrate stores, be of any benefit?

The rationale behind these fasting diets is that exercising in a fasted state leads the body to burn more fat (known as fat oxidation). This is interesting for runners, since using fat as a fuel can also save muscle carbohydrate stores (known as glycogen) for later use – like that sprint to the finish line. Some studies also show improved muscle metabolism, which might improve your endurance and help you recover better. In the last few years, new research has focussed on the impact on muscle recovery and athletic performance when training in a fasted state or with low energy stores.

What happens to your body when you fast?

Before we get into what the research says about fasting and running performance, let’s have a look at how the body reacts to fasting.

The body’s main energy source – glycogen, or stored carbohydrate – is stored mostly in the muscles, with the rest stored in the liver. Between meals and while we’re sleeping, the body draws energy mainly from glycogen stored in the liver to maintain glucose supply to the brain and tissues. During exercise, most glycogen comes from the muscles. Usually, the higher the exercise intensity, the greater the dependence on muscle glycogen for fuel.

Although mental and physical fatigue can set in at this stage, exercising with low liver glycogen stores is usually easier than exercising with low muscle glycogen stores (which is why many runners feel comfortable going for their early-morning run without eating first). But continuing to fast for more than 12 hours eventually leads to reduced muscle glycogen and increased use of fat to supply energy to the body.

Relying on fat for fuel is exactly what many endurance runners are trying to accomplish in altering their diet and how much carbohydrate they eat, but most studies do not show any improvement in race times as a result.

The various methods and periods of fasting impact glycogen stores and endurance performance differently, which is why studies seem to show conf licting results. Let’s review the evidence, and whether these popular fasting diets make you run faster.