Running on Empty: The Pros And Cons of Fasting Runs

Last week, after a big breakfast and about a dozen cups of coffee, I started my Saturday long run. I was planning to spend two and a half hours leisurely running at a relaxed pace. I don’t usually bring gels on runs under three hours, but I packed a few on this one. I started fading 30 minutes into the run so reached for a gel. It helped for a short time, but as the miles ticked off, my tank emptied. After another gel, my fatigue and hunger faded for a few minutes, only to return shortly after with a vengeance.

It was a routine two and a half hour run—something I should’ve been able to handle with ease. Still, I was bonking. In response, my coach assigned fasted morning runs. In other words, after hearing that gnawing hunger and plummeting energy ruined my run, he told me the answer was to eat less. Seems counter-intuitive, right?

Runners often view bonking as a rite of passage. They think that if you don’t hit the wall during training then you’ll never get to your personal best. But we may be able to skirt bonks altogether. A nutritional practice known as fasting runs could let endurance athletes train their bodies to use fuel more efficiently. Here’s how it works.

Your Fuel

Successful endurance athletes use fat, not carbohydrates, as fuel. The more readily you can burn fat while running at a high intensity, the longer your glycogen stores last. You’ll have energy throughout the race and are less likely to hit the wall.

Your body has a limited supply of glycogen available to fuel your working muscles. You can run about two hours at high intensity before you run out of glycogen. Even the fastest endurance athletes in the world rely on more than just their own glycogen stores to fuel their goal.

But eating during the run won’t replace all the glycogen you burn. Mid-race fueling is limited by how quickly your digestive system can deliver glycogen to your bloodstream. And during a race, the stomach is not very efficient.

Training the Body to Burn Fat

By not having readily available muscle glycogen to burn, your body is forced to burn fat. When you perform long runs on low glycogen stores, the theory holds, your body will become more efficient at using fat as a fuel source.

A New Zealand study of cyclists found that when participants rode in a fasted state early in the morning, they improved muscle glycogen stores by as much as 50%. That study and other research suggests occasional fasting before exercise can improve glycogen storage and endurance performance.

But while fasting is helpful when used in occasional workouts, endurance athletes don’t appear to benefit from training with low glycogen levels during extended periods such as training blocks, as other studies indicate that prolonged carbohydrate depletion impairs performance without enhancing fat utilization.