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Can Running Fasted Improve Performance?

If you’re someone who runs first thing in the morning, you likely have debated whether you should eat something before your run, or run on an empty stomach.

Running without eating anything before or during a run is known as fasted running. The theory is that running when carbohydrates are depleted, helps train the body to use fat as the main source of energy, thereby improving endurance and promoting weight loss. This is also referred to as “training low” or being “fat adapted.”

But what impacts does fasted running have on the body and does it actually improve performance? 

Claims of Fasted Running and Potential Risks

1. Increased fat burning: When you run in a fasted state, your body's glycogen (carbohydrate) stores become depleted quickly. As a result, your body relies on more fat for energy. Fat is a very slow energy source. This means you cannot run as fast when running primarily off of fat. While running fasted for a short, low-intensity run may not be an issue, as the duration and intensity of a run increases, a review of 46 studies found that consuming enough carbohydrates prior to and during a run is needed to keep energy levels up and improve performance.

2. Metabolic adaptations: Regularly running without fuel may promote metabolic adaptations by recruiting more muscle fibers. Tired muscles starve off energy causing more muscle fibers to be recruited. This happens when the body is depleted of carbohydrates. The downfall of starting a run with tired muscles and no fuel is increased risk of injury.

3. Weight management: One of the most common assumptions is that fasted running will help you lose body fat. While the body does use more fat when running in a fasted state this doesn’t automatically equate to long term fat loss. Overall fat loss and body composition changes are dependent upon total daily calorie intake and energy balance over several days and weeks.Also related, those who exercise in a fasted state and without a post-run fuel, consumed fewer calories overall but were found to feel more hungry and ended up burning fewer calories compared to those who were fed before and after exercise.


If you’re looking to maximize your running performance, the evidence is clear that those who run fasted do not outperform those who are fueled. Consider the big picture, if you’re the type of runner who is injury prone, experiences fatigue, or struggles to recover, make fueling before every run a priority. Let’s move away from this restrictive mindset of “how little can I eat to get by” and instead have a growth mindset of “how can I best fuel my body to feel and perform my best.”

This article orginally appeared in the The Connection, TCM's weekly e-newsletter. Subscribe here.

Photo by Ben Garvin.


Kristy Baumann, RD, LD, is a Registered Dietitian who specializes in working with runners. She is one of the team of Motion Experts TCM has gathered to help its subscribers and participants get the most out of their running. Have a question for Kristy: [email protected], Instagram: @marathon.nutritionist or website:


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