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The Rising Popularity Of Fasted Running

A registered dietitian explains the rising trend of fasted running.

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Running On Empty

Along with the ketogenic diet and intermittent fasting, another current nutrition trend in the running world is fasted running, or running on an empty stomach. The thought behind fasted running is that it reduces the body’s available glycogen (stored carbohydrates) to help train the body to burn more fat during endurance events, like in the final miles of a marathon. Essentially, it teaches the body to adapt to low carbohydrate availability with the thought of improving endurance capacity. Other terms for fasted running include “training low,” or being “fat adapted.”

However, fasted running goes deeper than just running on an empty stomach. Let’s consider fat vs. glycogen stores. While our glycogen muscle stores are finite, we basically have unlimited stores of fat. We store carbohydrates in both our liver and muscles. The liver glycogen is responsible for keeping our blood sugar stable throughout the day and night, and providing energy for our cells and brain. Our muscle glycogen primarily provides energy for our working muscles when exercising. Those who train more generally store more glycogen.

There are essentially two ways to deplete glycogen and achieve low carbohydrate availability. The first is after an overnight fast, or waking up to run on an empty stomach. In this situation, the liver glycogen is low, but the muscle glycogen levels still remain high (our muscles don’t consume glycogen when we sleep). When doing this, we don’t see many benefits of “training low” since our muscles still have glycogen available.

The alternate option for depleting carbohydrates is by undertaking two bouts of exercise in close proximity without an adequate opportunity for refueling after the first session (i.e. training at night, and not replenishing with carbohydrates before training again in the morning). While both of these methods are commonly used by athletes, it still remains unknown if a specific threshold exists where glycogen depletion increases cellular signaling adaptations or affects performance.

The Role Of Carbohydrates

There are many reasons our bodies need carbohydrates when exercising. Carbohydrates are the preferred fuel source for our muscles and are readily utilized. Aside from providing quick fuel, carbohydrates also play a role in recovery, immune function and training adaptations. In fact, carbohydrate supplementation has the strongest scientific support for reducing post-exercise stress and inflammation and improving immunity.

Exercising while carbohydrate-depleted incurs extra stress on the body. There is significant evidence that the performance of intermittent high-intensity or prolonged and sustained exercise is enhanced by tactics that maintain high carbohydrate availability and match glycogen stores to fuel needs for exercise.  While training with limited carbohydrate availability may lead to some metabolic adaptations during training, it hasn’t been linked to performance improvements. Alternatively, training in a fasted state or with limited carbohydrate stores may impair training intensity and duration. The depletion of these stores is associated with fatigue, reduced work rates, impaired skill and concentration and increased perception of effort.

Is Fasted Running Worth it?

Ultimately, consider the price you want to pay. Is achieving some small metabolic adaptation benefits worth it, when comparing other factors like compromised speed and intensity, fatigue, hydration, immunity and stable energy? Are you looking to achieve a personal best, or just run longer without hitting the wall?

As is with pretty much anything in nutrition, consider individual variability. What works for one person may not necessarily work well for you. If you thrive on carbohydrates now, this may not be right for you and will probably just leave you feeling sluggish and irritable. However, any metabolic adaptations may be helpful for longer, ultra-endurance events where a compromised intensity is less important. If you’re interested in trying this approach, try eating low carb or avoiding fuel during your easy long runs, rather than your hard workouts.

Hopefully, future research will show if periodizing carbohydrates during training will enhance future performance.

Sarah Schlichter is a registered dietitian and marathon runner based in Charlotte, N.C. She works as a nutrition consultant and in private practice, where she writes the blog, Bucket List Tummy, sharing nutrition posts, healthy recipes, running tips and everything on her bucket list.


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