Running on an Empty Stomach: A Guide to Fasted Running

Whether on road, trail, or gravel, running on an empty stomach is common for those who run first thing in the morning, but is it better to run on an empty stomach? It depends on your goals and preferences. Before deciding whether to run fasted, consider session duration and intensity to ensure proper fueling and maximize the benefits of running in the morning or any time of day.

Fasted running: pros and cons

Fasted running or running on an empty stomach means starting out with less fuel stored in your body, specifically from carbs. Carbs, once ingested, are converted into glycogen and stored in the muscles as the body’s main energy source. Despite this, some people still prefer running before breakfast. Here are some pros and cons of fasted running to help you decide how to plan your run.

Pros:

• Fasted runs can boost performance later by prompting more efficient energy storage and conversion of carbs into fuel.
• Cortisol levels increase during fasted runs, a stress response that some runners report helps them focus.

Cons:

• Increased cortisol can add physical stress for some.
• During higher-intensity efforts, glycogen-depleted muscles fatigue faster, and falling blood-sugar levels can cause lightheadedness.

While it’s not bad to run on an empty stomach, you may “hit a wall” if you run out of energy due to a lack of glycogen. To avoid this, align your fueling strategy to your workout goals and use fasted runs only when advantageous.

When running on an empty stomach is the right call

Salomon athlete Courtney Dauwalter has mentioned starting race mornings with coffee alone, skipping solid food until well into the effort. Even on regular training days, running on an empty stomach simplifies your routine. Just put on your running shoes and head out the door. If you’re wondering, “Should I run on an empty stomach?,” consider the time of day you want to run and the type of run you’re planning.

Running first thing in the morning

There are plenty of benefits to jogging or running in the morning. For example, you may want to take advantage of cooler temperatures when running in hot summer months. Fasted running is also easier in the morning because overnight fuel reserves can typically support 60-90 minutes of low-intensity work before daily activity.

Easy runs and recovery sessions under 60 minutes

A slower, easier effort requires less energy, making it better suited to a fasted run. During these efforts, your heart rate stays in an aerobic zone, your body relies more heavily on fat as a fuel source, and blood sugar should hold steady enough to avoid any dizziness.

When running on an empty stomach isn't the right choice

Eating before running becomes less optional when duration and intensity increase. If you’re unsure what to eat before a morning run or evening training, be aware that some workouts may require additional fuel.

Long runs over 90 minutes

Once a run passes the 90-minute mark, stored fuel will run low regardless of pace, and fat alone typically can't meet your energy demands, potentially leaving you fatigued during the final stretch. Runners training for long races, from a half marathon to 100K, who skip fuel on extended runs often report hitting a wall in the final third. If you need to refuel during a longer effort, running bags and belts are a great place to store a bar or energy gel. It’s also always a good idea to carry water; flasks and reservoirs help you stay hydrated on the trail or road.

High-intensity intervals and tempo runs

Intervals and tempo work rely on carbs stored as fuel for anaerobic energy production. Even if you’re able to finish the workout as a fasted run, power output, pacing consistency, and recovery may all be affected without available glycogen. Aim for a small carbohydrate source 30-60 minutes before pace-based sessions meant to help you run faster.

You may also consider additional fuel if you’re already feeling tired or sore. Running at altitude or with delay-onset muscle soreness can compound fatigue, making fasted running even harder.

Fasted running and fueling basics

You should also factor in digestion time when deciding whether to eat before or after a run. A low-fiber snack close to a workout should be fine, but a full meal needs two to three hours to clear your system, and running on a full stomach can be unpleasant. As for what you should eat before a run, a good starting point is 20-30 grams of easily digested carbohydrates (such as a banana or slice of toast) 30-60 minutes beforehand.

Common questions about fasted running

Is it better to run on an empty stomach?

Again, this depends on the workout type and your ultimate goals. Fasted runners often post similar times for short, easy distances, and fasted training may cause metabolic adaptations in some runners. However, pace and power output decline once energy stores are depleted.

Is it okay to run 10k fasted?

If you’re training for a 10K, rest assured that this distance at an easy to moderate pace (typically finished in 45-65 minutes) falls within the one-hour window that fasted glycogen stores can generally support.

Do professionals run fasted?

Many elite athletes incorporate fasted running into their training routine, reserving pre-run fueling for race-pace or longer workouts.

Knowing when to run on an empty stomach

Fasting and running can be safely combined as long as you adjust your fueling strategy to your workout timing, duration, and intensity. Short, easy efforts and early-morning runs lend themselves to fasted running, while longer, higher-intensity efforts later in the day call for prior fuel intake. Balance all of this advice with your own personal preferences, and use your training to determine what works best for you, whether that’s running before breakfast or fueling up before heading out.