At the start of a run, the pace feels comfortable, breathing is steady, and the legs respond well—everything feels smooth and controlled. However, somewhere in the middle of the run, the heart rate begins to rise and the same pace suddenly requires more effort to maintain.
Sometimes, runners interpret this as a sign that they lack endurance. In many cases, though, the explanation is simpler: the body is running low on readily available fuel.
To understand why you might lose energy mid-run, it helps to look at how the body generates energy during exercise.
Why you lose energy halfway through a run
Many runners lose energy halfway through a run because their glycogen stores—the body’s main source of running fuel—begin to run low.
During running, the main source of energy comes from carbohydrates stored in the muscles and liver as glycogen. These stores allow the body to produce energy quickly and support a stable pace. As the run continues, glycogen levels gradually decline.
Once these carbohydrate stores begin to fall, the body starts to rely more on other fuel sources, especially fat. This shift happens naturally during longer runs; still, fat cannot supply energy as quickly as carbohydrates. As a result, maintaining the same pace begins to require more effort: breathing and heart rate increase, and the run starts to feel heavier.
Research in endurance physiology consistently shows that carbohydrate availability strongly influences performance during sustained exercise.
When glycogen levels are already low at the start of a run, the available supply runs out sooner. That is why the drop in energy often appears in the middle of the run: the body reaches the point where carbohydrate support becomes limited.
This pattern is especially common during morning runs or any session that begins under-fueled. At first, the body still has enough stored energy to support the pace. As the run progresses, however, the buffer becomes smaller. Even if the pace stays the same, the effort rises.
For many runners, this moment leads to frustration: they may want to push harder or assume that endurance needs more training. Yet the real cause may appear much earlier: the run simply began with less fuel than the body needed to keep energy steady from start to finish.
This often occurs during early morning runs, when it feels easier to head out the door without breakfast. So, what actually happens in the body overnight that makes this such a common problem?
Why running on an empty stomach drains your energy
During the night, the body continues to use glycogen to maintain stable blood glucose levels. The brain depends on this supply, so the liver steadily releases glucose into the bloodstream while you sleep.
By morning, liver glycogen stores are often significantly lower than they were the evening before.
Muscle glycogen usually remains relatively intact, but the overall amount of readily available carbohydrate is still reduced. That matters because carbohydrates are the primary fuel for running at moderate and faster paces.
Related: Should You Run On an Empty Stomach or Eat Breakfast First?
When a run begins without eating, the body starts the workout with lower carbohydrate availability. For short, easy runs, this may not cause any noticeable problem. However, during longer or moderately hard runs, the available glycogen can become limiting much earlier.
This is why runners who skip breakfast often notice their energy dropping sooner than expected. The body has less carbohydrate available to support sustained effort.
A small amount of carbohydrate before the run can significantly change this picture. Even a light pre-run snack can help maintain steadier energy through the middle of the run.
Here are several options that work well before a run.
What to eat before a run for steady energy
The goal of a pre-run snack is to provide a small amount of easily digestible carbohydrate that the body can use quickly.
Large meals or heavy foods can slow digestion and make running uncomfortable, so, in most cases, you will do best with simple, familiar foods eaten about 30–60 minutes before heading out:
• A banana
One of the simplest and most widely used pre-run snacks. A medium banana provides about 25–30 grams of carbohydrate and is easy to digest for most runners.
• A toast with a small amount of jam
A slice of white or sourdough toast with a thin layer of jam or honey provides quick carbohydrate without much fiber or fat.
• An energy bar
Many runners prefer half a bar before the run and save the rest for later. This provides a small carbohydrate boost without feeling too heavy.
• A small bowl of oatmeal
A modest portion of plain oatmeal can work well before longer runs. It provides steady carbohydrates and is easy to customize with a little honey or fruit.
• A handful of pretzels or crackers
Pretzels or plain crackers digest quickly and provide carbohydrate without much fat or fiber.
• Applesauce or a fruit pouch
These are surprisingly popular among runners because they are light, portable, and easy to digest.
• A sports drink or half a gel
When there is little time before the run, liquid carbohydrate can be the simplest solution.
A small amount of carbohydrate before running can help stabilize energy levels and delay the drop in pace that many runners experience halfway through a run. Over time, you will develop a short list of pre-run foods that work reliably for you.
When fasted running can still work
Skipping food before a run does not automatically lead to problems. In some situations, fasted running can work well and even feel comfortable—for example, before short, easy runs.
Fasted running can work well in situations such as:
- Easy runs lasting about 30–45 minutes
At this duration and intensity, most runners still have enough glycogen available to support the effort. - Very relaxed recovery runs
If the goal is to move the legs and promote circulation, fueling beforehand is usually unnecessary. - Early morning runs when appetite is low
Some runners prefer to run first and eat afterward. As long as the run is easy and not too long, this approach can work well.
However, once the run becomes longer or harder, fasted running becomes much less practical. Long runs, as well as tempo runs, intervals, or any other speed work, require more carbohydrate to maintain pace. Without fuel beforehand, you may notice your energy dropping much earlier than expected.
Other fueling mistakes that can drain energy during a run
Starting a run under-fueled is one of the most common reasons runners lose energy midway. A few other fueling habits can lead to the same problem.
- Waiting until you feel tired before taking your first gel during a long run
- Eating mostly protein or fat before the run, with little carbohydrate
- Having a large meal too close to the start of the run
- Relying only on water during long runs instead of adding carbohydrates
- Taking in most of your calories late in the run instead of spreading them out
- Starting a long run after many hours without eating
- Trying new foods or gels on race day or during an important long run
Once fueling becomes more consistent, the middle miles of a run often feel noticeably easier.
Related: 7 Most Common Reasons Runners DNF—And How to Finish Strong
How to prevent the mid-run energy crash
In many cases, the drop in energy during a run does not come from lack of fitness but from fueling habits. A few small adjustments before and during the run can make a noticeable difference in how steady your pace feels.
First, make sure the run does not begin with empty energy stores: a small amount of carbohydrate before heading out can increase available fuel and help stabilize pace during the middle part of the run.
For longer runs, it also helps to think about fueling during the workout rather than waiting until fatigue appears. Taking small amounts of carbohydrate earlier and more consistently can support a steadier pace.
Equally important is keeping pre-run meals easy to digest and primarily carbohydrate-based, as they tend to work best before running.
When you fix your fueling habits, many runs that once felt unpredictable begin to feel much more controlled.
FAQ: Running energy and fueling
Why do I lose energy halfway through a run?
Many runners lose energy halfway through a run because their glycogen stores—the body’s main source of running fuel—begin to run low. When carbohydrate availability drops, maintaining the same pace requires more effort.
Should I eat before a run?
For most runs longer than about 45–60 minutes or runs with higher intensity, a small carbohydrate snack beforehand can help maintain steady energy levels.
Is running on an empty stomach bad?
Not necessarily. Fasted running can work well for short, easy runs. However, longer runs, tempo runs, and speed workouts usually feel better when some carbohydrate is available.
What should you eat before a run?
A small, easily digestible carbohydrate snack—such as a banana, toast with jam, or a small energy bar—can help stabilize energy during a run.
