Is It Smart to Run a Race on an Empty Stomach?

October 18, 2025 •

5 min read

For endurance events over 90 minutes, carbohydrate loading can boost muscle glycogen stores by 30% or more, highlighting the importance of fuel for performance. For most athletes, this makes deciding whether to run a race on an empty stomach a high-stakes question with significant consequences for their race day outcome.

Quick Summary

Running a race, especially long or high-intensity events, on an empty stomach is not recommended due to the high risk of premature fatigue, muscle loss, and impaired performance. Proper fueling strategies are essential for sustained energy and optimal race execution.

Key Points

Performance Risk: Running a race unfueled significantly increases the risk of decreased performance and early fatigue due to depleted glycogen stores.
Distance Matters: While easy, short runs can sometimes be done fasted, long or high-intensity races absolutely require proper fueling for sustained energy.
Fueling Prevents 'The Wall': Consuming carbohydrates before and during a long race prevents the glycogen depletion that causes runners to 'hit the wall'.
Practice is Key: Race day nutrition should never be an experiment. Practice your entire fueling strategy, including pre-race meals and mid-race gels, during training.
Avoid Muscle Breakdown: Fasted high-intensity running can cause the body to break down muscle tissue for energy, negatively impacting strength and recovery.
Hydration is Crucial: Regardless of your eating strategy, staying well-hydrated with fluids and electrolytes before and during a race is essential.

The Science of Fuel: Fasted vs. Fed States

To understand the right strategy for race day, it's crucial to know how your body uses fuel. When you haven't eaten for an extended period, such as after an overnight fast, your body's glycogen (stored carbohydrate) levels are low. In this fasted state, your body relies more heavily on fat stores for energy, a process known as fat oxidation. This might sound like a great idea for fat loss, and for low-intensity training, it can offer some metabolic benefits by improving the body's ability to utilize fat.

Conversely, a fed state means you've consumed carbohydrates that are readily available in your bloodstream as glucose and stored as glycogen in your muscles and liver. This is the body's preferred and most efficient energy source for high-intensity or prolonged exercise. Eating a pre-race meal ensures these crucial glycogen stores are topped up, providing the fuel needed for sustained effort and preventing early burnout.

The Risks of Running a Race on Empty

For a true race, where the goal is performance, the risks of running on an empty stomach far outweigh any perceived benefits. These risks include:

Decreased Performance: Without a quick-burning fuel source like carbohydrates, you will feel sluggish and lack the power needed for speed and endurance, especially in longer events. A 2018 review of 46 studies found that exercise after eating improves prolonged aerobic performance.
Risk of 'Hitting the Wall': This infamous phenomenon occurs when your glycogen stores are depleted, forcing your body to switch to less efficient fat-burning. The result is a sudden, extreme drop in energy, fatigue, and the inability to maintain pace. Proper fueling prevents this altogether.
Hypoglycemia and Injury: Low blood sugar (hypoglycemia) can cause dizziness, lightheadedness, and weakness. In a race environment, this can lead to poor form, a lack of focus, and an increased risk of falling or injury.
Muscle Breakdown: When your body is starved of glycogen, it can begin to break down muscle tissue for protein to use as an energy source, which is counterproductive to building strength and endurance.

Tailoring Your Fueling Strategy by Race Distance

Your fueling plan should always be based on the length and intensity of the race. The longer the race, the more critical proper fueling becomes.

Short Distance (5K or 10K)

For shorter, high-intensity races, your body relies primarily on stored glycogen and blood glucose. While a fasted morning run might work for easy training, race day is different. You need peak energy. A light, easily digestible carbohydrate snack 30-60 minutes before the start is a good strategy.

Pre-Race Snack Examples:
- Small banana
- Energy gel or chews
- Slice of toast with jam

Mid-Distance (Half-Marathon)

This distance depletes glycogen stores more significantly, making mid-race fueling necessary. Practice your fueling strategy during long training runs to ensure your stomach can handle it. A common strategy involves consuming a small meal 2-3 hours before the race and then supplementing during the race with gels or chews.

Long Distance (Marathon or Ultra)

For races exceeding 90 minutes, fueling is not optional—it's essential. A comprehensive carb-loading strategy over the 1-3 days before the race, along with consistent fueling during the event, is vital to prevent glycogen depletion. Most experts recommend consuming 30-60g of carbohydrates per hour after the first hour of running. For longer ultras, this can increase to 60-90g per hour.

Comparison: Fasted vs. Fueled Racing


Feature	Fasted Race Strategy	Fueled Race Strategy
Energy Source	Primarily fat stores; inefficient for high-intensity efforts.	Primarily glycogen and blood glucose; efficient, quick-burning fuel.
Performance Impact	Significant risk of decreased speed, power, and early fatigue.	Enhanced endurance and sustained energy to perform at your best.
Stomach Comfort	Can potentially reduce pre-race GI issues for sensitive stomachs.	Can cause stomach upset if not practiced; proper timing is key.
Mental Clarity	Some report initial clarity, but this can give way to mental fog and low motivation.	Brain function is maintained with a steady supply of glucose, preventing mental fog.
Recovery	Slower recovery time due to muscle protein breakdown and low energy.	Faster recovery time; proper fuel intake helps replenish glycogen and repair muscles.
Race Suitability	Not suitable for high-intensity or long races. Extremely risky.	Ideal for all race distances and intensities, especially when aiming for peak performance.

Your Race Day Fueling Plan: A Guide

For an optimal race day, especially for distances beyond a 5K, a strategic fueling plan is your best asset. Here is a step-by-step approach based on common practices among runners:

Practice in Training: The first and most important rule. Never try a new food or gel on race day. Use your long training runs to experiment with different foods and timings to see what your body tolerates best.
Carb-Load (for longer races): In the 1-2 days before a half-marathon or longer, increase your carbohydrate intake while reducing fat and fiber. This tops off your glycogen stores. Think white rice, pasta, or potatoes.
The Pre-Race Meal (3-4 hours before): This meal should be familiar, high in carbohydrates, low in fat, and moderate in protein. Good examples include oatmeal with a banana or a bagel with peanut butter.
The Pre-Race Snack (30-60 minutes before): A quick, high-carb snack like an energy gel or a few energy chews can top off blood glucose levels right before the start.
Fuel During the Race (for long races): Consume 30-60 grams of carbohydrates per hour, starting around 30-40 minutes into the race. Use gels, chews, or sports drinks. Carry what you practiced with.
Hydrate Strategically: Drink water or an electrolyte drink throughout the day before the race. During the race, follow a hydration schedule based on your sweat rate.

Conclusion: Fuel to Succeed

While the concept of fasted cardio holds a place in some training regimes, deliberately choosing to run a race on an empty stomach is a strategy that puts performance at severe risk. For any competitive or high-intensity race, your body needs readily available carbohydrates to maintain energy, power, and focus. Failing to fuel properly can lead to decreased performance, injury, and the dreaded 'wall.' The most reliable path to a strong race is a practiced, well-executed fueling plan that includes carbohydrate-rich meals and hydration before and during the event. Listen to your body, train your gut, and fuel for success on race day.

Learn more about fueling for endurance events by exploring detailed nutrition resources, such as those published by the National Institutes of Health.

Frequently Asked Questions

While it's possible to complete a 5K on an empty stomach, particularly for less competitive runners, it is not recommended for optimal performance. A light, easily digestible carbohydrate snack 30-60 minutes beforehand can provide an energy boost without causing stomach upset.

Fasted exercise does increase fat oxidation during the workout itself. However, it does not necessarily lead to greater long-term weight loss compared to fed exercise, and can negatively impact performance and recovery.

Prior to a marathon, you should carb-load over the 1-3 days leading up to the event by increasing your carbohydrate intake. On race morning, eat a familiar, high-carb, low-fat, and moderate-protein meal 2-3 hours before the start.

Feeling dizzy is a symptom of hypoglycemia, or low blood sugar. When your body doesn't have enough glucose from food, your blood sugar levels can drop, affecting both your energy and your brain function.

Hitting 'the wall' is a term for the severe fatigue that occurs when your body runs out of stored glycogen during a long race. To avoid it, practice proper carbohydrate loading before the race and consume energy gels or chews during the race to replenish your stores.

For races lasting over 60-90 minutes, energy gels, chews, or sports drinks are highly recommended to provide the continuous carbohydrates your body needs to sustain performance. For shorter races, they can be used for a quick boost.

Practice your entire fueling strategy, including your pre-race meal and mid-race snacks, during your long training runs. This allows your digestive system to adapt and helps you find what works best for you before the actual race.