To Fast or Not to Fast: Pros and Cons of Running on an Empty Stomach

Ahhh, breakfast. The highly controversial meal in our post-industrial modern society. Some love it, some hate it, some can't live with it and some can't live without it. For us runners, the question of whether to run on an empty stomach or fuel up beforehand is a question that's unavoidable at some point in our journeys. Here, we weigh the pros and cons of running fasted, then provide guidance for different types of runners. (Disclaimer: we're not doctors, and this ain't medical advice.)

Running on an Empty Stomach: The Pros

Here are a few reasons why you might want to break your fast after hitting the tarmac.

1. Fat Burning

Running on an empty stomach forces the body to tap into its fat stores for energy. When glycogen levels are low, the body resorts to burning fat as fuel. This can be advantageous for runners aiming to improve their body composition or enhance their fat-burning capabilities.

2. Improved Insulin Sensitivity

Fasting before running can enhance insulin sensitivity. When you exercise without consuming carbohydrates beforehand, your body becomes more efficient at utilizing insulin and regulating blood sugar levels. This can be particularly beneficial for individuals with insulin resistance or those aiming to prevent the onset of type 2 diabetes.

3. Mental Toughness

Running on an empty stomach can be a mental challenge. By pushing through the discomfort and completing a workout without external fuel, it's possible that the added challenge can help you develop mental resilience and discipline (as long as you have enough gas to push it). This can translate to improved focus and determination during races or challenging training sessions.

Running on an Empty Stomach: The Cons

Here are some reasons why you might want to have some pre-run breakfast before getting after it.

1. Decreased Performance

Running without fuel can lead to decreased performance, especially during high-intensity or long-duration workouts. Carbohydrates are the body's primary source of energy, and without an adequate supply, fatigue can set in earlier, leading to decreased speed, power, and endurance.

2. Risk of Muscle Breakdown

When the body lacks readily available carbohydrates, it may start breaking down muscle tissue for energy. This can be detrimental to runners aiming to preserve muscle mass (as all of us should be) or those engaged in strength training alongside running (aka tryna get swole).

3. Increased Risk of Hypoglycemia

Running on an empty stomach can lead to low blood sugar levels, resulting in hypoglycemia. This can cause dizziness, lightheadedness, weakness, and even fainting. If you have a medical condition, definitely proceed with caution and consult your doctor before attempting a fasted run. 

Which Types of Runners Should Run on an Empty Stomach?

If you're an advanced runner or looking to lose weight, you may want to consider running fasted. Here's why.

1. Experienced Endurance Runners

Seasoned endurance runners who have adapted to running long distances and have a well-established aerobic base can benefit from occasional low intensity fasted runs. Their bodies are more efficient at utilizing fat as fuel, and they can tap into their fat stores without experiencing a significant drop in performance. You can also experiment with having a light breakfast and then doing your 2-hour long run without any gels or in-flight fuels to get some of the same benefits with a less extreme approach.

2. Those Aiming for Fat Loss

People with a primary goal of shedding excess body fat may find running on an empty stomach beneficial—as long as you properly fuel the day before. By engaging in fasted workouts, you can create a calorie deficit and potentially enhance fat-burning capabilities. However, it's important to strike a balance and not compromise performance or overall health.

Which Types of Runners Should Eat Before Running?

Here's our take on who should fuel before lacing up.

1. Beginners

Novice runners who are just starting their running journey or have limited experience should prioritize fueling before workouts. Adequate carbohydrates provide the necessary energy to sustain workouts and help prevent premature fatigue.

2. Intense or Long-Distance Runners 

Runners engaging in high-intensity interval training, speed workouts, or long-distance running should fuel their bodies before exercise—i.e., if you're ripping 800 repeats with the boys on the local elementary school track before work, definitely eat beforehand. These types of workouts demand a higher energy output, and carbohydrates are essential for optimal performance and recovery.

3. Individuals with Medical Conditions

Runners with diabetes, hypoglycemia, or other medical conditions affecting blood sugar regulation should consult with their healthcare providers before running on an empty stomach. These conditions require careful nutrition management best left to the pros.