Fasted Running Explained: The Pros, Cons, and What to Know

Fasted running is a hotly debated topic in both the health and running communities.

Advocates claim it’s a game-changer for fat burning and performance, while critics argue it can harm your body and hinder results.

So, should you lace up and hit the pavement before eating?

What is Fasted Running?

Fasted running involves running without eating for 8–14 hours beforehand, typically after an overnight fast.

To qualify as a fasted run, it must occur before consuming any calories, meaning morning runs after a night of sleep are the most common form.

However, running on an empty stomach later in the day, after hours without eating but after a prior meal, does not count as fasted running.

The Theory Behind Fasted Running

Proponents of fasted running suggest it improves fat oxidation, the process by which your body burns fat for energy. When glycogen stores are low, as they are during fasting, your body shifts to burning fat. This adaptation is believed to:

• Improve endurance by increasing reliance on fat during lower-intensity efforts.
• Optimize body composition by burning more fat.

However, fat oxidation becomes less relevant at intensities near or above your aerobic threshold (e.g., marathon pace or faster). At these intensities, your body shifts back to primarily burning carbohydrates for energy.

The Science of Fasted Running

Research offers conflicting insights into fasted running. Here’s what we know so far:

Potential Benefits

1. Increased Fat Oxidation
   This study shows that fasting before exercise increases fat utilization. This adaptation can be beneficial for ultra-distance events where fat becomes a primary energy source.
2. Convenience
   For some runners, fasted runs are simply easier to schedule, especially for early-morning sessions when eating beforehand might feel uncomfortable.

The Risks and Downsides

1. No Performance Gains
   A 2021 study published in Nutrients found no improvement in 10K times among runners who practiced intermittent fasting, even though they experienced fat loss. The study theorized that low carbohydrate availability negated any benefits from improved body composition.
2. Muscle Breakdown
   Fasting can increase protein breakdown in muscles, according to a 2011 article in Strength and Conditioning Journal. This breakdown may lead to a loss of strength and a higher risk of soft tissue injuries.
3. Compromised Endurance
   A 2018 meta-analysis in the Scandinavian Journal of Medicine and Science in Sports concluded that eating before exercise improves aerobic endurance performance compared to fasted states.
4. Increased Hunger and Poor Appetite Regulation
   In a 2012 study, participants who exercised after eating had better appetite regulation than those who ran fasted. Post-run hunger spikes after fasted runs may lead to overeating or poor nutritional choices.
5. Lower Exercise Ceiling
   At higher intensities, such as interval sessions or tempo runs, the body relies primarily on carbohydrates. Fasting depletes glycogen stores, lowering your capacity for hard efforts.

Special Considerations: Gender and Energy Availability

Fasted running can pose unique risks for female runners. Poor energy availability during exercise increases the risk of menstrual irregularities and reduced bone density.

Fasted runs create extended periods of low energy availability, which can exacerbate these issues.

For both men and women, relative energy deficiency in sport (RED-S) is a real concern.

Symptoms include fatigue, frequent injuries, and impaired performance. Fasted running can contribute to RED-S in athletes with high energy demands.

When (and If) to Use Fasted Running

Fasted running isn’t inherently harmful, but it’s not the magic bullet some believe it to be. Here’s how to decide if it’s right for you:

When to Skip Fasted Running

• You have specific performance goals (e.g., training for a PR).
• You’re prone to injuries or muscle breakdown.
• You experience menstrual irregularities or suspect RED-S.
• You feel sluggish or underperform during fasted runs.

When Fasted Running May Be OK

• For convenience on easy runs under 60 minutes.
• When training for ultra-distance events that emphasize fat oxidation.
• If you don’t have performance goals and prioritize general fitness.

How to Implement Fasted Runs Safely

If you decide to try fasted running, follow these guidelines to minimize risks:

1. Limit to Easy Runs: Fasted running should only be done at low intensities where fat oxidation is sufficient to fuel your efforts. Avoid fasted long runs, interval workouts, or tempo sessions.
2. Stay Hydrated: Drink water before and during your run to prevent dehydration.
3. Observe Your Body: Monitor how you feel during and after fasted runs. If you notice fatigue, irritability, or poor performance, consider adding a pre-run snack.
4. Start Small: Begin with short, easy runs of 20–30 minutes and gradually extend as needed.

Pre-Run Fueling: A Better Alternative

For most runners, eating a small, easily digestible snack before running will improve performance and recovery.

Ideal pre-run snacks include:

• A banana;
• A handful of dried fruit;
• A slice of toast with honey;
• A stroopwafel.

These provide the quick energy you need without causing digestive issues.

Final Verdict: Should You Do Fasted Runs?

Fasted running has its place for some runners, but it’s not a universal solution.

While it may improve fat oxidation, it doesn’t necessarily translate to better performance, especially for shorter races or high-intensity efforts.

For most runners, the risks muscle breakdown, poor energy availability, and potential injury outweigh the potential rewards.

Prioritizing proper fueling before runs will likely lead to better training outcomes and long-term health.