When it comes to marathon or ultramarathon training, some athletes wonder if they can push their bodies to new limits by skipping pre-race meals or running while fasted. The belief is that by training or racing on an empty stomach, the body will adapt to burning fat more efficiently, ultimately boosting performance. But is “running while fasted” actually beneficial, or does it put endurance athletes at a disadvantage? Let’s dig into the science behind fasted running and the potential risks it can pose to marathon and ultramarathon performance.

Understanding Fasted Running

First, let’s define what it means to run while fasted. Fasted running typically means that an athlete hasn’t consumed calories in the hours leading up to a workout, usually due to skipping breakfast or timing a workout first thing in the morning. For some, it’s simply a matter of convenience, while others see it as a strategic move to enhance fat adaptation, allowing the body to rely more on stored fats for fuel.

However, this method has gained attention in endurance sports, where the demands on energy are far greater than in shorter, less intense workouts. The trend of racing while fasted has grown alongside the popularity of ketogenic diets and intermittent fasting, promising athletes leaner physiques and improved endurance. But does this approach truly hold up when it’s time to perform, or are we sacrificing athletic potential for a misguided theory?

The Science of Fueling: Why Carbohydrates Are Essential for Endurance

When tackling endurance events like marathons and ultramarathons, energy needs skyrocket. For instance, during a marathon, the body primarily relies on glycogen—stored carbohydrates in the muscles and liver—as the primary source of fuel. This is because glycogen is the most accessible energy source for high-intensity and prolonged activities.

Yet, when glycogen stores are depleted (usually around the 90-minute mark in endurance activities), athletes hit what’s known as “the wall.” This phenomenon, where exhaustion, reduced coordination, and mental fatigue set in, is notorious among marathoners. The simplest solution? Consume carbohydrates during the race to keep glycogen stores high and avoid the wall.

By running while fasted, however, athletes deprive their bodies of these critical glycogen reserves. While the body will indeed adapt to burning fat when glycogen runs out, fat is a less efficient energy source during high-intensity activities. This forces the body to work harder, which, during a race, is the last thing an athlete needs.

How Running While Fasted Impacts Marathon and Ultramarathon Performance

In the podcast episode “The Zero Calorie Ultra,” I discuss the trend of zero-calorie endurance events, where athletes take in nothing but electrolytes, avoiding any source of calories altogether. This concept might seem revolutionary, but it can be highly problematic, especially for endurance athletes.

Avoiding calories in long races can have severe consequences, including:

• Rapid glycogen depletion: Without replenishing glycogen, athletes experience fatigue sooner, impairing their ability to sustain the pace they’ve trained for.
• Increased likelihood of ‘bonking’: The wall becomes almost inevitable without caloric intake, as the body has no immediate energy stores to draw from.
• Mental fatigue and impaired decision-making: The brain relies on glucose for fuel, so a lack of carbohydrates can impair cognitive functions, essential for pacing, navigating the course, and making quick decisions.

Fasting may work in shorter, lower-intensity workouts where glycogen stores aren’t as taxed, but in the context of marathons and ultramarathons, it can lead to a sharp decline in performance, making an athlete’s hard work feel counterproductive.

The Myth of Enhanced Fat Adaptation

A common argument for fasted running is the idea that it enhances “fat adaptation,” meaning the body becomes more efficient at burning fat for fuel. While it’s true that fasted training can increase fat oxidation rates to some extent, the body’s capacity to burn fat is finite. As training intensity increases, so does the body’s demand for quick, efficient fuel sources—carbohydrates. Fat is a slower-burning fuel and cannot sustain the rapid energy needs of a race without causing significant fatigue.

Moreover, while fat adaptation may benefit ultra-distance athletes to some extent, even they will eventually need carbohydrates to maintain optimal performance. Carbohydrate sources like energy gels or drinks provide the fast-acting fuel necessary to keep going strong in high-demand environments like marathons.

The Dangers of Zero-Calorie Endurance Events

Fasted racing can exacerbate health issues and lead to disordered eating patterns. Endurance athletes are already at risk of inadequate fueling due to high energy demands, and adding calorie restriction to the mix can lead to a harmful spiral. Specifically, running while fasted or pursuing zero-calorie events can:

1. Disrupt natural hunger cues: Skipping meals before and during exercise can interfere with the body’s signals for hunger, leading athletes to ignore their body’s genuine need for nutrients.
2. Increase the risk of Relative Energy Deficiency in Sport (RED-S): When athletes do not consume enough energy to meet their training demands, they risk RED-S, a condition that can lead to weakened immunity, poor recovery, hormonal imbalances, and even loss of bone density.
3. Encourage an unhealthy relationship with food: For athletes prone to perfectionism, extreme practices like fasted racing or zero-calorie events can lead to obsessive thoughts about weight, caloric intake, and performance, ultimately reducing enjoyment of the sport.

Focusing on performance over calorie restriction is essential for a healthy and sustainable approach to endurance sports.

Why Fueling Properly Is the Key to Long-Term Performance

The benefits of proper fueling extend beyond race-day performance. Athletes who prioritize adequate nutrition recover faster, train more effectively, and experience fewer injuries. Fueling well allows the body to perform at its best, while fasted running adds unnecessary stress that can reduce an athlete’s potential over time.

Endurance athletes should remember that marathon and ultramarathon events are particularly demanding. Just as they invest in quality shoes and training plans, athletes should prioritize nutrition that supports the rigors of the race. While trends like running fasted may seem appealing, the science overwhelmingly supports fueling well as the smarter, safer, and more effective choice.

This is something I teach my athletes within the Flight Collective Sports Nutrition Membership Program. 

The Bottom Line on Running While Fasted for Marathon and Ultramarathon Athletes

The risks of glycogen depletion, impaired decision-making, and increased likelihood of bonking far outweigh any potential benefits of running while fasted. While fasted training might offer marginal gains in fat adaptation, the body’s need for carbohydrates during high-intensity, prolonged events is undeniable.

Instead of taking the extreme route of running without calories, endurance athletes can benefit more from a well-rounded approach to fueling. By incorporating balanced pre-race meals, using carb periodization in training, and practicing steady fueling strategies, athletes set themselves up for success on race day.

Final Thoughts

Running a marathon or ultramarathon is a significant accomplishment, requiring both mental and physical strength. Sacrificing calorie intake may seem like a path to peak performance, but it often leads athletes down a path of decreased energy, mental fog, and potentially harmful eating habits. By respecting the body’s need for fuel, athletes can train harder, recover faster, and reach the finish line stronger.

If you’re looking to make the most of your marathon or ultramarathon training, consider working with a sports dietitian (like me!). I can help you find a fueling strategy that aligns with your body’s needs, training demands, and performance goals. Rather than running on empty, let’s embrace running on energy. Your body—and your race times—will thank you.