Why Zero-Calorie “Depletion” Runs Aren’t Worth the Hype

Is a depletion run some magical marathon training tool – or are they an outdated and dangerous training technique? Zero-calorie runs cyclically trend in popularity. People claim these improve performance or simply do them out of fear of stomach upset. However, these claims are not supported by the research. In this article, we’ll look at the myths behind depletion runs and how learning to fuel your runs is the magic bullet for improving your performance.

How Depletion Runs Work

Whether you call them depletion or zero-calorie runs, the practice is the same: one deliberately forgoes any carbohydrates (or calories) during a long run.

There are two theories behind depletion runs. The first theory is that depletion runs improve fat oxidation at higher relative intensities. The second theory is that zero-calorie runs enhance AMPK signaling. 

When you eat carbohydrates before or during exercise, you increase your plasma glucose availability. Generally, this is favorable. Glucose provides an efficient substrate for running. Low plasma glucose is associated with fatigue during long runs. 

During a depletion run, your plasma glucose levels are lower. To compensate, your body oxidizes more fat (in the form of free fatty acids and intramuscular triglycerides) and amino acids. 

Depletion runs enhance AMPK signaling. AMPK is a protein responsible for various metabolic adaptations, including mitochondrial biogenesis and glucose uptake. Training intensity and duration can both promote AMPK signaling; fasted runs are not the only nor the most effective way to stimulate this pathway. 

In short, depletion runs attempt to hack two training adaptations that would already occur with appropriate training. A higher volume of truly easy running would improve fat oxidation; likewise, training at both high intensities and long distances enhances AMPK signaling.

The Myth of Zero-Calorie Runs

Zero-calorie runs are not a magic bullet for running a faster marathon. Contrary to the previously-held belief, withholding carbs on runs doesn’t make fueling more effective on race day. Nor does being fat-adapted does not make you faster. In fact, most registered sport dietitians recommend against depletion runs. 

A 2022 review on carbohydrate research summarizes it well: 

“Increasing fat oxidation rates as such should not come in front of the goal to improve one’s aerobic capacity, especially in the light of evidence demonstrating that training with low carbohydrate availability can result in a reduced ability to exercise at high intensities due to reduced ability to utilize carbohydrates.”

A 2021 review of “train low” approaches reached a similar conclusion. The researchers found that depletion runs did not lead to any improvements in performance when compared to high-carbohydrate availability in well-trained athletes. 

As the researchers point out, zero-calorie runs often result in a reduced exercise capacity – in terms of intensity and duration. Even at moderate intensities, your performance will decline on a depletion run – resulting in compromised performance and training adaptations. 

Even if you finish a zero-calorie long run, you will likely be slogging through the end with poor running economy. Your fatigue resistance will be worse during zero-calorie runs. You will have a higher heart rate but a slower pace if you skip all running fuel on a long run. 

Virtually all recreational runners will benefit more from training with carbohydrate availability since fueling allows them to accrue more intensity and volume in training. Additionally, you will build your confidence for races better if you perform well in training runs. 

Let’s take a moment to understand exercise metabolism. Even if you improve your fat oxidation, your performance ceiling will remain lower. Fat oxidation only supports so much intensity. Once you are working at marathon pace and faster, you need to be able to oxidize carbohydrates efficiently. 

Additionally, depletion runs do not rely strictly on fat. Exercise metabolism does not work that way! When you run low on glycogen, your body will try to get glucose from other sources – including your muscles. A process called gluconeogenesis breaks down your muscle protein to convert it into glucose.

The breakdown of muscle protein for energy results in additional muscle damage during long runs. Your recovery will be longer from these sessions and your injury risk will increase. 

>>Related: How to Avoid Bonking in Your Marathon

Nutritional Needs for Runners

Runners need more carbohydrates than the sedentary population. The amount of carbohydrates you need depends on your training load. The higher your training volume, the more carbohydrates you need – a concept known as “fueling for the work required.” 

You don’t want to do depletion runs – nor do you want to overwhelm your gut with 90 grams of carb on a 60-minute easy run. Instead, your carbohydrate needs on runs will scale based on the duration. 

The recommended carbohydrate needs on runs:

• Under 60-75 minutes: no carbohydrates needed or sport drink only
• 1-2 hours: 30-60 grams of carbs per hour
• 2+ hours: 60-90 grams of carbs per hour

>> Learn More: How to Fuel Your Long Runs

The Risks of Depletion Runs

Carbohydrate restriction on long runs can have cascading effects on the body. A 2020 review in Nutrients enumerates negative outcomes of no-carbohydrate training, including low energy availability, hormonal dysfunction, poor immune function, and decreased training quality. Other risks include low iron and increased risk of injury. 

As demonstrated in a 2020 study in Nutrients, low carbohydrate availability on long runs is linked to low iron status. Inadequate carbohydrate availability alters hepcidin production, which in turn alters your body’s ability to absorb iron. 

It is a well-established fact that low iron status impairs endurance performance. Low iron impairs oxygen delivery during exercise, effectively lowering your exercise capacity. 

Similarly, depletion runs could increase the risk of bone stress injuries. A 2024 study in the European Journal of Sport Science concluded that athletes who practiced fasted runs were 160% more likely to develop a stress fracture than their counterparts who fueled during runs. 

Needless to say, a bone stress injury is not performance-enhancing. Bone stress injuries typically involve several weeks, if not months, off of training.  

None of the purported benefits of doing a depletion run outweigh the risks. Even if zero-calorie runs did enhance performance, it would not outweigh the performance impairments brought forth by low iron, low energy availability, increased risk of bone stress injury, and so on. 

>>Keep Reading: How to Prevent Stress Fractures

The Benefits of Fueling Workouts Properly

In contrast, properly fueling your long runs offers several benefits – with virtually no known downsides. These benefits include improved performance, quicker recovery, better gut tolerance, and a lower risk of low energy availability/REDs. 

Improved Performance

It is well established that carbohydrate intake during runs improves performance at any intensity. Even in easy-pace training runs, improved performance matters – you will get a better training response. 

Performance scientist Jonah Rosner demonstrated this well in a recent Instagram reel. When he compared oxygen consumption after three hours without fuel vs three hours with fuel, he was 6% more efficient after fueling with 90 grams of carbs per hour. As a result, marathon pace is faster!

Quicker Recovery

When you properly fuel your long runs, you recover more quickly. A 2023 study in Medicine and Science in Exercise and Sports examined the differences in next-day endurance exercise after cyclists trained for 2.5 hours with either a carbohydrate drink or a placebo. The carbohydrate group experienced less metabolic stress, muscle protein degradation, and fatigue, which led to better performance in next-day exercise. 

Why does that matter? Quicker recovery allows better training quality. Instead of needing a rest day after your long run, you can run some easy miles the next day (which may improve fatigue resistance). You will be fresher for your following quality sessions a few days later, further enhancing training. 

Fewer GI Issues 

Many runners avoid taking in fuel on runs because of fear of stomach issues. Counterintuitively, underfueling can actually increase the risk of GI issues. Research shows that training the gut with high carbohydrate intake during runs results in increased absorption of carbohydrates in the gut, improved stomach comfort, and reduced GI symptoms. 

You don’t start with large amounts of gels. Rather, start with 30 grams of carbs per hour, and build up to 60 grams of carbs per hour over five to ten weeks. 

Additionally, GI issues are a common symptom of low energy availability and relative energy deficiency in sports. As counterintuitive as it sounds, ensuring you fuel your body appropriately will improve your gut health over time.

>> Learn More: How to Train Your Gut

Lower risk of low energy availability

Low energy availability (LEA) is a real risk for athletes. LEA leads to performance impairment, increased risk of overtraining and injury, higher occurrence of illness and infection, and negative changes to daily life and well-being. 

 LEA can occur intentionally or unintentionally. Your risk of LEA will be higher if you withhold carbohydrates during long runs – because you are expending energy in a prolonged period of no energy intake. 

>>Listen to the Tread Lightly Podcast: Low Energy Availability

Skip Glycogen Depletion Runs and Fuel for Performance

Glycogen depletion runs promise improved performance, but they lead to worse performance, poorer recovery, and increased risk of injury and other issues. If you really want to improve your marathon time, ensure you fuel well for quality long runs, quicker recovery, and faster race times.