The Hidden Dangers of Fasting for Endurance Runners: A Running Coach's View on Why It Could Be Hurting Your Health and Performance
With the growing popularity of fasting diets, it’s no surprise that runners, like everyone else, are tempted to try it. Whether it's intermittent fasting or prolonged fasts, the concept promises fat loss, improved energy, and mental clarity—all things runners often strive for. But as a coach who’s worked with athletes of all levels, I’ve seen firsthand how fasting can take runners down a dangerous path, leading to non-functional overreaching (NFO) and illness. Here’s what every runner should know about the impact of fasting on performance and why caution is crucial.
1. Energy Deficit and Non-Functional Overreaching
Running places a high demand on our bodies, requiring substantial energy and nutrient support to fuel those intense workouts, recover properly, and build resilience. Fasting disrupts this balance by creating an energy deficit. When you train with insufficient fuel in the tank, you’re depleting glycogen stores faster than you can replenish them. This forces your body to turn to muscle tissue and fat for fuel. Over time, this wear on the system leads to non-functional overreaching—a stage of chronic fatigue, physical strain, and mental burnout that even rest days struggle to fix.
Unlike functional overreaching, which athletes can recover from with short-term rest, non-functional overreaching drags on for weeks or months. It affects the nervous system, reduces muscular power, and cuts down endurance capacity. Fasting can unintentionally push runners into this state because the body just isn’t getting what it needs to function, let alone improve. The irony here is that while fasting might feel like it’s ‘leaning you out,’ it’s likely slowing down your progress and setting you up for fatigue and injury.
2. Hormonal Imbalances and Increased Illness Risk
Our bodies are finely tuned machines with delicate hormonal balances that regulate energy, mood, recovery, and immunity. One of the most critical players here is cortisol, a stress hormone that naturally spikes in the morning to help us wake up and tackle the day. When fasting, cortisol levels tend to remain high for extended periods, especially if you’re training hard on an empty stomach. This can lead to a ‘fight or flight’ response, putting the body in a heightened stress state. Chronic elevation of cortisol is linked to several negative effects, from sleep disturbances to immune suppression.
Runners who fast frequently may find themselves more susceptible to illness or longer recovery times after a cold or injury. The body is too busy managing stress to focus on immunity. Plus, without an adequate intake of protein, vitamins, and minerals, the body’s ability to repair tissue and fight infections is compromised. In other words, fasting may leave you more vulnerable to getting sick or staying sick for longer, derailing your training entirely.
3. Glycogen Depletion and Impaired Endurance
For runners, glycogen is the gold standard in fuel—it’s fast, efficient, and key to maintaining high energy outputs. However, glycogen stores are limited, and when you’re fasting, these reserves are quickly used up. Relying on fats alone just doesn’t cut it when you’re trying to hit a tempo pace or go for a long run. The quick, explosive energy you need isn’t there, and that can lead to "bonking" or hitting the wall early on in workouts or races.
Additionally, frequent training in a low-glycogen state can blunt the body’s natural ability to store and utilise glycogen effectively. Over time, this hampers performance and can leave runners feeling sluggish even on easy runs. While some athletes adopt ‘fasted’ runs to improve fat utilisation, this should be done sparingly and with expert guidance. For most runners, the downsides far outweigh any minor metabolic advantage that fasting may offer.

4. Mental Burnout and Mood Instability
It’s worth mentioning that nutrition affects not just physical performance but also mental well-being. Glucose (blood sugar) is the brain's preferred energy source, and fasting tends to drop blood glucose levels, which can lead to brain fog, irritability, and poor concentration. For runners, these symptoms can turn into more than just distractions; they affect motivation, consistency, and even enjoyment of the sport.
Non-functional overreaching doesn’t just exhaust the body; it wears down the mind. When your brain is struggling without adequate fuel, the cumulative mental stress can lead to burnout. You’re mentally and physically too drained to continue performing, and that’s a real risk of prolonged fasting when combined with endurance training. Remember that running well requires not only muscle strength but mental resilience, which is why balancing training, nutrition, and rest is essential.
5. Compromised Recovery and Muscle Loss
After a hard run or race, your body enters a state of repair, breaking down and building up muscle tissue stronger than before. Fasting interferes with this process by limiting the availability of amino acids, the building blocks of protein. In a fasted state, protein synthesis—the body’s process of rebuilding muscle—is reduced, which not only leads to diminished recovery but also risks muscle loss.
If muscle mass decreases, so does power and speed, making it harder to reach your potential in training and races. The last thing a runner needs is to lose the muscle they’ve trained hard to gain. Additionally, when muscle repair is slowed, it increases the likelihood of soreness, inflammation, and injury—all setbacks to performance.

The Bottom Line: Fast at Your Own Risk
As a running coach, I’m all for personal experimentation and finding what works best for each individual. But fasting is something I recommend approaching with extreme caution, especially for runners. For most athletes, the risk of overreaching, illness, and burnout far outweighs any potential benefits. Endurance sports already push the body’s limits, and by adding fasting to the mix, you risk crossing the line into a state of non-functional overreaching that can undo months of progress.
If you’re considering fasting or adjusting your diet to align with training goals, I highly recommend seeking advice from a qualified sports nutritionist or coach. The goal should be long-term performance, not short-term trends, and balancing proper fuelling with strategic training is the best path to sustainable, enjoyable, and healthy running.
Written By: Joshua Schofield, Head Coach Running Coach and Founder at PGC1-Coaching. Alongside his coaching qualifications Joshua holds a Masters degree from Loughborough University in Sport and Exercise Nutrition and helped contribute to published research alongside his studies.
Josh provides nutritional support to endurance athletes alongside his coaching plans. The main focus of his work is helping empower athletes to make nutritional choices that enhance training. Having worked with athletes from beginners to internationals Josh uses information from his academic studies alongside practical research with his own running and athletes to help guide you.
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