What Happens If You Run Without Eating Enough?

January 8, 2026 •

4 min read

According to a study of Boston Marathon runners, 42% of female and 18% of male runners showed indicators of Low Energy Availability (LEA). This phenomenon can lead to serious health issues, so it's crucial to understand what happens if you run without eating enough and how to avoid underfueling your training.

Quick Summary

Running without adequate fuel can lead to depleted glycogen stores, compromised performance, and fatigue. It triggers muscle breakdown, increases injury risk, and disrupts hormonal balance, which can cause long-term health problems like reduced bone density. Proper nutrition is essential for energy, recovery, and overall runner health.

Key Points

Glycogen Depletion: Running without sufficient food rapidly depletes muscle glycogen, leading to premature fatigue and a loss of running performance.
Muscle Breakdown: In a state of low energy availability, the body breaks down muscle tissue for fuel, hindering recovery and causing muscle loss over time.
Hormonal Disruption: Chronic underfueling can cause significant hormonal imbalances in both men and women, affecting reproductive health and overall physiological function.
Increased Injury Risk: Underfueling compromises bone density and repair, substantially increasing the likelihood of stress fractures and other musculoskeletal injuries.
Impaired Immune Function: The stress response from inadequate fueling suppresses the immune system, making runners more prone to illness and infection.
Mental Fog: The brain's need for glucose is unmet, resulting in poor concentration, irritability, and a lack of motivation during and after runs.
Fueling is Faster: Proper fueling, rich in carbohydrates and protein, is proven to improve performance, aid recovery, and reduce injury risk, contrary to the myth that 'lighter is faster'.

The Immediate Impact of Underfueling on Your Run

When you head out for a run without consuming sufficient calories, your body’s immediate resources are compromised. Carbohydrates, stored as glycogen in your muscles and liver, are your body's primary fuel source for high-intensity or prolonged exercise. If these stores are low, your body is forced to look for alternative fuel sources, like fat and, eventually, protein from your muscle tissue.

This lack of readily available energy leads to a cascade of negative effects that you will feel almost instantly. Your pace will likely slow, your perceived effort for a given speed will increase, and you will hit the dreaded “wall” much sooner than expected. The central nervous system also suffers, as the brain relies heavily on glucose for proper function. This can lead to brain fog, poor focus, and a general lack of mental sharpness, making it difficult to maintain proper form and stay motivated. In simpler terms, running becomes significantly less enjoyable and effective.

The Body's Survival Response

As your body's energy reserves plummet, it shifts into a survival mode. The stress hormone cortisol levels rise, which, in high concentrations, can promote muscle protein breakdown. Instead of repairing and strengthening your muscles post-run, your body begins to cannibalize them for energy, leading to muscle loss and weakness over time. This state of physiological stress also weakens your immune system, leaving you more susceptible to illness and infection, particularly respiratory tract infections.

Long-Term Consequences of Chronic Underfueling

Repeatedly running in an underfueled state, a condition known as Low Energy Availability (LEA), has far-reaching and serious health consequences. Over time, these issues can lead to the more severe condition of Relative Energy Deficiency in Sport (RED-S).

Hormonal Disruption

Insufficient caloric intake can wreak havoc on your endocrine system. For female runners, low energy availability can suppress estrogen levels, leading to irregular or absent menstrual cycles (amenorrhea). Since estrogen is crucial for bone formation, this hormonal imbalance can cause premature bone mineral density loss. For male athletes, insufficient calorie intake can lower testosterone levels, resulting in reduced muscle mass and strength, and decreased libido.

Compromised Bone Health

Running is a high-impact sport that places significant stress on the bones. When properly fueled, this stress stimulates the bones to rebuild stronger. However, with chronic underfueling, the body lacks the necessary energy and nutrients to repair and rebuild bone tissue effectively. This creates a dangerous imbalance where bone breakdown outpaces bone formation, increasing the risk of stress fractures and, in the long term, osteopenia and osteoporosis. Shockingly, changes in bone formation have been observed in as little as five days of underfueling.

Underfueled vs. Properly Fueled Running: A Comparison


Aspect	Underfueled Running	Properly Fueled Running
Energy Levels	Early fatigue, sluggishness, and feeling “gassed” early in the run.	Consistent, sustained energy for the duration of the run.
Performance	Reduced endurance, slower pace, and lower intensity capacity.	Improved performance, ability to maintain a desired pace and intensity.
Recovery	Prolonged muscle soreness, delayed repair, and increased fatigue.	Faster muscle recovery and adaptation to training.
Injury Risk	Higher risk of stress fractures and muscle injuries due to compromised bone and muscle health.	Reduced risk of injury due to better tissue repair and stronger bones.
Hormonal Health	Disrupted hormone balance (e.g., low estrogen or testosterone).	Stable hormonal function supporting performance and overall health.
Mood	Irritability, mood swings, difficulty concentrating, and lack of motivation.	Improved mood, better mental focus, and enhanced enjoyment.

Strategies to Avoid Underfueling

To prevent the negative consequences of underfueling, runners must adopt a conscious and consistent fueling strategy. This involves understanding your body’s needs and planning your nutrition around your training schedule.

Eat Enough Carbohydrates: Carbs are a runner's primary energy source. Focus on high-quality, complex carbs like whole grains, fruits, and starchy vegetables for sustained energy throughout the day, with simpler carbs appropriate before or during runs.
Prioritize Timing: Aim to consume a meal with carbs and protein 2-3 hours before a run, or a small, easily digestible snack (like a banana) 30-60 minutes beforehand. Replenish glycogen stores and repair muscles by consuming a carbohydrate-to-protein snack or meal within 30-60 minutes after your run.
Hydrate Strategically: Staying hydrated is as important as eating. Drink water and consider an electrolyte beverage, especially during longer or more intense runs, to replace lost minerals.
Listen to Your Body: Pay attention to early warning signs like persistent fatigue, increased hunger, or an inability to complete workouts that were once easy. If you notice these symptoms, it's a clear signal to reassess your energy intake.
Consult a Professional: A sports dietitian can help you create a personalized nutrition plan based on your individual needs, activity level, and goals, ensuring you are adequately fueled for optimal performance and health.

Conclusion: Fueling is Performance

Running without eating enough is not an efficient or sustainable strategy for performance or health. While some might pursue it for perceived benefits like weight loss, the data shows that underfueling leads to decreased performance, slower metabolism, and an increased risk of serious short- and long-term health issues. A balanced, consistent approach to nutrition is the foundation of a successful and healthy running journey. Remember that 'fuelling is faster,' and fueling your body properly is the smartest investment you can make in your athletic and personal well-being.

Frequently Asked Questions

Carbohydrates are the primary fuel source for runners, stored in the body as glycogen in the muscles and liver. When you run, your body uses this stored glycogen for energy, especially during high-intensity or long-distance efforts.

Hitting the wall is the feeling of sudden, profound fatigue that occurs when your body's glycogen stores are completely depleted. It is often accompanied by a sudden loss of energy, a heavy feeling in the legs, and significant mental and physical slowdown.

While some studies suggest fasted running may increase fat oxidation during the workout, this often leads to decreased overall performance and fewer total calories burned because you can't run as hard or as long. It is not a proven strategy for effective long-term weight loss.

RED-S is a medical condition caused by chronic low energy availability, where caloric intake is insufficient to meet the energy demands of training. It has widespread negative effects on health, hormones, bone density, and performance.

Significant disruption to bone formation can occur in as few as five consecutive days of underfueling. This rapid loss of bone density increases the risk of stress fractures and long-term issues like osteoporosis.

For optimal performance, eat a meal rich in carbs and protein 2–3 hours before a run. If you are short on time, a smaller, easily digestible carbohydrate snack (like a banana) can be eaten 30–60 minutes beforehand.

Within 30–60 minutes post-run, consume a snack or meal with a good balance of carbohydrates and protein. This helps to replenish glycogen stores and repair muscle tissue for faster recovery. Examples include chocolate milk, a turkey sandwich, or Greek yogurt with fruit.