What Happens If You Don't Eat Enough as a Runner?

October 8, 2025 •

4 min read

According to the American Sports and Performance Dietitians Association, inadequate fueling is a widespread phenomenon among athletes, particularly endurance runners, leading to Low Energy Availability (LEA). This article explores what happens if you don't eat enough as a runner and the profound impact it has on your performance and long-term health.

Quick Summary

Chronic underfueling can compromise a runner's performance and health, causing fatigue, poor recovery, increased injury risk, and hormonal imbalances. This state of low energy availability forces the body to prioritize vital functions over exercise and recovery, leading to significant negative consequences for the athlete.

Key Points

Performance Decline: Chronic underfueling leads to depleted glycogen stores, causing fatigue, reduced endurance, and slower recovery, effectively stalling progress.
Injury Risk: A low energy state impairs bone health, increasing the risk of stress fractures and other bone-related injuries, a common symptom of RED-S.
Hormonal Imbalance: Underfueling disrupts hormonal functions in both male and female runners, leading to low testosterone in men and menstrual issues in women.
Metabolic Slowdown: The body's survival instinct kicks in, suppressing the resting metabolic rate to conserve energy, which can ironically hinder weight management efforts.
Cognitive and Mood Changes: Insufficient fuel can cause brain fog, irritability, mood swings, and increase anxiety, affecting mental sharpness both on and off the run.
Weakened Immunity: Underfueled athletes are more susceptible to illness, frequently catching colds and infections due to a compromised immune system.
Balancing Fuel is Key: Optimizing a runner's diet involves consuming enough carbohydrates, protein, and healthy fats to match their training demands, a personalized approach best guided by a sports dietitian.

The Hidden Dangers of Underfueling for Runners

Many runners, driven by the desire to improve performance or manage weight, unknowingly fall into the trap of underfueling. While a calorie deficit might seem like a direct route to leanness, it can be severely counterproductive for an athlete. The body’s response to inadequate energy intake is not a simple matter of weight loss; it involves complex physiological adaptations that ultimately compromise both performance and health. This cascade of negative effects is often referred to as Low Energy Availability (LEA) and, in its more serious form, Relative Energy Deficiency in Sport (RED-S).

Decreased Performance and Endurance

When a runner doesn't consume enough calories, their body's primary fuel stores—glycogen in the muscles and liver—become depleted. This leads to a severe energy disadvantage and can significantly impair performance. The sensation of 'hitting the wall' during a long run is a classic sign of this energy crisis, as the brain signals a shutdown to conserve limited fuel. Endurance suffers as the body struggles to maintain pace and intensity, and even perceived effort increases significantly, making runs feel much harder than they should. Furthermore, without sufficient energy, the body can't effectively repair and rebuild muscle fibers after a workout, which stalls training adaptations and prevents strength gains. Elite rowers, for example, demonstrated worsened recovery and poorer 5,000m performance after a period of energy restriction. This creates a vicious cycle where a runner trains hard but fails to improve, a frustrating plateau that many mistake for overtraining.

Increased Risk of Injury and Illness

Chronic underfueling severely compromises the body's ability to maintain bone density and support a robust immune system. Research has shown a strong link between low energy availability and an increased incidence of stress fractures, a common and debilitating injury for runners. This is because the body, in a state of energy conservation, reduces bone formation, leaving the skeletal system vulnerable to the repetitive impact of running. In one study of elite distance runners, those with low testosterone (a marker of low energy availability) had a 4.5 times higher incidence of bone stress injuries. Frequent illnesses, such as respiratory tract infections, are also common among underfueled athletes. The compromised immune system struggles to fight off pathogens, leading to more missed training days and slower recovery from sickness.

Hormonal Disruption and Metabolic Slowdown

The body's survival response to a prolonged energy deficit involves downregulating non-essential bodily functions. For both men and women, this includes the reproductive system. Women may experience menstrual dysfunction, including irregular or missed periods (functional hypothalamic amenorrhea), a clear indicator of low energy availability. In men, underfueling can cause a significant drop in testosterone levels, which can lead to low libido and further compromise bone health and muscle repair. Moreover, the body's resting metabolic rate (RMR) can be suppressed, meaning it burns fewer calories at rest. This is a desperate attempt to conserve energy, but it can make weight loss efforts even more difficult and lead to feelings of being constantly cold. This metabolic slowdown is a direct consequence of the body prioritizing survival over performance.

Comparison Table: Well-Fueled vs. Underfueled Runner


Characteristic	Well-Fueled Runner	Underfueled Runner
Energy Levels	High and sustained energy throughout runs and the day.	Persistent fatigue, sluggishness, and 'hitting the wall'.
Performance	Consistent improvement, strength gains, and adaptation to training.	Stagnation, decreased endurance, and increased perceived effort.
Recovery	Rapid recovery, minimal soreness, and feeling ready for the next workout.	Delayed recovery, persistent muscle soreness, and heavy legs.
Injury Risk	Lower risk of stress fractures and overuse injuries due to strong bones.	Higher risk of bone stress injuries and frequent nagging injuries.
Hormonal Health	Stable hormone levels supporting reproductive and metabolic health.	Hormonal disruptions, such as menstrual irregularities or low testosterone.
Immune System	Strong immune function, less frequent illnesses.	Compromised immune system, more frequent colds and sickness.
Mental State	Positive mood, high motivation, and clear concentration.	Irritability, mood swings, brain fog, and obsessive thoughts about food.

The Importance of a Balanced Approach

Effective fueling is not about eating restrictively; it is about providing the body with the right balance of macronutrients—carbohydrates, protein, and fat—to support training and health. Carbohydrates are the primary fuel source for runners and are essential for replenishing glycogen stores. Protein is crucial for muscle repair and adaptation. Healthy fats are necessary for hormone production and energy. A balanced diet, adjusted to the specific demands of your training, is key to avoiding the pitfalls of underfueling. Working with a registered sports dietitian can help runners, especially those with high training loads, navigate these complexities and ensure they meet their nutritional needs.

Conclusion: Fuel Your Body to Go the Distance

The consequences of not eating enough as a runner extend far beyond feeling tired during a workout. They encompass a wide range of serious health and performance issues, culminating in conditions like Relative Energy Deficiency in Sport (RED-S). By understanding the signs of underfueling—such as persistent fatigue, poor recovery, and recurring injuries—and taking proactive steps to improve nutrition, runners can safeguard their health and unlock their full athletic potential. Proper fueling is an investment in long-term performance, resilience, and overall well-being. Ultimately, a well-nourished body is a stronger, faster, and healthier one.

Reference: Understanding the female athlete triad, and its broader implication as RED-S.

Frequently Asked Questions

Early signs include persistent fatigue, feeling sluggish during runs that used to be easy, prolonged muscle soreness, a plateau in performance, and increased irritability or difficulty concentrating.

Yes, chronic underfueling compromises bone mineral density and immune function. This can lead to frequent injuries like stress fractures, slower healing, and more common illnesses.

The Female Athlete Triad describes three interrelated conditions in female athletes: disordered eating, menstrual dysfunction (amenorrhea), and low bone mineral density. It is now understood as a subset of the broader Relative Energy Deficiency in Sport (RED-S), which can affect all genders.

Male runners are also at risk. Chronic underfueling can cause low testosterone levels, decreased libido, reduced bone density, and impaired muscle repair, with similar performance and health consequences as those seen in female athletes.

Intense exercise can sometimes suppress appetite by altering hunger hormones like ghrelin. This can be problematic if the runner already struggles with low energy availability and mistakenly relies solely on their blunted hunger cues.

While symptoms overlap, underfueling is a core cause of many issues. If you are training consistently but seeing no improvement, experiencing a drop in performance, or feeling unusually tired, it's essential to assess your nutritional intake. A sports dietitian can help determine if the issue is a fuel deficit rather than simply an excessive training load.

Runners should focus on a balanced diet rich in carbohydrates (whole grains, fruits, vegetables), lean protein (meat, eggs, dairy, beans), and healthy fats (nuts, seeds, avocados). Prioritizing nutrient-dense foods and timing meals and snacks around training sessions is crucial.