What Happens If You Don't Eat Enough When You Workout?

November 14, 2025 •

4 min read

According to a study published in the British Journal of Sports Medicine, athletes with low energy availability are at a significantly higher risk for requiring medical support during a marathon compared to those who are properly fueled. So, what happens if you don't eat enough when you workout, and what are the consequences for your health and performance?

Quick Summary

This article explores the wide-ranging negative effects of underfueling, including decreased performance, significant muscle loss, metabolic slowdown, and severe hormonal imbalances. It details the physiological and psychological impacts of consistently expending more energy than you consume while exercising.

Key Points

Performance Decline: Undereating depletes muscle glycogen stores, leading to decreased endurance, strength, and overall athletic performance during workouts.
Muscle Loss: In a consistent energy deficit, your body may break down muscle tissue for fuel, leading to a loss of lean muscle mass rather than fat.
Metabolic Slowdown: Chronic underfueling causes your metabolism to slow down in an attempt to conserve energy, making weight management more difficult.
Hormonal Disruption: Significant undereating can lead to hormonal imbalances, including suppressed reproductive and thyroid hormones and elevated cortisol levels.
Increased Injury Risk: Poor fueling leads to compromised bone health and increased fatigue, significantly raising the risk of injuries like stress fractures.
Mental Health Effects: The psychological consequences include increased irritability, anxiety, mood swings, and a preoccupation with food.
Impaired Recovery: Without proper fuel, your body cannot effectively repair and rebuild muscle tissue, leading to prolonged soreness and poor adaptation to training.

The Immediate Impacts on Your Workout

When you begin a workout session without sufficient energy, your body's ability to perform is immediately compromised. The glycogen stored in your muscles and liver, which is your body's primary fuel source for high-intensity exercise, becomes quickly depleted. This leads to a cascade of negative effects that manifest during and immediately after your training session.

Decreased Performance and Endurance: You will find it harder to maintain intensity and duration. Your lifts may feel heavier, and your stamina will drop, a phenomenon often referred to as 'bonking' or 'hitting the wall'.
Increased Perceived Exertion: The effort required for a given exercise intensity feels much harder. A moderate workout can feel like a high-intensity session, leading to increased fatigue.
Poor Concentration and Coordination: Your brain requires a steady supply of energy. A lack of fuel can lead to 'brain fog,' making it difficult to focus on your form or make sound decisions during your workout.
Elevated Injury Risk: Poor form, fatigue, and muscle weakness significantly increase your chances of sustaining an injury, from minor strains to more serious stress fractures over time.

The Chronic Physiological Consequences

Consistently underfueling creates a state of 'Relative Energy Deficiency in Sport' (RED-S), impacting numerous bodily systems. The body enters a survival mode, conserving energy by slowing down non-essential processes, which can have severe long-term health implications.

Hormonal Imbalances Under-eating and over-exercising significantly disrupt the endocrine system. The body perceives the energy deficit as a stressor, leading to a host of hormonal changes:

Cortisol Levels: The stress hormone cortisol becomes chronically elevated, which can contribute to fatigue, poor recovery, and insomnia.
Reproductive Hormones: In women, this can cause menstrual dysfunction or a complete loss of the menstrual cycle (amenorrhea). Men may experience a reduction in testosterone levels, affecting bone density and libido.
Thyroid Hormones: A suppressed metabolism, a key sign of undereating, is mediated by a decrease in thyroid hormone output, making weight management more challenging.

Muscle Loss and Compromised Metabolism In a caloric deficit, your body may start to break down muscle tissue for energy instead of just relying on fat stores. This is counterproductive for fitness goals and overall health.

Muscle Breakdown: Without adequate protein and carbohydrates, the body can't repair the micro-tears in muscle fibers caused by resistance training. Instead of rebuilding stronger muscles, it catabolizes existing muscle mass for fuel.
Metabolic Slowdown: A reduction in metabolically active muscle tissue, combined with the body's energy-saving response, leads to a suppressed metabolism. This makes it harder to lose weight and easier to regain it once normal eating resumes.

Bone Health Deterioration Chronic underfueling, particularly with associated hormonal imbalances like reduced estrogen, can negatively impact bone mineral density. This significantly increases the risk of stress fractures and osteoporosis later in life. The body prioritizes immediate survival functions over the long-term health of the skeletal system.

Comparison Table: Properly Fueled vs. Underfueled


Aspect	Properly Fueled	Underfueled (Chronic)
Energy Levels	Sustained and high for both daily life and exercise.	Chronic fatigue, lethargy, and mental exhaustion.
Training Adaptation	Promotes muscle growth and increased strength and endurance.	Impaired muscle protein synthesis; leads to decreased strength and no progress.
Hormonal Balance	Healthy and stable hormone production, including sex hormones.	Disrupted reproductive and thyroid hormone function, elevated cortisol.
Metabolism	Maintains a healthy metabolic rate, supporting body composition.	Suppressed basal metabolic rate as the body tries to conserve energy.
Recovery	Faster recovery from workouts, reduced muscle soreness.	Prolonged muscle soreness, slow healing, and increased inflammation.
Mood & Mental State	Enhanced mood, motivation, and mental clarity.	Increased irritability, anxiety, mood swings, and difficulty concentrating.

Psychological and Behavioral Impacts

The effects of undereating are not limited to physical health. The mental toll can be just as significant.

Food Preoccupation: A constant state of energy deficit can lead to obsessive thoughts about food, making it difficult to focus on other aspects of life.
Emotional Dysregulation: Undereating can cause significant mood shifts, increased anxiety, and irritability due to changes in brain chemistry and nutrient deficiencies.
Loss of Libido: Hormonal imbalances, particularly reduced sex hormones, can lead to a significant drop in sex drive.
Disordered Eating Patterns: Chronic caloric restriction increases the risk of developing disordered eating behaviors, such as binge-eating and fear around certain food groups.

The Solution: Fuel Your Body for Success

The path to recovering from underfueling involves a multi-pronged approach that focuses on restoring adequate energy availability. A key is to eat a balanced diet that includes enough carbohydrates for energy, protein for muscle repair, and healthy fats for hormone production. Reconnecting with and honoring your body's natural hunger cues is crucial, as is prioritizing consistent and sufficient sleep. Adjusting your training intensity and frequency can also give your body the rest it needs to recover and adapt properly. Working with a sports dietitian or nutritionist can help create a personalized fueling strategy that aligns with your fitness goals and promotes overall health and well-being. The message is clear: instead of adopting the potentially harmful mindset that "lighter is faster," embrace the reality that "fueling is faster" for sustainable performance and health.

Conclusion

Not eating enough when you workout can have severe and interconnected consequences for both your physical and mental health. From compromising workout performance and triggering muscle loss to inducing hormonal imbalances and a metabolic slowdown, the risks of underfueling are extensive. The consistent energy deficit forces the body into a survival state, deprioritizing crucial functions like reproduction, bone health, and mood regulation. To see real progress, protect your health, and perform at your best, it is imperative to fuel your body adequately and listen to its signals. A balanced diet and strategic fueling plan are not just optional—they are foundational to achieving your fitness goals and maintaining long-term well-being.

Frequently Asked Questions

Underfueling, or low energy availability (LEA), is a state where your caloric intake is insufficient to cover your body's daily energy needs and the energy you expend during exercise. Signs include chronic fatigue, frequent illness, persistent muscle soreness, decreased performance, mood changes, and, for women, irregular or lost menstrual cycles.

Yes. After high-intensity exercise, your muscles need protein and carbohydrates for repair and recovery. If you don't eat enough, especially protein, your body may resort to breaking down existing muscle tissue for energy, leading to muscle loss over time.

When you consistently undereat, your body adapts by slowing down your metabolic rate to conserve energy. This is a survival mechanism that makes it harder to lose weight and easier to regain it, effectively sabotaging your weight loss efforts.

Yes, chronic underfueling can negatively impact bone mineral density. The hormonal disruptions, such as decreased estrogen, can lead to increased bone reabsorption and reduced new bone formation, raising the risk of stress fractures and osteoporosis.

Absolutely. Inadequate nutrition can lead to nutrient deficiencies and changes in brain chemistry that affect mood-regulating hormones like serotonin and dopamine. This can result in increased irritability, anxiety, and mood swings.

RED-S is a syndrome caused by prolonged low energy availability. It affects numerous bodily functions, including metabolic rate, menstrual function, bone health, immunity, and cardiovascular health, ultimately impairing health and performance.

To fuel properly, ensure you have a balanced diet with sufficient carbohydrates for energy and protein for muscle repair, especially before and after workouts. Listening to hunger cues, avoiding skipping meals, and consulting a sports dietitian for personalized guidance can be very helpful.