What Happens If You Exercise and Not Eat Enough?

January 9, 2026 •

4 min read

According to a study on Relative Energy Deficiency in Sport (RED-S), failing to eat enough to cover exercise demands can impair athletic performance and cause serious health problems. This comprehensive guide explores what happens if you exercise and not eat enough, detailing the physical and mental consequences of underfueling your body.

Quick Summary

This article explains the dangers of exercising without sufficient caloric intake. It covers the metabolic and hormonal imbalances that can lead to muscle loss, decreased performance, and long-term health complications like stress fractures. Learn why proper nutrition is vital for active individuals and how to avoid severe underfueling.

Key Points

Muscle Loss: Insufficient calories force the body to break down muscle tissue for energy, a process known as catabolism, hindering strength and growth.
Metabolic Slowdown: Chronic underfueling can trigger adaptive thermogenesis, lowering your metabolic rate and making future weight loss more difficult.
Hormonal Disruption: Low energy availability can lead to imbalances in reproductive and thyroid hormones, causing issues like irregular menstrual cycles and reduced libido.
Reduced Performance: Without adequate fuel, endurance, strength, and overall exercise intensity decrease, making workouts less effective.
Increased Injury Risk: Impaired bone health due to hormonal changes and nutrient deficiencies can increase susceptibility to stress fractures and other injuries.
Psychological Impact: Symptoms like mood swings, irritability, and brain fog can arise from the mental stress and glucose imbalances caused by underfueling.
Prioritize Nutrient Timing: Consuming a balanced intake of carbohydrates and protein before and after exercise is crucial for performance, recovery, and overall health.

The Immediate Impact of Underfueling

When you engage in physical activity, your body demands energy, primarily sourced from carbohydrates and fats. If your caloric intake is insufficient to meet this demand, your body's survival mechanisms kick in, leading to immediate and noticeable consequences. One of the most common signs is rapid fatigue and a significant drop in energy levels. This isn't just regular post-workout tiredness; it's a persistent, draining sensation that can affect your entire day and subsequent workouts. Glycogen stores, the muscle's primary fuel source, become depleted, forcing your body to look for alternative energy sources.

The Role of Blood Sugar and Hormones

Exercising in a fasted or underfueled state can cause a sudden and sharp drop in blood sugar levels. This can trigger unpleasant symptoms such as dizziness, lightheadedness, and nausea. In some cases, this can lead to fainting, which poses a serious injury risk, especially during intense workouts. Hormonal responses are also immediately affected. The body releases stress hormones like cortisol in response to the energy deficit. While beneficial in short bursts, chronically elevated cortisol can impair recovery and promote fat storage.

The Long-Term Consequences of Chronic Underfueling

While the immediate effects are uncomfortable, the long-term consequences of consistently exercising on an energy deficit are more severe and can cause lasting damage. This can lead to a serious condition known as Relative Energy Deficiency in Sport (RED-S), which affects both men and women and impacts multiple physiological systems.

Common effects of chronic underfueling include:

Muscle loss (catabolism): With insufficient carbohydrates and fat, the body begins breaking down muscle tissue for energy, a process known as catabolism. This negates the purpose of strength training and can lead to a softer, less toned physique over time.
Metabolic slowdown: Your body adapts to the lower calorie intake by decreasing its metabolic rate, a process called adaptive thermogenesis. This makes weight loss plateaus more likely and can cause weight regain more easily when normal eating resumes.
Hormonal dysfunction: Prolonged energy restriction can disrupt the production of key hormones, including thyroid hormones and reproductive hormones like estrogen and testosterone. In women, this can lead to menstrual dysfunction (amenorrhea), and in both sexes, it can decrease libido and affect mood.
Decreased bone density: For women, low estrogen levels resulting from underfueling can negatively impact bone mineral density, increasing the risk of stress fractures and osteoporosis. Even with strength training, adequate nutrition is required to build and maintain bone health.
Compromised immune function: The body's immune system is also suppressed, leaving you more susceptible to illness and infection. Frequent colds or infections can disrupt your training schedule and overall health.

Psychological and Mental Effects

Beyond the physical toll, exercising without eating enough can significantly impact your mental health. Your brain, which relies on glucose for energy, can experience "brain fog" or poor concentration. Chronic underfueling is also associated with mood swings, irritability, and in severe cases, anxiety and depression. Food fixation and disordered eating patterns can also develop, as the body's survival instincts prioritize finding food.

Underfueling vs. Fueling for Performance: A Comparison


Feature	Underfueled Training	Properly Fueled Training
Energy Source	Muscle and fat stores	Carbohydrates and fats from recent meals
Performance	Decreased endurance and strength	Improved endurance and power
Muscle Mass	Muscle atrophy (loss)	Muscle protein synthesis and repair
Metabolism	Slows down to conserve energy	Remains optimal, supporting energy expenditure
Hormones	Disrupted reproductive and thyroid hormones	Balanced and responsive to training
Injury Risk	Increased risk of stress fractures and soft tissue injuries	Minimized risk, faster recovery
Mental State	Mood swings, irritability, brain fog	Improved mood, better concentration, mental clarity

How to Fuel Your Body Properly

To maximize your workout benefits and safeguard your health, proper nutrient timing is essential. This involves eating a balanced diet with an emphasis on carbohydrates for energy and protein for muscle repair.

Key strategies for proper fueling:

Eat a balanced meal before and after workouts: Aim for a meal with a mix of carbohydrates and protein 2-3 hours before exercise. If time is short, a smaller snack with easily digestible carbs and protein 30-60 minutes prior is effective.
Focus on nutrient-dense foods: Prioritize whole grains, lean proteins, fruits, and vegetables to ensure your body gets a wide range of vitamins and minerals.
Prioritize post-workout nutrition: Consume carbohydrates and protein within 1-2 hours after a moderate-to-hard workout to replenish glycogen stores and support muscle repair.
Stay hydrated: Drink plenty of fluids throughout the day, as dehydration can mimic symptoms of fatigue and impact performance.
Adjust intake to training intensity: Listen to your body and adjust your caloric and macronutrient intake based on your activity level. High-intensity or prolonged exercise requires more fuel.

Conclusion: Fuel Your Performance, Not Your Deficit

Exercising without eating enough is a counterproductive and harmful practice that can lead to significant health issues and undermine your fitness goals. It not only reduces your performance and recovery but can also cause long-term damage to your metabolism, hormones, and bones. Prioritizing proper fueling, including carbohydrates and protein around your workouts, is the key to sustaining high energy levels, building muscle effectively, and maintaining overall physical and mental well-being. By focusing on nourishing your body, you can train smarter, feel better, and achieve lasting, healthy results. For more information on athlete nutrition and avoiding energy deficiencies, consult resources like those from reputable sports medicine organizations.

Frequently Asked Questions

Immediate signs include lightheadedness, dizziness, fatigue, and nausea due to low blood sugar levels. You may also experience a noticeable drop in performance and energy during your exercise session.

Exercising in a fasted state may increase the short-term use of fat for fuel, but it also increases the risk of losing muscle mass. For long-term health and sustainable fat loss, fueling your body properly before and after workouts is more effective.

Yes, chronic underfueling can cause your metabolism to slow down as a survival mechanism, leading to easier weight gain when you do eat more calories. It can also cause a shift in body composition, with more fat and less muscle.

RED-S stands for Relative Energy Deficiency in Sport, a syndrome affecting both male and female athletes when their energy intake is insufficient to cover the demands of training and daily life. It leads to health consequences impacting metabolism, hormones, immunity, and bone health.

Signs of a slowed metabolism include persistent fatigue, weight loss plateaus despite a calorie deficit, increased cold sensitivity, and difficulty losing fat even while undereating.

The key is overall calorie and nutrient intake relative to your activity level. For those with a busy schedule, eating a main meal 2-3 hours before exercise and a smaller carb-and-protein-rich snack closer to the workout can be effective. It is important to find a consistent approach that works for your body.

After a moderate-to-hard workout, consuming a meal or snack containing both carbohydrates and protein within 1-2 hours is ideal. This combination replenishes glycogen stores and provides amino acids for muscle repair and growth.