What Happens If You Don't Eat Enough Calories While Working Out?

October 13, 2025 •

4 min read

According to sports dietitians, chronic underfueling during exercise can force your body to use its own muscle tissue for energy, hindering recovery and performance. Understanding what happens if you don't eat enough calories while working out? is crucial for anyone pursuing their fitness goals safely and effectively.

Quick Summary

Exercising in a severe energy deficit can undermine your progress, leading to muscle loss, metabolic slowdown, hormonal disruption, and an increased risk of injury. This approach ultimately impedes long-term health and athletic performance, rather than enhancing it.

Key Points

Performance Decline: Inadequate calories deplete muscle glycogen, leading to reduced endurance, strength, and overall workout quality.
Muscle Loss: In a severe energy deficit, the body breaks down muscle tissue for fuel, counteracting strength training and slowing metabolism.
Metabolic Slowdown: Chronic underfueling triggers metabolic adaptation, causing the body to conserve energy and making sustained weight loss difficult.
Hormonal Imbalances: Low energy availability can disrupt hormones, affecting thyroid function, reproductive health, and increasing stress.
Increased Illness and Injury: A weakened immune system and brittle bones resulting from poor nutrition make you more prone to illness and stress fractures.
Mental Health Impact: The stress of underfueling can lead to brain fog, irritability, anxiety, and a dysfunctional relationship with food.
Slower Recovery: Without enough calories and nutrients, the body's ability to repair and rebuild after exercise is impaired, leading to prolonged soreness.

The Physiological Toll of a Caloric Deficit

When you work out, your body expends energy to fuel muscle contractions, maintain core functions, and repair tissue damage. This energy comes primarily from calories consumed through your diet. If your caloric intake is insufficient to meet your body's total energy needs—a state known as low energy availability (LEA)—your body enters survival mode. In this state, it reallocates energy away from 'non-essential' functions to conserve resources, which has a devastating effect on your fitness and overall health.

Prioritizing Survival Over Performance

One of the first signs of underfueling is a significant drop in athletic performance. Your body's preferred and most efficient energy source for high-intensity exercise is glycogen, which comes from carbohydrates. When glycogen stores are depleted due to a lack of calories, workouts feel harder, endurance plummets, and your ability to maintain intensity is severely diminished. You may experience dizziness, lethargy, and a feeling of being 'zapped' of energy mid-session. This directly sabotages your training efforts and prevents you from achieving the necessary stimulus for muscle growth and strength gains.

The Vicious Cycle of Muscle Loss

Contrary to the common belief that exercising on an empty stomach maximizes fat burning, underfueling can lead to muscle catabolism, or muscle breakdown. When your body lacks enough calories from fat and carbohydrates, it turns to protein, including the protein stored in your muscles, for fuel. This process not only hinders the muscle repair and rebuilding essential for progress but also decreases your overall muscle mass. Since muscle tissue is metabolically active, this loss also slows down your metabolism over time, making it harder to manage your weight in the future. A vicious cycle ensues where a slower metabolism and reduced muscle mass make it easier to regain fat after dieting.

Beyond the Gym: Systemic Health Consequences

The negative effects of underfueling extend far beyond the gym, impacting several critical bodily systems.

Metabolic Damage and Hormonal Chaos

Long-term, chronic low energy availability can trigger metabolic adaptation, where your body lowers its basal metabolic rate (BMR) to conserve energy. This makes it significantly more challenging to lose weight and easier to regain it once you stop restricting calories, a phenomenon known as the 'yo-yo effect'. This metabolic slowdown is accompanied by hormonal imbalances:

Increased Cortisol: Chronic stress from under-eating and over-exercising elevates cortisol, the stress hormone, which can lead to inflammation and increased fat storage.
Thyroid Dysfunction: A low-calorie intake can decrease T3 hormone levels, further slowing your metabolism.
Suppressed Reproductive Hormones: In women, this can lead to amenorrhea (loss of menstruation) and reduced bone density. Men can experience decreased testosterone, affecting libido and bone health.

Immune System Suppression

An undernourished body lacks the resources to maintain a robust immune system. As a result, you become more susceptible to frequent illnesses, colds, and infections, and your recovery from sickness is prolonged. For athletes, this means more time spent on the sidelines rather than training, further derailing progress.

Short-term Effects vs. Long-term Dangers

Underfueling presents a range of issues, from immediate discomfort to severe, long-term health problems. A comparison helps illustrate the gravity of the risks.


Aspect	Optimal Fueling	Underfueling
Energy Levels	Sustained energy throughout workouts and the day.	Chronic fatigue, lethargy, and mid-workout energy crashes.
Muscle & Strength	Consistent muscle gain, strength improvements, and efficient recovery.	Muscle loss (catabolism), plateaued or decreased strength, and impaired repair.
Metabolism	Maintains a healthy, functioning metabolic rate.	Metabolic slowdown (adaptive thermogenesis), making weight management difficult.
Hormonal Balance	Supports stable hormone production and healthy bodily functions.	Disrupts hormones (cortisol, thyroid, sex hormones), leading to health complications.
Injury Risk	Lower risk of injuries due to stronger bones and proper recovery.	Increased risk of stress fractures and recurring injuries due to bone and muscle weakening.
Mental State	Improved mood, better concentration, and overall well-being.	Irritability, brain fog, anxiety, and obsession with food.

How to Ensure Adequate Fueling

To prevent the negative consequences of underfueling, focus on a balanced, strategic approach to your diet. Consider the following steps:

Eat Regular Meals: Do not skip meals, especially around your workouts. Aim for balanced meals containing carbohydrates, protein, and healthy fats.
Prioritize Carbohydrates: As your body's primary fuel, carbohydrates should be a key part of your diet. Before a workout, have a source of fast-digesting carbs like a banana or toast. After a workout, replenish glycogen stores with a mix of carbs and protein.
Include Enough Protein: Ensure adequate protein intake to support muscle repair and growth. For many, this means around 1g of protein per pound of bodyweight.
Listen to Your Body: Pay attention to hunger cues, fatigue, and other signals. If you're constantly tired, irritable, or sore, it's a sign you may need more fuel.
Seek Expert Advice: If you are unsure about your caloric and nutritional needs, consulting a registered dietitian or sports nutritionist can help tailor a plan to your specific activity level and goals.

Conclusion: Fuel Your Body, Not Just Your Workout

Undereating while working out is a common but dangerous mistake that can sabotage your health and fitness goals. The short-term weight loss that might be achieved comes at a steep price, leading to muscle loss, metabolic damage, and systemic health problems. Instead of focusing solely on calorie restriction, a sustainable approach involves properly fueling your body to support your workouts and overall well-being. By providing your body with the right energy and nutrients, you can build a stronger, healthier, and more resilient physique, making lasting progress without compromising your health. For more on the dangers of underfueling, a detailed overview can be found on the Equip Health website.

Frequently Asked Questions

While you may see initial weight loss, this often includes significant muscle mass and water weight. The severe calorie deficit will trigger metabolic slowdown, making sustained fat loss much more difficult and increasing the likelihood of regaining the weight.

Early signs often include chronic fatigue, plateauing performance, longer recovery times after workouts, increased irritability, difficulty concentrating, and a constant preoccupation with food.

Yes, chronic underfueling forces your body into 'survival mode,' where it slows down your metabolism to conserve energy. This phenomenon, known as metabolic adaptation, makes it harder to lose weight and easier to regain it.

Yes. If your calorie deficit is too large, your body may resort to breaking down muscle tissue for energy, a process called catabolism. This can happen even with strength training, as your body lacks the nutrients needed for proper repair and growth.

Your calorie needs depend on factors like age, sex, weight, height, and activity level. You can use an online TDEE (Total Daily Energy Expenditure) calculator to get an estimate and then adjust your intake based on your fitness goals. For personalized advice, consult a professional.

RED-S is a syndrome affecting many bodily systems when an individual does not consume enough calories to match their energy expenditure from exercise. Symptoms include impaired metabolic rate, bone health, immune function, and hormonal issues.

Focus on a balanced diet with regular meals containing sufficient carbs, protein, and healthy fats. Pay attention to your body's hunger signals and plan meals or snacks around your workouts to provide consistent energy and support recovery.