What happens if you don't eat enough calories when working out?

October 15, 2025 •

5 min read

Did you know that up to 58% of athletes may experience low energy availability, a condition where not enough calories are consumed to meet exercise demands? So, what happens if you don't eat enough calories when working out, and could it be sabotaging your fitness goals rather than helping?

Quick Summary

Under-fueling for exercise can cause muscle loss, a slower metabolism, fatigue, hormonal imbalances, and a weakened immune system, ultimately hindering performance and overall health.

Key Points

Muscle Catabolism: Working out with a severe calorie deficit forces your body to break down muscle tissue for energy, undermining strength gains and lowering your resting metabolism.
Metabolic Slowdown: In response to low calories, your body enters 'adaptive thermogenesis,' slowing your metabolism to conserve energy, which makes future weight loss more difficult.
Poor Performance and Fatigue: Insufficient fuel leads to reduced workout intensity, fatigue, and poor endurance, causing performance to plateau or decline significantly.
Hormonal Imbalances: Undereating can disrupt hormone production, affecting mood, sleep, stress response, and reproductive health, potentially leading to conditions like RED-S.
Weakened Immune System: A lack of nutrients and energy can compromise your immune system, making you more prone to illness and increasing recovery times from both sickness and injury.
Diminished Recovery: Without enough calories and protein post-workout, your muscles cannot repair and rebuild effectively, leading to prolonged soreness and limited growth.
Mental Fog and Mood Swings: Insufficient energy for your brain can cause poor concentration, irritability, anxiety, and other mood-related issues.

For many, the simple math of 'calories in vs. calories out' is the only factor considered for weight management. While this equation is a fundamental principle, it fails to account for the body's complex physiological and hormonal responses, especially when exercise is added to the mix. When you consistently don't eat enough calories to meet your energy expenditure from daily life and exercise, your body goes into survival mode. It starts to make some critical changes to conserve energy, many of which can have significant negative effects on your health, athletic performance, and long-term goals.

The Immediate Impact: Performance and Energy

Undereating has an immediate and direct impact on your workout performance. The food you eat provides the fuel your body needs to power through training sessions. Carbohydrates, in particular, are the body's preferred and most easily accessible source of energy, stored as glycogen in the muscles and liver.

Fatigue and Decreased Output: Without sufficient energy stores, your body simply cannot perform at its best. You may find yourself feeling tired, weak, and lacking the stamina to complete your workouts with the same intensity. This can lead to stalled progress in strength and endurance, creating a frustrating plateau.
Bonking or Hitting the Wall: Endurance athletes, in particular, are familiar with the phenomenon of 'bonking'—a sudden and severe fatigue that occurs when glycogen stores are completely depleted. This energy crisis can lead to a drastic drop in performance, a risk that becomes much higher when you are chronically under-fueled.
Poor Recovery: Proper recovery is just as crucial as the workout itself. After a strenuous session, your muscles need amino acids from protein and glycogen from carbohydrates to repair and rebuild. When you don't eat enough calories, you deprive your muscles of the essential building blocks they need, leading to slower recovery times and prolonged muscle soreness.

The Long-Term Consequences: Health and Metabolism

Beyond the gym, persistent under-fueling can wreak havoc on your overall health, triggering a series of metabolic and hormonal changes.

Metabolic Slowdown (Adaptive Thermogenesis): When your body perceives a state of starvation, it slows down its metabolic processes to conserve energy. This is known as adaptive thermogenesis. Your body becomes more efficient at using fewer calories, and your resting metabolic rate (RMR) decreases. This can make weight loss much more difficult and can lead to weight regain once you return to a normal eating pattern.
Hormonal Imbalances: Chronic low energy availability can disrupt your endocrine system. Hormones like cortisol (the stress hormone), leptin (which signals fullness), and ghrelin (which signals hunger) are all affected. For women, it can suppress reproductive hormones, leading to irregular or absent menstrual cycles, a key symptom of Relative Energy Deficiency in Sport (RED-S).
Weakened Immune System: Insufficient calorie and nutrient intake can compromise your immune function, making you more susceptible to illness and infections. Increased stress hormones and systemic inflammation can further suppress the immune response.
Loss of Bone Density: Hormonal imbalances, particularly in women, and a lack of essential nutrients like calcium and vitamin D can lead to decreased bone mineral density. This increases the risk of stress fractures and, over time, osteoporosis.

The Paradox: Why Undereating Can Stall Your Fat Loss

Many people restrict calories to lose fat, but extreme or prolonged deficits can be counterproductive. As the body enters survival mode, it prioritizes keeping you alive over burning fat.

The Vicious Cycle of Undereating

Energy Scarcity: You consume too few calories relative to your exercise level.
Muscle Breakdown: Your body, needing fuel, breaks down metabolically active muscle tissue, not just fat, for energy. This is called catabolism.
Metabolism Slows: With less muscle mass, your resting metabolic rate (RMR) decreases. You now burn fewer calories at rest.
Plateau: Your body adapts, and weight loss stalls, even though you are eating very little.
Rebound Weight Gain: If you eventually increase your calories, your now-slower metabolism causes you to regain weight, often in the form of fat, more easily.

Eating Enough vs. Not Eating Enough: A Comparison


Feature	Eating Enough Calories (Properly Fueled)	Not Eating Enough Calories (Under-fueled)
Workout Performance	Stronger, more consistent energy, improved endurance, and strength gains.	Fatigue, decreased power, and stalled progress. Risk of 'bonking' or hitting a wall.
Body Composition	Supports muscle growth and preservation, leading to a higher resting metabolism.	Causes muscle loss as the body uses muscle protein for fuel. Leads to a lower resting metabolism.
Metabolism	Maintains a healthy metabolic rate. Efficiently uses calories for energy and muscle repair.	Forces adaptive thermogenesis, slowing down metabolism to conserve energy. Makes fat loss harder.
Recovery	Muscles have the necessary nutrients (protein, carbs) for efficient repair and recovery after exercise.	Slow and impaired recovery due to insufficient nutrients, leading to prolonged soreness.
Hormonal Health	Supports balanced hormones for overall health, including reproductive and thyroid function.	Can cause hormonal imbalances, including disrupted thyroid and reproductive hormones. Risk of RED-S.
Mental State	Better energy, focus, and mood regulation. Lower anxiety and irritability.	Brain fog, mood swings, irritability, and potential for depression or anxiety.
Long-Term Health	Stronger bones, robust immune system, and reduced risk of injury.	Weakened immune system, digestive issues, and risk of bone density loss and stress fractures.

Practical Steps for Proper Fueling

Determine Your Needs: Use an online TDEE (Total Daily Energy Expenditure) calculator to get an estimate of your maintenance calories, factoring in your activity level. From there, you can determine a moderate and sustainable deficit (or surplus for muscle gain).
Prioritize Protein: Ensure you are consuming enough protein, especially with each meal and after workouts. This is crucial for muscle repair and minimizing loss during a deficit. Aim for 1.2 to 2.0 grams of protein per kilogram of body weight daily.
Don't Fear Carbs: Carbohydrates are essential for fueling your workouts and replenishing glycogen stores. Choosing healthy, whole-food carbohydrate sources is key.
Meal Timing Matters: Eat balanced meals and snacks regularly throughout the day. Fueling before a workout provides energy, while consuming a meal with carbs and protein within 1-2 hours after exercise optimizes recovery.
Listen to Your Body: Pay attention to signals like chronic fatigue, irritability, poor sleep, or stalled progress. These are often signs that you need to increase your calorie and nutrient intake. For serious concerns, consult a sports dietitian.

Conclusion

Ultimately, a restrictive calorie deficit when combined with a demanding workout schedule is a recipe for physical and mental setbacks. While it may seem like a fast track to weight loss, the consequences—including muscle loss, a slower metabolism, and compromised health—are detrimental and ultimately hinder your long-term success. By shifting your mindset from restriction to proper fueling, you can support your body's needs, optimize performance, and achieve your fitness goals in a sustainable and healthy way. Prioritizing adequate calories and a balanced diet is not a setback, but a necessary investment in your overall health and well-being.

For more in-depth information on Relative Energy Deficiency in Sport (RED-S), visit the National Institutes of Health (NIH) website.

Frequently Asked Questions

Yes, you will likely lose weight, but not in a healthy or sustainable way. Your body may lose a mix of fat, water, and valuable muscle mass. This also slows your metabolism, making it harder to maintain weight loss and easier to regain it later.

Metabolic slowdown, or adaptive thermogenesis, is your body's survival response to low calorie intake. It lowers your resting metabolic rate, meaning your body burns fewer calories at rest to conserve energy, which can make weight loss efforts less effective.

Yes, when you are in a large calorie deficit, your body looks for alternative fuel sources. It will begin to break down muscle tissue to get the protein and energy it needs, hindering your ability to build or even maintain muscle mass.

RED-S is a syndrome affecting athletes who don't consume enough energy to support their exercise levels. It can cause a range of health issues, including a suppressed metabolism, hormonal problems, bone density loss, and impaired immune function.

Key signs include chronic fatigue, plateauing performance, prolonged soreness after workouts, mood swings, difficulty sleeping, frequent illness, and, in women, loss of menstrual cycle.

No. Drastically cutting calories is not a sustainable or healthy long-term strategy. It can lead to severe side effects like muscle loss, a slower metabolism, and hormonal disruptions. A moderate, consistent deficit is much safer and more effective for long-term fat loss.

Before your workout, a meal with easily digestible carbohydrates provides fuel. After a workout, a mix of carbohydrates and protein is ideal for replenishing glycogen stores and repairing muscle tissue. Examples include a fruit smoothie, yogurt with berries, or whole-grain toast with eggs.