Understanding the Consequences: What happens if you don't eat enough calories while exercising?

October 30, 2025 •

4 min read

According to a 2024 meta-analysis, approximately 45% of athletes have 'low energy availability', a state of underfueling. Understanding what happens if you don't eat enough calories while exercising is crucial for preventing negative health and performance consequences, from metabolic slowdown to hormonal imbalances.

Quick Summary

Insufficient calorie intake while exercising triggers a survival response in the body, leading to a cascade of negative effects. The body sacrifices muscle tissue for energy, slows the metabolism, and disrupts hormone production, harming both athletic performance and long-term health.

Key Points

Metabolic Slowdown: A large and persistent calorie deficit forces your body into 'starvation mode,' lowering your metabolism and making fat loss more difficult.
Muscle Loss: When glycogen stores run out during exercise, your body breaks down muscle protein for fuel, compromising strength and body composition.
Hormonal Disruption: Underfueling can cause hormonal imbalances, affecting menstrual cycles in women, lowering testosterone in men, and increasing stress hormones like cortisol.
Performance Decline: Without sufficient energy, your endurance, strength, and power output will decrease, leading to subpar workouts and increased fatigue.
Increased Injury Risk: Poor bone density, slower recovery, and compromised immune function resulting from underfueling increase your susceptibility to injuries and illness.

The Immediate Impact on Your Body

When you engage in physical activity, your body requires energy to perform. Your primary fuel source is glucose, derived from carbohydrates stored as glycogen in your muscles and liver. When you don't eat enough calories to match your energy expenditure, especially while exercising, your body has to find an alternative power source.

Fueling Your Workout

In the short term, this leads to a rapid depletion of stored glycogen. Once these reserves are exhausted, your body turns to its next available fuel: muscle protein. This process, known as catabolism, directly counteracts your fitness goals. Instead of building strength and muscle, your body is actively breaking it down. This can result in several immediate and noticeable signs, including:

Chronic Fatigue: Feeling constantly drained, not just during or after your workout, but throughout the day.
Reduced Performance: You may hit a wall sooner during your workouts, experience decreased power output, or find it difficult to complete your usual routines.
Dizziness or Lightheadedness: Low blood sugar levels can make you feel weak or faint, especially during intense exercise.
Increased Hunger and Cravings: The body's primal instinct to acquire fuel can lead to intense cravings, often for sugary foods, setting up a cycle of overeating or yo-yo dieting later on.

The Long-Term Consequences of Chronic Underfueling

Persistently undereating while exercising can move from a temporary inconvenience to a serious threat to your health. Your body adapts to the state of low energy by slowing down non-essential functions, leading to long-term issues.

Metabolic Slowdown

The 'starvation mode' is a real physiological response. When you consistently deprive your body of adequate calories, your metabolism slows to conserve energy. This makes it harder to lose weight and easier to regain it once you return to normal eating habits. This can lead to a frustrating weight-loss plateau or even unintentional weight gain over time, a counterintuitive but common result of severe caloric restriction.

Hormonal and Reproductive Health

Chronic low energy availability has profound effects on the endocrine system, impacting vital hormones in both men and women.

Cortisol: Levels of the stress hormone cortisol increase, which can lead to inflammation, fat storage, and a higher risk of illness.
Reproductive Hormones: In females, low energy availability can lead to hormonal imbalances, causing irregular or absent menstrual cycles (amenorrhea). This is a key indicator of Relative Energy Deficiency in Sport (RED-S). In men, it can decrease testosterone levels, affecting libido and overall strength.
Thyroid Function: Thyroid hormone production can decrease, further slowing metabolism and contributing to fatigue.

Weakened Immune System and Bone Health

Proper nutrition is essential for a robust immune system. In a state of low energy availability, your body's immune response can be suppressed, making you more susceptible to infections and illnesses. Furthermore, the hormonal disruptions caused by underfueling lead to reduced bone mineral density. In women, low estrogen is particularly damaging to bone health, increasing the risk of stress fractures and potentially leading to early-onset osteoporosis.

The Health Risks of Underfueling Versus Fueling Properly


Feature	Insufficient Calorie Intake	Sufficient Calorie Intake
Energy Levels	Chronic fatigue, lethargy, midday crashes	Sustained energy, improved endurance, less fatigue
Metabolism	Metabolic slowdown; body hoards calories	Efficient metabolism; body burns calories effectively
Muscle Mass	Muscle protein breakdown, strength regression	Muscle protein synthesis, strength and hypertrophy gains
Bone Health	Reduced bone mineral density, increased fracture risk	Stronger bones, reduced risk of injury
Hormonal Balance	Disruptions to reproductive and thyroid hormones	Stable hormonal function, balanced mood
Immune System	Weakened immune response, frequent illness	Stronger immunity, better protection from illness
Psychological State	Mood swings, irritability, anxiety	Improved mood, better mental clarity
Weight Management	Plateaued weight loss or unintentional weight gain	Sustainable fat loss and healthy body composition

Understanding and Preventing Relative Energy Deficiency in Sport (RED-S)

RED-S, or Relative Energy Deficiency in Sport, is a syndrome caused by low energy availability that impairs physiological function and health. It encompasses numerous health and performance issues that arise from a mismatch between dietary intake and exercise expenditure. This affects multiple body systems, not just bone and menstrual health. The symptoms can be physical, mental, and performance-related, including gastrointestinal issues, cardiovascular problems, and mental health concerns.

Strategies for Proper Fueling

To prevent the dangers of underfueling, consider the following strategies:

Calculate Your Needs: Use a reliable online calculator or, even better, consult with a registered dietitian or sports nutritionist to determine your individual daily energy needs based on your activity level, age, sex, height, and weight.
Prioritize Nutrient Timing: Focus on fueling your body before and after workouts. Consuming a combination of carbohydrates and protein post-exercise is crucial for replenishing glycogen stores and repairing muscle tissue.
Eat Consistently: Avoid skipping meals, especially around your workout times. Distribute your calorie intake evenly throughout the day to provide a steady supply of energy.
Listen to Your Body: Pay attention to signs of underfueling like persistent fatigue, mood changes, or declining performance. They are a clear signal that your body needs more fuel.
Focus on Whole Foods: Emphasize a balanced diet rich in complex carbohydrates, lean protein, and healthy fats. This ensures you get a wide range of essential vitamins and minerals, not just calories.

For more detailed information on RED-S and its impact on athletes, you can refer to the informative resources provided by organizations like the International Olympic Committee.

Conclusion

Exercising while chronically undereating is a dangerous practice that can have severe short-term and long-term consequences for your health. While a small, temporary calorie deficit is necessary for weight loss, a substantial and prolonged deficit can harm your metabolism, sacrifice muscle mass, disrupt hormones, and weaken your immune system. By understanding the science behind underfueling and implementing mindful eating strategies, you can ensure your body is properly fueled to perform at its best, recover efficiently, and achieve your health and fitness goals safely and sustainably.

Frequently Asked Questions

Performance can be impacted almost immediately. A single underfueled workout can feel difficult and leave you feeling depleted. With chronic underfueling, a noticeable decline in strength, speed, and endurance becomes apparent within weeks.

While exercising in a fasted state can increase fat calorie burn in the short term, it also risks significant muscle loss, which can hinder long-term metabolic health. The body prefers burning readily available carbs first, and once those are gone, it will break down muscle for fuel.

RED-S is a syndrome caused by low energy availability, a state where calorie intake is insufficient for the energy expended during exercise. It can impair metabolic rate, bone health, reproductive function, immunity, and overall health in athletes of all genders.

Feeling cold is a common symptom of underfueling. Your body reduces non-essential processes, including maintaining core body temperature, to conserve energy. This is a survival mechanism to prioritize vital functions.

Signs include chronic fatigue, frequent illness, persistent hunger, mood swings, digestive issues like constipation, and a plateau or decline in your athletic performance. In women, irregular or absent menstrual cycles are a clear red flag.

This can trigger a severe metabolic slowdown, hormonal imbalances, and significant muscle mass loss. It also increases your risk of injury due to weakened bones and delayed recovery, ultimately compromising your health and fitness goals.

Yes, in most cases, it is possible to reverse these effects by increasing calorie intake to match energy needs and adopting a balanced diet. However, some damage, like significant bone loss, may not be fully reversible, highlighting the importance of early intervention.