What Happens When You're Not Eating Enough Calories?

November 17, 2025 •

4 min read

According to research, long-term calorie restriction can significantly lower your resting metabolic rate and make weight loss efforts harder over time. This happens because when you're not eating enough calories, your body goes into survival mode, triggering a series of physiological and psychological changes to conserve energy. Understanding these effects is crucial for anyone trying to manage their weight or improve their health.

Quick Summary

Long-term undereating triggers the body's survival mechanisms, slowing the metabolism and breaking down muscle for energy. This leads to fatigue, nutrient deficiencies, mood swings, and other serious health complications. It can also disrupt reproductive function and compromise the immune system, making it more difficult to achieve sustainable health and weight management goals.

Key Points

Metabolic Slowdown: Chronic undereating triggers adaptive thermogenesis, where your metabolism slows down to conserve energy, stalling weight loss.
Muscle Loss: The body will break down lean muscle tissue for fuel when calories are too low, further reducing your metabolic rate and strength.
Nutrient Deficiencies: Severe calorie restriction often leads to a lack of essential vitamins and minerals, impacting everything from bone density to immune function.
Mental and Emotional Toll: Insufficient fuel for the brain can cause mood swings, irritability, anxiety, and obsessive thoughts about food.
Hormonal Disruption: Hormonal balance is easily disturbed by low energy intake, which can lead to reproductive issues and other health problems.
Fatigue: A lack of sufficient calories results in constant fatigue, low energy levels, and impaired daily performance.
Sustainable Alternative: Focus on a moderate calorie deficit combined with nutrient-rich foods and resistance training to lose weight sustainably while protecting your metabolism and muscle mass.

The Body's Survival Response: Adaptive Thermogenesis

When you consistently consume fewer calories than your body needs, it perceives this as a state of famine. As a survival mechanism, it initiates a process called adaptive thermogenesis, or "starvation mode". The primary goal is to conserve energy and prevent fat loss by reducing the number of calories burned. Your body prioritizes vital functions like breathing and heartbeat, while slowing down non-essential processes. This can be a major reason for weight loss plateaus, where progress stalls despite continued calorie restriction.

Physical Consequences of Insufficient Fuel

Chronic Fatigue and Low Energy: Calories are the fuel for all bodily activities. Without enough of them, your energy levels plummet, leaving you feeling tired and sluggish all the time. Even simple daily tasks can feel exhausting.
Muscle Atrophy: When carbohydrate and fat stores are depleted, your body turns to its most metabolically active tissue—muscle—as a source of energy. This catabolic state leads to muscle loss and a further decrease in your metabolic rate, making it harder to manage your weight in the long run.
Nutrient Deficiencies: Restricting food intake severely limits your consumption of essential vitamins and minerals. For instance, a lack of calcium and vitamin D can weaken bones, increasing the risk of osteoporosis. Iron deficiency can cause anemia, leading to fatigue and cold sensitivity.
Digestive Issues: Undernutrition can slow down the gastrointestinal tract, resulting in constipation, bloating, and other discomforts. This is because your body allocates fewer resources to digestion when in survival mode.
Feeling Cold: Your body generates heat by burning calories. When calorie intake is too low, this process slows down to save energy, causing you to feel perpetually cold.
Compromised Immune Function: A deficiency in key nutrients like protein and vitamins weakens your immune system, making you more susceptible to infections and slowing down recovery from illness.

Psychological and Hormonal Effects

Beyond the physical toll, undereating profoundly impacts mental health and hormonal balance.

Mood Swings and Irritability: Low blood sugar from inadequate caloric intake can lead to irritability, anxiety, and mood swings, a state commonly known as being "hangry". Your brain needs a consistent supply of glucose to function properly.
Food Obsession: Long-term calorie restriction can cause the brain to fixate on food as a survival response, leading to obsessive thoughts about eating and cravings. This can contribute to unhealthy eating patterns or disordered eating behaviors.
Hormonal Disturbances: Insufficient energy can disrupt hormone production. For women, this can lead to irregular menstrual cycles or amenorrhea (the absence of a period), and potentially infertility. In men, low testosterone levels can cause fatigue and decreased libido.

Short-Term vs. Long-Term Calorie Deficit Comparison


Aspect	Short-Term, Moderate Calorie Deficit	Long-Term, Severe Calorie Deficit
Weight Loss Rate	Safe, gradual loss (e.g., 1-2 lbs per week).	Rapid initial weight loss, followed by a plateau or stall.
Metabolism	Minimal impact on metabolic rate; sustainable.	Significant metabolic slowdown (adaptive thermogenesis).
Body Composition	Primarily fat loss with muscle preservation (especially with adequate protein and exercise).	Significant muscle loss as the body burns lean tissue for fuel.
Energy Levels	Generally stable, with sufficient energy for daily activities and workouts.	Chronic fatigue, lethargy, and reduced performance.
Nutrient Intake	Balanced and nutrient-rich, minimizing deficiency risk.	High risk of vitamin and mineral deficiencies.
Mental State	Positive mood and motivation from seeing progress.	Increased irritability, anxiety, and obsessive food thoughts.
Sustainability	Easily maintained as part of a healthy lifestyle.	Difficult to maintain and increases risk of weight regain (yo-yo dieting).

What To Do Next

If you recognize the signs of undereating, it's essential to address the issue properly. Instead of continuing a severe calorie restriction, focus on sustainable practices. Gradually increase your calorie intake with nutrient-dense foods to bring your body out of survival mode. Prioritize lean protein and healthy fats to support muscle mass and hormonal health. Incorporate strength training to build muscle, which will help increase your metabolic rate over time. If you're concerned about your relationship with food or have been undereating for a long period, consulting a registered dietitian or a healthcare professional can provide guidance on a safe path forward. The goal is a balanced, nourishing approach to health, not a race to the lowest number on the scale. For additional resources and support on mindful eating, consider exploring authoritative sources like the National Eating Disorders Association.

Conclusion

In summary, intentionally or unintentionally restricting calories too severely can trigger a cascade of negative effects on your body and mind. While a moderate calorie deficit is necessary for weight loss, going too far can slow your metabolism, destroy muscle, and create nutrient deficiencies. The psychological and hormonal impacts can also lead to mood swings, food obsession, and a difficult cycle of weight regain. The key to sustainable health is to nourish your body with the right amount and type of food, rather than forcing it into a state of deprivation.

Frequently Asked Questions

Common signs of insufficient calorie intake include chronic fatigue, constant hunger, feeling cold, irritability, hair loss, constipation, and a plateau in weight loss.

Yes, in the long term, severe calorie restriction can slow your metabolism so significantly that your body conserves energy and stores fat more readily. This can cause a weight loss plateau or even weight regain, especially when returning to a normal diet.

When you undereat, your body lowers its basal metabolic rate (BMR) to conserve energy, a process known as adaptive thermogenesis. This makes your body more efficient at using fewer calories and burning less fat, which hinders weight loss.

Yes, undereating can cause significant mental health issues. It can lead to mood swings, increased anxiety and depression, and obsessive thoughts about food due to insufficient fuel for the brain.

While the term 'starvation mode' is often sensationalized, the underlying physiological response is very real and is more accurately called metabolic adaptation or adaptive thermogenesis. It’s your body’s natural response to conserve energy during prolonged calorie restriction.

A safe approach involves a moderate calorie deficit of 300–500 calories, combined with eating a balanced diet rich in protein, whole grains, fruits, and vegetables. Incorporating exercise, particularly strength training, helps preserve muscle and boosts metabolism.

Long-term effects include nutrient deficiencies, bone health issues like osteoporosis, hormonal imbalances, impaired immune function, muscle loss, and an increased risk of developing gallstones.