What Happens if You Eat Less Than Maintenance Calories?

October 15, 2025 •

4 min read

According to the National Institutes of Health, a moderate calorie deficit of 500 calories per day is often recommended for safe, sustainable weight loss. If you consistently eat less than your maintenance calories, your body will tap into stored energy, leading to a host of physiological and psychological changes.

Quick Summary

This guide explores the various effects of a calorie deficit, outlining how the body adapts to conserve energy and the potential health risks associated with inadequate long-term calorie intake. It covers the mechanisms behind weight loss, metabolic slowdown, and the importance of a balanced approach.

Key Points

Metabolic Slowdown: Consistently eating below maintenance calories forces your body to slow its metabolism to conserve energy, a survival mechanism called metabolic adaptation.
Nutrient Deficiencies: Severe calorie restriction increases the risk of developing nutritional deficiencies, as it becomes difficult to consume enough essential vitamins and minerals.
Muscle Atrophy: An aggressive calorie deficit can cause your body to break down muscle tissue for energy, which further decreases your metabolic rate.
Hormonal Disruption: Chronic undereating can lead to hormonal imbalances, affecting mood, energy, and fertility, especially in women.
Long-Term vs. Short-Term Effects: While a moderate deficit promotes steady, healthy weight loss, a severe deficit can lead to serious long-term health problems and is often unsustainable.
Psychological Impact: Eating too few calories can cause irritability, anxiety, and obsessive thoughts about food, potentially contributing to eating disorders.

Short-Term Effects of Eating Less Than Maintenance Calories

When you first begin eating below your body's maintenance level, the effects are relatively immediate as your body adjusts. The purpose of a calorie deficit is to force your body to use stored energy (fat) for fuel, resulting in weight loss.

Initial Metabolic Response

In the short term, the body's primary response is to increase feelings of hunger to prompt you to eat more. As you persist with the deficit, your body's survival instincts kick in. It begins by using stored carbohydrates (glycogen) for energy. This is often accompanied by a significant initial drop in weight, much of which is water weight bound to the glycogen stores.

Common Side Effects

During the first few weeks, you may experience several side effects as your body acclimates to the lower energy intake:

Fatigue and Low Energy: With less fuel from food, your body may feel sluggish and less energetic. You may find that intense workouts become more challenging.
Irritability and Mood Changes: Often described as being "hangry," hunger can directly affect your mood and make you more irritable.
Headaches and Nausea: Low blood sugar levels can lead to headaches, dizziness, and feelings of nausea.
Constipation: Reduced food volume and intake of fiber can lead to infrequent bowel movements.
Feeling Cold: Your body may lower its core temperature to conserve energy, making you feel cold more often.

Long-Term Consequences of Chronic Undereating

While a moderate, short-term calorie deficit is a standard and effective approach for weight loss, a prolonged and severe deficit can lead to more serious and detrimental health consequences. This is because the body will shift its priorities from optimal function to pure survival, leading to a state of metabolic adaptation.

Metabolic Slowdown

Long-term calorie restriction forces the body into a state of metabolic adaptation, often referred to as "starvation mode". The body adjusts by slowing its basal metabolic rate (BMR), the number of calories it burns at rest, to conserve as much energy as possible. This means that over time, you will burn fewer calories simply by existing, which makes further weight loss more challenging and can contribute to weight regain once you return to a higher calorie intake.

Nutrient Deficiencies and Health Risks

Insufficient calorie intake makes it difficult to consume all the necessary micronutrients, such as vitamins and minerals. Over time, this can lead to:

Compromised Immune System: A lack of essential nutrients can weaken your immune response, making you more susceptible to illness.
Hormonal Imbalances: Hormones that regulate appetite, stress, and fertility can be significantly disrupted. For women, this can lead to irregular menstrual cycles or amenorrhea.
Muscle Loss: When calorie deficits are too steep, the body may begin breaking down muscle tissue for energy, particularly if protein intake is inadequate. Losing muscle further lowers your metabolic rate.
Bone Density Loss: Chronic low energy availability can have a negative impact on bone health, increasing the risk of osteoporosis.
Gallstones: Rapid weight loss is a known risk factor for developing painful gallstones.

Psychological and Behavioral Effects

The mental toll of chronic undereating can be substantial:

Increased Anxiety and Depression: Constant hunger and nutrient deprivation can negatively affect mood and mental well-being.
Obsessive Thoughts about Food: Calorie restriction can lead to preoccupation with food, meal planning, and exercise, which can contribute to disordered eating patterns.
Social Withdrawal: Fatigue and mood changes can lead to a decrease in interest in social activities.

Comparison: Moderate vs. Severe Calorie Deficit


Aspect	Moderate Calorie Deficit (e.g., 300-500 kcal/day)	Severe Calorie Deficit (e.g., >750 kcal/day)
Rate of Weight Loss	Slow and steady (approx. 0.5-1 lb/week), primarily fat.	Rapid initial weight loss, but often includes significant muscle mass.
Metabolic Adaptation	Minimal and temporary slowdown.	Significant metabolic slowdown, persistent for years after weight loss.
Nutrient Intake	Easier to meet micronutrient needs through food choices.	High risk of nutritional deficiencies due to lower food volume.
Energy Levels	Generally manageable, with minor fluctuations.	Prone to fatigue, lethargy, and low energy levels.
Sustainability	Highly sustainable, focusing on long-term habit changes.	Very difficult to sustain, leading to eventual weight regain.
Health Risks	Low risk of negative health consequences when monitored.	High risk of serious health issues like gallstones, muscle atrophy, and hormonal problems.
Psychological Impact	Fosters mindful eating and positive habits.	Increases risk of disordered eating, anxiety, and obsession with food.

Conclusion

While eating less than your maintenance calories is the fundamental principle of weight loss, the severity and duration of the calorie deficit are crucial determinants of its impact on your health. A moderate, sustainable deficit, achieved through a combination of reduced intake and increased physical activity, can lead to healthy and long-lasting results. In contrast, severe and prolonged calorie restriction triggers a survival response that slows your metabolism and increases the risk of serious nutritional, hormonal, and psychological problems. The key to healthy weight management is to find a balanced approach that nourishes your body while allowing for gradual, consistent progress. For best results, consult with a healthcare professional or registered dietitian before starting any significant dietary changes.

Get Help if You Need It

For those who suspect they may be struggling with disordered eating as a result of severe calorie restriction, resources are available. For more information and support, the National Centre for Eating Disorders provides valuable resources on the effects of under-eating.

Frequently Asked Questions

A moderate deficit is typically 300-500 calories below your maintenance level and leads to slow, sustainable weight loss, mostly fat. A severe deficit is a much larger reduction, leading to faster initial weight loss but increasing the risk of muscle loss, metabolic slowdown, and nutrient deficiencies.

Yes, as a survival mechanism, your body will adapt to a lower calorie intake by reducing its metabolic rate. This is known as metabolic adaptation and can make weight loss more challenging over time.

The duration depends on your weight loss goals, but it is not meant to be a permanent state. For significant weight loss, it's common to alternate periods of a calorie deficit with phases of eating at maintenance to help prevent metabolic slowdown and plateaus.

Yes, it is possible. Strategies like portion control, focusing on nutrient-dense whole foods, increasing physical activity, and reducing sugary drinks can all help create a calorie deficit without needing to meticulously count every calorie.

Initial signs of undereating include increased fatigue, frequent hunger, irritability, headaches, and feeling colder than usual. These are your body's initial signals that it needs more energy.

No, very low-calorie diets carry significant health risks, including nutrient deficiencies, muscle loss, and gallstones. Rapid weight loss is also more difficult to sustain long-term and often results in weight regain.

Maintenance calories, or Total Daily Energy Expenditure (TDEE), can be estimated using formulas like the Mifflin-St Jeor equation, which accounts for your age, sex, weight, height, and activity level. Many online calculators can help with this.

In a way, yes. While a calorie deficit is necessary for weight loss, chronic and severe restriction can trigger metabolic adaptation, making your body cling to fat stores and slowing future weight loss. When you eventually resume a higher calorie intake, your slowed metabolism may cause rapid weight regain.