What happens if you eat under the recommended amount of calories?

November 22, 2025 •

5 min read

According to the U.S. Department of Health, adult females typically require around 1,600–2,400 calories daily to maintain weight, while males need 2,000–3,000. So, what happens if you eat under the recommended amount of calories?

Quick Summary

Chronic undereating can cause serious side effects, including metabolic slowdown, nutrient deficiencies, fatigue, and hormonal imbalances. This guide outlines the short- and long-term health risks associated with inadequate calorie intake and offers advice for healthy weight management.

Key Points

Metabolic Slowdown: Chronic undereating causes your body to reduce its basal metabolic rate to conserve energy, making weight loss more difficult in the long run.
Nutrient Deficiencies: Restricting calories often leads to a lack of essential vitamins and minerals, which can weaken the immune system and cause issues like hair loss and brittle nails.
Hormonal Imbalances: Inadequate energy intake can disrupt hormones that regulate metabolism, appetite, and reproduction, potentially leading to menstrual irregularities or infertility.
Loss of Muscle and Bone Mass: Without sufficient fuel, the body will break down muscle tissue for energy and may compromise bone density, increasing the risk of osteoporosis.
Mental Health Effects: Undereating can cause fatigue, irritability, and brain fog due to the brain being deprived of necessary energy, and can increase the risk of developing disordered eating patterns.
Cardiovascular Issues: Severe, long-term calorie restriction can damage the heart muscle, leading to a dangerously slow heart rate and low blood pressure.
Prioritize Health Over Speed: A moderate, sustainable calorie deficit (300-500 calories) is safer and more effective for long-term weight management than a drastic, extreme one.

The concept of a calorie deficit is simple: consume fewer calories than your body burns. While a small, controlled deficit is necessary for weight loss, drastically or consistently eating under the recommended amount of calories can trigger a survival response in your body with significant negative consequences. Your body, sensing a famine, prioritizes essential functions and conserves energy, impacting your metabolism, hormonal balance, and overall physical and mental health.

The Short-Term Effects of Undereating

When you first begin restricting calories, your body taps into its energy reserves. The immediate effects can often be mistaken for normal dieting side effects, but they are your body's initial distress signals.

Common short-term side effects include:

Fatigue and Low Energy: Calories are fuel, and a significant lack of them will leave you feeling sluggish and tired throughout the day.
Irritability and Mood Swings: Your brain relies on a steady supply of glucose from food for proper function. When this is restricted, emotional regulation becomes difficult, leading to mood swings and increased irritability.
Constant Hunger and Cravings: Hormones like ghrelin, which stimulate appetite, increase in response to calorie restriction. This can lead to persistent feelings of hunger and intense cravings for high-calorie foods.
Digestive Issues: Undereating can slow down the digestive tract, leading to problems like constipation, bloating, and gastroparesis.
Brain Fog and Poor Concentration: The brain needs energy to function optimally. A lack of sufficient fuel can result in difficulty concentrating, poor memory, and a general feeling of mental fogginess.

The Long-Term Consequences of Chronic Undereating

Persisting with a significant calorie deficit beyond the short term forces your body into a deeper state of preservation, leading to more serious and potentially irreversible health problems.

Here are some of the critical long-term effects:

Metabolic Slowdown: To conserve energy, your body lowers its basal metabolic rate (BMR). This means you burn fewer calories at rest, making weight loss more challenging and increasing the likelihood of regaining weight once you resume a normal calorie intake.
Hormonal Imbalances: For women, chronic undereating can disrupt the menstrual cycle, leading to irregular periods or amenorrhea (loss of menstruation) and increasing the risk of infertility. It can also impact other hormones, including stress hormones like cortisol.
Nutrient Deficiencies: Severely restricting calories often means missing out on vital nutrients, including vitamins, minerals, and protein. This can lead to hair loss, brittle nails, a weakened immune system, and an increased risk of infections.
Loss of Muscle Mass and Bone Density: When fat stores are depleted, the body begins breaking down muscle tissue for energy, a process known as catabolism. The lack of proper nutrition also impairs bone health, leading to osteopenia and a higher risk of fractures.
Cardiovascular Damage: The heart is a muscle, and chronic undereating can cause it to lose muscle mass, leading to a slowed heart rate (bradycardia) and low blood pressure. In severe cases, this can lead to heart failure.

Comparison of Short-Term vs. Long-Term Effects


Feature	Short-Term Effects	Long-Term Effects
Energy & Metabolism	Fatigue, lethargy, and a slight decrease in metabolic rate.	Significant metabolic slowdown, weight regain resistance, and muscle loss.
Mood & Mental State	Irritability, mood swings, cravings, and brain fog.	Increased risk of depression, anxiety, food obsession, and potentially irreversible cognitive decline.
Hormonal Health	Mild fluctuations in hormone levels.	Severe hormonal imbalances, reproductive issues (e.g., amenorrhea), and increased cortisol levels.
Physical Appearance	Initial weight loss, potential for dry skin or brittle nails.	Hair loss, fine downy body hair (lanugo), extreme skin dryness, and reduced bone density.
Internal Health	Digestive upset (constipation, bloating).	Organ damage, severe cardiovascular complications, nutritional deficiencies, and compromised immune function.

How to Avoid the Dangers of Undereating

It is crucial to approach weight management with a focus on sustainable, long-term health rather than quick fixes through extreme restriction. To lose weight safely, you need to create a calorie deficit, but this should be done in a moderate, controlled way.

A healthy approach involves:

Calculate Your Needs: Use a reliable online calculator (like the Mifflin-St Jeor equation) to determine your basal metabolic rate (BMR) and total daily energy expenditure (TDEE). This provides a baseline for your needs based on your age, sex, weight, and activity level.
Create a Moderate Deficit: A daily deficit of 300-500 calories is generally considered safe and sustainable for gradual weight loss. This avoids triggering your body's severe survival responses.
Prioritize Nutrient-Dense Foods: Fill your meals with whole foods like fruits, vegetables, lean proteins, and whole grains to ensure you receive essential nutrients, even with fewer calories.
Don't Forget Protein: Adequate protein intake is vital for preserving muscle mass during weight loss. It also helps you feel fuller for longer.
Listen to Your Body: Pay attention to hunger and fullness cues. If you're constantly fatigued, irritable, or hungry, it may be a sign that your deficit is too large.

Conclusion

While a calorie deficit is the fundamental principle behind weight loss, eating too far under the recommended amount of calories is a dangerous and counterproductive strategy. The human body is remarkably resilient but will prioritize survival over optimal function when starved of adequate energy. The side effects, from metabolic slowdown and hormonal disruption to serious organ damage, far outweigh the short-term goal of rapid weight loss. Sustainable, healthy weight management is achieved by creating a moderate calorie deficit, focusing on a nutrient-rich diet, and listening to your body's signals, not ignoring them. For personalized guidance, consulting a healthcare professional or a registered dietitian is always the safest course of action.

External Resource

For more information on the impact of severe caloric restriction, particularly in the context of eating disorders, visit the National Centre for Eating Disorders, a UK-based organization offering valuable insights and resources.

Note: The content provided is for informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition.

Frequently Asked Questions

A healthy calorie deficit is a moderate and controlled reduction in calories, typically 300-500 below your maintenance level, to promote gradual fat loss while preserving muscle and overall health. Undereating, by contrast, is an excessive or chronic calorie restriction that forces the body into a state of deprivation, leading to negative health consequences.

Undereating can cause a metabolic slowdown, a phenomenon known as metabolic adaptation, where your body becomes more efficient at using energy. While it can persist for a long time, it is not always permanent and can often be reversed by gradually increasing calorie intake and focusing on strength training.

Initially, yes, you will lose weight, but much of this can be water weight and muscle mass, not just fat. Over time, your body will adapt by slowing your metabolism, and the weight loss will likely plateau. This is unsustainable and can lead to weight regain and health problems.

Mental effects of undereating include irritability, fatigue, mood swings, anxiety, depression, and 'brain fog' or poor concentration. This is because the brain needs a steady supply of energy from food to regulate emotions and function properly.

Common signs of undereating include persistent fatigue, constant hunger, irritability, digestive problems, a constantly cold body temperature, and irregular menstrual cycles in women. If you experience these symptoms, it's wise to re-evaluate your intake.

If you suspect chronic undereating, you should consult a healthcare professional or a registered dietitian. They can help you develop a safe refeeding plan to restore your nutrient levels and metabolic function without causing additional strain on your body.

An occasional day of very low calorie intake (e.g., due to illness or a busy schedule) is unlikely to cause significant harm for most healthy individuals. However, this should not become a regular habit, and the body's nutrient needs should be met the following day.

Yes, inadequate calorie intake, especially coupled with low calcium and vitamin D, can decrease bone mineral density over time. This can lead to weakened bones (osteopenia) and eventually osteoporosis, increasing the risk of fractures.

Yes, chronic undereating can suppress your immune system, making you more susceptible to infections and illnesses. It can also prolong the recovery time from sickness or wounds.

Gradually increase your calorie intake by adding nutrient-dense foods to your diet rather than jumping straight to high-calorie junk food. Focus on balanced meals with protein, healthy fats, and complex carbohydrates. Professional guidance from a dietitian can be very helpful.