Understanding Why: Does Your Body Use Up More Energy When You're Sick?

September 28, 2025 •

4 min read

According to research, fighting off an infection can increase your body's metabolic rate by as much as 20% or more, depending on the severity of the illness. The answer to Does your body use up more energy when you're sick? is a resounding yes, driven by the intense demands of your immune system.

Quick Summary

Your body significantly increases its energy expenditure to power the immune system's aggressive response against pathogens. This heightened metabolic activity, especially when accompanied by a fever, is the primary reason you experience profound fatigue and weakness during illness.

Key Points

Immune Response: Your immune system's aggressive activation to fight off pathogens is the primary driver behind the increase in your body's energy consumption when sick.
Increased Metabolism: Illness, particularly when accompanied by fever, significantly increases your metabolic rate, causing you to burn more calories even at rest.
Fatigue is a Tool: The deep fatigue you feel serves a purpose—it forces you to rest and conserve energy so your body can allocate its resources to fighting the infection.
Proper Fueling: Despite a potential loss of appetite, it's crucial to consume nutrient-dense foods and fluids to provide your body with the energy needed for recovery.
Listen to Your Body: Rest is not a sign of weakness but a necessary part of the healing process. Pushing through illness can deplete your body further and prolong recovery.
Fever's Effect: A fever is a deliberate energy-intensive process that helps combat infection by making the body a less favorable environment for pathogens.

The Immune System's High-Energy Demands

When you fall ill, your body shifts its priorities. Instead of focusing on normal day-to-day functions like digestion or physical activity, it channels immense amounts of energy toward the fight against invading viruses or bacteria. This redirection of resources is crucial for a successful immune response, but it comes at a cost to your overall energy levels.

Your immune system is not a passive defense. It's a complex, multi-layered army that requires substantial fuel to function. The production of immune cells, such as T-cells and B-cells, is a metabolically expensive process. These cells need energy to multiply rapidly and travel to the site of infection. Additionally, the creation and release of chemical messengers, known as cytokines, require significant resources to orchestrate the body's defensive strategy. Think of it as your body's emergency response team, which demands a massive surge of power to operate at peak capacity. This is a fundamental reason does your body use up more energy when you're sick? and why you feel so drained.

The Role of Fever and Inflammation

One of the most noticeable and energy-intensive responses to illness is a fever. A fever is not an accidental side effect; it is a deliberately triggered defense mechanism. By raising your body's core temperature, your body creates a less hospitable environment for pathogens. This process is energy-intensive, and research indicates that for every one-degree Celsius increase in body temperature, your metabolic rate can increase by about 13%. This explains why you feel hot and sweaty when sick and why high fevers lead to such intense fatigue.

Inflammation also plays a critical role. This localized response, characterized by swelling, heat, and pain, is how your body recruits immune cells to the affected area. The inflammatory process itself requires energy, as it involves a complex cascade of cellular and chemical reactions. For conditions like the flu, this can lead to widespread inflammation and body aches, contributing to the overall feeling of being run down.

Why Sickness Makes You Feel So Tired

Fatigue is not just a symptom; it is a protective strategy. The profound exhaustion you feel is your body's way of forcing you to rest, conserving energy for the immune system. When your body is in an energy deficit, it prioritizes the most critical tasks, and fighting illness is at the top of the list. By minimizing physical activity, your body ensures that precious resources are not diverted elsewhere.

This is why even a relatively mild cold can leave you feeling completely wiped out. The energy required to fight off the infection, combined with the body's natural push for rest, creates a powerful feeling of lethargy. Ignoring this signal and trying to push through can prolong your recovery and put unnecessary stress on your body.

Supporting Your Body During Illness

While your body is working overtime, you can help it along by providing the right fuel and support. This does not mean overeating, especially if your appetite is low, but rather focusing on nutrient-dense foods and staying hydrated. Proper nutrition is essential for providing the building blocks for immune cells.

Here are a few tips for supporting your body's energy needs while sick:

Stay hydrated by drinking plenty of water, herbal tea, and broth.
Opt for easily digestible foods like soup, toast, and bananas.
Prioritize rest and sleep to conserve energy for your immune system.
Listen to your body and don't push yourself to exercise.
Consider nutrient-rich smoothies if your appetite is very low.

Energy Expenditure: Sick vs. Healthy

Here is a simple comparison to illustrate the difference in energy use.


Aspect	Healthy State	Sick State
Energy Allocation	Balanced across daily activities, digestion, and maintenance.	Prioritized for immune response; other functions reduced.
Metabolic Rate	Basal metabolic rate (BMR) is stable.	Elevated, potentially by 10-20% or more, especially with a fever.
Immune System Activity	Low, routine surveillance.	High, with rapid production of immune cells and inflammatory chemicals.
Physical Feeling	Energetic, able to perform normal tasks.	Fatigued, weak, and lethargic.
Appetite	Normal, consistent food intake.	Often suppressed, but calorie needs are higher.
Primary Goal	Daily function and long-term health.	Fight off infection and recover.

Conclusion: Listen to Your Body

The next time you feel completely exhausted from being sick, remember that it's a sign that your body is doing exactly what it's supposed to do. The energy drain is a direct result of your immune system's tireless work to get you back to health. By listening to your body's signals, resting, and providing it with the right nutrition and hydration, you can support its natural processes and speed up your recovery. Ignoring these signals can prolong the illness and make recovery more difficult. For more information on health and wellness, consult authoritative sources like Verywell Health Do You Burn More Calories While You're Sick?. Understanding this fundamental aspect of sickness empowers you to make better choices for your recovery journey.

Frequently Asked Questions

Yes, a fever significantly increases your body's energy expenditure. The metabolic rate can increase by approximately 13% for every one-degree Celsius rise in body temperature. This heightened energy use is part of your body's defensive strategy to create an inhospitable environment for pathogens.

The fatigue you feel when sick is your body's way of conserving energy. The immune system is working overtime, and this process is metabolically demanding. By making you feel tired, your body forces you to rest, which ensures that energy is redirected toward the fight against the infection rather than being used for other activities.

While your body's energy needs increase, your appetite may decrease. It's important to listen to your body and focus on consuming nutritious, easily digestible foods. Staying hydrated with broths and drinks rich in electrolytes is often more important than forcing yourself to eat large meals.

Yes, your body burns more calories at rest when you are sick. This is because your basal metabolic rate (BMR) is elevated due to the increased activity of your immune system. The extra energy is required for producing immune cells, triggering inflammation, and sustaining a fever.

While your body does burn more energy, losing weight during illness is not a healthy or sustainable strategy. The weight loss is often due to a combination of increased metabolism and decreased appetite, and can involve the loss of both fat and muscle mass. Prioritizing recovery with proper rest and nutrition is more important than focusing on weight during this time.

Energy expenditure typically returns to normal as your body recovers. The duration depends on the severity and length of your illness. It may take several days or even weeks for your energy levels to fully normalize, especially after a more serious infection, as your body still needs energy to repair and rebuild.

Yes, the amount of energy your body expends varies with the type and severity of the illness. A more intense infection or one that causes a higher fever will demand significantly more energy from your immune system than a milder cold or infection.