Does Your Body Need Calories to Fight Infections? The Truth About Sickness and Metabolism

October 15, 2025 •

5 min read

When you have a fever, your metabolism can accelerate significantly, sometimes burning hundreds of extra calories per day. This increased metabolic rate provides a clear answer to the question: Does your body need calories to fight infections? In fact, neglecting your body's nutritional needs during sickness can weaken your immune system and delay recovery.

Quick Summary

The body increases its metabolic rate to fuel the immune response during illness, requiring a higher intake of calories and nutrients. This extra energy supports the production of immune cells, antibodies, and helps regulate bodily functions like fever. Proper nutrition is crucial for fighting infection and preventing the body from breaking down its own muscle and fat stores.

Key Points

Sickness Increases Calorie Needs: Your metabolism accelerates significantly during illness, especially with a fever, burning extra calories to fuel the immune response.
Protein is Crucial for Immune Defense: Protein provides the building blocks for creating immune cells, antibodies, and for repairing damaged tissues, preventing muscle loss during sickness.
Carbohydrates Provide Rapid Energy: Immune cells use carbohydrates as a quick source of fuel to support their high metabolic demands during an active infection.
Micronutrients Act as Immune Cofactors: Vitamins A, C, D, and minerals like zinc and selenium are essential for various immune functions, and deficiencies can severely weaken your body's defenses.
The "Starve a Fever" Myth is False: Whether you have a cold or a fever, your body needs adequate nutrition and hydration to recover effectively, making the old folklore dangerously inaccurate.
Fluid Intake is Critical: High fluid intake is essential to prevent dehydration, which can be exacerbated by fever and other illness-related symptoms.
Small, Frequent Meals Can Be Best: For those with a reduced appetite, eating smaller, nutrient-dense meals throughout the day is often more manageable and effective than trying to consume larger portions.

The Immune System's Energetic Demands

During an infection, your immune system is activated and works in overdrive, which is an incredibly energy-intensive process. The body prioritizes energy for this immune response, a state sometimes called "sickness metabolism".

The Role of Increased Metabolism and Fever

One of the most obvious signs of your body's increased energy expenditure is a fever. A fever is a controlled increase in body temperature, which helps create an unfavorable environment for viruses and bacteria. To raise and maintain this higher temperature, your metabolism speeds up, burning more calories. In fact, shivering, a common symptom with fever, also burns extra energy to produce heat. In severe cases of infection, like sepsis, the energy expenditure can be exceptionally high, demonstrating just how demanding the immune response can be.

How Immune Cells Use Energy

Immune cells, such as macrophages and lymphocytes, have a high metabolic turnover and require a constant supply of energy to function effectively. Upon activation, they can rapidly reprogram their metabolism to meet these demands. This energy is used for several critical tasks:

Proliferation: The immune system needs to create new white blood cells rapidly to combat the invading pathogen.
Cytokine Production: The body produces cytokines, which are messenger proteins that help immune cells communicate and coordinate the defense response.
Phagocytosis: Phagocytes, a type of immune cell, engulf and destroy pathogens, a process that requires a significant amount of energy.

The Power of Macronutrients

While the body needs overall caloric energy, it also needs specific types of fuel, or macronutrients, to support the immune system.

Protein: The Building Blocks

Protein is perhaps the most crucial macronutrient during illness. It provides the amino acids necessary to build the core components of the immune system.

Antibodies: These specialized proteins are created to target and neutralize specific pathogens.
Immune Cells: Protein is essential for the structure and function of white blood cells.
Tissue Repair: When illness damages tissues, protein is needed for repair and recovery. When your body doesn't get enough protein from your diet during sickness, it can start to break down your muscle and other body tissues to meet this high demand, leading to muscle loss and prolonged weakness.

Carbohydrates: The Preferred Fuel

Carbohydrates are the body's primary and most readily available source of energy. Activated immune cells often switch to a process called aerobic glycolysis to generate energy quickly from glucose. Having an adequate intake of carbohydrates ensures this rapid fuel is available and prevents the body from diverting protein away from its vital functions. Simple, easily digestible carbohydrates found in rice, toast, and crackers can be a good option when appetite is low.

Fats: Regulating the Response

Fats, especially essential fatty acids like omega-3s, play a crucial role in regulating the immune response and inflammation. They are a core component of cell membranes and influence how immune cells communicate and signal. While moderation is key, healthy fats are important for a balanced and effective immune system.

Comparison: Nutritional Needs During Health vs. Illness


Nutrient Type	During Health	During Illness	Key Difference
Calories	Maintain body weight and basal functions.	Increased requirements due to higher metabolic rate, fever, and immune cell proliferation.	Energy is diverted to the immune system.
Protein	Build and maintain muscle mass, tissues, and enzymes.	Increased demand for building antibodies, new immune cells, and repairing tissue.	Greater need to prevent muscle wasting.
Carbohydrates	Primary energy source for daily activities.	Critical fuel source for activated immune cells; supports rapid energy production.	Higher demand, especially for simple carbs when appetite is low.
Micronutrients	Meet recommended daily allowances for normal function.	Higher demands for specific vitamins and minerals that act as cofactors for immune processes.	Specific nutrients (Vit C, D, Zinc) are used more rapidly.
Hydration	Crucial for overall bodily function.	Even more critical to combat fluid loss from fever, sweating, and vomiting.	Prevention of dehydration is a top priority.

The Role of Micronutrients

Beyond macronutrients, a wide array of vitamins and minerals are critical cofactors for the immune system. Deficiencies can severely impair immune function and increase susceptibility to infection.

Vitamin C: A powerful antioxidant that supports the function of white blood cells and helps maintain tissue barriers.
Vitamin D: Plays a key role in regulating both the innate and adaptive immune responses.
Zinc: Essential for the development and function of immune cells. A deficiency can weaken immune response.
Vitamin A: Helps maintain the integrity of mucosal barriers in the gut and respiratory tract, which act as a first line of defense.
Selenium: An antioxidant that supports the production of antibodies and enhances natural killer cell activity.

Practical Dietary Advice When You're Sick

Even with a decreased appetite, which is common during illness, it's vital to focus on easily digestible, nutrient-dense options to provide your body with the fuel it needs. The old adage "feed a cold, starve a fever" is a dangerous myth; proper nutrition is needed in both cases.

Here are some practical tips:

Stay Hydrated: Drink plenty of fluids like water, herbal tea, and broth to prevent dehydration, especially with a fever. Broth can also provide electrolytes and some calories.
Prioritize Easy-to-Digest Foods: When solid foods are unappealing, opt for soft, bland foods. Think of things like scrambled eggs, soups, yogurt, bananas, rice, or toast.
Eat Smaller, More Frequent Meals: Don't pressure yourself to eat large meals. Small, nutrient-rich snacks every few hours can be more manageable and provide a steady stream of energy.
Include Protein: Add lean protein sources like chicken, eggs, or fish to your diet as you're able. Protein shakes can also be a good option for a quick, concentrated source.
Don't Forget Your Gut: The gut is a major site of immune activity. Fermented foods like yogurt or kefir can provide probiotics to support gut health.

Conclusion

In conclusion, the belief that you should restrict calories during an infection is a misconception with potentially harmful consequences. Your body's immune system requires a significant amount of energy, and therefore, a higher caloric and nutrient intake to mount an effective defense. Supporting this process with a balanced diet rich in proteins, carbohydrates, healthy fats, and essential micronutrients is not just helpful—it's crucial for a faster and more complete recovery. Proper nutrition fuels the production of antibodies and immune cells, helps manage fever, and prevents the breakdown of your own body tissue, proving that a well-nourished body is a resilient one.

For more information on the intricate relationship between nutrition and immune function, consult the National Institutes of Health.

Frequently Asked Questions

When you are sick, your immune system becomes highly active to fight off the infection, which is a very energy-intensive process. Your body also burns more calories to create a fever, increasing your metabolic rate significantly and requiring more fuel.

No, the old adage to "starve a fever" is a myth and potentially harmful. Whether you have a fever or a cold, your body needs adequate nutrition and hydration to fuel your immune system and support your recovery.

If you don't consume enough calories and nutrients, your body will be forced to break down its own muscle and fat stores for energy. This can weaken your immune response, prolong your illness, and lead to muscle loss and general weakness.

Focus on easily digestible, nutrient-dense foods and fluids. Options like soups, broths, scrambled eggs, yogurt, toast, rice, and bananas can provide essential calories, protein, and electrolytes even when you don't feel like eating.

Protein is vital during an infection because it provides the amino acids needed to build critical components of your immune system, including antibodies and new immune cells. It also helps with tissue repair and prevents muscle wasting.

Yes, many vitamins and minerals act as essential cofactors for the immune system and are used more rapidly during an infection. It's important to consume adequate amounts of nutrients like Vitamin C, Vitamin D, and Zinc to support your body's defenses.

Good hydration is critical. Options include water, herbal tea, broths, and electrolyte-replenishing drinks. Avoiding caffeinated beverages and alcohol is recommended, as they can contribute to dehydration.