The Truth Behind the Myth: Does Your Body Burn More Calories When Sick?

October 1, 2025 •

4 min read

Research indicates that for every 1°C increase in body temperature, a person's energy expenditure can rise by 10-13%. So, does your body burn more calories when sick? The short answer is yes, as your immune system mobilizes to fight off infections.

Quick Summary

The body's metabolic rate increases during illness, particularly with a fever, as the immune system demands more energy to fight infection. This heightened energy need underscores the critical importance of proper hydration and nutrient intake for a strong recovery.

Key Points

Immune System Activation: When you are sick, your body's immune system activates and works harder, requiring extra energy in the form of calories to produce immune cells and combat pathogens.
Fever Increases Metabolic Rate: A fever significantly increases your basal metabolic rate (BMR) because your body requires more energy to raise and maintain a higher core temperature.
Nutrient Intake is Critical: Despite a potential loss of appetite, consuming enough calories and nutrients is essential to prevent muscle breakdown and provide the necessary fuel for your immune system to function effectively.
Hydration is Vital: Increased fluid loss from fever and sweating makes staying hydrated crucial for all illness and recovery.
Recovery, Not Weight Loss: The temporary increase in calorie burn is a byproduct of healing, not a sustainable or healthy strategy for weight management. Focus on recovery, not on exploiting illness for weight loss.
Muscle Preservation is Key: During illness, the body can break down muscle protein for energy if calorie intake is too low. Eating protein-rich foods helps to preserve muscle mass.
Severity of Illness Matters: The number of extra calories burned depends on the severity and duration of the illness; a fever will have a more significant effect than a mild cold.

The Energetic Cost of Fighting Infection

When you are under the weather, your body initiates a sophisticated immune response to combat invading pathogens like viruses and bacteria. This process is highly energy-intensive and leads to an increase in your basal metabolic rate (BMR), which is the number of calories your body needs to perform its basic life-sustaining functions. Your body enters what scientists call a "sickness metabolism," a state in which energy is strategically redirected to the immune system to optimize the body's fighting capability.

The energetic cost is not just about immune cell production. When your body fights an infection, it's a systemic effort that impacts many different processes. This includes upregulating cellular activities, promoting healing, and responding to inflammatory signals—all of which require a significant caloric investment. The intensity and duration of this increased calorie burn are highly dependent on the severity of the illness.

The Role of Fever and Inflammation

Among the most calorie-intensive aspects of an illness is fever. A fever is not an accident; it is a deliberate and evolutionarily conserved strategy by the body to create a hostile environment for pathogens. To elevate your core temperature, your metabolism goes into overdrive. For every 1°F increase in body temperature above normal, your BMR can increase by about 7%. For a person with a BMR of 2,000 calories, a 4°F fever could lead to an extra 623 calories burned in a day.

Another metabolic booster is inflammation. When the immune system is activated, it releases chemical messengers to orchestrate the defense. This inflammatory response requires additional energy to maintain, contributing to the overall increase in metabolic activity. Symptoms like shivering, which is the body's attempt to generate heat through rapid muscle contractions, can also temporarily increase calorie burn significantly.

Why You Need to Eat When You're Sick

Contrary to the centuries-old saying, "starve a fever," modern science has shown that adequate nutrition is crucial for recovery. Your body requires an increased supply of nutrients and calories to fuel its fight against illness. When appetite is low, and calorie intake drops below the body's elevated needs, it is forced to tap into its energy reserves.

Unfortunately, during severe infections, up to 30% of the calories burned can come from the breakdown of protein, including muscle tissue. This is different from healthy fat loss and can lead to a loss of muscle mass, making recovery slower and potentially compromising the immune system further. Proper protein and calorie intake help preserve muscle mass and support immune function during illness.

Comparison of Calorie Use

To put the metabolic shift during sickness into perspective, here is a comparison of calorie usage in different health states.


Factor	When Healthy	When Sick (Mild Cold)	When Sick (High Fever)
Immune System Activity	Normal baseline energy use.	Increased activity requiring additional calories.	Significantly increased, requiring a substantial energy boost.
Metabolic Rate	Standard basal metabolic rate (BMR).	Slightly elevated due to immune response and symptoms like coughing.	Significantly elevated (e.g., 7% increase per 1°F).
Energy Source	Fuel from food and modest use of fat reserves.	Primarily from food, but may tap into reserves if intake is low.	High demand; may utilize significant protein from muscle if calorie intake is insufficient.
Physical Activity	Regular daily activities contribute significantly to calorie burn.	Often reduced due to lethargy and fatigue, which lowers overall calorie expenditure.	Severely limited (bed rest).
Primary Goal of Metabolism	Maintain daily functions and energy balance.	Focus on mounting an effective defense against pathogens.	Prioritize survival functions and defense against severe infection.

Practical Nutrition and Recovery Tips

Supporting your body with proper nutrition during illness is vital. Here are some practical tips:

Prioritize hydration: Drink plenty of fluids, including water, broth, or herbal teas, to combat the fluid loss that occurs with fever and sweating. Electrolyte drinks can also be beneficial.
Eat small, frequent meals: If your appetite is low, try eating smaller portions more often instead of three large meals.
Choose easy-to-digest foods: Opt for nutrient-dense foods that are gentle on your stomach, such as broths, soups, and the BRAT diet (bananas, rice, applesauce, toast) if experiencing stomach issues.
Focus on protein: Consume adequate protein to help preserve muscle mass, which can be broken down during illness.
Embrace comfort foods, but with caution: A bowl of chicken soup can be both soothing and nourishing, providing protein, electrolytes, and hydration. However, avoid excessive sugar and greasy foods, which can cause inflammation.
Get plenty of rest: Rest allows your body to focus its energy on fighting the infection rather than other activities. Forcing yourself to be active while sick can hinder recovery.

The Final Word on Illness and Calories

Yes, your body burns more calories when sick, but this is a sign of your immune system working hard, not an opportunity for weight loss. Attempting to restrict calories during illness is counterproductive and can prolong recovery and lead to unhealthy muscle loss. The focus should always be on nourishing your body to help it heal as effectively as possible.

For more insight into the complex physiological processes that occur during illness, consult scientific literature on the topic, such as this review on sickness metabolism: The immunology of sickness metabolism. When you feel unwell, remember that the most effective way to help your body is by providing it with the energy it needs through proper nutrition, hydration, and plenty of rest.

Conclusion

In summary, the body's metabolic rate increases during sickness to fuel the immune system's fight against infection. This effect is most pronounced with a fever but also occurs with milder illnesses. Understanding this physiological process is key to supporting your body effectively. By focusing on proper nutrition, hydration, and rest, you can provide your body with the resources it needs to recover and get back to full health. Relying on illness for weight loss is dangerous and undermines the healing process.

Frequently Asked Questions

Yes, having a fever burns a significant number of additional calories. Your body's metabolism increases by approximately 7% for every 1°F increase in body temperature above normal, as it expends more energy to generate heat and fight infection.

Yes, you should try to eat enough to fuel your body's recovery, especially since your metabolic rate is higher during illness. Focusing on nutrient-dense foods and staying hydrated is more important than forcing a high volume of food if your appetite is low.

A cold generally results in a minimal increase in calorie burn compared to the flu. This is because flu symptoms often include a fever, which dramatically increases metabolic rate, while a cold typically does not.

No, it is neither a safe nor sustainable weight loss strategy. The weight loss you experience is often a combination of fluid loss and muscle breakdown, which can harm your long-term health and slow down your recovery.

You feel tired because your body is diverting a large amount of energy to the immune system to fight the illness. This leaves fewer resources for other activities, leading to lethargy and fatigue.

Nutrient-dense, easily digestible foods like chicken soup, bone broth, fruits high in vitamin C, and simple carbohydrates like toast are excellent choices. These foods provide the necessary fuel without taxing your digestive system.

No, a low appetite is common when ill, but you should not avoid eating entirely. Focus on consuming smaller, more frequent meals and prioritize fluids. If your low appetite persists for several days, consult a healthcare provider.