How Many Calories Are Burned When You're Sick?

December 20, 2025 •

4 min read

Research suggests that for every 1°C increase in core body temperature, your energy expenditure can rise by approximately 10-13%. When ill, your body works harder to fight infection, which directly influences your metabolism and calorie burn, especially if you have a fever.

Quick Summary

The body's immune system elevates your metabolic rate to combat illness, increasing calorie expenditure, particularly when accompanied by fever. Factors like appetite loss and reduced activity also significantly impact total energy balance during sickness.

Key Points

Metabolism Increases: Sickness, especially with fever, elevates your body’s metabolic rate to power the immune response, causing higher calorie burn.
Weight Loss Mix: Sickness-related weight loss is often a combination of water, muscle, and some fat, as the body uses stored energy during periods of low food intake.
Rest is Vital: While calorie burn increases, physical activity decreases, so focusing on rest and nourishment is more beneficial than calorie counting.
Fever Multiplier: A fever significantly boosts calorie expenditure; a 0.5°C rise can increase your BMR by about 7%.
Nourish for Recovery: Prioritize nutrient-dense foods, fluids, and electrolytes to fuel your immune system and prevent muscle breakdown, aiding a faster recovery.
Appetite Matters: Decreased appetite and reduced food intake are major contributors to the energy deficit and subsequent weight loss when sick.

The Science Behind Calorie Burn When Ill

When you get sick, your body's immune system kicks into overdrive, triggering a complex, energy-intensive response to fight off the invading pathogens. This heightened state of biological activity requires a substantial amount of fuel, which is where the increased calorie burn comes from. The body enters what is sometimes referred to as a "sickness metabolism," prioritizing energy allocation to the immune response over other functions.

The Role of Fever and Inflammation

One of the most significant drivers of increased calorie burn during illness is a fever. A fever is a deliberate act by your body to create a hostile environment for viruses and bacteria, which thrive less effectively at higher temperatures. To raise your core body temperature, your metabolism accelerates, burning more calories to generate the necessary heat.

Fever: For each 0.5°C (0.9°F) increase in body temperature, your basal metabolic rate (BMR) can rise by about 7%. This is why a sustained fever, even a moderate one, can have a noticeable effect on your overall energy expenditure.
Shivering: If you experience chills, your body is using rapid, involuntary muscle contractions to generate heat. Shivering can dramatically increase your calorie burn for short periods, amplifying the effect of the fever.
Inflammation: Beyond fever, the inflammatory response itself is a high-energy process. Your immune cells release chemicals called cytokines and interleukins to orchestrate the defense, and this systemic response requires significant energy.

The Impact of Other Sickness Factors

While fever and the immune response increase calorie burn, other sickness-related factors can influence your overall energy balance, sometimes leading to an overall calorie deficit and weight loss. This isn't always the result of a higher metabolic rate alone.

Key factors that affect your energy balance:

Decreased Appetite: A common symptom of illness is a reduced appetite due to nausea, congestion, or fatigue. Eating less creates a calorie deficit, forcing your body to use its stored energy reserves.
Reduced Physical Activity: Most people become sedentary or bedridden when they are sick, which decreases the calories burned through daily movement. While the resting metabolic rate is higher, the total daily energy expenditure can be lower than an active, healthy day.
Nutrient Absorption: Some illnesses, particularly gastrointestinal ones, can affect your body's ability to absorb nutrients, further complicating your energy intake.

Understanding Sickness-Related Weight Loss

Many people lose weight when they are sick and assume it's simply a sign of burning more calories. However, this weight loss is complex and can include more than just fat loss. In a state of low calorie intake and high energy demand, the body first depletes its glycogen stores. After that, it turns to stored energy, and while some fat is used, the body may also break down muscle tissue to convert its protein into glucose, a process called gluconeogenesis. Losing muscle mass is not an ideal form of weight loss and can weaken the body, potentially slowing recovery. This is why adequate nutrition, especially protein, is crucial during and after illness.

Comparison Table: Calories Burned When Healthy vs. Sick


Factor	Healthy State	Sick (with fever)	Implications
Basal Metabolic Rate (BMR)	Normal, stable rate.	Elevated by approximately 7% for each 0.5°C temperature increase.	The body's energy factory works harder at rest.
Physical Activity Level	Varies based on lifestyle (active, sedentary).	Significantly reduced, often sedentary or bedridden.	Reduced movement can offset some of the increased resting calorie burn.
Energy Source	Primarily dietary intake and readily available glycogen.	Dietary intake often reduced; body uses stored energy (glycogen, fat, and muscle).	Risk of muscle loss and weakened state if not nourished properly.
Immune System Demand	Baseline maintenance levels.	High-demand, requiring significant energy and nutrients to fight infection.	This is the primary driver of the caloric increase.
Appetite	Normal intake.	Often suppressed, leading to reduced calorie consumption.	Contributes to the overall energy deficit experienced during illness.

Nutritional Needs During Illness

Fueling your body with the right nutrients is more important than worrying about calorie burn during sickness. Rest and proper nutrition are the best recovery tools. A varied diet, even if small portions, ensures your immune system has the resources it needs.

Key Nutrients:

Fluids: Staying hydrated is critical to replace fluids lost through sweating, vomiting, or diarrhea. Water, broths, and electrolyte drinks are essential.
Protein: Lean protein sources help preserve muscle mass and support the creation of immune cells. Good options include chicken soup, yogurt, or fish.
Vitamins and Minerals: Vitamins A, C, D, and Zinc are vital for immune function and can become depleted during illness. Consuming fruits, vegetables, and fortified foods can help replenish these stores.

Conclusion: Focus on Recovery, Not Calories

It is true that your body burns extra calories when you are sick, mainly due to the fever and the energy-intensive work of your immune system. However, this is not a healthy or sustainable method for weight loss. The body's priority during illness is healing, and a calorie deficit caused by poor appetite and a higher metabolic rate can lead to the breakdown of precious muscle tissue. Instead of focusing on the numbers, prioritize listening to your body. Provide it with the hydration, nourishing foods, and rest it needs to recover efficiently. For more information on nutritional support during recovery, consult health resources like Cleveland Clinic's nutrition guidelines.

What to Eat and Drink

When dealing with a bug, your appetite might wane, but staying hydrated and getting nutrients is non-negotiable. Here's a list of beneficial options:

Chicken Noodle Soup: Provides fluids, electrolytes, and easy-to-digest protein.
Broths: Excellent for hydration and soothing sore throats.
Ginger Tea: Can help with nausea and provides comforting warmth.
Bananas: Easy on the stomach and provides potassium to replenish electrolytes.
Yogurt with Probiotics: Supports gut health, which is a major component of the immune system.
Avocados: Contain healthy fats and nutrients while being soft and easy to eat.

Remember to choose foods that you can tolerate and focus on small, frequent meals if your appetite is low.

Frequently Asked Questions

For every 0.5°C (0.9°F) increase in body temperature above normal, your basal metabolic rate (BMR) can increase by approximately 7%. This means the number of extra calories burned depends on how high and for how long your fever lasts.

Yes, but the effect is minimal. Your body still requires extra energy to fuel the immune system's activity and repair tissues, even without a fever. However, this increase in calorie burn is generally quite modest compared to an illness with fever.

If you are already at a healthy weight, losing weight when sick can be detrimental. In a calorie deficit, the body may break down muscle for energy, which can prolong recovery. The focus should be on proper nutrition to fuel healing, not weight loss.

This is a myth. The body needs nourishment and hydration for both colds and fevers. Instead of restricting food, listen to your appetite and focus on consuming nutritious, easily digestible foods and plenty of fluids to aid recovery.

The most important nutrients include plenty of fluids and electrolytes to prevent dehydration, along with protein to support immune function and preserve muscle mass. Vitamins and minerals like A, C, D, and Zinc are also crucial for a robust immune response.

Yes, shivering burns calories. It's your body's way of generating heat through rapid muscle contractions. This process requires energy, and in combination with a fever, it further increases your overall calorie expenditure.

When you have a reduced appetite, your calorie intake is lower than usual. This creates an energy deficit, forcing your body to draw from its stored energy reserves (glycogen, fat, and protein), which can contribute to weight loss.