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When you catch the flu, a cold, or another infection, you probably notice changes in your body: fatigue, loss of appetite, sweating, or even weight fluctuations. These changes often spark the question, do you burn more calories when sick? The short answer is yes—your body often requires more energy to fight illness—but the long-term impact on weight and health is more complex than most people realize.

Do You Burn More Calories When Sick?

Yes, illness can raise calorie expenditure. When you’re sick, your immune system works harder, and your body temperature may rise. This triggers an increase in your basal metabolic rate (BMR), which is the number of calories your body burns at rest. Research suggests that for every 1°C increase in body temperature, calorie burn can rise by up to 10%.

However, not all sicknesses affect metabolism equally. Some increase calorie needs significantly, while others have only a minimal effect.

How Fever Drives Energy Use

Fever is one of the most important factors in illness-related calorie burn. When your body raises its temperature to fight infection, it spends extra energy. For instance:

• A normal BMR of 1,600 calories per day might rise to 1,760 calories with a fever.
  
  

• High fevers lasting several days can lead to substantial extra caloric expenditure.
  
  

• Children and individuals with higher muscle mass may burn even more during fever episodes.
  
  

This energy use is critical for fighting off bacteria and viruses, but it can also strain your system if you’re not eating enough.

Metabolic Response During Illness

Illness doesn’t just increase calorie burn through fever. Other immune responses also consume energy:

• Production of white blood cells and antibodies
  
  

• Repairing damaged tissues
  
  

• Removing dead cells and waste
  
  

• Regulating inflammation throughout the body
  
  

This explains why people often feel so drained during infection—the body is diverting energy to healing.

Do All Illnesses Burn More Calories?

Not every illness has the same effect. Here’s a breakdown:

• Cold without fever: Minimal calorie increase.
  
  

• Cold with mild fever: Slightly higher energy expenditure.
  
  

• Flu with high fever: Significant rise in caloric burn.
  
  

• COVID-19: Varies by case severity; moderate to high increase possible.
  
  

• Allergies: Rarely increase calorie burn, though they cause other discomforts.
  
  

Why You May Lose Weight While Sick

It’s common to see the scale drop when you’ve been ill for several days. But what’s really happening?

• Fluid Loss: Sweating, fever, or diarrhea reduce water weight.
  
  

• Appetite Loss: Eating fewer calories lowers intake.
  
  

• Muscle Breakdown: Lack of protein may force your body to use muscle as fuel.
  
  

• Increased Burn: Fever slightly raises daily energy expenditure.
  
  

This type of weight loss is often temporary and not equivalent to healthy fat loss.

Real-Life Example

Consider someone with a daily burn of 2,200 calories. If illness raises their burn by 200 calories and they only consume 1,500 calories due to poor appetite, they’re at a deficit of nearly 900 calories per day. Over five days, that’s about 4,500 calories, or a little over one pound of body tissue. But much of that may come from water and muscle, not fat.

Key Factors That Influence Illness-Related Calorie Burn

• Fever severity and duration
  
  

• Immune system strength
  
  

• Nutritional intake during illness
  
  

• Body size and muscle mass
  
  

• Level of physical activity while recovering
  
  

• Hydration status
  
  

Illness Calorie Burn vs. Exercise Calorie Burn

• Illness: Calorie burn comes from internal processes like fever and immune activity.
  
  

• Exercise: Burn comes from controlled activity and supports fat loss, fitness, and health.
  
  

While both increase energy expenditure, only exercise provides long-term metabolic and physical benefits.

Supporting Your Body’s Needs When Sick

When your body is burning more calories due to illness, it’s important to provide support:

• Drink fluids frequently to prevent dehydration
  
  

• Eat light but nutrient-rich foods (broths, fruits, vegetables, whole grains)
  
  

• Include protein to help preserve muscle
  
  

• Get plenty of rest to allow healing
  
  

• Resume exercise gradually after recovery
  
  

FAQs

Do you burn more calories when sick with a fever?

Yes. Fever can raise metabolism by 7–10% per degree Celsius above normal.

Do you burn more calories when sick without fever?

Slightly. The immune system still uses energy, but the increase is smaller.

Do you burn more calories when sick with COVID-19?

In moderate to severe cases, calorie burn may rise significantly due to fever and immune stress.

Do you burn more calories when sick with the flu?

Yes, the flu usually raises metabolism more than a common cold.

Is sickness a good weight-loss strategy?

No. Most illness-related weight loss is temporary and unhealthy, often involving water and muscle rather than fat.

Takeaways

• Illness often increases calorie burn, especially with fever.
  
  

• The immune system uses energy for repair and defense.
  
  

• Most weight loss during illness is short-term, not fat loss.
  
  

• Proper nutrition, hydration, and rest are essential for recovery.
  
  

Final Thoughts

So, do you burn more calories when sick? Yes, but this process is part of your body’s natural defense system, not a method for healthy weight management. Illness-related calorie burn may lead to temporary weight loss, but it comes with downsides like dehydration and muscle breakdown. For long-term fat loss, focus on regular exercise, balanced nutrition, and lifestyle habits rather than relying on sickness to do the work for you.