How Many Calories Does a Fever Burn?

Fever is your body's way of fighting infection. To make your body temperature go up, your metabolism goes on overdrive, which requires calories. How many calories a fever burns depends on several factors that are individual to you, so it varies from person to person.

While a fever does burn calories and increase metabolism, the exact number of calories burned depends on your basal metabolic rate, and, therefore, your genetics, sex assigned at birth, weight, height and body composition, among other factors.

You might think having a fever is bad for you, but that's not true. According to the U.S. National Library of Medicine (NLM), a fever is your body's way of trying to kill invading viruses or bacteria. These invaders do not survive as well at higher body temperatures, so your body raises your temp to fight them. Fever also triggers your immune system into action, per the NLM.

The average normal body temperature is about 98.6 degrees Fahrenheit (37 degrees Celsius). A fever is present when your body temperature is at 100.4 F (38 C) or higher, according to Harvard Medical School.

When your body's defense system detects a foreign invader like a virus, an area of your brain called the hypothalamus is alerted to raise your body temperature, like turning up the thermostat in your home, according to the Mayo Clinic.

That increase in the thermostat requires energy from your body's furnace, called your metabolism, says Oscar Morey Vargas, MD, an endocrinologist at the Cleveland Clinic.

"Body temperature has an effect on the number of calories burned," Dr. Vargas says. "Fever, for example, is associated with higher metabolic requirements." So, when you have a fever, your body is working harder.

​Read more:​ How to Get an Accurate Temperature With a Digital Thermometer

Your metabolism is the process your body uses to make energy from the calories you eat or drink, according to the Mayo Clinic. Oxygen combines with these calories to release energy.

But your metabolism is unique to you, per the Mayo Clinic, and it's based on a calculation called your basal metabolic rate (BMR), or the energy your body needs to carry out basic functions.

How many calories that requires depends on many factors, and everybody's BMR is different. According to the the American Council on Exercise (ACE), your BMR is affected by your:

• Genes
• Sex assigned at birth
• Age
• Weight
• Height
• Body composition
• Glands
• Diet
• Exercise habits

The bigger your body, the more calories you burn at rest, according to the Mayo Clinic.

People assigned male at birth (AMAB) tend to burn more calories than people assigned female at birth (AFAB) because people AMAB tend to have more muscle. Similarly, older adults tend to burn fewer calories because they lose muscle over time.

As far as your metabolism and fever, "when patients are having fever, they may start shivering for the purpose of rapidly increasing the production of heat by the muscles," Dr. Vargas says. "When this occurs, further increments in energy consumption are expected."

All those energy needs mean nutrition needs. You should forget about the old saying "feed a cold and starve a fever," according to NorthShore University HealthSystem.

The right way to eat when you're sick is to feed them both. A cold needs calories to help your immune system stay strong, and a fever is using up calories your body needs.

According to Harvard Medical School, that means drinking plenty of fluids to avoid dehydration and eating foods that are easy to digest.

If you're looking for ideal foods, NorthShore recommends trying some chicken soup for calories and fluid replacement for hydration and to help keep mucus loose and draining.

​Read more:​ The 7 Best Foods to Eat When You Have a Fever (and 3 to Avoid)