Yes, your body burns more calories when you’re sick. A fever alone increases your resting metabolic rate by about 7% for every degree Fahrenheit above normal, and the immune response driving that fever demands additional energy on top of that. For a typical illness with a moderate fever, you could be burning 10% to 25% more calories than usual just lying in bed.
How Fever Raises Your Calorie Burn
The most direct way illness increases calorie expenditure is through fever. Your body deliberately raises its internal temperature to create a hostile environment for invading pathogens, and maintaining that higher temperature costs energy. Research published in JAMA found that heat production rises an average of 13% for each degree Celsius of fever, which works out to about 7.2% per degree Fahrenheit. So if your normal temperature is 98.6°F and your fever hits 101.6°F, your resting metabolic rate jumps roughly 21% above baseline.
To generate that extra heat, your body uses shivering and a process called non-shivering thermogenesis, where specialized fat tissue burns calories purely to produce warmth. Shivering can temporarily push your metabolic rate to two or even five times its resting level during intense episodes, though those peaks are short-lived. The chills you feel at the onset of a fever are your muscles rapidly contracting to generate heat, and that process is surprisingly energy-intensive.
Your Immune System Is an Energy Hog
Fever is only part of the picture. The immune response itself is metabolically expensive. When your body detects a pathogen, immune cells release signaling molecules, primarily IL-1, IL-6, and TNF-alpha. These molecules don’t just trigger fever. They ramp up the production of white blood cells, antibodies, and other defensive proteins, all of which require raw materials and energy to build.
These same immune signals increase resting energy expenditure through several mechanisms at once. They promote the conversion of white fat into a more metabolically active form that generates heat. They stimulate the release of stress hormones like cortisol, which mobilizes stored energy. And they trigger the breakdown of muscle protein to supply amino acids for immune cell production and tissue repair. The net effect is that your body is running in a higher gear across multiple systems simultaneously.
Where the Extra Energy Comes From
Here’s the catch: while your calorie needs go up when you’re sick, your appetite typically drops. That’s not a coincidence. The same immune signaling molecules that cause fever also act on appetite centers in the brain to suppress hunger. IL-1 in particular targets a region near the base of the brain that regulates food intake, actively dialing down your desire to eat. This is why food becomes unappealing when you’re feverish, even though your body technically needs more fuel.
When calorie intake drops and energy demands rise, your body starts pulling from its own reserves. It breaks down stored carbohydrates first, then increasingly turns to fat and muscle protein. During a short illness like a cold or flu, this is manageable. You might lose a pound or two, mostly from water and glycogen, and regain it quickly. But during prolonged or severe illness, the muscle breakdown becomes significant. Critically ill patients can lose substantial muscle mass because stress hormones like cortisol simultaneously accelerate protein breakdown and suppress new protein building.
How Much Extra Are You Actually Burning?
For a common cold without much fever, the increase is modest, probably 5% to 10% above your normal resting rate. That might mean an extra 100 to 200 calories per day, roughly the energy in a banana and a glass of juice. You likely won’t notice this as weight loss.
A flu with a fever of 102°F to 103°F is a different story. Between the fever itself, the immune response, and the energy cost of symptoms like coughing, you could burn 300 to 500 extra calories daily. Over several days, that adds up, which is why people often feel noticeably thinner after a bad bout of flu. Most of the weight lost during acute illness is water, though. Fever, sweating, and reduced fluid intake cause dehydration that accounts for much of what the scale shows.
Severe infections or extended illness push the numbers higher still. Sustained inflammatory states can raise resting energy expenditure well beyond 25% above normal, and the body increasingly cannibalizes its own muscle tissue to keep up with demand.
Why You Lose Weight but Not Much Fat
If you’re hoping illness might at least have a silver lining for weight loss, the reality is less encouraging. The calories burned during sickness come disproportionately from carbohydrate stores and muscle protein rather than fat. Dehydration also shifts how your body fuels itself, pushing it toward burning more carbohydrates and less fat. One study found that dehydrated conditions increased carbohydrate use by about 8% compared to a well-hydrated state.
The muscle lost during illness is metabolically active tissue. Losing it actually lowers your resting metabolic rate after you recover, meaning you burn fewer calories at baseline than before you got sick. Combined with the appetite rebound that typically follows recovery, this can lead to gaining back more fat than you lost. The net result of a significant illness is often a temporary dip on the scale followed by a body composition that’s slightly worse than before: less muscle, same or more fat.
Eating and Recovery
The old advice to “feed a cold, starve a fever” doesn’t hold up well. Because your body is burning more calories across the board when you’re sick, adequate nutrition supports a faster recovery. You don’t need to force-feed yourself, but eating what you can tolerate gives your immune system the raw materials it needs and reduces the amount of muscle your body breaks down for fuel.
Protein matters most during illness, since your body is actively dismantling muscle to harvest amino acids. Even small amounts of protein-rich food help slow that process. Fluids are equally critical because fever and sweating increase water losses substantially, and dehydration forces your body into less efficient metabolic pathways. The fatigue and weakness you feel after recovering from an illness aren’t just from the infection itself. They partly reflect the energy deficit and muscle loss your body accumulated while fighting it off.