When you're feeling unwell, you might assume your body is in rest mode, using less energy while you stay in bed. But the reality is more complex. The body’s response to illness is active, not passive and it takes energy to fight off infection. So, do you burn more calories when sick? In many cases, yes. Depending on the type and severity of the illness, your metabolism may increase as your immune system works overtime to get you back to health.

Fever and Calorie Burn

One of the clearest ways illnesses increase calorie burn is through fever. When you have a raised body temperature, your body uses more energy to maintain that higher internal heat. For every degree Celsius your body temperature rises, your resting energy expenditure can go up by around 10 to 12 percent. That means someone with a fever of 39°C could be burning several hundred more calories per day than usual, even if they’re lying in bed.

This increase in energy use is the body’s way of fighting infection. Heat helps slow down the reproduction of viruses and bacteria, and the immune system becomes more active at higher temperatures. However, this process is energy-intensive and leads to more calories being burned, even when you feel like you're doing nothing.

Inflammation and the Immune Response

Even without a fever, your body burns more calories during an illness due to the immune response. Fighting off a viral or bacterial infection involves producing antibodies, activating white blood cells, and running internal systems that repair tissue and regulate inflammation. All of this requires fuel and that fuel is calories.

Conditions like the flu, bronchitis, or strep throat may not cause high fevers but still trigger immune activity that subtly increases your basal metabolic rate (BMR). While the increase may not be huge, it’s still noticeable in the way you feel fatigued, hungry, or drained during recovery.

Loss of Appetite vs. Higher Energy Demand

Interestingly, when you're sick, your appetite often drops, even though your body may be burning more calories. This mismatch is why some people lose weight during extended illnesses. The body is working harder, using more energy than usual, but you’re taking in less food. This can lead to muscle breakdown and slower recovery if the body doesn’t get enough protein and calories to support immune function and tissue repair.

That’s why even if you don’t feel like eating much when you're ill, it’s important to try to consume small, frequent meals, especially those rich in fluids, protein, and micronutrients.

The Type of Illness Matters

Not all illnesses affect calorie burn equally. Viral infections, fevers, and respiratory illnesses typically raise calorie expenditure more than things like mild colds or stomach bugs. Gastrointestinal illnesses, like food poisoning or stomach flu, may lead to dehydration and calorie loss through vomiting or diarrhoea, but don’t always trigger an immune response strong enough to raise metabolism significantly.

Chronic illnesses such as autoimmune conditions or long-term infections can also lead to increased baseline calorie needs over time due to ongoing inflammation and repair processes.

Rest vs. Activity

Although your body might burn more calories internally when you're sick, overall calorie burn may still decrease because your physical activity level drops. You're likely not exercising, moving around less, and spending more time resting. This reduction in movement offsets some of the internal energy increase from illness.

So while you may burn slightly more calories at rest, the total daily energy expenditure might remain the same or only slightly increase. It's not a reliable method of burning calories, nor should it be seen as a weight-loss opportunity.

Should You Eat More When Sick?

If your body is burning more calories during illness, especially with fever then yes, eating more can help support recovery. However, the priority should be quality of food over quantity. Nutritious, easy-to-digest foods that support the immune system such as broths, soups, fruit, toast, eggs, or yoghurt can help fuel recovery without putting stress on the digestive system.

Protein is especially important. Your body may be breaking down muscle tissue for energy, and eating enough protein can help preserve lean mass and reduce fatigue. Hydration is equally essential, especially during fever or diarrhoea.

Short Illness vs. Long Illness: Calorie Burn Works Differently

A short-term illness like a cold or flu can cause a temporary spike in calorie use, especially if there’s a fever. But during longer illnesses like chronic infections, autoimmune conditions, or long COVID, the calorie demands can stay elevated for weeks or even months, slowly draining your energy reserves and muscle mass. That’s why people often lose weight during prolonged illness, even without dieting. It’s not just from eating less, the body is in a constant low-grade fight mode, which uses more energy than usual.

The Role of Shivering and Chills

If you’ve ever had the flu and couldn’t stop shivering, that’s your body burning through calories fast. Shivering is an intense muscular process that generates heat when your body is trying to raise its internal temperature. It’s one of the fastest ways the body increases calorie burn some studies suggest it can double your resting energy expenditure temporarily. Even if the shivering only lasts an hour, that’s a significant bump in energy use.

Weight Loss During Illness Isn’t Always Fat Loss

When you’re sick and eating less, especially during a fever, most of the initial weight loss is water and glycogen, not fat. But if the illness stretches on and you’re still under-eating, your body can start breaking down muscle for fuel. That’s why muscle soreness and weakness are common during recovery and why protein intake is critical even if you’re not hungry. Maintaining muscle takes energy, and when your body is starved for fuel, it goes for muscle first, not fat.

Post-Illness Hunger Is Real

Once you start feeling better, many people report a surge in appetite and that’s no coincidence. Your body is trying to refill the tank and repair tissue damage from the illness. That post-sickness hunger can feel intense, and it’s your body’s way of playing catch-up. This is when nutrient-dense, higher-calorie meals can help you bounce back faster, rather than relying on ultra-processed, low-nutrient snack foods that only fill the gap temporarily.

Calorie Burn Varies by Infection Type

Different types of infections have different metabolic effects. Viral infections, like influenza or COVID-19, often involve higher fevers and inflammation, which drive up calorie use more than bacterial infections. Respiratory illnesses in general like pneumonia or bronchitis tend to use more energy than digestive ones, unless there’s severe dehydration or vomiting involved. Skin infections, UTIs, or sinus infections, by contrast, often have minimal impact on metabolic rate unless complications develop.

Metabolism May Stay Elevated After You Recover

Even after symptoms clear, your metabolism might stay slightly elevated for a few days or even weeks. The immune system continues to clean up, rebuild cells, and restore normal function — all of which burn energy. That’s why some people feel drained for days after a bad cold or flu. It’s not just mental fatigue your body is still working in the background, even if you’ve technically “recovered.”

Recovery Nutrition Is Often Underrated

A lot of people stop worrying about food the moment they feel better but recovery nutrition is just as important as what you eat when you're ill. This is the ideal time to prioritise iron, zinc, vitamin C, vitamin D, and protein, all of which help rebuild what was lost and support immune strength going forward. Skipping this step can lead to feeling run-down for longer than necessary.

Summary

You do burn more calories when sick, particularly if your illness involves a fever or a strong immune response. The body uses more energy to fight off infection, regulate temperature, and repair damaged tissues. However, this increase in calorie burn is often subtle and may be balanced out by lower physical activity. Eating nourishing, protein-rich foods even in small amounts supports your body’s natural healing processes and can help you recover faster. So while being sick might raise your calorie burn, the goal should always be recovery, not energy deficit.

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