When your body is fighting an illness, your metabolism changes and you may find yourself burning more calories than usual. This is because your immune system ramps up to fight infection, which requires extra energy. However, the amount of additional calories burned varies based on the type and severity of illness.
Quick answer
Yes, your body typically burns more calories when you’re sick with certain illnesses like the flu or a fever. This is because your immune system works harder to fight infection, raising your metabolic rate and energy expenditure. However, the increase in calorie burn is usually moderate, not dramatic. Severe infections or illness may actually lower calorie expenditure due to fatigue and appetite loss.
How illness affects metabolic rate and calorie burn
Several factors influence the metabolism changes that occur when you’re sick:
- Fever – Each degree increase in body temperature corresponds to around a 10-13% increase in metabolic rate and calorie burn.
- Immune response – Ramping up the immune system is metabolically demanding and requires extra energy.
- Infection-fighting processes – Increased heart rate, respiration, and circulation during illness raise energy needs.
- Stress hormones – Levels of cortisol, adrenaline, and noradrenaline increase, accelerating metabolism.
Research suggests that common illnesses like colds and the flu may increase daily energy expenditure by about 8-15%. More severe infections like pneumonia may boost calorie burn by up to 40%.
However, studies show calorie burn tends to peak within the first 24 hours of illness and then gradually declines as the body starts to rest and recover. Prolonged, severe illness may eventually lower calorie expenditure due to fatigue and lack of physical activity.
How many extra calories are burned when sick?
It’s difficult to provide exact numbers on how many extra calories are burned when sick since it depends on your baseline metabolic rate as well as the type/severity of illness. However, some general estimates for an average adult:
- Common cold – An additional 100-300 calories per day.
- Influenza (flu) – An extra 300-400 calories daily during acute illness.
- Strep throat – Around an extra 350 calories per day.
- Sinus infection – Up to an additional 500 calories burned daily.
- Bronchitis – Can increase daily calorie burn by 250-350 calories.
- Pneumonia – May burn 500-600 more calories than usual each day.
Again, calorie expenditure tends to decrease and normalize as you recover. But during the peak of illness, the above estimates give a rough idea of the potential extra calorie burn compared to your normal healthy state.
How long does the elevated calorie burn last?
Research indicates the spike in calorie expenditure is temporary and usually lasts:
- 24-48 hours for colds and mild flu.
- 3-5 days for more severe flu, bronchitis, or sinusitis.
- 1-2 weeks for prolonged illnesses like pneumonia.
Some studies have observed up to 21 days of slightly elevated metabolism after severe infections like sepsis. But in general, the majority of extra calorie burn occurs within the first few days of acute illness and then gradually decreases as you recover.
Tips for managing weight and nutrition when sick
Here are some tips to keep in mind regarding your weight and nutrition if you’re burning extra calories due to illness:
- Don’t drastically cut calories – Your body needs fuel to fight infection. Only reduce intake slightly below normal levels.
- Stay hydrated – Drink plenty of fluids like water, broths, and diluted juices.
- Choose nutritious foods – Opt for brothy soups, yogurts, soft fruits and vegetables.
- Supplement if appetite decreases – Try nutritional shakes or electrolyte drinks if eating less.
- Rest and recover – Don’t exercise or overexert yourself, which may prolong illness.
- Manage medications – Be aware some drugs like steroids may increase appetite and calorie intake.
Focus on supporting your body and getting well rather than targeting weight loss during illness. The extra calorie burn is usually temporary and modest in scope.
The bottom line
Your metabolic rate does increase when your immune system is activated by illness, leading to extra calorie burn, especially in the first few days. However, the degree of elevated calorie expenditure depends on the type and severity of infection. Make sure to provide your body adequate fuel and nutrients during illness without emphasizing weight loss or calorie restriction.
Your metabolism will return to normal levels once you’ve fully recovered.
Summary
| Illness | Extra Calories Burned Per Day | Duration of Elevated Burn |
|---|---|---|
| Common Cold | 100-300 calories | 24-48 hours |
| Influenza | 300-400 calories | 3-5 days |
| Strep Throat | Around 350 calories | 3-5 days |
| Sinus Infection | Up to 500 calories | 3-5 days |
| Bronchitis | 250-350 calories | 5-10 days |
| Pneumonia | 500-600 calories | 1-2 weeks |
As shown, the degree of extra calorie burn depends on the type of illness. More severe infections like pneumonia can increase calorie expenditure significantly. But even milder illnesses like colds temporarily rev up metabolism and energy needs.
The duration of elevated calorie burn also depends on illness severity. It lasts only 1-2 days for colds but may persist up to 2 weeks for more serious infections like pneumonia that require longer recovery.
In all cases, the extra energy expenditure is temporary and declines as the illness resolves and the body returns to its normal state.
Factors that influence calorie burn during illness
Several key factors impact how many extra calories your body will burn when you’re sick:
- Severity of infection – More severe illness places greater demand on your immune system and metabolism.
- Type of pathogen – Viruses, bacteria, and fungi provoke different immune responses.
- Location of infection – Respiratory vs gastrointestinal vs skin infections.
- Baseline metabolism – Higher resting metabolism = greater calorie burn response.
- Age – Children and adolescents burn more calories during illness.
- Medications – Antibiotics and other drugs may affect calorie expenditure.
Additionally, the degree of fever and inflammation, hydration status, glucose control, and nutrition intake can all impact the body’s energy needs and calorie burn during sickness.
Changes in appetite and eating habits when sick
Illness often changes your normal appetite cues and eating patterns, which affects calorie intake. Common changes include:
- Decreased hunger signals – Lack of appetite due to nausea, fatigue, or congestion.
- Increased taste sensitivity – Food may taste bland or metallic.
- Difficulty eating – Problems chewing, swallowing, digesting.
- GI issues – Diarrhea, constipation, indigestion.
- Sore throat – Difficulty swallowing due to pain.
- Congestion – Reduced sense of smell and taste.
- Medications – Some drugs suppress appetite or cause nausea.
Due to these factors, food intake often declines during illness, sometimes cancelling out the extra calorie expenditure. However, the drop in calories tends to be temporary until normal appetite returns during recovery.
Does increased calorie burn lead to weight loss when sick?
It’s possible to lose some weight during illness if the spike in calorie burn exceeds the intake decrease. However, multiple factors influence the impact on weight:
- Pre-illness weight – People with more fat reserves tend lose more weight.
- Severity and duration – Mild illness causes minimal loss; severe illness may lead to muscle loss.
- Type of illness – GI infections linked to more weight loss.
- Change in appetite – Bigger intake decline = more weight loss.
- Hydration status – Dehydration can falsely elevate weight loss.
- Medications – Steroids stimulate appetite and weight gain.
Research finds that most weight lost while sick is regained once the illness resolves and normal appetite/metabolism returns. Therefore, illness-related weight loss is usually minor and temporary.
Can certain illnesses lead to increased calorie needs?
Although many common infections like colds and flu raise calorie burn moderately, some more severe illnesses substantially increase the body’s energy needs and requirements for nutritional support:
- Sepsis – Serious bloodstream infections can dramatically accelerate metabolism and calorie expenditure. Providing increased nutritional support has been shown to improve outcomes in sepsis patients.
- Burns – Severe burns significantly elevate calorie needs in order to heal damaged tissue and fight infections. High-calorie nutrition protocols are a critical part of burn patient care.
- Traumatic injuries – Major trauma from events like car accidents amplifies the body’s metabolic demands and calorie requirements during recovery.
- Cancer – Many cancers raise energy expenditure while also impairing appetite and intake. Nutritional support helps prevent cachexia (wasting).
For illness and injuries that substantially increase caloric needs, clinical nutrition strategies are implemented to provide adequate fuel and nutrients during healing and recuperation.
The effects of different types of infections
Here’s a more in-depth look at how some common infections influence calorie burn and metabolic rate:
Colds and flu
Studies show respiratory infections like influenza temporarily increase resting energy expenditure by 8-15%. Calories burned at rest were elevated 13% on day 1 and 8% on day 2 in one flu study.
Strep throat
Research on strep throat, caused by streptococcus bacteria, demonstrated a 12% higher metabolic rate during acute illness compared to after recovery. Calorie burn averaged 329 more calories daily.
Pneumonia
A study in elderly pneumonia patients found resting energy expenditure increased 20% on illness day 1 and remained 14% higher on day 7. Daily calorie burn was elevated by about 500 calories.
Sepsis
Severe sepsis may double resting energy needs. One study found septic patients burned 1000 more calories daily – an increase of 57% from pre-sepsis levels.
Malaria
Research suggests malaria parasites increase metabolism by up to 56% during acute fever episodes. Findings vary based on parasite load and malaria severity.
Tuberculosis (TB)
Active TB is linked to a 29% average increase in resting metabolic rate. More severe lung involvement may correspond to greater calorie expenditure.
HIV/AIDS
Advanced HIV infection can raise resting energy needs 20-30% higher than normal. Opportunistic infections may further increase calorie requirements.
SIRS/sepsis
A systematic inflammatory response to trauma or infection can substantially accelerate metabolism and calorie burn. Providing increased nutritional support has been shown to improve outcomes.
The impact of medications on calorie burn
Certain medications used to treat illness can also influence metabolic rate and calorie expenditure:
- Decongestants like pseudoephedrine are linked to 3-5% higher resting energy expenditure.
- Inhaled beta-agonists such as albuterol may increase calorie burn by 2-15%.
- Corticosteroids used to reduce inflammation and immune response tend to increase appetite and lead to weight gain.
- Antibiotics by themselves don’t appear to significantly impact energy needs or calorie burn.
- Over-the-counter cold/flu drugs don’t substantially influence metabolic rate once fever is accounted for.
So while some respiratory medications may mildly raise calorie expenditure, steroids tend to stimulate appetite and weight gain, counteracting the elevated calorie burn of illness.
Should you exercise when sick?
No, most health experts recommend avoiding exercise while ill with an infection. Reasons to rest and recover include:
- Energy needs are already elevated fighting illness, so exercise is an unnecessary added stress.
- Exercise can worsen and prolong symptoms like fever, fatigue, and body aches.
- Heavy exertion may suppress some immune processes trying to fight infection.
- Respiratory infections raise risks with strenuous cardio exercise.
- Overexertion can lead to exhaustion, poor recovery, and muscle loss.
While light activity like walking is usually fine, most doctors advise taking a full break from strenuous workouts while sick. Let your body’s increased metabolism target fighting the illness rather than trying to also metabolize an intense exercise session.
Key takeaways
- Illness can temporarily increase resting metabolism and daily calorie needs, especially early on.
- But the degree of elevated calorie burn depends on the type and severity of infection.
- The extra energy expenditure tends to peak within 24-48 hours and decline as you recover.
- Factors like fever level, immune response, and medications influence calorie burn.
- Increased calorie needs are most dramatic with serious infections like sepsis or trauma.
- Weight loss is typically minor and reversible once normal appetite/metabolism resume.
- Avoid exercise while sick – rest is critical to conserve energy for healing.
