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When you get sick, either due to an infection or illness that lasts for 3–5 days, you may notice some amount of weight loss even though you have not done anything to burn calories.
While this could be because you ate lesser food during the period of illness, the more likely reason is that the body burns more calories than normal when you get sick.
How Does the Body Burn Calories When Sick?
You may develop a fever when you get sick, which is a normal response of the body to fight off the foreign disease-causing microorganisms. Clinically, fever is the elevation of body temperature well above the normal 37°C. (1)
Studies have found that with every single unit rise of fever, energy expenditure increases by 10%. (2)(3) This rise in energy expenditure means the body needs more fuel, which can come in the form of calories.
But if you find yourself averse to eating meals, then the body ends up using stored energy in the form of fat or muscle mass to use as fuel. (4)
Multiple case studies in adults have reported weight loss associated with fever as a result of any illness. (5)(6)(7)
How Is Fever Beneficial?
Even though fever may be unsettling and causes great discomfort, it is essential to human survival. Fever can:
- Act as an alarm to activate immune cells that will fight the infection and destroy the disease-causing microorganisms (8)
- Cause the proteins in the cell of the microorganism to undergo changes so that it cannot multiply in the body to prolong the illness. (1)
Warning Signs Associated With Fever
A certain rise in temperature is fairly tolerable by the body and can be managed with medication. However, an excessive rise or one that doesn’t come down despite medication can be a cause for concern.
Fever is categorized as: (8)
- Low grade: till 38°C
- Moderate grade: 38.1–39°C
- High grade: 39.1– 41°C
- Hyperthermia: Over 41°C, which requires immediate medical attention
Assessing care will depend on how hot or cold you normally may be. (8) However, if you have taken medication and the fever doesn’t reduce within 30 minutes to an hour, you may want to consult a doctor, especially in the case of children.
Can I Recover the Lost Weight After a Sickness?
Yes, recovery after an illness can happen fairly quickly. Ensure you eat a healthy diet with plenty of protein after getting well. Avoid high-calorie foods such as chips, sodas, packaged juices, and deep-fried foods.
Practical Takeaways
- When you get sick, you often get a fever, which is the body’s normal physiological reaction to any foreign pathogen.
- Fever increases the core body temperature beyond the normal 37°C to alert the immune system and also changes the proteins in the microorganism’s cells to prevent them from multiplying and prolonging the illness.
- This rise in temperature increases the energy expenditure of the body.
- Increased energy expenditure burns more calories either via food or by using stored reserves from fat and muscles.
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