When sick, the internal energy expenditure of your body generally increases. This change relates to the difference between Basal Metabolic Rate (BMR) and Total Energy Expenditure (TEE). BMR is the minimum number of calories needed to maintain basic functions while at rest. TEE is the total energy used daily, including BMR, physical activity, and digestion. During illness, BMR rises due to the body’s defense mechanisms, but TEE is influenced by other factors.
The Energy Cost of the Immune Response
The primary reason for the increase in resting calorie burn is the intense, high-demand activity of the immune system. When the body detects a foreign invader, it initiates an organized and energetically costly defense requiring substantial energy to fuel the rapid production and activation of immune cells. Inflammation is an energy-intensive process demanding significant resources. Immune cells, like neutrophils and macrophages, must quickly multiply and mobilize to the site of infection.
This cellular proliferation and the synthesis of signaling molecules use Adenosine Triphosphate (ATP), the body’s energy currency. This heightened state means the body must reallocate energy to prioritize fighting the illness, naturally increasing the resting metabolic rate even before a fever develops.
Fever and Metabolic Rate
Fever, an elevated body temperature, represents a distinct and quantifiable increase in metabolic rate separate from the general immune response. When the body’s thermostat in the hypothalamus is reset, the body works harder to generate and maintain that new set point through increased heat production, known as thermogenesis.
This temperature-related calorie burn follows a predictable physiological rule. For every one-degree Celsius (1.8 degrees Fahrenheit) rise in core body temperature above the normal range, the basal metabolic rate increases by approximately 13%. This phenomenon is partly due to the van’t Hoff law, which suggests that the rate of chemical reactions, including metabolism, is accelerated by rising temperature.
For example, a two-degree Celsius fever could increase the BMR by about 26%. This additional demand is the cost of maintaining the elevated temperature. Shivering, sometimes associated with fever, represents a further, temporary spike in energy expenditure as muscles rapidly produce heat.
The Impact of Symptoms on Total Calorie Burn
While the BMR rises significantly during sickness, the Total Energy Expenditure (TEE) often does not increase as dramatically, due to compensating behavioral changes. The feeling of fatigue and the desire to rest, collectively known as sickness behavior, are evolutionary mechanisms that conserve energy. By reducing physical activity, the body frees up calories that can then be diverted to the immune system.
Reduced physical activity means a sharp decrease in the energy burned through daily movement, or Non-Exercise Activity Thermogenesis (NEAT). For many people, the calories saved by simply lying on the couch or sleeping for most of the day can nearly negate the extra calories burned by the immune response and the fever. This is why a short-term illness may not lead to significant weight loss despite the heightened metabolic state.
A decreased appetite is another common symptom that affects the overall calorie picture. Eating less reduces the Thermic Effect of Food (TEF), which is the energy required to digest, absorb, and process nutrients. Lower food intake combined with reduced physical activity and the energetic demands of fighting the illness contribute to a negative energy balance, meaning the body is consuming more energy than it is taking in.