When the body encounters an infection or injury, it requires a substantial shift in energy use. Metabolism is the complex process by which the body converts food into the energy required to power every cellular function. This process is highly dynamic, constantly adjusting in response to internal and external demands. The body’s response to an invading pathogen or tissue damage requires an immediate shift in how and where energy is allocated.
The Metabolic Cost of Illness
Being sick causes the body to burn extra calories compared to its resting state. This increased energy usage is reflected in a temporary rise in the resting metabolic rate (RMR), the number of calories burned while the body is at rest. This acceleration occurs because the body is actively working to maintain internal stability and fight off the illness.
The magnitude of this increase correlates directly with the severity of the condition. For a common cold, the increase in RMR may be modest. However, in cases of severe infection or significant injury, metabolic demands can increase dramatically, sometimes by 20% to 60% above the normal resting rate.
A significant factor influencing the energy spike is the presence of a fever. For every degree Fahrenheit the body temperature rises above its normal set point, the metabolic rate increases by approximately 7%. This higher rate reflects the heightened activity taking place internally as the body mobilizes its defenses.
Mechanisms Driving Increased Energy Use
The primary driver of the increased metabolic rate is the activation and sustained function of the immune system. Launching an effective defense against pathogens is an energy-intensive process requiring the rapid creation of new immune cells, such as specialized leukocytes. These cells also produce signaling molecules, known as cytokines, which coordinate the immune response, and this synthesis consumes considerable energy.
Inflammation, a necessary defense mechanism, is a high-energy process that contributes significantly to the caloric burn. Immune cells switch to a less-efficient, faster pathway called aerobic glycolysis to generate the rapid cellular energy supply they need. This metabolic shift ensures the immune response is swift but results in higher overall energy expenditure.
Maintaining an elevated body temperature during a fever requires continuous energy expenditure to hold the new, higher thermal set point. The heart also works harder to circulate blood faster, often resulting in a higher heart rate, which further increases caloric demand. These combined efforts—immune cell production, cytokine signaling, inflammation, and temperature regulation—are the core machinery demanding extra fuel.
Fueling the Recovery Process
Since the body burns more calories while sick, ensuring adequate fuel intake is necessary to support recovery. Despite the common loss of appetite accompanying illness, maintaining sufficient caloric intake prevents the body from breaking down its own tissues for energy. When energy intake is too low, the body uses stored protein, which can lead to muscle wasting and slower recovery.
Along with adequate calories, protein intake is important because protein is the building block for new immune cells and tissue repair. Protein requirements can increase substantially during illness, and adequate consumption helps preserve muscle mass and support immune function. For individuals with severe illness, the need for protein can be over 1.2 grams per kilogram of body weight daily.
Hydration is a major consideration, as illness often increases fluid loss through fever, sweating, or symptoms like vomiting and diarrhea. Dehydration can strain the body and slow metabolic processes. Consuming sufficient fluids and electrolytes is important to maintain metabolic function and support recovery.