When the body encounters an infection or illness, it initiates an energy-intensive response, leading to a temporary increase in the number of calories burned. This increased expenditure is distinct from the Resting Metabolic Rate (RMR), which is the baseline amount of calories needed to perform fundamental life-sustaining functions like breathing and maintaining organ function. While RMR is typically stable, disease signals the body to divert and increase energy resources to the immune system, effectively raising the total metabolic demand. This metabolic shift is driven by specific biological mechanisms, the severity of the illness, and the nutritional support required for recovery.
The Immune System and Increased Energy Demands
The primary reason for increased calorie burn during sickness is the mobilization of the immune system, which requires substantial energy to operate effectively. Fighting off a viral or bacterial invasion is a highly metabolic process involving rapid cell production and complex signaling. The body must quickly generate and deploy millions of specialized immune cells, such as white blood cells, requiring significant caloric input for their proliferation and activation.
This energy drain is further amplified by the body’s inflammatory response, a coordinated effort to isolate and eliminate the pathogen. Inflammation involves the release of signaling proteins called cytokines, which direct immune cells and alter the body’s internal environment, all demanding energy. Immune cells, particularly activated T cells and macrophages, switch to demanding metabolic pathways, such as increased glucose consumption, to fuel their rapid functions.
A significant element of elevated calorie expenditure is the presence of a fever, which is the body’s attempt to create an inhospitable environment for pathogens. Maintaining a higher core body temperature requires the body to speed up its internal chemical reactions, inherently burning more energy.
Illness Severity and Metabolic Rate Fluctuations
The extent to which calorie burn increases is highly variable and depends on the specific nature and intensity of the disease. While a mild common cold may cause a slight, short-lived elevation in energy expenditure, the metabolic impact is minimal. However, illnesses accompanied by high fever or systemic infection trigger a substantial and measurable increase in energy use.
Research indicates that for every one degree Celsius (1°C) rise in core body temperature, the resting metabolic rate can increase by approximately 10 to 13%. A high fever sustained over multiple days can lead to a significant cumulative increase in caloric demand. More severe conditions, such as influenza or bacterial pneumonia, often lead to a prolonged hypermetabolic state, sometimes raising the resting expenditure by 20% or more.
While the metabolic rate at rest increases, the total daily calorie expenditure might not rise dramatically for every person. This is because severe illness often causes extreme fatigue and reduced physical activity, meaning the energy saved from lying still can offset some of the internal metabolic increase. Nevertheless, the energy required for the body’s self-repair mechanisms remains elevated, creating an internal energy deficit if not supported by adequate intake.
Fueling Recovery: Nutritional Strategies
Given the increased internal demand for energy and the potential for a calorie deficit, maintaining adequate nutritional intake is important for a successful recovery. If the body does not receive enough calories to meet the needs of the immune system, it may begin to break down muscle tissue for protein and energy, a process known as catabolism. Prioritizing consistent caloric intake helps prevent muscle loss and provides the fuel needed for the immune response without compromising other body functions.
Macronutrient balance is important during this time, as protein needs often increase significantly to support tissue repair and the production of new immune cells. Daily protein requirements can rise from baseline levels to over 0.8 grams per kilogram of body weight, and even higher in severe illness, to maximize muscle protein synthesis and support recovery. Complex carbohydrates should also be consumed to provide a steady supply of glucose, the preferred fuel source for many activated immune cells.
Hydration is also a key component of the recovery strategy, especially when a fever is present. Elevated body temperature and increased respiratory rate accelerate fluid loss, making dehydration a concern. Consuming plenty of fluids, including water, broth, or electrolyte-rich beverages, helps regulate body temperature, aids in transporting nutrients, and supports overall physiological function.