Fever is a controlled elevation of the body’s core temperature, orchestrated by the hypothalamus, usually in response to infection or inflammation. This temperature increase is triggered by chemical messengers called pyrogens, which signal the body to raise its internal temperature set point. Maintaining this defense mechanism requires significant metabolic effort, confirming that the body burns more calories to maintain this elevated state.
The Link Between Fever and Increased Calorie Use
A fever significantly increases the body’s overall calorie expenditure, directly impacting the basal metabolic rate (BMR). When a fever is present, this resting energy expenditure rises measurably as the body works harder to combat the pathogen and regulate its temperature. This increase is directly proportional to the temperature rise above the normal 37°C (98.6°F) set point. For every one-degree Celsius increase, the BMR increases by approximately 10 to 15 percent. A rise of one degree Fahrenheit typically corresponds to about a seven percent increase in calorie use.
Physiological Reasons for Higher Energy Demand
The heightened energy demand during a fever is a result of two primary, energy-intensive processes: the enhanced immune response and the active process of thermoregulation. The immune system requires massive amounts of energy to ramp up its defense mechanisms. This includes the rapid production and activation of immune cells, such as leukocytes, T-cells, and B-cells, which are responsible for identifying and neutralizing invading pathogens. Furthermore, the body must produce signaling molecules like cytokines, which coordinate the inflammatory and febrile response.
Energy is also consumed by the body’s effort to reach and maintain the new, higher temperature set point directed by the hypothalamus. To rapidly generate heat, the body employs mechanisms like metabolic activation and involuntary muscle contractions, commonly known as shivering. Shivering is an extremely energy-intensive process, as it uses muscle activity to quickly raise the core temperature.
Beyond these two core processes, the body’s metabolism shifts to meet the high energy cost. The body begins to break down muscle tissue to supply amino acids to the liver, a process called gluconeogenesis, to support the rapid production of glucose. This metabolic change can lead to increased use of body proteins as an energy source, which contributes to the feeling of weakness. The increased metabolic activity also necessitates a higher rate of oxygen consumption, heart rate, and respiratory rate.
Managing Hydration and Calorie Intake During Illness
Given the increased metabolic rate, managing fluid and calorie intake is important to support recovery and prevent further strain on the body. A fever leads to increased fluid loss through elevated respiration and sweating, which is the body’s mechanism to prevent the temperature from rising too high. Maintaining adequate hydration is a primary concern, often requiring more than the typical daily water intake to replace lost fluids and electrolytes.
The increased caloric demand means that the outdated advice to “starve a fever” can be counterproductive, as the body needs fuel to support its immune function. Even with a decreased appetite, it is important to consume small, frequent amounts of nutrient-dense, easy-to-digest foods. Broths and soups are excellent choices, as they provide both hydration and calories, along with easily digestible nutrients. Ensuring a sufficient intake of protein and carbohydrates helps prevent the body from excessively breaking down its own muscle tissue for energy. Fluids containing electrolytes, such as sports drinks, help maintain the mineral balance lost through perspiration.