The word “calorie” represents a unit of energy used by the body to power fundamental life-sustaining processes like breathing, circulation, and cell maintenance. This baseline energy expenditure is formally known as the Basal Metabolic Rate (BMR) or the more commonly measured Resting Metabolic Rate (RMR). RMR accounts for the energy used while the body is at rest. When the body encounters an infectious illness, this foundational metabolic rate is significantly altered.
The Body’s Heightened Energy Demand During Illness
Yes, the body often requires substantially more energy than its baseline resting rate when sick. Fighting off an infection forces the body into a hypermetabolic state where energy consumption rises dramatically, proportional to the severity of the illness and the intensity of the defensive response.
For a mild illness, the Resting Metabolic Rate may increase moderately. In cases of severe infection, trauma, or widespread inflammation, energy expenditure can increase by 40% or more above the normal RMR. This metabolic shift mobilizes resources for survival and repair, requiring the breakdown of stored energy to fuel intense cellular activity.
The increased energy demand is driven by the activation and proliferation of immune cells. The inflammatory response requires substantial energy to initiate and maintain, signaling the body to prioritize defense mechanisms and elevating the overall rate of calorie consumption.
Specific Physiological Processes That Increase Calorie Burn
The generation and maintenance of a fever is one of the most potent drivers of increased calorie burn during illness. The body intentionally raises its core temperature (thermoregulation) to create an environment less favorable for pathogens, requiring significant energy to generate and maintain the heat.
For every one-degree Celsius increase in body temperature, the metabolic rate can rise by approximately 10% to 13%. This energy cost is associated with the muscular activity of shivering and the general increase in cellular activity.
The immune system’s cellular response is highly energy-intensive. The rapid production and mobilization of white blood cells, such as neutrophils and lymphocytes, demand a substantial supply of energy. Signaling molecules, like cytokines, which coordinate the inflammatory response, also contribute to the increased caloric burn.
Illness places a greater workload on major organs, increasing energy consumption. The heart and lungs work harder to deliver oxygen and nutrients to immune cells and clear metabolic waste. To support these accelerated demands, the body breaks down stored fat and muscle protein to create new glucose (gluconeogenesis), adding to the total energy expenditure.
Meeting Increased Nutritional Needs While Sick
Given the body’s hypermetabolic state, maintaining adequate caloric and nutrient intake becomes paramount, even when appetite is suppressed. A failure to consume enough energy forces the body into a catabolic state, where it breaks down its own muscle tissue for fuel. This muscle wasting can significantly delay recovery and contribute to overall weakness.
Consuming protein is particularly important because it provides the amino acid building blocks necessary for immune cell production and tissue repair. Individuals with severe illness may require protein intake exceeding 1.2 grams per kilogram of body weight to support intense healing and defense processes. Prioritizing protein-rich foods helps to mitigate the loss of lean muscle mass during the recovery period.
Hydration is also a major concern, as fever and increased respiratory rate can lead to significant fluid loss. Drinking plenty of fluids, including water and electrolyte-containing beverages like broths, helps to maintain blood volume and support the heightened metabolic processes. Broths offer the dual benefit of providing both hydration and some calories and protein in an easily digestible form.
To manage a low appetite, consuming small, frequent meals throughout the day is generally more effective than attempting large meals. Focus on nutrient-dense foods that are easy to tolerate, such as soups, soft fruits, and simple sources of carbohydrates, to ensure a steady supply of energy. Supporting the body with consistent nutrition allows it to focus its remaining energy on fighting the infection and facilitating a quicker return to health.