Fighting a viral infection like COVID-19 triggers a significant physiological cascade that directly increases the body’s energy demand, leading to a higher metabolic rate. The basal metabolic rate (BMR) is the energy required to sustain life at rest, while the resting metabolic rate (RMR) accounts for the energy expenditure of light activity. During an acute illness, the RMR becomes substantially elevated as the immune system shifts into a highly active, energy-intensive state to combat the invading virus.
Metabolic Drivers of Increased Energy Expenditure
The most immediate driver of increased calorie expenditure during COVID-19 is the presence of a fever. Fever is an energetically expensive process, requiring the body to actively raise its core temperature; physiologically, the Resting Metabolic Rate (RMR) increases by an estimated 10 to 15% for every one-degree Celsius rise in body temperature above the normal range.
Beyond the heat generation of fever, the systemic inflammatory response itself demands a large amount of energy. The body is in a state of hypermetabolism, meaning its energy needs are significantly higher than normal resting levels. This hypermetabolic state is fueled by the rapid production, mobilization, and activity of immune cells, such as T-cells and macrophages, which are energetically demanding processes.
The release of pro-inflammatory signaling molecules, known as cytokines, further accelerates metabolic processes throughout the body, increasing the speed of cellular reactions and requiring more fuel. In severe cases of COVID-19, this hypermetabolism is compounded by respiratory distress and the increased effort required for breathing. Critically ill patients may display highly variable and elevated resting energy expenditure that often exceeds standard predictive estimates.
The increased metabolic demand is a double-edged sword because the body often breaks down its own tissues for fuel in a process called catabolism. This is why individuals with severe infections commonly experience a rapid loss of lean muscle mass. The overall energy required to sustain the fight against the virus, even while lying in bed, is substantially greater than the energy expenditure of a healthy person at rest.
Supporting Nutritional Needs During Illness
Given the body’s elevated energy and protein requirements, supporting nutritional needs is essential during the acute phase of the illness. Prioritizing fluid intake is paramount, as fever and increased respiration lead to greater water loss, increasing the risk of dehydration and electrolyte imbalance. Consuming electrolyte-rich fluids helps replace minerals lost through excessive sweating and supports cellular function.
The hypercatabolic state means that the body is actively breaking down protein to meet its energy demands and to supply the building blocks for new immune cells. Therefore, adequate protein intake is necessary to mitigate the rapid loss of muscle mass that accompanies the infection. Easily digestible sources of protein should be offered, even if a person’s appetite is diminished.
Although appetite loss, nausea, and fatigue are common symptoms, the body still requires sufficient calories to prevent a significant energy deficit. Focusing on nutrient-dense foods that are simple to prepare and consume is helpful, including those rich in micronutrients like Vitamin D and Zinc, which support immune cell function. Nutritional support, sometimes involving oral supplements, ensures the body has the fuel it needs to maintain the immune defense and limit tissue wasting.
Post-Recovery Metabolic Adjustments
The metabolic state does not instantly revert to normal once the acute infection subsides. Many individuals experience a period of metabolic adjustment following recovery, which can manifest as persistent fatigue or sluggishness. This lingering exhaustion is sometimes attributed to a temporary hypometabolic state, where the body’s energy-producing organelles, the mitochondria, may not be fully recovered or functioning optimally.
The muscle wasting, or catabolism, that occurred during the acute illness must be reversed through a focused recovery effort. The body requires a sustained intake of protein and calories, combined with physical rehabilitation, to rebuild lost muscle mass and strength. Restoring lean tissue is an energy-intensive process that can take weeks or months after the fever and inflammatory markers have returned to baseline.
For a significant number of people, the metabolic disruption extends beyond the initial recovery phase, contributing to the condition known as Long COVID. Persistent symptoms like chronic fatigue and “brain fog” have been linked to ongoing metabolic dysfunction, including issues with insulin sensitivity and glucose regulation. Insulin resistance, a metabolic state that impairs the body’s ability to use glucose for energy, can persist even after the virus is cleared. This continued metabolic dysregulation underscores the complexity of post-viral recovery and the need for medical consultation if symptoms related to energy balance or fatigue persist.