The common experience when ill is a loss of appetite, often called the anorexia of infection, yet some people find themselves suddenly hungry or intensely craving specific foods. This phenomenon is a normal, though less-discussed, response to the physiological stress of being sick. The body is initiating a complex biological and psychological response to mobilize energy stores and seek comfort. This increased hunger occurs because illness forces the body to dramatically shift its energy demands and hormonal signaling.
The Metabolic Cost of Fighting Infection
Fighting off an illness requires a substantial increase in the body’s energy expenditure. The immune system is highly energy-intensive, and activating its full defense mechanisms is a costly endeavor. This heightened activity translates directly into an increased basal metabolic rate (BMR), which is the number of calories the body burns at rest.
The most dramatic metabolic increase occurs when a person develops a fever. For every 1 degree Celsius rise in core body temperature, the metabolic rate can increase by 10% to 12.5%. This extra heat is produced through processes like shivering and the release of hormones, such as norepinephrine, to stimulate thermogenesis. This energy powers the immune cells, including white blood cells, which must rapidly replicate and travel to sites of infection.
The body’s energy needs also change in terms of fuel source. While healthy, the body primarily uses glucose, but during a severe infection, metabolism may switch to breaking down fat and protein stores for energy. This breakdown of protein, particularly muscle tissue, highlights why consuming calories while sick is important. The demand for readily available fuel creates a genuine caloric deficit, which the body signals by increasing the sensation of hunger.
Hormonal Drivers of Increased Appetite
The body translates the metabolic demand for fuel into the conscious sensation of hunger through specific hormone signals. One such signal is Ghrelin, often called the “hunger hormone,” which is typically released from the stomach and stimulates appetite. While the immune response can sometimes suppress Ghrelin, the physiological stress of illness and the energy deficit can cause its levels to rise, urging the body to eat.
Another major player is Cortisol, a glucocorticoid hormone released by the adrenal glands in response to physical and psychological stress, including illness. Cortisol stimulates appetite, specifically for highly palatable, energy-dense foods high in fat and sugar. This hormonal push for quick energy is a survival mechanism, as these foods provide the most calories per bite to meet the high metabolic demand.
The elevation of Cortisol during sickness may explain intense food cravings for specific items that deliver rapid caloric intake, beyond general hunger. This hormonal shift, coupled with an imbalance in other appetite-regulating hormones, creates a powerful biological drive to consume “comfort foods.” The endocrine system is effectively overriding usual checks and balances to prioritize energy acquisition for survival and recovery.
The Role of Stress and Comfort Eating
Beyond the purely physical and hormonal drives, psychological factors contribute significantly to increased eating during illness. Being sick is a form of acute physical stress, which can trigger emotional or comfort eating as a coping mechanism. The anxiety, discomfort, and malaise associated with illness motivate a person to seek familiar sources of relief.
Many people have a learned association between certain foods and feelings of care or well-being, often rooted in childhood memories of being looked after while ill. Consuming these specific foods can provide a temporary emotional boost and a sense of psychological comfort. This expectation of a positive emotional outcome is a powerful driver, separate from the body’s metabolic need for calories.
Confinement due to illness often leads to boredom, which is a known trigger for increased food intake. When usual activities are restricted, eating becomes an accessible source of stimulation and reward. This behavioral response, combined with the body’s genuine physiological need for fuel, completes the picture of why appetite can increase when fighting an infection.