Losing appetite is a common experience when feeling unwell, whether from a mild cold or a more serious infection. This reduction in the desire to eat, often termed anorexia of illness, is a universal symptom accompanying various sicknesses. While uncomfortable, this response is a complex biological process involving the immune system and the brain.
The Body’s Biological Response
When the body encounters an infection or illness, the immune system initiates a coordinated defense. This involves the release of signaling molecules called cytokines, such as interleukin-1 (IL-1), interleukin-6 (IL-6), and tumor necrosis factor-alpha (TNF-alpha). These pro-inflammatory cytokines, produced by immune cells, travel through the bloodstream to the brain.
Once in the brain, these cytokines interact with specific regions that regulate appetite, such as the hypothalamus. The hypothalamus contains specialized neurons, like the arcuate nucleus, which control feelings of hunger and fullness. Pro-inflammatory cytokines can disrupt the balance of appetite-stimulating and appetite-suppressing signals within the hypothalamus, leading to a decreased desire to eat.
Beyond the hypothalamus, other brain areas also play a role. The bed nucleus of the stria terminalis (BNST) is activated by inflammatory signals. This activation can lead to a reduction in feeding behavior. Neurons in the BNST can inhibit activity in the lateral hypothalamus, a region known to promote hunger, further contributing to appetite suppression.
Neurotransmitters, the brain’s chemical messengers, are also affected. Normally, BNST neurons projecting to the lateral hypothalamus are activated by glutamate, an excitatory neurotransmitter. This activation results in the release of gamma-aminobutyric acid (GABA), an inhibitory neurotransmitter, which calms activity in the lateral hypothalamus, allowing for normal appetite. However, during illness, an immune molecule called IL-18 can interfere with this system by reducing glutamate release, leading to less activation of these BNST neurons, decreased GABA signaling, and ultimately, a loss of appetite.
The Adaptive Purpose
The reduction in appetite during illness is an adaptive response, thought to help the body fight off pathogens. One theory suggests that by reducing food intake, the body conserves energy that would otherwise be used for digestion and nutrient absorption. This conserved energy can then be redirected to fuel the immune system’s heightened activity, allowing it to more effectively combat the infection.
Limiting food intake may also reduce the availability of certain nutrients that pathogens need to thrive and multiply. Some microorganisms depend on specific nutrients from the host, so a temporary reduction in nutrient availability could hinder their growth and replication.
A decreased appetite might also be a way to avoid consuming potentially harmful substances or foods that could exacerbate the illness or introduce new pathogens. In ancestral environments, a general aversion to food during sickness could have offered a protective advantage.
Other Factors Affecting Appetite
While acute infections are a common cause, many other factors can also lead to a reduced appetite.
Medications
Certain medications, such as antibiotics, can disrupt the gut microbiome, which influences appetite. Chemotherapy drugs frequently cause nausea and appetite loss.
Mental Health Conditions
Stress, depression, and anxiety often lead to changes in appetite, with some individuals experiencing a decrease in hunger. These conditions can alter neurotransmitter levels and influence brain regions involved in appetite regulation, impacting the desire to eat.
Chronic Illnesses
Chronic illnesses, such as kidney disease, cancer, or heart failure, frequently involve persistent inflammation. This can continuously trigger the biological mechanisms that suppress appetite, leading to sustained appetite loss and, in some cases, significant weight loss.
Sensory Changes
Sensory changes, such as altered taste and smell perception due to illness or medication, can also make food unappealing.
When to Be Concerned
Although a temporary loss of appetite is a normal part of being sick, there are situations when it warrants medical attention.
Seek medical advice if:
Appetite loss persists for more than a few days (typically beyond three to five days for an acute illness), as it could indicate a more serious underlying issue or lead to complications.
You experience significant unintentional weight loss, defined as a loss of 5% or more of body weight over 6 to 12 months.
Dehydration is a concern, especially if accompanied by vomiting or diarrhea. Signs include inability to keep fluids down, extreme fatigue, dizziness, or reduced urination, which require immediate medical evaluation.
Appetite loss is accompanied by other severe symptoms, such as high fever, severe pain, unexplained bleeding, or difficulty breathing.
Maintaining hydration by sipping water, clear broths, or electrolyte solutions is important even when solid food is unappealing.