Your body burns measurably more calories during an infection, but the answer to whether you should eat more is not a simple yes. Fever alone can push your resting energy expenditure up by roughly 8 to 14 percent, and the immune system’s demand for fuel on top of that makes illness one of the most metabolically expensive things your body does. Yet your appetite typically crashes at the same time, and a growing body of research suggests that suppressed hunger during acute illness is not a bug in the system but a feature. The real story involves a tug-of-war between your body’s increased energy needs and its evolved strategies for fighting off whatever is making you sick.
How Fever and Infection Drive Up Energy Expenditure
Raising your body temperature is expensive. Every degree Celsius of fever increases your energy expenditure by roughly 11 percent, according to measurements taken in febrile patients on controlled nutrition.1PubMed. The metabolic cost of fever That cost comes from the sheer thermodynamic work of generating and maintaining extra heat, plus the behind-the-scenes activity of immune cells ramping up their output. A fever of 39°C (about 102°F) sustained for several hours can raise your baseline calorie burn by a quarter or more compared to your normal resting state.
Even mild infections that don’t produce a dramatic fever still cost energy. A study measuring resting metabolic rate in men who had picked up natural infections found an average increase of about 8 percent, with some participants seeing jumps above 14 percent.2PubMed. Toward quantifying the usage costs of human immunity: Altered metabolic rates and hormone levels during acute immune activation in men These weren’t people laid up in hospitals; they had everyday infections. The takeaway is that mounting an immune response is genuinely costly in calorie terms, even for a run-of-the-mill cold.
Where do those extra calories go? Immune cells dividing rapidly, antibodies being assembled, inflammatory signaling molecules flooding the bloodstream. All of these are protein- and energy-intensive processes. The immune system is sometimes described as a metabolically silent organ when you’re healthy, but when it’s activated it behaves more like a sprinting muscle, suddenly demanding a disproportionate share of available fuel.3Nature Reviews Immunology. Getting enough energy for immunity
Why Your Appetite Crashes When You Need Energy Most
If sickness raises your energy needs, you might expect evolution to have given us ravenous hunger during a fever. Instead, the opposite happens. The nausea, loss of appetite, and general disinterest in food that come with being sick are collectively part of what researchers call “sickness behavior,” a coordinated set of changes driven by inflammatory signaling molecules called cytokines. Fatigue, social withdrawal, sleepiness, and appetite loss all cluster together, and they appear across virtually every species studied, from insects to mammals.
This behavioral package appears to be adaptive rather than accidental. Sickness behavior conserves energy by shutting down activities that aren’t immediately useful for survival, like foraging or socializing, and redirects that energy toward the immune response.4PubMed Central. Depression and sickness behavior are Janus-faced responses to shared inflammatory pathways The appetite suppression piece is particularly interesting because it doesn’t just save you the effort of finding food. It may actively help fight the infection. Reduced food intake during sickness shifts your metabolism away from recently eaten carbohydrates and toward stored fuel, particularly fat, which generates ketone bodies. Those ketone bodies are not just backup fuel; they appear to have direct signaling effects on immune cells.5Nature Metabolism. Fasting as key tone for COVID immunity
In animal models, the metabolic shift that comes with reduced food intake during infection mobilizes amino acids from skeletal muscle to fuel immune activity and ketone production, causing weight loss primarily from lean body mass rather than fat stores.3Nature Reviews Immunology. Getting enough energy for immunity That may sound alarming, and for prolonged illness it genuinely is a problem, but in the short term it’s the body prioritizing immune defense over muscle maintenance. The fact that this trade-off is so consistent across species suggests it works well enough to have been preserved over hundreds of millions of years of evolution.
It Depends on What’s Making You Sick
One of the more surprising findings in this area is that the “right” nutritional approach during illness may depend on whether you’re fighting a virus or a bacterium. A landmark study in mice found that glucose supplementation during influenza infection improved survival, while the same glucose supplementation during a bacterial infection with Listeria actually increased death rates.6Cell. Sickness Behavior Associated with Anorexia Regulates Host Defense by Modulating Cellular Adaptation of Metabolic Stress The effect wasn’t subtle. Blocking glucose utilization was lethal in viral inflammation but protective in bacterial inflammation.
The mechanism appears to involve how cells handle the stress of each type of immune response. During viral infections, glucose helps cells avoid a self-destructive stress response in their protein-folding machinery. During bacterial infections, the body benefits from ketone production, which requires limiting glucose availability, because ketones help mop up damaging reactive oxygen species generated by antibacterial inflammation.6Cell. Sickness Behavior Associated with Anorexia Regulates Host Defense by Modulating Cellular Adaptation of Metabolic Stress In other words, the metabolic environment your body creates by not eating may be part of the weapon system against certain pathogens.
This is a mouse study, and translating it directly to human dietary advice would be premature. But it offers an intriguing framework for understanding why appetite suppression during illness might not always be something to override. The old folk wisdom of “feed a cold, starve a fever” has been around for centuries, and while it was never grounded in molecular biology, it accidentally maps onto something real: colds are usually viral, and fevers often accompany bacterial infections. A small human study found that eating shifted immune responses in one direction while fasting shifted them in the other, each potentially better suited to different types of pathogens.7PubMed Central. Feed a cold, starve a fever? Researchers have hypothesized that the saying may reflect a legitimate behavioral strategy people stumbled on through trial and error long before anyone understood T cells.8Medical Hypotheses. “Starve a fever and feed a cold”: feeding and anorexia may be adaptive behavioral modulators of autonomic and T helper balance
What Your Body Burns Through During Infection
When you don’t eat enough to cover the increased energy demand of illness, your body starts breaking down its own tissues. The most significant casualty is skeletal muscle. During infection, inflammatory cytokines trigger a state of accelerated protein breakdown that sends amino acids to the liver, where they’re used to build acute-phase proteins, fuel immune cells, and support gluconeogenesis.9PubMed. The role of cytokines in the catabolic consequences of infection and injury The result is negative nitrogen balance, which essentially means you’re losing protein faster than you can replace it.
For a short bout of the flu, this muscle loss is minor and you bounce back within a week or two. But for more serious or prolonged infections, the protein deficit accumulates quickly. This is why people who’ve been through a bad illness often notice their strength has dropped noticeably, even after the infection itself has cleared. The muscle was literally consumed as fuel for the immune system.
Sickness-associated anorexia can be understood as a trade-off that makes sense only for acute, short-duration illness.10Metabolism. Starvation and infection: The role of sickness-associated anorexia in metabolic adaptation during acute infection When an infection drags on for weeks or months, or when someone enters illness already malnourished, the adaptive benefits of reduced appetite get swamped by the damage of ongoing tissue wasting. Conditions like cancer, chronic heart failure, and chronic infections can produce a severe wasting syndrome called cachexia, where the combination of appetite loss and metabolic disruption leads to dangerous losses of muscle and fat. At that point, the equation flips entirely and adequate nutrition becomes critical to survival.
Why Forcing Too Much Food Can Backfire
If illness increases calorie needs, it seems logical that pushing more calories should help. But evidence from critically ill patients tells a more complicated story. In intensive care settings, where patients can be fed precisely controlled amounts through tubes, researchers have found that aggressive early feeding often makes outcomes worse, not better. One randomized trial found that patients receiving low-calorie feeding achieved readiness for discharge sooner and had fewer gastrointestinal complications than patients fed at higher calorie levels.11PubMed Central. How to avoid harm with feeding critically ill patients: a synthesis of viewpoints of a basic scientist, dietitian and intensivist A separate observational analysis suggested early full feeding was associated with higher mortality at 28 days.
The harms of overfeeding during acute illness are well documented. Pushing too many calories can cause insulin resistance, impaired liver function, worsened blood sugar control, and increased risk of infectious complications. In severe cases, overfeeding has been linked to delayed recovery from mechanical ventilation and longer ICU stays.12PubMed Central. A New Perspective on Overfeeding in the Intensive Care Unit (ICU): Challenges, Dangers and Prevention Methods The body’s own metabolic adaptations during the acute phase of illness, including the shift toward ketone production and the activation of cellular recycling pathways like autophagy, appear to serve protective functions. Flooding the system with calories suppresses those adaptive responses.13Frontiers in Nutrition. Protective nutrition strategy in the acute phase of critical illness: why, what and how to protect
These findings come from critically ill patients in hospital, and they shouldn’t be extrapolated directly to someone at home with a head cold. But the principle they illustrate is relevant: during the acute phase of illness, more food is not automatically better. Your body has its own metabolic program running, and overriding it with aggressive caloric intake can interfere with immune function and cellular repair processes that depend on a fasted or semi-fasted state.
Iron and Nutritional Immunity
The idea that restricting nutrients can fight infection isn’t limited to calories. One of the body’s oldest immune strategies is iron sequestration: actively pulling iron out of the bloodstream and locking it away in storage tissues so that invading bacteria can’t use it to grow. Iron is essential for bacterial metabolism, and during an acute infection the body deliberately creates a state of iron scarcity in the blood.14PubMed Central. Sequestration and scavenging of iron in infection This process is so fundamental that it’s been found in organisms as distant from us as fruit flies, where the same inflammatory signaling pathways that drive mammalian immunity also trigger iron withdrawal from circulating fluid into fat storage tissue.15PubMed Central. Iron sequestration by transferrin 1 mediates nutritional immunity in Drosophila melanogaster
Researchers call this strategy “nutritional immunity,” and it explains some puzzling clinical observations. People with iron overload conditions are more susceptible to certain bacterial infections, because there’s too much circulating iron for the body to hide effectively.16Molecular Aspects of Medicine. Iron in infection and immunity The practical implication is that loading up on iron supplements when you’re fighting a bacterial infection could, in theory, be counterproductive. Your body is trying to starve the bacteria of iron, and supplementing works against that effort. This doesn’t mean you should avoid iron if you’re genuinely deficient, but it’s another example of how the intuition that “more nutrients equals better healing” oversimplifies a much more nuanced biological reality.
Your Gut Microbiome Gets Involved Too
Reduced food intake during illness doesn’t just change your metabolic state. It reshapes the community of microorganisms living in your gut, and those changes may contribute to fighting infection. In mice, fasting altered the composition of the gut microbiome in ways that strengthened resistance to an invading bacterial pathogen, Salmonella Typhimurium. The fasted mice’s gut bacteria blocked the pathogen from invading the intestinal lining, preventing full-blown gastroenteritis. When the experiment was repeated in germ-free mice that lacked gut bacteria, the protective effect of fasting disappeared, confirming that the microbiome was doing the heavy lifting.17PubMed Central. Fasting increases microbiome-based colonization resistance and reduces host inflammatory responses during an enteric bacterial infection
Interestingly, even respiratory infections change your gut microbiome, and the mechanism is appetite loss rather than the virus reaching the gut directly. Research on mice infected with respiratory syncytial virus or influenza showed that the weight loss during infection was driven by reduced food consumption, and that fasting alone, without any infection, produced similar shifts in gut microbial composition.18PubMed Central. Respiratory Viral Infection Alters the Gut Microbiota by Inducing Inappetence This suggests that the loss of appetite during a respiratory illness isn’t just about redirecting energy from digestion; it’s reshaping the microbial landscape of your gut in ways that may support overall immune readiness.
Practical Advice for Common Illness
Most of the research above comes from animal models and critically ill hospital patients. If you’ve got a cold or a stomach bug, the practical takeaways are simpler than the underlying science might suggest.
For short-term illnesses lasting a few days, trust your appetite. If you’re not hungry, you don’t need to force a full meal. Your body is running its metabolic program, and there’s reasonable evidence that mild caloric restriction during acute illness serves a purpose. Sip fluids, eat when you feel like it, and don’t worry that skipping a day of normal eating will weaken your immune system. The muscle loss from a three-day fever is trivial and rebounds quickly.
That said, complete starvation isn’t the goal either. Some caloric intake, particularly from easily digested sources, helps maintain blood sugar and provides substrate for immune cell production. If you have a respiratory virus, the mouse evidence leans toward glucose and carbohydrates being helpful rather than harmful. Simple foods like toast, broth with noodles, or fruit juice are fine choices and are the foods most cultures have independently converged on as “sick food” for good reason.
Hydration matters more than solid food in the short term. Fever, sweating, and respiratory infections all increase fluid loss, and dehydration impairs immune function and makes you feel worse. Water, broth, electrolyte drinks, and diluted juice all work. The specific fluid matters less than the volume.
Watch for red flags that suggest the short-term playbook no longer applies: illness lasting more than a week, inability to keep any food or fluids down, significant weight loss, or any chronic condition that makes malnutrition a baseline risk. In those situations, getting adequate nutrition becomes a priority that overrides the body’s appetite suppression signals, and consulting a doctor is the right move.
Recovery Is When the Calories Really Count
The phase most people underestimate is the aftermath. Once the infection clears and your appetite returns, your body has repair work to do, rebuilding the lean tissue it burned through, restoring depleted nutrient stores, and returning organ function to baseline. Data from recovery studies suggest that calorie needs during this phase are substantially higher than normal. Research drawing on the landmark Minnesota Starvation Study found that after significant weight loss, a healthy person required roughly 4,000 to 5,000 calories per day for months to fully regain lost muscle mass and weight.19PubMed Central. Tailoring nutrition therapy to illness and recovery While most everyday illnesses don’t produce that degree of wasting, the principle holds: your body needs a surplus during recovery to rebuild what was lost.
This is where protein intake becomes especially important. The muscle breakdown during illness was driven by the need for amino acids, and replacing them requires protein-rich food. Eggs, dairy, meat, fish, legumes, and similar foods help replenish what the immune response consumed. People who go back to their normal eating patterns after a significant illness without accounting for the rebuilding phase often find that their strength and energy take weeks longer to return than expected.
For older adults, the recovery window is especially important. Age-related declines in muscle synthesis efficiency mean that every episode of illness-related muscle loss is harder to reverse. An older person who loses muscle mass to a week of pneumonia may never fully recover that tissue unless they deliberately eat above their normal calorie and protein intake during the weeks after the infection resolves. The common pattern of “eating light” during recovery out of habit or lingering appetite suppression works against the body’s rebuilding needs at exactly the wrong time.
When Illness Becomes Chronic
Everything discussed so far applies to acute illness, the kind that arrives, peaks, and resolves over days or weeks. Chronic illness rewrites the rules. Conditions like cancer, advanced heart failure, chronic obstructive pulmonary disease, and long-term infections like HIV can produce ongoing inflammation that keeps the body in a catabolic state for months or years. The appetite suppression and tissue wasting that make evolutionary sense for a five-day fever become life-threatening when sustained indefinitely. Cachexia, the severe wasting seen in these conditions, is responsible for a significant fraction of deaths in cancer patients and in trauma patients who develop prolonged sepsis.
In chronic illness, the metabolic disruption goes beyond simple calorie deficit. Carbohydrate, fat, and protein metabolism are all altered in ways that resist correction through normal eating.9PubMed. The role of cytokines in the catabolic consequences of infection and injury Appetite alone doesn’t explain the wasting; even patients who eat adequately can continue to lose weight because inflammatory signaling keeps shunting energy toward immune activation and away from tissue maintenance. For people living with chronic inflammatory conditions, working with a dietitian or physician to establish a nutrition plan that accounts for elevated energy expenditure and altered metabolism is far more important than for someone riding out a cold on the couch.