Fighting off a cold does increase the number of calories your body burns, though the bump is smaller than most people assume. Research on men experiencing mild illnesses found roughly an 8% rise in resting metabolic rate during active infection, with some individuals seeing increases above 14%.1PubMed. Toward quantifying the usage costs of human immunity: Altered metabolic rates and hormone levels during acute immune activation in men That extra burn comes mainly from immune cells ramping up their activity, plus the energy cost of raising your body temperature if a fever develops. But the story gets more complicated when you factor in appetite changes, reduced physical activity, and the type of pathogen your body is fighting.
How Much Extra Energy a Cold Actually Costs
For someone with a baseline resting metabolic rate around 1,400 to 1,600 calories per day, an 8% increase works out to roughly 110 to 130 extra calories burned at rest. That is the equivalent of a medium banana or a handful of almonds. A subset of men in the same study burned more than 14% above their baseline, which would push the extra expenditure closer to 200 calories per day.1PubMed. Toward quantifying the usage costs of human immunity: Altered metabolic rates and hormone levels during acute immune activation in men These were mild, naturally acquired illnesses, not severe infections requiring hospitalization, so the numbers offer a reasonable picture of what happens during an ordinary cold or respiratory bug.
An important counterpoint comes from a study that measured resting metabolic rate after influenza vaccination. Despite triggering a measurable immune response, the vaccine did not produce a statistically significant change in metabolic rate at either two or seven days afterward.2PLoS ONE. Effect of influenza vaccination on resting metabolic rate and c-reactive protein concentrations in healthy young adults The takeaway is that the immune system has to be working hard enough, fighting actual replicating pathogens, to meaningfully move the needle on calorie burn. A mild immune activation like a vaccination does not seem to cut it.
Where the Extra Calories Go
Your immune system is not a passive barrier. When it detects a pathogen, it mobilizes an army of cells that need fuel. White blood cells called lymphocytes, for example, dramatically increase their glucose uptake when activated. B cells responding to an infection need so much glucose that roughly 90% of the glucose they consume goes toward producing and assembling antibodies.3Portland Press (Clinical Science). Essential metabolism required for T and B lymphocyte functions: an update T cells go through a similar metabolic shift, switching from a relatively quiet energy state to an active, glucose-hungry mode.
This cellular fuel demand is part of why you feel tired and sometimes crave quick-energy foods when you are sick. Your body is diverting resources toward immune function. The calorie cost is real, even if it does not show up on a bathroom scale in any noticeable way during a week-long cold.
Fever Is the Biggest Metabolic Expense
The single most energy-intensive thing your body can do during an illness is raise its temperature. Generating and maintaining a fever requires extra metabolic heat production, and this has been documented through direct calorimetry going back decades. The magnitude of that increased heat production depends on several factors: how high and how long the fever runs, your nutritional status going into the illness, whether you are taking fever-reducing medications, and even the temperature of the room you are in.4PubMed Central. The metabolic cost of fever
A long-standing rule of thumb in clinical medicine holds that metabolic rate increases by about 10 to 13% for each degree Celsius of fever above normal. For a typical cold, many people run low-grade fevers (if they develop one at all), so the metabolic hit stays modest. Influenza or a secondary bacterial infection that produces higher, more sustained fevers will burn considerably more. This is one reason the flu leaves people feeling so depleted compared to an average cold: the fever alone can push daily calorie burn up by hundreds of calories, on top of what the immune system itself consumes.
The vaccination study mentioned earlier reinforces this connection. The participants who received a flu shot showed increases in C-reactive protein (a marker of inflammation), confirming their immune systems responded to the vaccine, but they did not develop fevers, and their metabolic rates stayed flat.2PLoS ONE. Effect of influenza vaccination on resting metabolic rate and c-reactive protein concentrations in healthy young adults Immune activation without fever appears to cost far less metabolically than immune activation with fever.
Why You Probably Will Not Lose Weight From a Cold
The extra calorie burn from a cold sounds like it should lead to weight loss, but the math rarely works out that way. An extra 100 to 200 calories per day over a five- to seven-day cold adds up to maybe 500 to 1,400 extra calories total. Meanwhile, you are likely spending most of the day on the couch instead of walking, commuting, or exercising, which easily cancels out the metabolic bump and then some. A person who normally burns a few hundred calories a day through daily movement and exercise may actually end up in a net calorie surplus if they maintain their usual food intake while sitting still all week.
There is also the appetite factor. Some people eat less when they are sick (especially when congested, since food tastes bland), but others reach for comfort foods, hot soups with bread, crackers, juice, and other calorie-dense staples that can more than compensate for the modest increase in metabolic demand. The net effect on body weight from a typical cold is essentially a coin flip, driven more by changes in eating and activity patterns than by the immune system’s calorie cost.
Being Cold and Being Sick at the Same Time
There is a reason colds feel worse when it is actually cold outside, and it is not just psychological. Research on house finches infected with a respiratory pathogen found that the metabolic costs of fighting infection and maintaining body temperature in cold conditions were additive. The combined energy cost of thermoregulation and infection together was measured at about 4.71 kilojoules per night, a figure within the range of the daily energy requirements those birds use for growing new feathers during molt.5Functional Ecology. Additive metabolic costs of thermoregulation and pathogen infection In other words, fighting infection while also trying to stay warm is a double hit to your energy budget, not a case where one cost absorbs the other.
For humans, this has practical implications during winter. If you are sick and spending time in a poorly heated home or going outside in the cold, your body has to burn extra fuel both to fight the pathogen and to keep your core temperature stable. This compounding effect helps explain why being bundled up and warm while sick is not just about comfort. It reduces one of the two competing drains on your energy reserves, leaving more for your immune system.
Feed a Cold, Starve a Fever?
The old saying gets trotted out every flu season, and researchers have actually tried to test whether the type of infection should change your eating strategy. The short answer is that the evidence remains thin and inconsistent. A review of nutritional strategies during bacterial versus viral infections concluded that the macronutrient and micronutrient needs during infection depend more on the specific organism involved than on whether the pathogen is a virus or a bacterium. Factors like age, baseline nutritional status, immune health, and the severity and duration of the illness all play a larger role in outcomes than whether you eat more or less.6SpringerLink / Current Nutrition Reports. “Feed a Cold, Starve a Fever?” A Review of Nutritional Strategies in the Setting of Bacterial Versus Viral Infections
From a calorie-burn standpoint, deliberately undereating during a cold is a bad idea. Your immune system needs glucose and amino acids to build antibodies and fuel white blood cells. Severely restricting food during a time of increased metabolic demand can make the calorie deficit worse and may prolong recovery. The practical advice is straightforward: eat when you are hungry, focus on foods that are easy to digest and nutrient-rich, and do not force-feed yourself to “fuel your immune system” either. Your appetite during illness is a decent guide, though staying hydrated matters more than hitting a particular calorie target.
When the Body Slows Down Instead of Speeding Up
Not every infection triggers an increase in metabolic rate. In some cases, the body appears to do the opposite, entering a reduced-energy state that researchers have compared to a form of dormancy. Work in animal models has shown that energy-conserving hypometabolic states can actually promote disease tolerance during bacterial infection, helping the host survive not by killing the pathogen faster but by reducing the damage the immune response does to the body’s own tissues.7Cell. Energy-Conserving Hypometabolic States Promote Disease Tolerance during Bacterial Infection
This is a fundamentally different survival strategy. Instead of ramping everything up and burning through energy reserves, the organism dials down its metabolic rate and waits out the infection. The lethargy and sleepiness that accompany many illnesses may not just be side effects of inflammation; they may be part of a coordinated metabolic downshift that conserves energy for survival. For a mild cold, you are unlikely to enter a true hypometabolic state, but the principle helps explain why fighting illness is not always a straightforward “burn more calories” equation. The body sometimes chooses tolerance over resistance, and the metabolic signature of those two strategies looks very different.
Immune Trade-Offs and Why Rest Matters
The energy your body spends on fighting infection has to come from somewhere, and organisms, humans included, face real trade-offs in how they allocate that energy. Research in insect models has demonstrated this vividly: caterpillars infected with bacteria reduce one type of costly immune response (nodulation, a process where immune cells clump around invading bacteria) when they are able to use behavioral fever, raising their body temperature by moving to warmer spots, instead.8PubMed Central. Trade-Offs among Immune Mechanisms: Bacterial-Challenged Spodoptera frugiperda Larvae Reduce Nodulation Reactions during Behavioral Fever In other words, when energy is limited, the organism picks the cheaper immune strategy and drops the more expensive one.
Humans cannot crawl onto a warm rock to fever-regulate like a caterpillar, but the underlying principle still applies. When you are sick, energy is being diverted toward immune function. Activities that compete for that energy, intense exercise, stressful work, sleep deprivation, all pull from the same pool. This is the bioenergetic case for rest during illness: not that rest itself heals you, but that minimizing other energy demands gives your immune system more resources to work with. The extra 100 to 200 calories your immune system needs may not sound like much, but it represents a real reallocation of metabolic priority, and your body signals that reallocation through fatigue for a reason.
What Happens With More Severe Infections
Everything discussed so far applies primarily to a garden-variety cold or mild respiratory infection. The calorie math changes substantially with more severe illnesses. Hospitalized patients with serious infections can see metabolic rates climb by 30 to 50% or more above baseline, driven by high sustained fevers, systemic inflammation, and the massive energy demands of an immune system in overdrive. The metabolic cost of fever varies with the severity and duration of the insult, and in critically ill patients, the negative energy balance created by increased metabolic demand combined with reduced food intake can lead to significant muscle wasting within days.4PubMed Central. The metabolic cost of fever
This spectrum matters because people sometimes extrapolate from serious illness to a common cold or vice versa. A cold is at the low end of the metabolic cost curve. You burn a bit more, feel tired, and recover within a week. Influenza sits higher. Pneumonia or sepsis can push the body’s energy expenditure to levels that cause real nutritional emergencies. The same immune machinery is involved across all of these, but the scale differs enormously, and so do the practical implications for eating and resting.
Exercise While Sick
A common question that follows naturally from the calorie-burning question is whether you should exercise when you have a cold. The folk wisdom about the “neck check” (symptoms above the neck like a stuffy nose are fine, symptoms below the neck like a chest cough are not) has some logic behind it from an energy perspective. If your immune system is already drawing extra fuel from your metabolic budget, adding the energy cost of a vigorous workout on top of that can leave less for immune function. For a mild cold with no fever, light activity like a short walk is unlikely to be harmful and may even help with congestion. But pushing through a hard run or heavy lifting session when your body is already running a metabolic deficit due to infection is working against your own recovery.
Fever changes the calculation entirely. If your temperature is elevated, your metabolic rate is already cranked up, and exercise adds further heat production on top of that. The combination can push your heart rate and core temperature to levels that feel disproportionately miserable relative to the effort involved. More practically, exercising with a fever extends the recovery timeline in most people’s experience, though controlled studies on this specific question are scarce for obvious ethical reasons (you cannot randomly assign people to exercise through fevers).
The calorie-burn angle occasionally tempts people to view illness as a passive “workout” or an opportunity to lose weight. The metabolic increase from a cold is real but trivial compared to actual exercise, and the side effects, fatigue, reduced appetite, disrupted sleep, and muscle catabolism in more severe cases, make it a lousy weight-loss strategy by any measure. Your body is burning those extra calories to survive, not to improve your physique, and the resources it uses come partly from breaking down your own tissue stores when dietary intake drops.