Malnutrition does not directly cause fever, but it dramatically increases your risk of infections that do. Severe malnutrition is the leading cause of secondary immune deficiency worldwide, and the infections that follow are responsible for an estimated 3.1 million child deaths each year. So while the malnutrition itself won’t raise your temperature, it creates the conditions that make fever-causing illness far more likely.
Paradoxically, malnutrition actually pushes body temperature in the opposite direction. A malnourished body tends to run cold, not hot, because it lacks the fuel to generate normal amounts of heat. Understanding this relationship helps explain why fever in a malnourished person is almost always a sign of infection rather than a consequence of poor nutrition alone.
Why Malnutrition Lowers Body Temperature
Your body produces heat through metabolism. When you’re not getting enough calories or nutrients, your metabolic rate drops as your body tries to conserve energy. This is an adaptive survival strategy: burn less fuel, stretch limited reserves further. But it comes at a cost. Core body temperature gradually falls, and the body loses its ability to warm itself effectively.
Animal studies show this clearly. Fasted subjects develop a progressive decline in core body temperature compared to well-fed counterparts. Their bodies suppress metabolic heat production and shift toward energy-saving behaviors, like seeking warmer environments, rather than generating warmth internally.
Specific nutrient deficiencies make this worse. Iron deficiency impairs thermoregulation through two separate pathways. First, iron-deficiency anemia reduces oxygen transport from the lungs to tissues, which limits the body’s ability to ramp up its metabolic rate in response to cold. Second, low tissue iron decreases the ability of muscles to produce energy for contraction, including the rhythmic contractions of shivering, one of the body’s primary warming mechanisms. Deficiencies in copper and zinc cause similar problems with heat production. When any of these minerals are chronically low, poor thermoregulation is a predictable result.
How Malnutrition Weakens the Immune System
The real connection between malnutrition and fever runs through the immune system. Undernutrition impairs nearly every branch of immune defense, creating gaps that bacteria, viruses, and parasites exploit.
Research shows that malnourished individuals have reduced numbers of almost all major immune cell types. T cells, B cells, and natural killer cells all decline in the bloodstream. Neutrophils and monocytes, the frontline responders that rush to infection sites, drop significantly in both absolute numbers and relative abundance. This isn’t just a matter of having fewer immune cells. The cells that remain work less effectively: T cells from undernourished subjects produce fewer infection-fighting signaling molecules, and their capacity to mount a coordinated response is blunted on a per-cell basis.
The bone marrow itself is affected. Malnutrition impairs the steady-state production of immune cells and, critically, also disrupts the emergency production that normally kicks in when an infection is detected. This means the body can’t surge its defenses when it needs them most. Even after refeeding begins, some of these immune deficits persist, leaving a window of continued vulnerability.
Infections That Follow
With weakened defenses, malnourished people, especially children, are highly susceptible to common infections that a well-nourished immune system would handle more easily. The increased mortality associated with childhood malnutrition is almost entirely driven by elevated risk of death from pneumonia, diarrhea, and bacterial bloodstream infections. These are the illnesses that produce fever.
Micronutrient deficiencies compound the problem. Zinc plays a critical role in maintaining the barrier cells that line the lungs, and its deficiency is considered a direct risk factor for respiratory infections. Vitamin A deficiency affects roughly 250 million children in low- and middle-income countries and is linked to higher rates of illness and death in children under five. When these specific deficiencies overlap with overall caloric shortage, infection risk multiplies.
Pneumonia is one of the most common reasons severely malnourished children are hospitalized, often alongside severe diarrhea. Both conditions produce fever, and both are far more dangerous in a malnourished body than in a well-nourished one.
Fever Looks Different in Malnourished People
Here’s an important nuance: even when a malnourished person does develop an infection, their fever response is altered. Because baseline body temperature is already lower than normal, a fever may not reach the numbers you’d typically associate with infection. In animal studies, fasted subjects injected with bacterial toxins still mounted a fever response, but their peak temperatures were consistently lower than those of well-fed subjects facing the same challenge. The first fever peak was both reduced (about half a degree Celsius lower) and delayed by roughly 45 minutes.
Researchers concluded that fasting doesn’t actually suppress the fever mechanism itself. The body still attempts to raise its temperature in response to infection. But because the starting point is lower and the metabolic machinery for generating heat is impaired, the absolute temperature reached during fever is diminished. This matters practically because it means a malnourished person with a temperature of 37.5°C (99.5°F) could be mounting the same level of immune response that would produce a 38.5°C (101.3°F) fever in someone well-nourished. A “low-grade” reading may actually represent a significant infection.
What This Means in Practice
If you or someone you’re caring for is malnourished and develops a fever, the fever is almost certainly caused by an infection rather than by the malnutrition itself. The malnutrition created the vulnerability, and an infectious agent moved in. This is the classic “malnutrition-infection cycle” that drives so much of the disease burden in undernourished populations: poor nutrition weakens immunity, infection takes hold, the infection worsens nutritional status through reduced appetite and increased metabolic demands, and the cycle deepens.
It’s also worth knowing that the absence of fever in a malnourished person doesn’t mean the absence of infection. Because the body’s heat-generating capacity is compromised, serious infections can be present with little or no temperature elevation. Hypothermia (abnormally low body temperature) is actually more common than fever in severely malnourished individuals and can itself be a sign of overwhelming infection.