The arrival of the winter season has brought a rise in flu cases in England, with reports indicating a higher incidence last month than seen in the previous four years, according to the UK Health Security Agency. This increase has reignited discussion around the old adage, “feed a cold, starve a fever,” and whether it holds scientific merit in managing respiratory infections such as colds and influenza.

Medical experts emphasize that symptoms like fever and runny nose are natural immune responses to infection. John Tregoning, a professor of vaccine immunology at Imperial College London, explains that fevers help the body fight off viruses and bacteria by slowing their growth, enabling the immune system to more effectively counter pathogens. Symptoms can escalate depending on infection severity and individual factors such as genetics or underlying respiratory conditions like asthma.

Research from Imperial College London in 2024 involving mice infected with the cold virus revealed that infection led to weight loss, likely through a mechanism involving interleukin-1 alpha, a protein that signals the brain to reduce appetite. This immune response elevates leptin hormone levels, which suppress hunger—a pathway thought to be similar in humans.

Supporting this, a 2022 study from the University of Bonn in Germany suggested that decreased food intake might encourage the body to metabolize fat stores, producing ketone bodies that serve as energy-rich molecules to fuel immune cells. Findings published in the journal Nature indicated that people with the flu exhibited elevated ketone body levels. Additionally, part of this study showed that ketogenic diets or ketone body supplementation enhanced immune cell function in mice infected with Covid-19.

However, some clinicians caution against deliberately reducing food intake during a fever. Dr. Praveen Molanguri, a consultant respiratory physician in Surrey, notes that fever raises metabolic rate and energy needs, making adequate nutrition important for recovery. He advises that if appetite diminishes, smaller, more frequent meals may suffice to maintain necessary calorie intake. Professor Nathan Davies, a biochemist at University College London, concurs that increased immune activity generally requires more energy, estimating that for every 1°C rise in body temperature, caloric expenditure may increase by 10 to 13 percent.

Nutritional content also plays a role. While some laboratory studies have suggested that foods like chicken soup, fresh ginger, or compounds found in broccoli may help reduce viral infections or improve recovery in animals, these findings cannot be directly extrapolated to humans, according to Professor Tregoning. Furthermore, while ketogenic diets may boost immune cell function in experimental settings, carbohydrates provide essential energy, underscoring the need for a balanced intake. Protein consumption is also critical, as it supports antibody production.

Hydration is another key component during illness. Fever and respiratory infections increase fluid loss through sweating and mucus production. This can impair lung function by reducing the surface area for oxygen exchange, which in turn causes faster breathing and additional fluid loss through exhalation, experts warn.

In summary, while the traditional recommendation to "feed a cold, starve a fever" contains elements rooted in physiological immune responses, medical advice stresses maintaining adequate nutrition and hydration as essential for supporting the body’s fight against infections. The complexity of immune system function and individual patient needs suggests that dietary approaches during illness should be personalized rather than dictated by popular sayings.