Can Fasting Fight the Flu or Common Cold? Understanding the Science

The adage "feed a cold, starve a fever" is an ancient piece of folk wisdom regarding infectious illness. Today, modern science is exploring whether voluntarily restricting food intake—or fasting—has a meaningful impact on fighting off viral infections like the common cold or influenza (flu). While clinical human trials on fasting during acute illness are limited, lab and animal studies suggest that withholding food can trigger profound metabolic and cellular changes that influence the immune system's response.

Always consult with a healthcare professional before making changes to your diet or fasting routine, especially when ill.

Fasting explained

Fasting, in the context of health and longevity, refers to a period of voluntary abstinence from food and drink (except for water and sometimes non-caloric beverages). The length and pattern of fasting can vary significantly, which impacts the physiological response.

Types of Fasting

• Intermittent Fasting (IF): Involves cycles of eating and fasting within a 24-hour period (e.g., the 16/8 method, where you fast for 16 hours and eat during an 8-hour window). This is generally done for long-term health benefits.
• Time-Restricted Feeding (TRF): A type of IF where the eating window is restricted to a specific time of day, often aligning with circadian rhythms.
• Prolonged Fasting: Extending fasting beyond 24 hours, often 36 - 72 hours. This is the type of fasting that triggers deeper cellular repair mechanisms but is not typically recommended during illness.

Metabolic Shift During Fasting

When food is restricted, the body shifts from burning glucose (sugar) for fuel to burning stored fat, producing ketone bodies. This metabolic state is characterized by several key changes that may influence the body's reaction to viruses:

• Reduced Glucose: Viruses require energy to replicate, and some evidence suggests that restricting glucose availability may slow down viral multiplication.
• Autophagy: A state of cellular "self-cleaning" where the body recycles old or damaged cell components. Fasting triggers autophagy, which is believed to help the body clear pathogens and damaged cellular debris.

Immune system effects

The impact of fasting on the immune system is complex and depends heavily on the duration of the fast and the type of infection being fought.

Acute Viral Response

Early research, primarily in mice models, suggests that the fed state (consuming carbohydrates) may favor the immune response against bacterial infections, while the fasted state may favor the response against viral infections. Specifically:

• Inflammation Reduction: Fasting can modulate the inflammatory response by reducing the production of certain pro-inflammatory cytokines. Since many flu and cold symptoms (fever, aches) are caused by the body's own inflammatory immune response, theoretically, reducing this inflammation could make symptoms feel less severe.
• Increased T-cell Production: Longer-term fasting has been linked to regeneration of the immune system. One study found that prolonged fasting could promote the removal of old, damaged immune cells and trigger the generation of new, potent immune cells (T-cells). However, this effect is often observed after a fast is broken (refeeding), not during the acute phase of illness.

The Fever Factor

The old saying "starve a fever" might have some basis in physiology. Fever is a powerful immune response. Some research indicates that restricting food may help the body more effectively raise its core temperature, which can inhibit the replication of heat-sensitive viruses. However, severe fever requires adequate hydration and energy to maintain, making prolonged fasting risky.

Role in recovery

While the initial phase of a viral infection (the fight) may potentially benefit from brief energy restriction, the recovery phase (the repair) requires substantial energy and building blocks.

The Importance of Energy

Fighting a virus is highly energy-intensive. When the body is symptomatic, it requires a steady supply of nutrients to power the immune cells, produce antibodies, and repair tissue damage caused by the infection. For the common cold, which is short-lived and generally mild (3 - 10 days), a brief period of reduced appetite (similar to a short fast) is generally tolerated.

• Protein for Repair: Recovery from any illness, especially one that causes muscle aches and fatigue, depends on protein intake to rebuild and repair tissues. Restricting protein can prolong the recovery period.
• Hydration: More important than food restriction is fluid intake. Fever, sweating, and mucus production all contribute to dehydration. Fasting while dehydrated can be dangerous, as electrolytes become imbalanced.

General Recommendation

For most people suffering from a routine cold or flu, medical advice does not endorse strict fasting. Instead, it supports listening to the body's natural signals. If your appetite is low during the peak of the illness, do not force yourself to eat heavy meals. If you feel weak or dizzy, consuming easily digestible, nutrient-dense foods is crucial for stabilizing blood sugar and supporting energy levels.

Other conditions

Fasting is generally contraindicated for individuals with certain health conditions, especially when they are acutely ill with a cold or flu.

• Diabetes: Fasting can destabilize blood sugar levels, leading to dangerous hypoglycemia or hyperglycemia. Individuals with diabetes should follow their doctor's advice on sickness-day management, which often includes maintaining a consistent carbohydrate intake.
• Elderly Individuals: Seniors are at a higher risk of malnutrition and dehydration. Fasting, even short-term, can accelerate muscle loss (sarcopenia) and worsen frailty during illness.
• Pregnant or Breastfeeding Individuals: Nutritional needs are elevated during these periods. Restricting calories could harm both the mother and the fetus/infant.

Foods that may help

Instead of strict fasting, focus on consuming foods that are easy to digest, support hydration, and provide essential vitamins and minerals needed for immune function.

• Bone Broth or Chicken Soup: These provide hydration, electrolytes, and easily digestible protein and collagen. The warmth helps clear nasal passages.
• Vitamin C-Rich Foods: Citrus fruits, bell peppers, and strawberries. While Vitamin C does not cure a cold, it is essential for immune cell function.
• Ginger and Garlic: Known for their anti-inflammatory properties, these can be consumed in tea or in broths to soothe symptoms.
• Honey: A natural anti-inflammatory and antiseptic, honey can soothe coughs and sore throats. (Not for children under one year old).
• Electrolyte-Rich Fluids: Sports drinks, coconut water, or diluted fruit juice can help replace electrolytes lost through fever or sweating.

When you have a cold or flu, the safest approach is to ensure adequate caloric intake to prevent the body from breaking down muscle tissue, while prioritizing fluids and easy-to-digest nutrients.