Does Fasting Speed Up Recovery from Illness? The Complex Science of Nutrition and Immunity

October 13, 2025 •

5 min read

During illness, the body's metabolic rate increases to fuel the immune response. This often leads to the question: Does fasting speed up recovery from illness, or does it hinder the healing process by depriving the body of necessary fuel? The answer is nuanced, depending on the type of illness, the fasting method, and individual health, with traditional wisdom often contradicting modern medical advice.

Quick Summary

Starving a sickness can be detrimental, and there is no conclusive human evidence that fasting improves recovery from common colds or flu. While fasting induces metabolic changes that can reduce inflammation, proper nutrition and hydration are critical for fueling the immune system and repairing tissues, and risks can increase with prolonged fasting.

Key Points

Debunk the Myth: The ancient advice to 'feed a cold, starve a fever' is not medically supported; the body needs nutrition to fight illness regardless of fever.
Understand the Fasting Mechanism: Fasting can induce metabolic shifts and produce anti-inflammatory ketones like BHB, but effects during acute illness are not well understood in humans.
Distinguish Illness Types: Animal studies suggest fasting could aid against bacterial infections but potentially worsen viral ones, a distinction that has not been confirmed in human trials.
Recognize the Risks: Fasting while sick can lead to dehydration, nutrient deficiencies, and serious complications, especially for those with chronic conditions like diabetes.
Prioritize Hydration and Nutrition: The most proven path to recovery is rest, staying hydrated with fluids like water and broth, and consuming nutrient-dense foods to fuel the immune system.
Seek Medical Advice: Always consult a healthcare professional, especially if you have an underlying medical condition, before attempting any fasting regimen while ill.

Debunking the Myth: 'Feed a Cold, Starve a Fever'

For centuries, the maxim "feed a cold, starve a fever" was a widely accepted piece of medical advice. The idea suggested that eating food would help generate heat to fight a cold, while fasting would help a fever break by cooling the body down. Today, medical professionals have debunked this old adage. Modern science reveals that the body needs nutrition and energy to power its immune system, regardless of whether you have a cold or a fever. Fasting during a fever can be particularly risky, as it can lead to dehydration when fluid intake is already insufficient. The body's immune response requires fuel to mount an effective defense, and restricting calories can deplete the energy needed to fight off an infection.

The Science Behind Fasting and the Immune System

Fasting prompts metabolic shifts in the body. When glucose, the primary energy source, is unavailable, the body turns to stored fat for fuel, producing byproducts called ketones. One ketone in particular, $\beta$-hydroxybutyrate (BHB), has been linked to anti-inflammatory effects. In some lab studies, BHB was shown to suppress the NLRP3 inflammasome, a protein complex that triggers inflammation. Fasting is also known to promote autophagy, a process where cells break down and recycle damaged components, which can regenerate immune cells.

However, the exact mechanisms are not yet fully understood in humans, and the effects observed in short-term lab settings or animal models do not always translate directly to clinical recovery from illness. While short periods of intermittent fasting may support healthy immune function in general by limiting inflammation, prolonged fasting (48 hours or more) has been shown to potentially increase inflammatory markers and malnutrition risk in some studies.

Bacterial vs. Viral Infections: A Critical Distinction from Animal Studies

Some of the most intriguing research on fasting and illness comes from animal studies, which suggest that the body's optimal response might depend on the type of infection. A 2016 study in mice, for example, found that fasting helped fight bacterial infections but potentially hindered the fight against viral infections, such as the flu. The researchers hypothesized that when fighting bacterial infections, the body's natural response may be to limit nutrient availability, whereas viral infections might require more glucose. This distinction adds a layer of complexity to the "starve a fever" concept, but it is critical to emphasize that these are animal studies, and the findings cannot be used as medical advice for humans.

Risks of Fasting While Sick

Ignoring the body's need for fuel and fluids can be counterproductive and even dangerous during illness. Here are some of the key risks:

Dehydration: Fever, vomiting, and diarrhea can all lead to significant fluid loss. Fasting from fluids increases the risk of dehydration, which can cause severe complications, including fatigue, dizziness, and delayed recovery.
Malnutrition: The immune system requires a wide array of nutrients, including vitamins (C, D), minerals (zinc, selenium), proteins, and healthy fats to function optimally. Fasting can lead to deficiencies, weakening the immune response and slowing the healing process.
Energy Depletion: Fighting an illness is metabolically expensive. Depriving the body of calories can leave it without the necessary energy reserves, prolonging symptoms like fatigue and muscle pain.
Underlying Health Conditions: Fasting can be particularly hazardous for individuals with pre-existing conditions. For example, people with diabetes who require regular medication and balanced blood sugar levels face significant risks. A doctor's consultation is essential for anyone with chronic health problems.

Fasting vs. Nutrition: A Comparison for Recovery


Aspect	Fasting During Illness	Optimal Nutrition During Illness
Energy Source	Shifts to fat and ketones; potentially insufficient for a robust immune response.	Provides a steady supply of energy (glucose and nutrients) to power immune cells.
Immune Support	May trigger anti-inflammatory processes and immune cell recycling, but potentially at the cost of overall immune strength.	Directly supplies the vitamins, minerals, and protein needed for immune function and cellular repair.
Hydration	Often leads to insufficient fluid intake, increasing the risk of dehydration.	Emphasizes abundant fluid intake (water, broth, tea) to combat fluid loss from fever and other symptoms.
Appetite Response	Ignores natural hunger cues, which may be the body's signal for needed energy.	Listens to appetite; small, frequent, nutrient-dense meals are recommended if the appetite is low.
Risks	Dehydration, nutrient deficiency, muscle loss, and complications for individuals with underlying health conditions.	Low risk for generally healthy individuals; focuses on safe, supportive healing.

What to Eat When You’re Ill

Instead of fasting, focusing on a strategic, nutrient-rich diet is the best course of action. When your appetite is low, listen to your body and prioritize fluids and easy-to-digest, nutrient-dense options.

Stay Hydrated: Drink plenty of water, clear broths, and herbal teas. Electrolyte solutions can help replace lost minerals due to fever, vomiting, or diarrhea.
Sip Warm Liquids: Chicken soup is a classic remedy for a reason; it provides fluids and electrolytes, and the steam can help clear congestion.
Eat Nutrient-Dense Foods: Lean proteins, fruits high in vitamin C (like citrus and berries), and probiotic-rich foods like yogurt can support a healthy immune response.
Prioritize Easy-to-Digest Options: If you feel nauseous, try bland foods like rice, toast, or potatoes, which are less irritating to the stomach.

Conclusion: Prioritize Hydration and Listen to Medical Advice

While research into fasting's general health benefits is ongoing, there is no conclusive evidence that fasting speeds up recovery from acute illnesses in humans, and it carries significant risks, including dehydration and nutrient depletion. The centuries-old myth of "starving a fever" has been disproven by modern medical understanding. The most effective strategy for speeding up recovery remains providing the body with the proper fuel it needs to fight infection: rest, hydration, and nutrient-rich foods. Always consult a healthcare professional before making drastic changes to your diet, especially when ill or if you have an underlying health condition. Listen to your body and give it the support it needs to heal safely and effectively.

Visit Healthline for more detailed information on nutrition during colds and flu.

Frequently Asked Questions

For most mild, temporary illnesses, short-term fasting (a few hours, perhaps due to a lack of appetite) is unlikely to be harmful, as long as you prioritize hydration. However, for more severe or prolonged illnesses, fasting is not recommended due to the body's increased need for nutrients and energy to recover.

When sick, it is crucial to stay hydrated. Good options include water, clear broths, herbal tea, and oral rehydration solutions to replenish lost electrolytes, especially if you have a fever, vomiting, or diarrhea.

During recovery, your body needs a robust intake of protein for tissue repair, healthy fats for reducing inflammation, complex carbohydrates for energy, and a variety of vitamins and minerals like Vitamin C, Vitamin D, zinc, and selenium to support immune function.

Animal studies have shown potentially different outcomes for bacterial and viral infections with fasting, but this has not been reliably demonstrated in humans. Relying on this distinction for self-treatment is medically unsound.

Some studies indicate that short-term fasting can produce ketones with anti-inflammatory properties. However, prolonged fasting may increase inflammation in some individuals, and the impact during acute illness is complex and requires more research.

No, you should not force yourself to eat if you have no appetite, as this can make you feel worse. The priority is staying hydrated. If your appetite doesn't return after a few days, consult a healthcare provider.

Medical experts recommend prioritizing rest, staying well-hydrated, and consuming a nutritious diet tailored to your appetite. These actions provide your body with the necessary energy and resources to effectively fight off the illness and recover.