Quick Answer: A mild cold: fasting is likely fine and may even support immune function. The flu or significant illness with fever, vomiting, or inability to eat: pause your fast and prioritize hydration and nutrition. The evidence leans toward "feed a cold, assess the flu."

Should You Fast When You Have a Cold or Flu?

The old saying is "feed a cold, starve a fever." Modern immunology has a more nuanced view — and the answer for intermittent fasting practitioners isn't simply "stop" or "continue."

Here's what the evidence actually says about fasting with a cold or flu, and a practical decision framework for when to keep your protocol and when to step back.

What Fasting Does to the Immune System

To understand whether fasting helps or hurts when you're sick, you need to understand how fasting affects immunity in the first place.

Short-term fasting (12–24 hours) appears to have several immune-relevant effects:

Reduces systemic inflammation: lower insulin levels reduce inflammatory cytokine production. This is relevant because excessive inflammation is responsible for much of the misery associated with colds and flu.
Triggers autophagy: the cellular cleanup process that eliminates damaged and infected cells. Autophagy is part of the body's innate immune defense against viruses.
Reduces immune energy competition: when the digestive system is at rest, more metabolic resources are theoretically available for immune function.

A 2016 study in Cell found that fasting-induced autophagy helped clear viral infections in animal models more rapidly.[^1] This is encouraging but not directly translatable to the specific context of cold or flu in humans.

However, prolonged fasting or severe caloric restriction suppresses immune function. The immune system needs adequate protein and micronutrients (zinc, vitamin C, vitamin D) to produce antibodies and mount an effective response.

Colds: The Case for Continuing IF

A common cold — runny nose, sore throat, mild fatigue, no fever — is generally not a situation that requires breaking your fast.

Reasons to continue IF during a mild cold:

Reduced Inflammation May Help

Cold symptoms are partly driven by your immune response — the inflammation intended to fight the virus also makes you feel awful. The anti-inflammatory effect of fasting could reduce the severity of symptoms, particularly nasal congestion and general malaise.

You Likely Don't Have Increased Energy Needs

A mild cold doesn't significantly increase metabolic requirements. Your body isn't working extraordinarily harder to manage a minor upper respiratory infection. The caloric deficit from fasting doesn't meaningfully impair the immune response to a cold.

Appetite Is Often Naturally Reduced

When you have a cold, you may not want to eat anyway. Honoring that signal by staying in your fasting window aligns with what your body is already telling you.

Practical approach for a mild cold:

Maintain your eating window
Prioritize nutrient-dense foods in your eating window: protein, zinc-rich foods, vitamin C sources
Increase fluids during fasting hours — plain water, herbal teas, plain broth (very low calorie is acceptable)
Monitor how you feel — if symptoms worsen, reassess

The Flu: A More Complex Picture

Influenza is a different situation from a common cold. The flu causes:

High fever (often 102–104°F)
Body aches
Significant fatigue
Often vomiting and diarrhea
Much higher metabolic demand from the immune response

When Flu Warrants Breaking Your Fast

If you have a high fever: See our full article on fasting with a fever, but the short version is that high fevers dramatically increase caloric and fluid needs. Maintaining a strict eating window during high fever is counterproductive.

If you can't eat enough in your window: The immune system needs protein to produce antibodies. If nausea limits what you can eat, forcing yourself into a strict eating window may result in total food avoidance. Eating small amounts throughout the day is better than eating nothing in a compressed window.

If you're vomiting or have diarrhea: Dehydration and electrolyte loss are the primary risks. These need to be managed freely, not constrained by fasting windows.

If you're taking medications that require food: See fasting while on antibiotics for specifics.

The Hydration Priority

Regardless of whether you maintain your eating window, hydration is the non-negotiable priority when you're sick.

During a cold or flu, you lose more fluid through:

Increased nasal secretions
Sweating (especially with fever)
Faster breathing
Vomiting or diarrhea

Plain water, herbal teas, and electrolyte solutions (without sugar) are all acceptable during fasting hours. Chicken or vegetable broth is a borderline case — it contains minimal calories but does have sodium and some amino acids. For a mild cold, plain broth during fasting hours is a reasonable compromise that supports recovery without fully breaking the fast.

Immune-Supporting Foods to Prioritize in Your Eating Window

When you're sick, what you eat in your eating window matters more than usual:

Zinc-rich foods: beef, oysters, pumpkin seeds, legumes — zinc has demonstrated antiviral properties and reduces cold duration[^2]

Vitamin C sources: citrus, bell peppers, strawberries, kiwi — supports immune cell function

Protein: chicken, fish, eggs, dairy — essential for antibody production

Ginger and garlic: both have demonstrated anti-inflammatory and antimicrobial properties

Bone broth or chicken soup: hydrating, contains electrolytes, and has traditional evidence supporting it (and some modern evidence showing it reduces upper respiratory inflammation)[^3]

The Decision Framework: Continue or Pause?

Use this framework to decide:

Continue fasting if:

Symptoms are mild (runny nose, mild sore throat, sneezing)
No fever above 100°F
You can eat adequately in your eating window
You're not on medications that require food
You have normal appetite and can stay hydrated

Pause or loosen fasting if:

Fever over 100°F
Significant nausea or vomiting limiting food intake
Symptoms are severe (high fever, intense body aches, significant fatigue)
You're taking antibiotics or other medications requiring food
You're not eating enough total calories to support recovery

"Pause" doesn't mean abandoning IF — it means switching to a more flexible window (14:10 instead of 18:6, for example) until you've recovered, then resuming your normal protocol.

How Long to Pause IF When You're Sick

Most colds resolve in 7–10 days. For mild illness, you may not need to pause at all. For moderate illness (flu-like symptoms), loosening your protocol for 3–7 days is appropriate. Resume your normal fasting protocol when:

Fever has been gone for 24+ hours
You can eat normally and have adequate appetite
Energy levels are returning to normal
You're off any antibiotics (or finished the course)

FAQ

Can fasting make a cold worse? Unlikely for a mild cold, and possibly helpful through anti-inflammatory and autophagy effects. It can be counterproductive for severe illness where adequate nutrition is needed for immune function.

Should I fast if I have COVID-19? COVID-19 varies enormously in severity. For mild cases, fasting may be fine. For moderate-to-severe cases with fever, fatigue, and respiratory symptoms, prioritize nutrition, hydration, and recovery. Follow your doctor's guidance.

Is chicken soup okay during a fast? Traditional chicken soup has enough calories to technically break a strict fast, but the amounts are minimal. If you're sick, the benefits of eating chicken soup during fasting hours likely outweigh the minor protocol deviation.

Will being sick break my fasting streak? Breaking your fast for a few days due to illness is not failure — it's appropriate judgment. Resume when you're recovered. The Fasted app tracks your history so you can see your overall pattern, including appropriate illness breaks.

Does fasting help the immune system fight colds faster? The evidence is preliminary. Autophagy induced by fasting may help clear viral infections, but this hasn't been definitively proven in human cold studies. Don't rely on fasting as a cold treatment — rest, hydration, and zinc are better-supported interventions.

[^1]: Alirezaei, M. et al. (2010). Short-term fasting induces profound neuronal autophagy. Autophagy, 6(6), 702–710. [^2]: Hemilä, H. (2017). Zinc lozenges and the common cold: a quantitative review of randomized controlled trials. JRSM Open, 8(5). [^3]: Rennard, B.O. et al. (2000). Chicken Soup Inhibits Neutrophil Chemotaxis In Vitro. Chest, 118(4), 1150–1157.