You may have heard the saying, “Feed a cold, starve a fever.” The phrase refers to eating when you have a cold and fasting when you have a fever.
Some people claim that avoiding food during a period of infection helps your body heal. Others say that eating gives your body the fuel it needs to quickly recover.
This article explores whether fasting has any benefits against the flu or the common cold.
Fasting is defined as abstinence from foods, drinks, or both for a period of time.
- Absolute fasting: not eating or drinking at all, usually for a short period
- Water fasting: allows intake of water but nothing else
- Juice fasting: also known as juice cleansing or juice detoxing and usually involves the exclusive intake of fruit and vegetable juices
- Intermittent fasting: an eating pattern that cycles between periods of eating and periods of fasting, which can last up to 24 hours
There are several ways to fast, and each has its own method of restricting the intake of foods and beverages.
While many forms of fasting exist, most research on fasting and the immune system looks specifically at intermittent and absolute fasting.
Fasting forces your body to rely on its energy stores to sustain typical function. Your body’s primary energy source is glucose, which is found circulating in your blood and is also stored as glycogen in your liver and muscles (
Once your glycogen is depleted, which generally occurs after 24–48 hours from prolonged endurance exercise or fasting, your body starts using fat and, to a lesser extent, amino acids for energy (
Stored body fat can be broken down into fatty acids, which will be used as a fuel source in a process known as lipolysis. Then, fatty acids can be metabolized to produce byproducts called ketones, which your body and brain can use as a source of energy (
In fact, researchers at the Yale School of Medicine observed that exposing human immune cells to BHB in amounts you’d expect to find in the body following 2 days of fasting resulted in a reduced inflammatory response (
It’s important to mention that the exact ways in which fasting affects the immune system are not yet fully understood. More studies are needed to understand which types of fasting, if any, can improve the body’s immune response to various conditions.
Short periods of fasting may support healthy immune function by promoting immune cell production and regeneration and limiting the inflammatory response.
Common cold and flu-like symptoms can be caused by either viruses or bacteria.
To be perfectly clear, cold and flu infections are initially caused by viruses, specifically the rhinovirus and influenza virus (
However, being infected with these viruses lowers your defense against bacteria, and this raises your chances of developing a bacterial infection at the same time. The symptoms of the bacterial infection are often similar to your initial symptoms from the virus (
Interestingly, it’s thought that the lack of appetite you often feel during the first few days of an illness is your body’s natural adaptation to fighting the infection.
During sickness, your immune system releases chemicals known as cytokines (e.g., IL-18), which increase inflammation. Due to the toll this takes on your body, it may increase feelings of tiredness and lack of hunger (
It’s also thought that, from an evolutionary perspective, lack of hunger eliminates the need to hunt for food and therefore preserves energy. It also reduces the energy needed for digestion. These effects may allow the body to focus solely on fighting off the infection (
Plus, some cold symptoms, such as nasal congestion, may impact your ability to taste and smell. This may decrease your desire to eat.
Finally, some suggest that the lack of appetite often accompanying an infection is a way to encourage autophagy — the removal of unhealthy cells and the production of new healthy cells (
While these theories are promising, there aren’t enough studies examining whether fasting or eating have any effects on the common cold or flu in the real world.
Many hypotheses attempt to explain how fasting can help promote healing, but more research is needed to confirm these effects in humans.
In addition to the potential benefits against infections, fasting may help with:
- Weight loss. Intermittent fasting may be effective in achieving weight loss, especially when paired with a regular exercise routine (
19, 20, 21, 22).
- Type 2 diabetes. Intermittent fasting may have positive effects on insulin resistance and blood sugar levels for some individuals (
- Oxidative stress. Intermittent fasting may help prevent disease by limiting oxidative stress and inflammation (
- Heart health. Intermittent fasting may reduce heart disease risk factors like body weight, total cholesterol, blood pressure, and triglycerides (
26, 27, 28).
- Brain health. Studies suggest that fasting may protect against cognitive decline and neurodegenerative diseases (
29, 30, 31).
While continued research is needed to better understand fasting and its role in health promotion, some of these health benefits may be due to the weight loss caused by fasting, as opposed to fasting itself. Therefore, more research is needed.
Either directly or indirectly, fasting may positively affect several medical conditions.
So far, there is only limited evidence that fasting improves symptoms or prevents the common cold or flu.
On the other hand, a number of studies suggest that eating certain foods may improve cold and flu symptoms.
Best foods to fight cold symptoms
Chicken soup is a common go-to meal when we’re sick. It’s a great source of fluids, electrolytes, protein, vitamins, and minerals (
Drinking warm beverages or broth may also help alleviate sinus congestion, though this is largely anecdotal. Staying hydrated makes mucus more fluid and easier to clear (
Best foods to fight flu symptoms
When trying to reduce stomach symptoms associated with the flu, it’s best to stick to eating bland, easily digested foods. Examples include clear soup broths or foods high in starches, such as rice or potatoes.
To ease an upset stomach, try staying away from irritants, such as caffeine and acidic or spicy foods. Also consider avoiding extremely fatty foods, which take longer to digest.
Finally, make sure to stay hydrated. Adding a pinch of salt to your fluids will also help replenish some of the electrolytes lost through sweat, vomiting, or diarrhea.
Best foods to prevent the common cold or flu
Nutrition can play a role in reducing your risk of developing a cold or flu and decreasing how long you have it for.
Surprisingly, your digestive system makes up over 70% of your immune system and is largely influenced by beneficial bacteria that reside in your gut, collectively known as your gut microbiome (
Yogurt with live cultures can help to ensure these beneficial bacteria continue to multiply. Also, make sure to favor a diet rich in prebiotics, such as bananas, garlic, onions, and dandelion greens (
Interestingly, one 2018 double-blind, randomized study showed a significant reduction in flu-like symptoms and incidence of upper-respiratory infection in those who took a probiotic-rich beverage for 12 weeks, compared with the placebo group (
Foods high in vitamin C — such as orange, mango, papaya, kiwi, and cantaloupe — may support a healthy immune system to reduce symptoms and length of colds. However, taking vitamin C supplements won’t cure an already existing cold and are unnecessary (
Instead, ensure that you are eating plenty of foods high in vitamin C as well as other nutrient-dense whole foods.
Consuming a nutritious diet may help support a healthy immune system and may reduce symptoms of the cold or flu.
Based on the current evidence, eating when you’re hungry seems to be a good idea. Yet there is no reason to force yourself to eat if you don’t feel hungry — especially when you’re sick.
If you notice that your hunger doesn’t come back after a few days, it’s best to speak with a healthcare professional. It’s also important to chat with them before trying intermittent fasting to make sure it’s right for you.
Until more research is available, it’s best to eat according to your personal needs and preferences.