Fasting could have some benefits, depending on the type of illness.
It is common for people to lose their appetite when they are sick. This could be the body’s natural response to the illness.
Below, we explore whether fasting can help the body recover. We also look into the possible adverse effects of fasting and foods that can boost healing.
A lack of evidence from studies in humans supports the idea that fasting can help people recover from illness.
Meanwhile, research in animals suggests that fasting may help fight off bacterial infections. The same research indicates that consuming glucose, or sugar, may help combat viral infections.
In a long-term study, 341 out of 404 participants with preexisting health problems reported an improvement in their condition after having fasted on vegetable juice and soup for 4–21 days. Overall, these participants consumed 200–250 calories per day.
Researchers believe that fasting may have some health benefits because of how the immune system responds to a lack of glucose.
When the body has no more energy stores of glucose, it starts using amino acids and fat as energy sources. Using fat as fuel causes the body to produce ketones. Certain ketones, such as beta-hydroxybutyrate, may benefit the immune system.
Other research has also suggested that fasting may enable the body to regenerate healthy immune cells.
Some scientists suggest that intermittent fasting, or eating only during restricted periods, is embedded in our physiology — that it constitutes an evolutionary adaptation and can trigger essential functions in cells, supporting overall health.
However, it is important to note that researchers still do not fully understand the effects of fasting on the immune system. Doing so will require further studies in humans.
A 2016 study in mice found that glucose has different effects on bacterial and viral infections.
In mice with a bacterial infection, glucose had a negative effect, but in those with a viral infection, the effect was positive.
Because the common cold is a viral infection, the positive effect of glucose may help explain the adage, “Feed a cold, starve a fever.”
When glucose is in short supply, such as during fasting, the body can use fat as a fuel source, producing ketones. Scientists have found that certain ketones can protect mice from bacterial inflammation.
There are various degrees of fasting and a range of approaches. A person may:
- eat only during restricted times (intermittent fasting)
- consume only broths or juices
- limit calories on certain days
Whether or not a person fasts, it is crucial to stay hydrated during illness and receive the right medical care.
People with some chronic conditions or who take medications on an ongoing basis should consult a doctor before fasting. It may otherwise be a good idea for a doctor to monitor a person’s fasting.
Research indicates that fasting may cause:
These symptoms usually occur in the first few days of fasting. If any symptom makes a sick person feel worse, it might not be a good idea to continue with fasting.
Some foods can help support the immune system and speed recovery from sickness. Examples of beneficial foods include:
- antioxidant-rich fruits and vegetables
- sources of probiotics, such as yogurt, kefir, some sauerkraut, and other fermented foods
- garlic, which has antimicrobial properties
- foods that may help combat inflammation, such as ginger and turmeric
Warm drinks, broths, and soups can be easy to digest, placing less strain on the stomach.
If a person with stomach symptoms has no appetite, they should still be sure to stay hydrated.
While some research in animals indicates that fasting could help with a bacterial infection, but not a viral infection, a person should always listen to their body.
Even if a person has no appetite, it is important to stay hydrated and rest. A doctor can provide more specific recommendations.
People with chronic conditions, such as diabetes, and people taking medications on an ongoing basis should consult a medical professional before fasting.
Scientists have carried out most research into the potential benefits of fasting during illness in animals. However, some results indicate that there could be some truth to the saying, “Feed a cold, starve a fever.”
Some types of fasting could benefit a person with a bacterial infection. In particular, intermittent fasting may benefit health and be a more convenient form of fasting.
However, this may not be the case with a viral infection. This would rule out fasting as a way to help recover from the common cold or the flu.
In particular, it may not be beneficial to fast during the viral SARS-CoV-2 infection that can develop into COVID-19.
Overall, to support the immune system, it can help to eat healthful foods that are rich in antioxidants and probiotics, and that combat inflammation.