During a fast, a person purposely abstains from food for a specific period of time. Fasting has been practiced throughout the ages for both religious and therapeutic purposes. A one-day fast is unlikely to cause any harm to a healthy body. Slightly longer fasts (two to three days) are also well-tolerated by most healthy people.

No matter how short the duration, fasting is unwise and potentially dangerous for some people, including pregnant and lactating women, people with cancer, diabetes, gout, hypoglycaemia, stomach ulcers, liver, kidney, or lung disease, or anyone with a compromised immune system. Some health experts caution against fasts lasting more than two to three days, even for healthy individuals—if longer fasts are practiced, they should be medically supervised.

During the first 24 hours of a fast, the body is able to utilise its stored carbohydrates—in the form of glycogen—to fuel essential body processes. When glycogen reserves are depleted, fat becomes the preferred energy source, so that protein (e.g., as found in muscle tissue) is partially spared.

However, some muscle tissue is often lost, even during short fasts. Weakness, nausea, headaches, and depression can also develop during a fast, because ammonia and nitrogen are released into the blood during the breakdown of muscle tissue. Ketones, byproducts of fat metabolism, are produced once the body switches from burning carbohydrates to burning fat. Elimination of ketones is accomplished by the kidneys. In extreme cases, extended fasts can lead to disturbances of heart rhythm and death.

“Modified” fasts, in which fruit or vegetable juices and herbal teas are consumed, are probably easier on the body than all-water fasts. Even so, a modified fast should be limited in duration; modified fasts lasting more than a week should be supervised by a healthcare professional.