Fasting is the act of willingly abstaining from some or all food and in some cases drink, for a period of time. Depending on the tradition, fasting practices may forbid sexual intercourse, masturbation, as well as refraining from eating certain types or groups of food (e.g. meat). Medical fasting can be a way to promote detoxification.

Fasting for religious and spiritual reasons has been a part of human custom since pre-history. It is mentioned in the Bible, in both the Old and New Testament, in the Mahabharata, in the Upanishads, and North American Indian religions, and in the Qur'an.

In natural medicine, fasting is seen as a way of cleansing the body of toxins, dead or diseased tissues, and giving the gastro-intestinal system a rest. Such fasts are either water-only, or consist of fruit and vegetable juices. Some results have been achieved while including fasting in the treatment of some kinds of cancer, autoimmune diseases, and allergies.

When food is not eaten, the body looks for other ways to find energy, such as drawing on glucose from the liver's stored glycogen and fatty acids from stored fat and eventually moving on to vital protein tissues. Body, brain and nerve tissue depend on glucose for metabolism. Once the glucose is significantly used up, the body's metabolism changes, producing ketone bodies (acetoactate, hydroxy-butyrate, and acetone). Even where this transition to alternative forms of energy has been made, some parts of the brain still require glucose, and protein is still needed to produce it. If body protein loss continues, death will ensue.

Short term fasting causes a starvation response that encourages the body to store fat once eating is resumed. This is one of the pitfalls of Yo-yo dieting.

After approximately three days of fasting, feelings of hunger usually become infrequent or disappear altogether. According to Herbert M. Shelton, N.D., N.D.Litt., D.C., a proponent of Natural Hygiene, whose 45-year-long career of promoting water-only fasts for up to ninety days was punctuated by being repeatedly arrested for practicing medicine without a license, the 'hunger' experienced during the first three days of a fast is "gastric irritation", and not "true hunger", which appears after all the body's resources are used up and when the fast must be broken to avoid permanent and irreversible organ damage.

Juice fasting is a type of fasting and detox diet in which the practitioner consumes only fruit and vegetable juices. Being available only in digestible carbohydrates, these foods are digested rapidly as the juice digestion process expends only a small amount of energy. People choose to undergo juice fasts for various reasons and via various methods. Juice fasts are often marketed together with supplies, supplements and support groups.

Juice fasts are commonly undergone with the intent of detoxification for greater health, the theory being that less energy is expended on digestion of foods; therefore more energy exists for the rest of the body to expel toxins. As toxins are believed to lie within many of the human body's eliminative glands and organs, different juice fasts target different sections of the body. For instance, a large portion of juice fasters believe that abstaining from solid food allows the body to recover and heal itself from damage and fatigue caused by the relentless stress of digestion. Others choose fasting because they want to target the liver, the kidneys, the urinary tract, the skin, the gallbladder, the brain, the immune system, etc.

Some loyal practitioners go so far as to partake in semi-annual week-long (or longer) periods of fasting in order to cyclically purify the body along with the nature's annual cycles. These semi-annual fasters and others may also partake in once-monthly, shorter (two or three days) periods of fasting. Some fasts involve a week-long trip to a spa resort, with Thailand being one especially popular destination.

Because pure juice contains little to no fiber, juice fasters often use an enema or an herbal or saltwater laxative during the time of fasting to efficiently expel waste from the intestines and colon.

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