Juice fasting (also known as "juice cleansing") is a controversial fasting method and a detox diet in which a person consumes only fruit and vegetable juices to obtain nutrition while otherwise abstaining from food consumption. The health benefits are unproven, with many health professionals considering them potentially dangerous.  Juice fasts may last anywhere from a few days to several weeks. The juice consumed during a juice fast is not the type commercially available, but rather that produced from freshly juiced fruits and vegetables. 
Reasons for fasting
Reasons to undergo juice fasting may include spiritual or religious reasons, detox, desire to lose weight, or attempts to stop habitual behavior such as smoking, drinking soda, overeating, caffeine addiction, etc. Participants may use juice fasting as an alternative medicine. Participants may believe juice fasting will cure chronic pain, cancer, depression, arthritis, severe infections that resisted antibiotics, autoimmune diseases, and many other incurable diseases.  One reason for juice fasting is to assist with other methods of gallstone passage. Others choose juice fasting because they believe they can focus on healing specific organs and systems.
Fasting therapy has a long history in Europe. Guidelines for therapeutic fasting have been formulated. Fasting has been demonstrated to have positive effects in rheumatological diseases, and preliminary observational studies suggest a therapeutic effect of medically supervised fasting in other chronic conditions such as metabolic diseases, pain syndromes, hypertension, chronic inflammatory diseases, atopic diseases, and psychosomatic disorders. Fasting can also be applied for preventing diseases in healthy subjects.
Some practitioners fast bi-annually in week-long (or longer) periods to attempt to purify the body in synchronization with what they believe to be annual cycles of nature. Fasts may even involve retreats and travel, such as with popular week-long spa-resort style trips to Thailand.
Because pure juice contains little to no fiber, juice fasters often use an enema or a herbal or saltwater laxative during the time of fasting to efficiently expel waste from the intestines and colon. Another method to achieve this effect involves mixing psyllium husks in with the juice. Because psyllium is not absorbed by the body, but greatly increases in volume upon water absorption, it creates the bulk necessary to facilitate evacuation. 
In a reduced calorie study, juice fasting for one week, without limitation of physical activity, has been demonstrated to reduce total free cholesterol, by the end of the fast.
Possible side effects
Fasters should take care to maintain their intake of vitamins and nutrients during fasting, though no specific side effects are associated exclusively with juice fasting. Medical conditions such as diabetes may be aggravated by excessive intake of certain juices.  Juice mixes containing grapefruit juice may also adversely interact with certain prescription drugs. 
As a detox diet
Scientists, dietitians, and doctors regard detox diets as less effective than water-fasting, and hence a waste of money. Catherine Collins, Chief Dietician of St George’s Hospital Medical School in London, England, states that "The concept of ‘detox’ is a marketing myth rather than a physiological entity. The idea that an avalanche of vitamins, minerals, and laxatives taken over a 2 to 7 day period can have a long-lasting benefit for the body is also a marketing myth."
Detox diets, depending on the type and duration, are viewed as potentially dangerous and can cause various health problems including muscle loss, decreased metabolism, and an unhealthy re-gaining of fat after the detox ends.
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