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Other common name(s): juice fast, dry fast

Scientific/medical name(s): none


Fasting generally means eating no food and drinking only water for a period of days, but there are different approaches. Sometimes a person doesn’t eat or drink anything at all (a dry fast) throughout the fast. A juice fast can mean drinking only water and juice for a day or several days. Tea, broth, or other special drinks may be part of fasting. Fasting is often promoted as part of a "detoxification" process in some types of metabolic therapies for cancer or other conditions.

Religious fasting has different requirements. One group fasts from food and drink, but only during daylight hours for a month out of the year. Another tradition includes a 24-hour dry fast on specific days. Yet another forbids certain foods (such as animal products and alcohol) for several weeks at a time, so that a person eats only certain types of foods and avoids others.

Here, unless otherwise specified, fasting will mean drinking adequate water but avoiding foods and caloric fluids for the duration of the fast.


Available scientific evidence does not support claims that fasting is effective for preventing or treating cancer in humans, although this is now being more fully investigated. Even a short-term fast can have negative health effects for some people, while fasting for a longer time could cause serious health problems and even death. Dry fasts can cause death from dehydration fairly quickly.

How is it promoted for use?

Practitioners of a type of alternative therapy called metabolic therapy (see Metabolic Therapy) believe the body has environmental toxins and other harmful substances that can be removed by fasting or detoxifying the body. They claim that fasting allows the body to focus energy on cleansing and healing itself. According to these practitioners, fasting helps the immune system work more efficiently, allows more oxygen and white blood cells to flow through the body, helps the body burn more fat, helps increase energy, and allows other healing functions to improve. Some supporters claim that fasting by a person who has cancer can "starve" a tumor, leading to cell death.

Other illnesses and conditions proponents claim can be treated by fasting include acne, allergies, arthritis, asthma, non-cancerous tumors, digestive disorders, fever, glaucoma, headaches, heart disease, high blood pressure, inflammatory diseases, pain, polyps, and ulcers. Fasting is also promoted to rejuvenate the body, lose weight or help maintain normal body weight, increase longevity and sex drive, and to improve mental clarity, self-awareness, and self-esteem. It is also said to be helpful in quitting or cutting back on tobacco use , alcohol, caffeine, or non-prescription drugs. Some practitioners claim it can heighten spiritual awareness.

What does it involve?

Short fasts, lasting from 1 to 5 days, are often done at home. Other than drinking only water or juice, fasting can also require a lot of rest. Sometimes other methods of detoxification, such as liver flushes or enemas, are recommended as part of the regimen (see also Juicing, Liver Flush, Colon Therapy). Longer fasts require professional supervision and often take place at a spa, resort, or similar facility. Medical fasts are sometimes done at clinics or hospitals.

What is the history behind it?

Ancient cultures believed fasting could purify the soul. The belief that fasting can also purify or cleanse the body is a fairly modern idea, gaining popularity in the second half of the 20th century.

What is the evidence?

Available scientific evidence does not support fasting as a cancer treatment in humans. Some studies in animals have suggested that long-term calorie restriction -- that is, consuming less than one's normal amount of calories each day -- may slow the growth of certain tumors, but this is not the same as fasting. In fact, some animal studies have found that actual fasting in which no food is eaten for several days could actually promote the growth of some tumors. But more recent animal studies have showed some tumor growth slowed down during fasting.

A study published in 2012 looked at rodents with cancer who were given chemotherapy after 48 to 60 hours of fasting (they were given only water). They had better effects from the chemotherapy than rodents who ate normally before treatment, and in fact some who didn’t get chemo had even improved. By itself, this does not mean the effect would be the same in humans. But studies are already under way to find out if fasting does help improve cancer treatment outcomes in humans.

In another report, 10 patients voluntarily fasted before (and some after) getting chemotherapy, and reported that their fatigue, weakness, and intestinal side effects were not as bothersome as when they got chemo while eating normally. Of note, they had different types of cancer and chemotherapy, and they fasted for different amounts of time, so it’s hard to say whether these results will hold up with further testing.

In 2011, one research group looked at the available studies of calorie restriction and health in animals and humans, including the practices of religious fasting and other types of fasting. Their summary discussed animal studies in which restricting calories over the life span resulted in longer life and less chronic disease, including cancer.

They also reviewed animal studies that compared calorie restriction to alternate-day fasting, which means no food or very little food one day, alternating with eating as much as they wanted the next day, over long periods of time. (Alternate-day fasting can result in lower calorie intake, but often did not, as both humans and animals tended to eat enough on their “feast days” to make up for the fast days.) Some positive effects were found in long-term animal studies, but human studies of alternate day fasting have been much shorter and show fewer benefits.

A brief fast (usually 8 to 12 hours) is often advised by medical professionals in preparation for certain diagnostic tests. In this case, the fast helps to produce more accurate test results. Fasting may also be advised for a period of time before and after surgery, especially if digestive system organs are involved. This is mainly to ensure the stomach and bowels are empty during surgery. This is important to avoid getting stomach contents into the lungs, since anesthesia disables the usual protections, such as swallowing and coughing, that keep a person from inhaling foreign matter into the lungs when he or she is awake. It also allows the intestines time to recover from anesthesia before reintroducing food.

As for reaching and maintaining proper weight, most experts recommend a combination of limiting portion sizes, choosing healthful foods, and being physically active instead of fasting.

Are there any possible problems or complications?

Fasting can have short-term side effects such as headaches, dizziness, feeling lightheaded, fatigue, abnormal heart rhythms, low blood pressure, and a fruity taste in the mouth. People who fast may have problems driving or operating dangerous machinery due to these effects. Fasting can also raise the risk of an attack in people with gout, and worsen symptoms of gallstones. Longer-term fasting can interfere with the immune system and vital bodily functions and can damage the liver, kidneys, and other organs. Fasting can be especially dangerous in people who are already malnourished, such as those with some forms of advanced cancer. Death results when fasts outlast the body’s supply of stored fuel or energy.

Dry fasting (no fluid intake) can quickly result in dehydration and death over a period of days, with the length of time varying by person and situation. Factors such as extreme heat, exertion, or illness can shorten it to a few hours.

Women who are pregnant or breast-feeding should not fast. People taking certain medicines can have problems with absorption, as well as changes in drug action and side effects.

Relying on this type of treatment alone and avoiding or delaying conventional medical care for cancer may have serious health consequences.

To learn more

More information from your American Cancer Society

The following information on complementary and alternative therapies may also be helpful to you. These materials may be found on our Web site (www.cancer.org) or ordered from our toll-free number (1-800-ACS-2345).

Dietary Supplements: What Is Safe?

The ACS Operational Statement on Complementary and Alternative Methods of Cancer Management

Complementary and Alternative Methods and Cancer

Placebo Effect

Learning About New Ways to Treat Cancer

Learning About New Ways to Prevent Cancer


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Note: This information may not cover all possible claims, uses, actions, precautions, side effects or interactions. It is not intended as medical advice, and should not be relied upon as a substitute for consultation with your doctor, who is familiar with your medical situation.

Last Medical Review: 02/24/2012
Last Revised: 02/27/2012