Prolonged fasting is an extremely effective–though not necessarily desirable–way to lose weight in a hurry. Much depends on the type of fast you undertake. Total fasts, which were popular in the 1960’s, promote the loss not only of body fat, but of muscle tissue and other body proteins as well. The modified fasts developed more recently solve this problem by allowing intake of protein. Total fasts also tend to produce unpleasant–sometimes even dangerous–side effects, another problem that the modified, protein-sparing diet generally eliminates. Both approaches, however, have one major drawback in common: Whatever weight you shed during the fast usually returns once it ends.


While the purpose of these long-term fasts is to help one shed unwanted pounds, many advocates of naturopathic medicine recommend shorter periods of therapeutic fasting to help alleviate diseases like arthritis, irritable bowel syndrome, eczema, depression, asthma, and psoriasis, and to “detoxify” the body from environmental poisons that have built up over the years (see also the profile on detoxification therapy). The value of this sort of fasting is much more dubious, although short fasts followed by a carefully restricted vegetarian diet have shown promise for relieving arthritis.

How the Treatments Are Done


Currently, two variations of the protein-sparing modified fast are used to assist in weight loss. Both are recommended only for people who are dangerously overweight. They are not for those who’ve gained a few pounds over the winter and are trying to get back into shape for the beach.

Version One calls for a daily intake of 1.5 grams of protein per 2.2 pounds of your ideal body weight. In practical terms, that means consuming about 75 grams daily if you’re a woman, or 100 if you’re a man–roughly the daily amount in the typical American diet. The protein should be high-quality (meat, fish, or fowl), and should be supplemented with a multiple vitamin tablet plus extra potassium and calcium.

Version Two substitutes a premixed formula rich in milk or egg protein for the meat and vitamins used in Version One. Such formulas usually provide about 50 to 70 grams of protein daily, 40 milligrams of carbohydrate, and a little fat. Critics fault them for failing to include enough protein, the requirement for which goes up as your calorie intake goes down. They also point out that it’s cheaper to buy the components separately than in premixed form. Aside from these minor differences, however, the two diets are much the same.

Typically, a modified fasting program lasts for several months, furnishing no more than 600 calories a day–an amount much lower than that needed to maintain weight. (A non-pregnant woman under age 50 usually needs between 1,600 and 2,500 calories. A man in the same age range needs 2,500 to 3,000.)


Some proponents of detoxification recommend brief, intense, water-only fasts. However, a three-day fast supplemented with vegetable and fruit juices is more typical. The procedure often begins with a “pre-fast.” On the day before the fast starts, you’re instructed to eat lightly, finishing the day with a meal of fresh fruits and vegetables. During the fast itself, you’re expected to drink 3 to 4 eight-ounce glasses of juice, preferably fresh squeezed. Most therapists also recommend avoiding coffee and soft drinks, and getting lots of rest. Often the entire procedure is scheduled for a weekend, when you can conserve energy and take naps as needed. It’s also best to avoid vigorous exercise, although short walks or stretching are considered acceptable.

You will notice that, as the fast progresses, your body temperature will fall and your blood pressure, pulse, and rate of breathing will slow down–all signs of a decline in your metabolic rate. Since the drop in body temperature may make you feel colder than those around you, be sure to wear extra clothing to keep warm. During the first day of the fast, you’ll lose 3 to 5 pounds. That’s a water loss that will come right back after you start eating again. To break the fast, experts suggest that you introduce solid foods slowly, in limited portions.

What Treatment Hopes To Accomplish

Although modified, protein-sparing fasts are considered a reasonably safe way for the dangerously obese to quickly bring down their weight, this approach is no longer favored by most doctors due to the abrupt rebound that almost invariably occurs when the patient resumes eating. Instead, most physicians now recommend a gradual weight-loss program accompanied by a sustained effort to modify bad eating habits.

Likewise, few doctors believe that fasting, in itself, has any lasting therapeutic effects. (Indeed, extended fasts can be quite harmful.) However, as part of a larger dietary modification program, some fasts have delivered encouraging results. During an experiment reported in the respected medical journal Lancet, Norwegian researchers put 27 patients with rheumatoid arthritis on a 7 to 10 day partial fast, followed by a special vegetarian diet free of refined sugar, citrus fruit, milk products, eggs, and gluten–the protein found in many grains.

The diet plan required participants in the study to slowly reintroduce foods, one by one, every second day, and to watch for the return of symptoms. If they reappeared within 48 hours, the food was eliminated permanently. At the end of 3 to 5 months, the participants were left on the resulting vegetarian diet.

Although the symptoms of arthritis often improve while a patient is fasting, they usually return as soon as eating resumes. In the Lancet experiment, however, the patients enjoyed lasting results. By the end of four weeks, patients on the experimental diet had significantly fewer swollen, tender joints, less pain, better grip strength, and improved lab-test results. Those volunteers who stayed off the regime didn’t fare nearly as well.

Some researchers attribute these results to the elimination of foods which, they theorize, act as “arthritic allergens,” causing the immune system to produce antibodies that attack the joints and produce symptoms. Others speculate that a vegetarian diet may alter the bacterial population in the large intestine, somehow relieving the disease. The truth of the matter has yet to be established; but if you suffer from arthritis, you might want to give the program a try regardless of the explanation.

Who Should Avoid This Therapy?

Prolonged, modified fasts should be avoided by all but the seriously obese, and should be undertaken only under the supervision of a physician who has plenty of experience and training in this kind of treatment. Experts also advise that even with adequate supervision, a modified fast should not extend beyond 16 weeks.

Shorter fasts are safer, of course, but for many people it’s wise to avoid even a three-day fast. Pregnant mothers, for instance, risk depriving their baby of much needed proteins, fats, and other nutrients whenever they fast. Similarly, nursing mothers compromise the quality of their breast milk. Others who should avoid any sort of fast are those with ulcers, diabetes, or diseases of the liver, heart, kidneys, and lungs, as well as anyone with advanced cancer.

Children are not very good candidates for even a one-day fast. In one study, children who fasted from dinner time until noon the next day had poorer scores on tests that measured memory and the ability to recognize various types of visual stimuli. As you might expect, those who were already undernourished did the worst.

It’s also wise to check with your doctor before attempting even a one-day fast if you are taking any prescription medications. Many drugs need to be taken with food to improve their absorption, or to minimize their harsh effects on the stomach and intestinal tract. In other instances, the biochemical changes that occur during fasting may alter the way the body handles the drug.

What Side Effects May Occur?

In the late 1970s, a popular “liquid protein” diet product was linked to a number of sudden deaths. The victims suffered fatal heart disturbances typically brought on by protein and mineral deficiencies, and it was discovered that the protein these folks were taking was actually collagen, which is useless as a source of nutritional protein.

The formulas available today are, needless to say, much safer and rarely cause any serious cardiac side effects (although they are still best reserved for those with severe weight problems and should always be taken under a doctor’s supervision). There are, however, other risks that continue to accompany any stringent weight-loss regime.

Potassium deficiency, which can cause severe irregularities in the heart rhythm, remains a threat. So does protein depletion. And the rapid burning of fat that occurs when the body is deprived of carbohydrates can lead to a build-up of byproducts called ketones, which, in turn, can interfere with elimination of uric acid, triggering a painful attack in those who suffer from gout.

Any rapid weight-loss program–including fasting–also poses a risk of gallstones. In addition, salt and water depletion can lead to light-headedness and dizziness when first standing up. Other potential side effects include constipation, dry skin, and menstrual irregularities.

If you are in good health and are doing a brief, 1-to-3 day fast, it’s not uncommon to experience some light-headedness, fatigue, or a headache. If you’ve been a heavy coffee drinker, for instance, and stop for 3 days, you’ll probably suffer a caffeine withdrawal headache. On the plus side of the equation, many fasters report mild euphoria or an increased sense of well-being and clearer thinking.

If you fast for only 24 hours, you may not experience the build-up of ketones that accompanies a longer fast. That’s because during the first day, your body is busy burning the stored carbohydrates referred to as glycogen. After that, as the body begins to metabolize fat stores, you may notice that your breath takes on a sweet or fruity aroma. That’s produced by acetone, one of the ketones produced when fat stores are burned. The symptom is harmless.

How to Choose a Therapist

If you are interested in a modified fast to help you lose weight, you’ll want to find a doctor who is specially trained in weight loss, a specialty called bariatrics. One of your best approaches would be to contact the American Society of Bariatric Physicians for a referral. (See the Resources section for details.)

If, on the other hand, you’re looking for a therapist to supervise a brief detoxifying fast, a licensed naturopathic physician might be a better choice. The American Association of Naturopathic Physicians maintains a list of qualified practitioners.

When Should Treatment Stop?

If you’re under the supervision of a qualified practitioner, consult with him or her before discontinuing your fast.

See a Conventional Doctor If…

Although most of the side effects you’ll encounter during a fast aren’t a cause for concern, an irregular heartbeat requires immediate medical attention. Call your doctor or go to an emergency room at once.