Liquid Diets



The first uses of liquid diets date back centuries. Ancient religious ceremonies often involved fasting, and many cultures served only broth to sick patients. Doctors have been prescribing liquid diets to patients before they undergo surgery for decades. Only in the past few decades have several medically monitored weight-loss programs, such as Optifast, and commercially available weight-loss programs, such as Slim Fast, become available.


Liquid diets refer to a broad category of diets that can be used for a number of different reasons. In essence, a liquid diet is any diet that replaces regular meals of solid foods with fluid drinks. For many medical procedures it is helpful, or even necessary, if patients consume only liquids before or after the operation. People might also consume only liquids during periods of fasting. When a person is diagnosed as seriously obese, a physician may decide that he or she should undergo a medically observed weight-loss program, such as Optifast. There are also several programs, like Slim Fast, that mimic the medically observed programs, but in a less severe way that can be followed without supervision.

Liquid diets for medical procedures

Before patients undergo certain medical procedures, their physicians may recommend a liquid diet. This is done to clear out the digestive system and decrease the strain on the digestive organs. It allows a patient to acquire the necessary calories, nutrients, and fluids, while minimizing the digestive impact. Tests that might require this include sigmoidoscopy, colonoscopy, magnetic resonance imaging (MRI), and certain x rays. Surgical procedures that can require a liquid diet include most types of serious oral surgery as well as almost any stomach or bowel surgery. Many surgical procedures, such as bariatric surgery, may also require that a patient follow a liquid diet after the operation while they regain the ability to digest solid foods.

Though guidelines will differ depending upon the procedure, following a liquid diet in preparation for a medical procedure will generally mean drinking only clear (translucent) liquids. Water, juice, broth, ice, and gelatin are usually acceptable. Soups that contain vegetables, noodles, meat, or rice are generally not allowed. While milk is usually acceptable, yogurt is usually restricted. When a physician prescribes a liquid diet, he or she will tell the patient the specific guidelines, including a time period during which the diet must be followed, and often provide literature that will describe the types of fluids that are allowed.


Many people carry out periods of fasting for a variety of reasons. While some fasts require the faster to only drink water, or to consume no liquid at all, fasting typically means to refrain from eating food, but not from drinking liquids. Most of the world's popular religions call for periods of fasting at certain times for tradition, for reasons of atonement, to clear the mind, as a way of mourning, for purification, or for other spiritual reasons. Jewish tradition says that fasting should be done during Yom Kippur. Many Christians fast during Lent. Muslims traditionally fast during the days of Ramadan. Many ascetic Buddhists and Hindus practice periodic fasting. Many people also fast for health-related reasons, because they believe that it can cleanse the body of toxins and some even believe it can cure disease. Historically, fasting has also been used for political reasons as a form of protest, like those carried out by Mohandas Gandhi in the 1920s and 1930s.

For whatever reason it is done, fasting should never be used for weight loss. Medical professionals disagree about whether fasting should be used for other reasons, but it is overwhelmingly accepted that fasting is not an effective way to lose weight and that it can be very dangerous. Not only does fasting slow down the metabolic processes, meaning that it can actually result in overall weight gain, it also weakens the immune system and can make people vulnerable to many serious diseases and conditions, including liver and kidney failure. People considering a fast should always consult with their doctor to make sure that they will not be posing any risks to their health.

Liquid diets for medical weight loss

One popular medically observed liquid diet is called Optifast. It is produced by the Swiss company Nestlé Health Science, which is also known for making Gerber baby food. The company reported that, in a study of 20,000 people who used the Optifast program for 22 weeks, the average person lost 52 pounds and decreased their blood pressure by 10%. The Optifast system is extremely expensive and is not intended for the typical dieter.

Commercially available liquid diets

Possibly because of the reputation for rapid weight loss in seriously obese patients, several less expensive liquid meal replacements have become commercially available for weight loss without medical supervision. These products are not usually intended to replace every meal or all solid foods. These products are intended to help dieters lose weight quickly, though they often do little to affect long-term lifestyle changes.

One of the more popular commercially available liquid meal replacement diets is called Slim Fast. The Slim Fast plan says dieters should eat one regular meal during the day and replace the rest with low-calorie shakes. The shakes each provide one-third of the daily recommendations for a healthy diet. Slim Fast is one of the few liquid replacement diets that defends its plan with controlled clinical studies. In a study done at the University of California, Los Angeles School of Medicine, 300 patients followed the Slim Fast diet for 12 weeks. They lost an average of 15 pounds, and 76% were able to keep at least 80% of the weight off by one year later. However, most dietitians still maintain that a liquid replacement diet is not an appropriate substitute for a healthy lifestyle.


Different liquid diets are intended for different functions. Many patients must follow a liquid diet before or after a medical procedure to clear out their digestive system. Fasting is done for religious, medical, and even political reasons. Physicians prescribe medically supervised liquid diets to seriously obese patients to lower their risk of medical consequences of obesity. Many people also purchase similar, but commercially available, meal-replacement diets to lose weight.

Diabetes mellitus—
A condition in which the body either does not make or cannot respond to the hormone insulin. As a result, the body cannot use glucose (sugar). There are two types (type 1 and type 2).
An inorganic substance found in the earth that is necessary in small quantities for the body to maintain health. Examples: zinc, copper, iron.
Defined in adults as having a body mass index (BMI) of 30 or greater; children are considered obese if their BMI is greater than the 95th percentile in their age and gender groups.
A nutrient that the body needs in small amounts to remain healthy but that the body cannot manufacture for itself and must acquire through diet.


The possible benefits to a liquid diet depend upon which sort of liquid diet a person is considering. A patient that is told by a physician to refrain from eating solid foods can prevent everything from vomiting during surgery to an ineffective test. Some people believe that fasting can have spiritual benefits or can help to remove toxins from the body.

The greatest health benefits of a liquid diet, however, are probably experienced by extremely obese patients who lose weight on a medically supervised meal replacement liquid diet. Obesity has been linked with many serious diseases such as diabetes, heart disease, kidney failure, liver failure, and cancer. Obese individuals who lose weight can drastically reduce their risk of getting these diseases and even reduce the severity of their symptoms if they already suffer from them. Health benefits can also be gained by people who lose weight using a commercially available meal replacement liquid diet.


Very low-calorie liquid diets should not be undertaken without close medical supervision. These are only intended for people who have large amounts of weight to lose, generally over 50 pounds, and are experiencing health risks because of their obesity. People considering any kind of meal replacement liquid diet should consult their physician to be sure the diet is safe for them.


Short-term liquid diets for use before or after a medical procedure carry few risks and are generally considered safe if the patient follows the prescribed guidelines and is sure to get enough caloric intake through juice, broth, or other clear liquids. Longer fasting carries many risks including possible damage to the intestinal tract, impaired liver or kidney function, and hypoglycemia. Fasting also impairs the body's immune system, which makes the body more vulnerable to communicable diseases such as influenza or streptococcus. Gaining fat is also a common risk of fasting, because although the body may use stores of fat during the fast, once the fast is over the body usually rebuilds these stores quickly and often rebuilds more than was originally available.

Medically supervised meal-replacement diets can carry their own risks, though these are usually outweighed by the benefits of weight loss for the extremely obese. Side effects can include gallstone formation, nausea, fatigue, constipation, and diarrhea. Commercially available liquid diets also have many risks, depending on the brand. Some are considered very low-calorie diets that can result in malnutrition. Many do not adequately replace the vitamins and minerals that would usually be supplied by solid foods. This can result in deficiencies that can cause problems—for example, if the body does not get enough calcium, the risk of osteoporosis and rickets increases.

Research and general acceptance

For certain medical procedures, it is generally accepted that patients must refrain from eating solid foods for at least 24 hours before the procedure. Most hospitals have prepared patient literature about the precise guidelines that should be followed for these procedures.


Doctors disagree about whether fasting can have health benefits, though most agree that it must be undertaken carefully and carries many risks. Most also agree that toxins do build up in the body when a person eats a diet that is high in processed foods and low in nutrients. However, the question of whether fasting can remove these toxins has yet to be conclusively answered. It is accepted that fasting is not a safe or effective method of weight loss.

Most medically supervised meal-replacement liquid diets are generally accepted. Some doctors question whether more traditional weight loss methods are better in cases of less extreme obesity, but it is generally believed that the risks and side effects of these programs are outweighed by the benefits for severely obese patients.

There are many commercially available liquid diets for weight loss, and their acceptance depends upon the brand and its program. Brands that include regular food, at least 1,200 calories each day, and some kind of exercise recommendations, like Slim Fast, are more accepted than programs that are very low in calories and do not include exercise, such as the Hollywood Celebrity Miracle diet.

See also Bariatric surgery ; Detoxification diets ; Juice fasts ; Malnutrition ; Optifast ; SlimFast .



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Helen Davidson
Revised by Laura Jean Cataldo, RN, EdD

  This information is not a tool for self-diagnosis or a substitute for professional care.