Table of Contents


Liquid diets is a term that encompasses a wide range of diets that serve a variety of functions. It can mean either partial or full meal replacement by either clear or non-clear fluids. Doctors often prescribe a liquid diet for before or after certain surgeries, or for patients who are medically obese. People also use them for fasting or weight loss.


The first uses of liquid diets date back centuries because ancient religious ceremonies often involved fasting, and many cultures served only broth to sick patients. Doctors have been prescribing a liquid diet to patients before they were to 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 is a broad category of diets that can be used for a number of different reasons. In essence, it means any diet which replaces regular meals of solid foods, with fluid drinks. For many medical procedures it is helpful, or even necessary, that patients consume only liquids before or after the operation. People 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, like 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 a physician 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 which might require this include sigmoidoscopy, colono-scopy, 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 liquids that can be seen through at room temperature. This means that water, juice, broth, water 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 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, as well as 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 also 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


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 or juvenile onset and type 2 or adult onset.

Fast—A period of at least 24 hours in which a person eats nothing and drinks only water.

Mineral—An inorganic substance found in the earth that is necessary in small quantities for the body to maintain a health. Examples: zinc, copper, iron.

Obese—More than 20% over the individual’s ideal weight for their height and age or having a body mass index (BMI) of 30 or greater.

Vitamin—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.

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 risking their health.

Liquid Diets for Medical Weight Loss

When a person is extremely obese, a physician may prescribe a medically monitored weight loss program that will usually involve replacing solid foods with a liquid substitute. The liquid substitute will usually supply between 500 and 800 calories each day, which means that it qualifies as a very low calorie diet. The liquid substitute will also supply all of the necessary vitamins and minerals that would normally be provided by solid food. Typically the liquid substitute comes in the form of a shake. Patients are told to drink a certain number of shakes every day, rather than eating, and to use that time period to break with old eating habits. After a number of weeks of rapid weight loss and frequent meetings with a physician, who monitors the health and progress of the patient, solid foods may be slowly reintroduced. The entire process is difficult and risky. It should only be undertaken when prescribed by a physician and it must be monitored by a medical professional! . Usually, this sort of liquid diet is only prescribed when serious health risks, caused by obesity, outweigh any risks from the program.

One popular medically observed liquid diet is called Optifast. It is produced by the Swiss company, Novartis Medical Nutrition Corporation, that is also known for making Gerber baby food. They report 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 percent. The Optifast system is extremely expensive and 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 replacements 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 plans 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 dieticians still maintain that a liquid replacement diet is not an appropriate substitute for a healthy lifestyle.


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 and others, including some health professionals, believe that fasting 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 and 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.


Anyone who has been prescribed a period of liquid diet because of a medical procedure should get as much information as possible about the specific guidelines and follow those guidelines precisely. Doing so will give the procedure its greatest chance of success. Anyone considering a fast should consult their physician and describe the nature of the fast to him or her so that it can be determined if the fast will carry serious risks. People with health problems should not engage in prolonged fasting.

Very low calorie liquid diets should not be undertaken without close medical supervision. These are only intended for people that 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


  • What sort of liquids should I drink on this diet?
  • How long should I go before I can eat solid foods?
  • Is fasting safe for me?
  • How will I know if my liquid diet is causing a problem?
  • Will I get proper nutrition from my liquid diet?
  • Is this liquid diet the best way for me to achieve my health goal?

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, though 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 which are likely to 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

It is generally accepted that, for certain medical procedures, it is necessary for patients to 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 the most procedures.

Most medically supervised meal replacement liquid diets are generally accepted. Some doctors question whether more traditional weight loss methods are better in some 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 those for whom they are usually prescribed.

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 1200 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.


The New Liquid Diets - For Oprah, maybe, but probably not for you Indianapolis, IN: Benjamin Franklin Literary And Medical Society, 2005.

Shannon, Joyce Brennfleck ed. Diet and Nutrition Source-book Detroit, MI: Omnigraphics, 2006.

Willis, Alicia P. ed. Diet Therapy Research Trends New York: Nova Science, 2007.


American Dietetic Association. 120 South Riverside Plaza, Suite 2000, Chicago, Illinois 60606-6995. Telephone: (800) 877-1600. Website: <>


“Liquid Diet” The Diet Channel 2007. <> (April 3, 2007).

Helen Davidson.

Low-carb diets see High-fat/low carb diets.