Liquid diets control calorie intake by restricting what you eat to mostly or all liquids. How they work varies from product to product. Some liquid diets are fluid only -- fruit or vegetable juices juices or shakes -- that replace all of your meals, taken three or four times a day. These programs are either do-it-yourself options sold over the counter, or medically supervised plans available only through doctors' offices or hospitals.

Other types of liquid diets replace just one or two meals (usually breakfast and lunch) with drinks, but let you eat a healthy, balanced dinner. These diets may also include snack bars for in-between meals.

Liquid diets contain a reduced -- and often significantly reduced -- number of calories. If you eat fewer calories than you burn off, you will lose weight. However, that weight loss may be short-lived. When you drastically reduce the amount of calories you consume, your metabolism slows to conserve energy. Unless you change your eating habits, you'll gain back the weight as soon as you return to your old diet.

Some liquid diets work better over the long term than others. Researchers have found that diets that include both food and liquids can help overweight people control the number of calories they eat with liquid meals and help keep the weight off for several years.

As for the claim by some liquid diets that they can 'detoxify' the system by cleansing it of impurities, there is no evidence to prove it. The body, say experts, is a very sophisticated machine that has its own system of detoxifying through the liver and sweat.

Ideally, liquid diet drinks should contain a balance of nutrients you need throughout the day, but that isn't always the case. Very low-calorie diets (400-800 calories per day) in particular can be lacking in these nutrients and should only be used under medical supervision. Missing out on essential nutrients can lead to side effects such as fatigue, dizziness, hair loss, gallstones, cold intolerance, electrolyte imbalance, and heart damage. A lack of fiber in your diet from not eating whole fruits and vegetables can lead to constipation and other digestive ailments. You also can lose lean body mass if you don't get enough protein in your liquid diet.

People who are about to undergo certain surgical procedures, such as colonoscopy and sigmoidoscopy, which call for little or no food in the intestines, might need to go on a liquid diet for a day or two before the test. Sometimes patients are put on a liquid diet for a few days after surgery or during certain medical treatments until their digestive tract is fully functioning again and they can digest food normally. Medically necessary liquid diets often include clear liquids such as soup, fruit juice, and Jell-o.

People who are obese and need to have surgery (including bariatric weight loss surgery) will sometimes go on a liquid diet to get down to a safer weight before the procedure. This type of liquid diet is supervised by medical professionals.

Some research suggests that liquid diets might help people with certain health conditions. For example, there is evidence that patients with Crohn's disease, which causes inflammation in the gastrointestinal tract, can benefit from a high-calorie liquid diet. By giving the intestines a much-needed rest, a liquid diet can help suppress the symptoms of Crohn's disease.

First, talk to your doctor about whether a liquid diet is appropriate for you. Certain people -- namely pregnant or nursing women and people with insulin dependent diabetes -- should skip liquid diets entirely.

If your doctor gives you the OK to go on a liquid diet, you should also see a registered dietitian, who can go over the diet with you and make sure that you're getting enough calories and nutrition. Your dietitian might recommend that you take a vitamin or nutritional supplement while you're on the liquid diet.

Before you choose a liquid diet plan, know what you're drinking. If considering one of the commercial diets, look at the daily values on the nutrition facts label. Be sure you're getting 100% of all the recommended vitamins and minerals. (The USDA has a guide to help you understand your Recommended Dietary Reference Intakes.)

So that you don't regain all the weight when you transition back to solid food, pick a diet that is not too low in calories and that lets you lose the weight gradually. Liquid diets that include a meal or two per day, or that teach you healthier eating habits, will be more likely to help you keep the weight off over the long term.

WebMD Medical Reference