Some plans, like Optifast and HMR, are so low in calories that they must be followed under medical supervision. For example, doctors may prescribe such diets to people who are obese and must quickly lose a certain amount of weight before undergoing surgery. Liquid diets generally fall into two broad categories. Some programs, like HMR and Optifast, offer prepackaged entrees and snacks to use for those real meals. Depending on which liquid diet you're following, you can choose from a variety of products, including liquid meal replacements and snack bars that are available in several flavors.
Calorie counts vary considerably.
Slim-Fast Shakes, on the other hand, average about 220 calories each. D., R.D., Director of Nutrition at the Rose F. Kennedy Center at Albert Einstein College of Medicine in New York City. The very low-calorie diet plans are the riskiest. For example, some high-protein liquid diets, providing as few as 400 calories per day, were even linked to deaths from irregular heart rhythms and cardiac arrest. That's why some, such as the very low-calorie liquid diet program with HMR, require medical supervision.