The Optifast Diet is an 18-week calorie-controlled weight loss program designed for maximum results. Around since the 1970s, the Optifast Diet claims to have helped over a million obese and overweight people -- including Oprah -- slim down.

During the first 12 weeks of the Optifast program, dieters are restricted to consuming liquids only, which come in ready-made meal replacements shakes and soups that are nutritionally balanced to ensure that they provide proper sustenance. During this liquid-only phase, dieters consume around 800 calories per day. After the first 12 weeks, dieters on the Optifast Diet enter a transition phase in which food is re-introduced into the diet, and daily intake during this phase does not exceed 1,200 calories. Dieters are told to expect to lose between two and three pounds per week. During the program, participants are medically supervised and an customized exercise program is also recommended.

While the Optifast diet has generated a fair amount of positive feedback, there are a number of criticisms of the program as well. Firstly, it's quite expensive -- the initial workup can cost around $600 and dieters might spend around $200 per week on meal replacement shakes. Furthermore, 800 calories a day is far below the recommended daily intake suggested by the USDA's Dietary Guidelines for Americans -- even if you're trying to lose weight. What's more, it's advised that dieters aim to lose only one to two pounds a week -- any more is considered rapid weight loss and can be harmful to the body. Finally, the weight loss is often not maintained because dieters aren't taught how to control their food intake.