Liquid Diets (or Juice Fasting) are used for a number of reasons - including detox and weight loss. However for weight loss, generally only fluid is lost - not fat.

A liquid diet consists of juices replacing every meal. Nutritionally this will provide virtually no protein or fat, and few calories. However vegetable and fruit juices will be rich in vitamins and minerals and cancer-fighting phytochemicals.

Liquid Diet for Weight Loss?
A Juice Fast should not be used as a weight loss technique. It is better to eat the fruit and vegetables whole - and therefore gain the fiber, and the increased metabolism resulting from digesting the whole food.

There are medically supervised liquid diets (such as Medifast and OPTIFAST) which are used for weight loss. Generally these (expensive) programs will cause quick weight loss - but they are not sustainable as a lifestyle.

Juice Diet Benefits
It may be beneficial to include vegetable and fruit juices in your diet - but not as a complete replacement of solid food. A number of people use a tonic or juice supplement in their diet to provide essential vitamins and minerals. Others claim there are detox benefits to be had from a juice fast. However you should not go beyond a one day juice fast without first consulting your doctor.

Meal Replacement Products
Meal Replacement Products (or MRP’s) are a general term describing products that have a full range of nutrients in them. Most of these are in powder form and when mixed with water or milk provide a liquid-based meal. These are often used by body builders or those looking to gain weight - they help provide macro nutrients in a convenient form, and are sometimes a lot easier than chewing your weight through lots of calories.

http://www.everydiet.org/diet/liquid-diets