It may work for a week or so, but you will eventually be tired and grumpy, and most likely hungry. Any liquid food gets absorbed very quickly. The body doesn’t have to work breaking it down, digesting, etc. The digestion process actually requires energy. Absorbing shakes – very little. This causes you to not be satisfied or feel full, and people get off these kinds of diets very quickly. It is also very difficult to get all the important nutrients, minerals and vitamins in a few shakes, so you’re running a danger of being nutrient deficient. If you need to lose weight really fast for some important event AND are able to find a nutritionally balanced shake: one that has enough good quality protein to spare your muscles, some good fat and carbohydrates to keep your energy levels up, as well as vitamins and minerals, go for it, BUT……only for a very short time. Depending on what’s in the shake already, you might want to take a multi-vitamin/multi-mineral supplement. Shakes, however, will never be able to compete with fresh, whole foods. And they will never be really satisfying. And that’s always a clue to being able to stick to any diet or food regimen – if you get bored with it, you are going to quit.
The exceptions here would be your post-workout recovery shakes, which are designed to enter the blood stream very fast to stimulate protein synthesis and glycogen storage in the muscles, as well as shakes used by people trying to gain big amounts of muscle. In these cases, they do come in very handy in addition to solid food, as it becomes very difficult to eat enough calories to support intense workouts and muscle growth. (Think Hillary Swank in “Million Dollar Baby”.) But these would be in addition to, and not instead of, meals.