I think our body has organs which releases various gastric juices to digest the food we eat. It is not only about how much calorie/protein one is consuming, there are also some useful by-products which helps in overall functioning of the body as well. A very simple analogy is only drinking fruit juice instead of eating them raw. We are not taking in fibers which helps in digestion, slow decomposition and good bowl movement.
I am very skeptical about approaches like this where we measure our food just in terms of calories, vitamins, protiens and then consume them directly in that format.