Sorry I forgot to get to this question...was looking over old questions I answered before and found this one. Hope this answers your question and makes sense.
Amino acid oxidation rates vary substantially when food intake is restricted, as they are then markedly influenced by the availability of other fuels, i.e., glucose, free fatty acids and ketone bodies (FLATT and BLACKBURN, 1976). This reflects the fact that maintenance of ATP levels takes priority in all cells, so that any available substrates will be used to regenerate ATP from ADP and phosphate.
I assume by liquid protein you mean injested protein of any kind? The body will use whatever nutrients are availale and the easiest. This is why it is imparitive to get 1.5 gr. of protein per lb. of bodyweight on a reduced calorie diet consisting of high protein/low carbs.
When only protein is consumed, nitrogen balances are less negative than during total food deprivation. Nitrogen balance can be approached even while the energy balance remains markedly negative, but substantially higher protein intakes are required. This situation is commonly encountered during a 'protein-sparing modified fast', where, after one week of adaptation, a dose of 1.5 g protein/kg body weight/d is generally sufficient to maintain nitrogen balance (LINDNER and BLACKBURN, 1976).