How I understand it, is that your body is going to try and get its energy from carbs and fatty acids. To get energy from protein it would have to convert the protein into a carb (glucose) via gluconeogenesis. So I am guessing if you just drank the shake mentioned above with no other source of energy, your body would break more of it down for energy instead of using it for your muscles and so if you mix it with water instead of milk you will probably want to eat something with it. So lets say I do mix it with a good amount of skim milk which adds about 100 calories from fat/carbs, how many actual calories am I going to be getting from the protein drink? It's not going to convert all of the protein into sugar, most if not all, I would think, would be broken down into amino acids to build muscle.
This was the protein drink I was looking at:
http://www.trueprotein.com/Product_Details.aspx?cid=22&pid=581
http://en.wikipedia.org/wiki/Protein_in_nutrition
Because the body is unable to store it, excess protein is broken down and converted into sugars or fatty acids. The liver removes nitrogen from the amino acids, so that they can be burned as fuel, and the nitrogen is incorporated into urea, the substance that is excreted by the kidneys. These organs can normally cope with any extra workload but if kidney disease occurs, a decrease in protein will often be prescribed.
http://www.ultracycling.com/nutrition/fueling_for_endurance.html
During endurance exercise, approximately 8-15% of the calories required should come from protein, with the remaining 85% from complex carbohydrates. Many Meal Replacement Drinks (MRPs) that are used as endurance fuels during exercise contain too much protein with very little carbohydrates. The human body, while able to handle 20-30 grams of protein in one intake, cannot tolerate that on an hourly basis. Too much protein fills the blood with too many amino acids. These excess amino acids are converted into carbon dioxide, water, and ammonia. This ammonia is toxic to the body and is a primary cause of premature fatigue. While the body is equipped to handle excess ammonia by converting it to urea then filtering it through the kidneys, too much puts a burden on the kidneys.
http://www.carbs-information.com/gluconeogenesis.htm
Once all the liver glycogen is used up (after 24 hours of fasting, or much sooner if you exercise) the liver has to make glucose from non-carbohydrate sources. The process is called gluconeogenesis. The liver can make glucose from lactic acid, if available, but its prime raw ingredient is protein. Fats cannot be turned back into glucose.
Gluconeogenesis - meaning, "the creation of a new form of glucose" - is the process of making glucose (sugar) from its own breakdown products or from the breakdown products of lipids (fats) or proteins. Gluconeogenesis occurs mainly in the liver or kidney. It's a "new form of glucose" because normally glucose-for-energy comes from carbohydrates.
Maintains Blood-Glucose Levels
Experts describe gluconeogenesis as the process by which the liver contributes to the maintenance of normal blood glucose levels (and thus energy metabolism) through synthesis of glucose. This process occurs in the fasting state, such as at night, and requires the presence of amino acids, lactate, and glycerol. The process is controlled, among other things, by the balance among various hormones - especially cortisol from the cortex of the adrenal glands and insulin from the pancreas.
When the liver engages in gluconeogenesis it burns fats (which are the energy for the gluconeogenic process), but is unable to fully metabolize them to carbon dioxide and water. These semi-metabolized fat products are then converted into ketone bodies. They are released into the blood, from where they are taken up and utilized by most tissues, where they are turned into carbon dioxide and water. The heart utilizes ketone bodies, so do the muscles, and also the brain which normally only burns glucose, although the brain cannot survive without glucose.
