I've had a lot of questions and comments about dietary protein and what happens to it, particularly when one is on "zero carbs". Here are some thoughts:
1) If your body is in glucose deficit and you are not getting enough dietary glucose, then gluconeogenesis will supply the glucose. Short term deficits, however, are met by making glucose from the glycogen in your liver, not by breaking down proteins. A simple fall in serum BG does not crank up the GNG machinery. GNG happens to replenish your glycogen, not after a single weight lifting session, just if there are no carbs by your next meal and your liver glycogen has been run down too far.
2) If glycogen stores are depleted and need to be rebuilt, they will be replenished from dietary glucose first. If there is not enough dietary glucose, dietary protein will be used (60% or 70% conversion ratio, pretty close) next. If there is not enough dietary protein (amino acids actually) then existing structural proteins in your body will be used to supply the amino acid feedstock for GNG to make the necessary glucose. Please remember that what is really happening is that proteins are constantly turning over into AAs in your body all the time. If you are deficient in dietary protein (no protein bolus at the next meal) , they are broken down and then are not being re-built. You are not directly scavenging your muscles, you are just failing to re-build them. Proteins and even fats are not static. Gaining or losing muscle or storage fat is a dynamic equilibrium process controlled by hormones, etc.
3) Protein eaten in excess of structural and enzymatic (our enzymes are proteins) needs, and in excess of GNG requirements if you eat no carbohydrates, can be burned as fuel for energy. This is why we say that protein has a caloric value of about 4 kcal/g, just like glucose. If it is burned as fuel, that is the energy density. There is no " every gram of protein turns into such and such" It depends on many variables.
4) Any macronutrient with caloric value (carb, protein or fat) can be burned for fuel. Any of these can also be stored as fat if they are in excess. Note I said, "can be" stored as fat. Insulin is the nexus of processes that end in fat storage. If insulin levels are lowered from your previous baseline, fat storage is inhibited (remember the equiibrium?) so you tend to lose fat down to a new equilibrium. You will tend to stay at that new equilibrium based on in my opinion, two things: spontaneously lower caloric intake and possible increased wastage or burning of any excess caloric value. (See your bible: GCBC by Gary Taubes)
The reason dietary proteins are less likely than carbs to end up as fat is the same reason dietary fat on VLC is less likely to end up as fat. It is not because they can't be converted to fat. Of course they can! It is because eating fat and protein and very little carbs keeps your insulin levels low, get it?
Once again, macronutrients influence hormones, and it's the hormones that direct fat storage.
Think how food affects hormones, not whether this or that food component "turns into fat".
Food only turns into fat at the direction of hormones.
Hormone levels (AUC or area under the curve) can be stable within a range of different macronutrient ratios. Your insulin levels in particular only drop so far. That is why I say repeatedly it is not biologically plausible that most people need to tightly define their macronutrient ratios, anymore than it is biologically plausible that you need to use a scale and a calculator to regulate your weight.
Like natural gas, oil and coal in the industrial environment, carbs fats and proteins have specific structural uses in our bodily environment, but each has a potential value (not required, but potential) as fuel to keep the metabolic fires burning. Natural gas is both a feedstock to make plastics and a fuel that can be burned. Oil can be used to coat your driveway or turned into gasoline for your car.
There is not any "typical" thing that always happens to a molecule of glucose, fat or protein. It depends on what the body needs.
As far as raising your insulin levels, glucose has the biggest effect, and fructose is bad because it induces insulin resistance, which requires you to produce higher levels of insulin to handle the same amount of glucose. Protein requires insulin but less than glucose and there is some insulin response to dietary protein. There is no insulin response to dietary fat. All three require some basal insulin for normal metabolism.