I'm not sure understanding the complexities of how protein gets metabolized, either as a substrate for bodily functions or converted to fuel, is necessary or, as the thread shows, just confusing.
In my mind, the average consumer should know:
1. Proteins have 4 calories per gram
2. Prioritize hitting your protein target when planning nutrition
3. You need a certain amount of healthy fats, too
4. How many carbs you eat can be driven by performance considerations or, if none, other preferences
5. Your net energy balance is the biggest rock in the room
Other things, like the thermic effect of food or the metabolic cycle for converting protein into fuel are either rounding errors in the big picture, or not necessary to really worry about.
It's also probably good to vary the types and sources of protein you consume.
Some excellent bullet points. To add:
To circumvent the problem of counting calories, don't.
Use approximations, over time. Go for trends over time, rather than daily precision. Both in terms of calorie and protein.
I agree too with
@Manuel Fortin points and also that it is a good question.
The reason why a calorie is a calorie....
Your weight is the total molecular mass of your body's constituent parts (mass) plus gravity.
Your mass (not weight) is is the sum total of what you've been eating and drinking. We are mostly oxygen, hydrogen and carbon...the carbon in food has an energy value. Energy is measured in Joules....and a calorie is a unit of energy, from the kiloJoule.
A fundamental law of physics: energy cannot be create or destroyed, it is converted from one form into another. So the carbon we eat from digestion to cellular production of energy goes through a sequence of energy exchanges and transfers to produce ATP - the energy molecule our cells use, aka your metabolism. In doing so it produces heat - it is not 100% efficient and we have different efficiencies - NOTE different efficiencies do not change the laws of physics.
So the mass in our food contains energy which produces ATP and the production of ATP fuels biological processes and work which produces heat and waste products....one being carbon dioxide which you expel in to the atmosphere. The equation between the energy you obtain from the mass in the food you eat to create your molecular mass to the energy you expel is: energy in = energy out = no change of mass, aka calories in = calories out. Balance.
However, efficiency differences and the rate at which you use energy can fluctuate, absolutely. Your metabolism can be more efficient - upto a point and it can certainly be less efficient and your command and control centres of your brain can turn down or up the rate at which you use fuel...your basal metabolic rate, or your idle speed. And you can be more or less active. Many variables feed into metabolism but the laws of physics do not change. A calorie is a calorie.
Does that help so far?
Protein is a carbon molecule with amino acids attached. Protein can be used for fuel depending on cellular energy needs. If you are well fed and not doing exhaustive exercise then it is less likely it will be used, but who knows.
All foods are either used, stored or booted out. The aminos may hang about for a while in your amino acid pool waiting for a signal to be used for biological processes, maybe they are broken down and used for fuel and/or pissed out with your beer. The fate? It depends.
It is due to other variables that create the appearance of the variability of calorie value. ...which I think relates to your question.
As inferred earlier, I have a rough idea of calorie values, certainly could not tell you any totals of protein. I can't be arsed counting calories. What I do know is that if I have a big meal and don't clean my plate, I rarely leave any protein. Well, I rarely leave anything. No waste!
If you train and in ball park energy balance I wouldn't quibble over a bit more protein, an extra steak, ahem, here and there. Even a mild surplus could be a good thing - entirely possible to gain muscle, be heavier, lose fat mass and be leaner. If you want to lose weight, then you ought to be in a negative energy state, if that is your primary goal. If you are in a negative state but nosh down on more protein, then go for it. Don't stay in negative energy balance for long periods though.....your basal metabolic rate will likely change. No such thing as a free lunch, eh?