Your first mistake is watching a show, movie, documentary or whatever and taking it at face value. All food has calories and will be used to fuel your body. That’s a basic building block. When you start getting into different macronutrients, that’s the next tier of understanding how nutrition works. It’s not worth considering part two until you understand part one.
Protein is added in TDEE not just for energy intake but for the essential amino acids too. Protein makes up 70% of our cells and we know cells in our body are replacing other cells. for example, we shed 200,000,000 skin cells in an hour. So Protein is an essential raw material of the cell is required for this. Not only this our body has to maintain fluid balance and protein plays a very important role in it. Several enzymes and hormones are also protein in nature.
You’re kind of right, OP. Our body is going to use the most efficient forms of energy first. Proteins are great and are absolutely necessary for some things (mostly structural-type uses in the body), but they are inefficient at turning protein into glucose. Fats are next, your body doesn’t mind using fats for energy, but they aren’t the easiest to use. Then there’s carbs. It’s easy to turns carbs into usable energy since there is so little that has to be done to them. So our bodies prefer carbs.
But your body can synthesize carbs from fat or protein. Carbs are entirely unnecessary from that standpoint.
The reason that we prioritize protein is that you have to ingest them to get them. Your body needs a certain amount of protein to function. Once you have that amount, then your body does what it does with everything else, it converts it into stored energy or it expels it.
I think any excess calories we consume in any form will be converted to energy and stored as fat. Basically I think the notion that protein is only converted to energy if glycogen stores are depleted is incorrect. Any excess protein that is not used for other functions such as structural repair will be converted to energy at some point.
My amateur opinion is that it is just convention/easier to approach energy needs from food intake point of view. Yes, depending on your cell maintenance needs, ingested protein might not contribute at all to daily energy expenditure, because it is all used for creating new proteins (and not energy). But since protein still burns in a caloriemeter and was shown to release 4 kcal of energy, and we need to consume protein as part of our regular diet, it was easier to approach the nutrition needs (which cover energy expenditure needs + protein need for maintenance) based on food input and not food usage.
When we try to measure effective energy contribution of different macronutrients, things get complicated. As mentioned, protein burn (in a calorie meter at least) for 4 kcal/g. But how much does ingested protein contribute to energy needs? For example, the absorption of protein in the intestines requires more energy than glucose and fat. So even if all absorbed protein would be used for energy needs, the net contribution is already lower than 4kcal/g, because energy was spent in the process to make that protein available for energy use in the first place. We just don’t have practical answers to these questions outside of very specialized lab settings.
It’s sorta hard to explain but amino acids can go through de-aminification (as it sounds it’s just breaking down amino acids further). It is harder to use protein for energy and it is harder to even store it as fat, BUT it will still be used for energy when needed. Situations where it can be used as fuel more is in marathon type training (excessive, long endurance), when you’re in a deficiency of calories, and such.Protein also is used as energy when being broken down as a part of basic chemical reaction. Basically, long story short, it still contributes to TDEE.
There’s an easy solution to this.
Eat your maintenance. Do you lose eating protein on maintenance? No. My TDEE is 1500 cal, I eat 100g protein a day. That’s 400 cal in protein. At 1100 calories I’d lose weight, at 1500 I don’t. So clearly your body uses the protein as energy and calories are consumed. People who eat primarily low calorie and high protein and eat at maintenance or a surplus also maintain or gain weight. Many people bulk by doing 1g per 1lb body weight, in there cases they’d be losing incredible amounts of weight during the bulk because it’s unused.
Protein is consumed and converted to energy just like every other macro. Anyone who tracks macros is evident of that because after 4 years of macro tracking, my protein isn’t magically a deficit. Protein isn’t a fiber, it gets broken down by the body and disposed of as such. It doesn’t get passed like dietary fiber would.