Order of energy sources

[Eurofusion]

I'm trying to understand the order in which particular foods are used for energy. I was under the impression that fat is not used for energy until ketosis sets in during a 100% carnivorous diet. So then, if I am eating enough carbs (small amounts of fruit) then this is my first source of energy, and then when that runs out, my muscles become the next source of fuel. Or does meat become the next source? (That would make more sense, but I have not read much about that). Any supporting text/links? I read once that fat was the first source of energy, but now I am confused.

[Raw Kyle]

Fat can be used for energy before keto-adaptation, just not all of the energy in the fat. I forget the biochemistry but the liberation of the glyceride part of the fatty acid gives you fuel, and maybe something with the chains themselves.

[Hannibal]

The fat is used as the main source of energy after several weeks being on low-carb diet
But remember - cells which do not contain mitochondria will always use glucose as the source of energy - biochemistry is clear on this subject