Proteins are a harder form of calorie to convert into energy then either carbs or fat and ideally your body would never have to. The process is a bit more involved and requires more energy to produce (food is broken down into ever simplified amino acid chains until eventually you get an individual amino acid that is then consumable by the body). Protein is far easier to use and store because unlike carbs, amino acid isn’t just a means of fueling muscles, it’s the core building block of the body, from DNA/RNA to muscles.