[eli5] Why does 1g of protein give 4 calories, while protein is used in muscle building, so if not taken beyond limit, all protein would be used in muscle building and not respiration to give off energy?

Question

Okay so this assumes that calories are basically energy from respiration, if im wrong please correct me as well, i know that a calorie is considered the energy to heat up 1g of water i think or something, but generally its energy , and the body currency of energy is ATP which is mainly from respiration right, so amino acids from protein are usually not used in respiration unless taken beyond a limit, but that isn’t taken into account when calculating its calories, every 1g of protein gives off 4 calories same amount as carbs, which is the main source of fuel for respiration lol, is there something i misunderstood about calorie concepts?

In: 13

Answer 1

1g of protein supplies 4 calories (on average) because the molecule is more oxidized then a fat. Fats supply 9 calories per gram because fats have more C-C and C-H bonds that release more energy when the bonds are broken. Proteins have more C-O and N-H bonds which are lower energy.

Metabolism is molecules to CO2. If you burn a protein, it should release 4 Calories per gram. If you burn a fat it should release 9 Calories per gram.

These calories only matter if they are oxidized and used for metabolism. If you store molecules in either muscle or fat – the caloric value is saved for later.