[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

ATP is the most famous molecule in a long and complex series of chemical reactions which harvest the energy from carbs to do work in the body like muscle contractions. The energy in carbohydrates that is harvested comes from the chemical bonds in the molecule, and when those bonds are broken by via a combustion reaction (AKA burning), that’s what releases the energy.

A similar thing can be done with protein, but it involves a different series of reactions. I’m not aware of whether ATP is also part of this process. But, you’re right, protein is primarily used as building blocks for your body so it would be a waste to use it as fuel. Your body only begins to burn protein for energy when it has critically low carb and fat reserves (you’re starving). However, we can still measure the energy available in the chemical bonds of protein the same way we measure it for carbs or fats: by burning it in a lab. That’s where the 4 calories per gram figure comes from.