[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?


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

Calories are a measure of energy, that’s it. You can use it to describe the quantity of energy the sun emits, the quantity of energy in food, the quantity of energy in a litre of fuel if you burn it.

As for how the body generates energy, it has multiple mechanisms. It can convert sugars to energy, it can convert fats to energy, and it can convert amino acids to energy.

What gets allocated to what depends on the body’s needs, and what’s available to it. If you need energy right away and you’re exercising, sugar is the way to go. You can get energy from it fast. If you need energy after exercising, fats are a great way to get it. You don’t need lots of energy in a short amount of time after all. If you’ve got a bunch of protein available, you don’t need them for building your own proteins after breaking them down, then it’s a good source of energy just lying there.

Those mechanisms also allow you to cannibalize your own body in case of starvation. It’s a great survival mechanism since it gives you more time to find sources of food.

EDIT: adding reference https://www.nature.com/scitable/topicpage/nutrient-utilization-in-humans-metabolism-pathways-14234029/ (dense reading ahead).

Calories are a of energy. Today it is defined as exactly 4.184 joules. The calories used for food energy are more exactly kilocalories so 1000 calories or 4183 joules. Heating up water is the original definition but is no longer used.

Proteins are needed to build all types of cells and other types of tissue not just muscles. Hair for example is primarily made up of protines of the keratin type.

We use proteins both to build up cells but also as a source of energy. To what degree you use proteins as energy depends on your diet. If you eat very little carbohydrate you need to use another energy source like proteins, fat.

If we stars we can use our own tissue as a source of energy. We use fat first, one of the functions of fat in the body is energy storage.

If we need to we can break down muscle and other tissue. Breaking down muscles is not good for you but better than the alternative of just dying.

The ability to break down proteins for energy is very advationous for all animals. Even if you just eat plants you eat a lot of proteins because all living cells contain it. Regular grass is 16-28% protein. That is the percentage of dry weight. Gras is around 83% of water so it is 2-5% protein wet weight


We are omnivores and can survive just by eating animals. It will be hard to get all you need like Vitamin C but it is possible if you need all of the animals and atleas part faw. Inuites have traidtialy a mostly meat-based diet. Whale skin is a source of Vitamine-C so it raw fish.

Just eating meat does not mean you just eat fat and proteins, there is carbohydrate in meat too.

So there is an obvious advantage to being able to use protein for energy.


The idea that we just used carbohydrates, fat, and proteins for energy is a bit of a simplification. we can extract energy from alcohol, organic acids, and other stuff too https://en.wikipedia.org/wiki/Food_energy

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.

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.

By reading through your replies to other comments I think I can see what has you confused.

You are conflating how many calories a particular food gives you, with how many calories you absorb from a particular food.

Let’s try a analogy.

Let’s say I have a heap of envelops

pink envelopes (protein) have $4 inside

Blue envelopes (carbs) have $4 inside

Green evolopes (fats) have $8 inside

Let’s say I give you a pink envelope.
I also give Ann and Bill a pink envelope each.

you owe John $2

Ann owes John $1

Bill doesn’t owe John anything

When someone gets a PINK envelope John takes what is owed to him before they even get to look inside the envelope. But only from the PINK envelopes

So when you look there is $2

When Ann looks there is $3

When Bill looks there is $4

Now let’s say there are boxes full of a mixture of different colour envelopes. (Food)

And Jim from the government has the job of trying to create a system to label them so everyone can determine what they will get from each box.

Jim can’t know who owns John what, it changes constantly and is different for every person.

How should Jim label the boxes?

They only way is to label how much is in the envelopes.

Not how much each person gets to keep from the envelopes. (Which is what your trying to do)