Calories are interchangeable like this percisely because a calorie is a unit of energy.

This “energy” we speak of is in stored as chemical potential energy of molecules.

When the human body digests foods, it breaks down molecules to build new ones through chemical reactions. Some such reactions release energy, while others require outside energy to happen. Some molecules are, likewise good stores of energy for the body because they take part in reactions that release energy.

But, at the end of the day, energy is energy. Another type of chemical reactions that release energy is burning. It just so hapoens to be much faster and easier to create and control than the work an ingestive tract does.

The only difference is that burning converts things into a slightly different set of molecules than digestion would (with burning releasing all energy and digestion leavinf some untapped), so energy released by burning isn’t 100% on par to the energy extractable to a human digesting it.

That being said, the difference between the “theoretical” energy (burning) and usable energy (ingestion) isn’t too important. You may put in the 1500 calories on the label, but you won’t utilize all of them. However, taking into account the fact that whenever energy is measured, it’s measured by burning we stay consistent. We may not be 100% percise, but we’re at least consistently wrong. And the amount of unavailiable energy is incredibly small - humans are actually more efficient than machines from an “energy efficiency” standpoint. Given the fact that each person has a different metabolism (and metabolism changes regularily throughout the day, year and with age), neither does trying to be 100% percise make sense, since your values for today will be different from your values for tomorrow.

About losing weight: Weight is lost when energy is taken in, and gained when it used.

Since a human uses about 2000 calories a day, 1500 was discovered as the best middle ground between starving and not gaining weight altogether.

It really doesn’t matter where the calories come from because the only important thing for tracking weight is net energy, gained or lost. 100 calories “trapped” in sugar is the same as 100 calories “trapped in fat”. With the human body being as efficient at sucking out energy out of stuff, the only real difference is in how long the process takes - energy in sugars is practically instantly availiable, while energy in protein takes some time to be extracted.

A net gain or loss of 200 calories is the same, wether it’s through sugars or proteins. But, for the body, it’s all the same. If it has a sufficit of energy it’ll store it (and you’ll have a net weight gain). If it has a deficit, it’ll seem you’ve lost weight, as that energy went into something other than your body’s reserves.

The others have put good descriptions of why calories are an accurate measurement for food energy.

However, you are absolutely correct that calories are not a perfect measurement, and different types of foods are not one to one replaceable. 1500 calories of sugar is NOT the same as 1500 calories of protein!

Burning the food produces a reasonable and useful approximation of the available energy.

Does the human body burn food? Of course not. We transform food into useful components and then pump them around the body to be used by cells.

If you eat 1500 calories of protein, your body will use some of those calories simply as proteins, rather than breaking them down into energy (via sugar). Which means you will have less food-energy in your system and are more likely to run a deficit.

Again with protein, the transformation of protein into sugars which can be used as energy takes energy, so you end up with a smaller amount of calories actually being available.

TL;DR Calories are not perfectly interchangeable. However, they are our best, and most useful, quick way of approximating energy intake from food.

There’s also the concept that the food we eat is more than just calories for energy. We also require nutrients to keep our biological machine tuned up. So just injesting calories from sugar isn’t going to give you the nutrients you need to keep your body working properly as you burn those calories.

“Calories” is actually two different things. The first thing is a unit of energy. In this sense, calories are very much interchangeable. Wood has calories, which is why we use it for fire. However, if you tried eating wood, you would mostly just be increasing the caloric value of your poop. This is not inherent to wood; if you were a termite and tried eating wood, you would actually get nutritionally relevant calories from it.

For nutritional purposes, we generally use some variant of the Atwater system. The core idea was to measure the caloric value of food, as well as the caloric value of the subjects feces and urine. This gives you a better estimate of how many nutritionally relevant calories there are.

Nowadays, we have standard values various core food components (e.g various fats, proteins, etc). By breaking down a food into its components, we can apply the standard conversion for each component and add up the results to get a value for the food as a whole.

This process is actually pretty bad. The digestibility of individual components does not perfectly predict the digestibility of a whole food. The measure of individual components is not perfect. The actual digestibility of some foods can vary significantly between people.

As a practical matter, “counting calories”, really just means eating less in a way that roughly measures food by effective energy content. It turns out that an accurate accounting of calories just isn’t super important or useful for this. There is even bigger variance in the “calories out” department (including the annoying tendency of bodies to become more energy efficient when less energy is available). Further, all of the errors in calorie counting tend to be consistent. If you reduce calories by reducing the quantity of food you eat, you are reducing actual metabolized calories, even in the exact measurement is wrong.

It is a little more complicated if you reduce calories by changing the composition of the food you eat, but broadly speaking lower reported calories are actually lower effective calories there as well. Further, if you are adjusting the composition of your food specifically enough for this to be a problem, then you are well past the point where you should be caring about other nutritional factors.

When you inhale you breath in oxygen, your body then adds carbon to that and you exhale CO2

With every exhale you’re literally losing carbon, when you eat you add carbon back into your body.

Burning wood on the fire works the same way, wood is carbon and when burnt in a fire it gets added to Oxygen and you get CO2

A fire and your metabolism both just take carbon and “burn” it with oxygen releasing CO2

You’re basically a big bucket of carbon and water with a slow leak.

So if you eat less “carbon” in a day than you breath out, you’ll lose weight

According to a book I am reading, diet science currently agrees that there is one way to loose weight: A calorie deficit.

Not actually true, broadly true, but not actually true.

For example, if I need 2000 kcal a day and eat only 1500 kcal a day, I will loose some weight over the next weeks/months.

I strongly recommend not doing a blind calorie deficit for body composition purposes. This non-fasting deficiency will reduce your resulting metabolic rate.

Calories are basically measured by burning food and measuring how much energy was set free.

Calories are measured by exploding items in a bomb Calorimeter and measuring the photons produced. This is a mechanical apparatus that tells us about combustion, but is not the human digestion system (there are real differences). For instance wood has amazing calorie density but doesn’t do much for you as a human if you eat it.

My question is: Why and how does it work so good and why are calories interchangeable?

It broadly works in that if your burning X calories per day but only supplying X-500 calories the deficiency will need to be mobilized from your stored fat (ideally), but the body can also solve this little math equation by reducing your metabolic rate reducing the burn, it certainly will increase your hunger and cravings.

Calories are 100% NOT interchangeable.

They key is that thermodynamics applies to closed systems without mass transfers. Humans are open systems that do mass transfers all the time pooping, eating, drinking, peeing, breathing (carbon, oxygen, water vapor), and while humans don’t violate the laws of thermodynamics it is nearly impossible to isolate the system to meaningfully

The far more clinically relevant viewpoint is the [Paper] The Carbohydrate-Insulin Model of Obesity - Beyond “Calories In, Calories Out” - 2018

Fat burning, requires the absence of insulin in the blood. People using CICO for weight control are entering a low insulin state between meals, when they sleep, etc. Any time carbohydrates are consumed, blood glucose rises, raising insulin - when insulin is elevated the body will not mobilize fat from adipose tissue, i.e. it pauses fat burning. So someone eating the standard 3 meals and 3 snacks with carbs is pausing their fat burning 6 times per day (for 2-4 hours per eating session depending on their inherit insulin resistance) - Which is almost the entire day.

This means a calorie from carbohydrates pauses fat burning for 2-4 hours, but a calorie from fat or protein does not. A calorie is not a calorie.

Why is there no difference (concerning weight loss), if I eat 1500 calories as pure sugar or eat them as pure protein (where I would assume the body needs more energy to break down the protein)?

There is a huge difference, as detailed above, the 1500 calories of sugar pauses the fat burning, the 1500 calories of protein does not. At the end of a day of eating sugar you will have less time actually burning stored fat.

The downvotes I get when I talk about the limitations of the CICO rule of thumb show how appealing it is to people, but the reality of the human system is rooted in hormones and they can’t be ignored.

It is possible to burn fat while consuming more then 1500 calories per day, body recomposition is documented in case studies of hyper caloric diets… this only works because of the insulin hormonal model.

Because thermodynamics exist. Food is compact nature way of storing energy. If you overeat, then your body will store excess energy in fat. Because it hasnt been all used. If you undereat energy is being used from for but rest of it will have to come from fat. Energy, measured in calories, had to come from something. You poop some stuff out too i would guess.

So calorieswise proteins, carbs, or fat - doesn’t matter. This matter when talking about proper diet, person needs protein to grow, fat to syntetise some hormones, and carbs as source if quick energy. For good poop you need fiber too.

You are correct, body uses up energy to process proteins and other matter. It is assumed overall that all of these processes are part of that body upkeep of 1400kcal daily.

I lost about 60 pounds over 6 months before by doing a simple calorie deficit of 1200 Calories/day but eating very lean and green. Mostly just tofu and broccoli along with an hour of exercise (30minutes cardio minimum) 5 times per week. It definitely doesn’t hurt to make sure you’re getting enough natural vitamins in you to keep your system from losing valuable nutrients.

To answer your question: a pound of fat is roughly 3,400 Calories so it doesn’t matter where they come from, only that they can be burned off through concentrated and sensible effort.

mass and energy are equals. it’s like if a car got smaller whenever it ran out of petrol to burn. it’s got to get energy from somewhere to carry on, so it converts mass (fat) into energy to burn it as fuel. if you overfill the tank (eat more calories than you’re burning), the petrol’s gonna pour into other parts of the car (fat is stored) to be used for later (and the car gets heavier). If you burn half the fuel in the car, the car’s going to be heavier, but if you push the car and burn more fuel than you’re putting in it’s going to get lighter

I don’t think I’ve seen the answer you’re chasing.

I also don’t have it but it’s a very intriguing question.

My assumption is that it is just close enough and we based the amount we’re meant to eat off of what worked and retroactively added the numbers in.

As for the interchangeability of it all, at the end of the day the body wants glucose ameno and fatty acids to make energy so it breaks things down into it. Most stuff is made of the same basic three building blocks.

They’re both related chemical reactions releasing bound energy in our dense fuel (food).

https://www.youtube.com/watch?v=d9hffYCwfEA

Also, it is inexact, just most bodies are more than happy to store the extra energy. And our digestive systems efficient I’m breaking the same chemical bonds that fire does.

Calories, in my opinion aren’t a thing necessarily. If you want to do some counting, count your carb gram intake. Reduce it and let fat be your source of energy. I do this myself, and I’m much healthier that way.