I know this is a basic question but please bear with me.

Why is it that total calories consumed matters so much for losing weight or gaining weight, no matter if it’s fat or muscle?

Calories measure the energy that goes in through your mouth. Weight gain or loss is a measure of the total mass (or weight) of your body. So why is the total energy in through your mouth the best indicator of whether your body will gain or lose mass?

Of course I do understand that if you’re not getting enough energy in through your mouth, then your body will burn energy stored in fat or muscle, and then that will decrease your weight. I get that.

But I what I don’t get is why these other factors are not significant:

  • the energy in through your lungs, by breathing
  • the mass in through your mouth, which would be measured in grams not calories
  • the mass that goes out of my body, when I poop
  • the energy that goes out of my body, when I poop
  • potential variation in the above two quantities, if for instance the human body “keeps” more or less of what it eats depending on its needs, its preferences, its general efficiency, etc..

If the conventional way of thinking based on calorie counting works (as it seems to do!), then as an empirical matter all these other factors I list must be negligible. But what I want to understand is why.

  • 1
    The answer is: because whatever you eat, the body can, within certain limits, simply convert it to whatever it wants. If you eat protein and your body doesn't need any protein right now for construction or repair tasks, it will convert it into energy, and if it doesn't need the energy right now it will store it for later (as fat). If you eat carbs these can be converted to proteins or to fat. A kilo of lettuce is very different from a kilo of butter. So calories are the most useful measure.
    – Gaius
    Jul 21, 2019 at 13:59
  • Andy Galpin's page goes into all the detail I wanted regarding the chemistry and physiology of weight gain / loss. Turns out you lose weight mostly by breathing the carbon out of your mouth, mainly! He has videos here: andygalpin.com/55-minute-physiology
    – algal
    Jul 30, 2019 at 21:42

3 Answers 3


I think you should still learn a bit about biology/chemistry/biochemistry and perhaps ask on another site, but I don't see why you shouldn't get an answer here. I'm not going to go into great detail and explain the inner workings of the universe to you here. If you want to learn more about biochemistry or so, it might be the best to ask someplace else. Let's go through your points:

The energy you consume through your lungs: Well, you actually consume A LOT of energy through your lungs, mainly through oxygen. Here is the thing: In order to build muscle or fat, you need carbon. You do not breathe in significant amounts of molecules containing carbon you can use (you are not a plant). What you will consume through your lungs will be more or less constant every day. Oxygen is really important, but well, neither can you really control how much oxygen you will consume nor does it vary so that it matters for muscle/fat loss.

The two mass things: Well, your body can use some chemicals, others not so much. The "calories" are contained in certain molecules, in others they are not. I'm going to be really basic here: If you want to know how much food there is on your plate, you substract the weight of the plate because you don't eat it. The same here: You do not really get energy from water, you won't get much from cellulose - two very important things that you eat. If you would eat a lof of sand, your body can't use any of it. If you would put a 100g steak on a 5kg plate, you don't get a 5.1 kg Steak, you will still only eat 100g of meat. Energy is contained in chemical bonds. Some chemical bonds can't be broken by your body, some are not broken in a way that gives you a lot of energy, some might even cost you energy (calories). If you want to know more about chemistry, perhaps head over to the chemistry stack exchange.

So what about the calories that leave your body as waste? Well, that's not so easy. First of all, it really depends and is kind of impossible to track. You only know what you put into your mouth, you cannot really measure what leaves your body. The water content will vary a lot, weighing it will lead you nowhere. You would have to send it to a lab every single time and that costs a lot of money. But you do not have to worry: What you lose on average is already considered in your calory goals. I'm not going to bore you with how math and science works, but it's almost impossible not to consider it. People have been looking at calories for a while now, don't worry, they haven't missed such an obvious thing. It might vary from day to day, but unless you have a disease or so, it will average out more or less. Just find out how much you need to consume on average for your goal and don't send every poop to the lab.

Variations. Well, your last point shows that you might not understand one thing: Gaining weight, losing weight, maintaining your weight isn't something you do once for one day in your life. It's something you do over months, years, decades. It might vary sometimes, but it will average out. But you are right, it can vary from person to person. Do not expect too much accuracy here. Your weight loss/gain will not depend on a single grain of rice or a miligram of olive oil. Of course, if you use a calculator online to check your caloric needs, the calculator wasn't developed specifically for you. People are different, but people are not insanely different. Assuming you are able to use such a calculator (or whatever you use) correctly, it will give you a decent estimate. It will be an average of course, but if you feel like you are losing too much weight, you can always adjust.

I'm going to say it again: You won't lose/gain muscle/fat over night. If you feel like you are not average but need to adjust a bit, do it. You will have plenty of time to correct for that.

  • In the context of this question, which essentially asks how the energy and mass we take in contribute to weight gain, it may be misleading to say that "you actually consume A LOT of energy through your lungs," because this may sound like breathing a lot can make you gain weight.
    – Jan
    Jul 18, 2019 at 15:48

But I what I don’t get is why these other factors are not significant:

the energy in through your lungs, by breathing

You do not actually get energy from breathing. You get oxygen, which is used by muscles, organs, etc in the continuous cycle of breaking down food or fat into usable energy or storage. (Think of it as gas for the body). The basic path is below, building blocks go into the energy cycle, and out the other side comes energy and waste products.

Food + water + oxygen --> Krebs cycle --> cell energy, water and carbon dioxide.

the mass in through your mouth, which would be measured in grams not calories

It's measured in both, actually. Food has a weight, as well as a calorie amount.

the mass that goes out of my body, when I poop

the energy that goes out of my body, when I poop

potential variation in the above two quantities, if for instance the human body “keeps” more or less of what it eats depending on its needs, its preferences, its general efficiency, etc..

Ok, at the very basic level, your body essentially runs on sugar. Everything your body does in a day to just stay alive requires a certain amount of energy. This energy total is called basal metabolic rate (BMR). If all you do is lay in bed and breathe all day (Other than bathroom breaks), you will burn X calories worth of your sugar storage (Glucose/glycogen).

If you get up and clean the house, you will burn an extra bit of calories. If you eat something, you burn calories chewing and digesting. If you dig a ditch, you will burn calories. Add all of that up, add it to your BMR, and you get the Total Daily Energy Expenditure (TDEE).

If the amount of potential energy (calories) that you eat is bigger then your TDEE on a consistent basis, you will gain weight. It might be fast or slow, but you will gain weight. Conversely, if you eat fewer calories than your TDEE, you will lose weight.

Now for your grams/calories, grams is a weight measure only. You can have 100 grams of broccoli, and it has the energy potential of 34 calories. Mostly because it is carbohydrates and undigestible fiber. Chips Ahoy cookies, 100 grams, has 480 calories of energy potential. Some of this is lost in the digestive process, but because of the fat and sugar content, it has a higher calorie count.

Finally, the reason people get fat when they eat too much too often, is that carbohydrates and proteins are approximately 4 calories per gram. Fat is approximately 9 calories per gram. You're getting a little more than double the energy for a gram of body fat. So, if you eat in excess of your TDEE, your body will store at least some of that extra energy as body fat. Conversely, if you eat less, your body will dip into body fat stores to make up the difference.

This is a very stripped down view of a very detailed topic, so let me know if you need any clarifications.


You get energy into your body by food that contains caloric nutrients (carbohydrates, proteins, fats, alcohol). Chemically speaking, you also get energy from breathing in oxygen, but this is not "additional" energy, because it is already calculated in the caloric values of the foods. So, the caloric value of the food is energy that is released when it reacts with oxygen (aerobic respiration). A small amount of energy can be also released from nutrients without the help of oxygen (anaerobic respiration).

Your body burns (loses) energy by muscular work and life-maintaining processes, such as your heart pumping the blood, your brain functioning, by digestion, etc.

The mass of food you eat is not directly associated with energy gain or weight gain. Water and undigestible fiber can give a lot of mass but no energy to the food, so they will also not give energy or mass to your body (they will, but only temporarily, until you excrete them).

The same way, the mass of stool and urine is not associated with any significant energy loss - because urine (water, urea and other wastes) and stool (water, bacteria and undigested food) do not contain any significant amount of usable energy. Also, the mass loss by stool and urine does not decrease your body weight in long term, because this mass comes from the food and not from your body.

How are calories in the food associated with body fat and muscle mass gain?

Let's say that one day you consume 2,500 Calories from carbohydrates, proteins, fats and alcohol, but you burn only 2,000 Calories that day. This leaves you with an excess of 500 Calories worth of nutrients, most of which will be converted into the body fat, because this is the main way how your body sets aside excessive nutrients. The nutrients will be converted into the muscle mass only when you actively use (train) the muscles.

In conclusion: Calories consumed with nutrients and burned are the only meaningful determinant of long-term weight gain or loss.


  • "the oxygen does not contain energy, but only allows nutrients to "burn" and thus release energy" is maybe the most incorrect thing I've ever seen in a SE answer
    – Raditz_35
    Jul 17, 2019 at 20:30
  • 1
    @Raditz_35, chemically speaking, oxygen does provide energy, so I edited my answer.
    – Jan
    Jul 18, 2019 at 15:45

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