When we talk about fat, we mostly refer to the accumulated energy in the body, primarily in the form of so-called triglycerides (triglycerides are fats that the body stores for later). The body uses them if you take in less energy (calories) than you burn, and this triggers a process where the stored fat can be broken down and used (if there is a need for it).

So what happens? A scientific study from Australia conducted by Ruben Meerman and Andrew Brown at the University of New South Wales states that if you lose 10 kg of fat, about 8.4 kg of it dissolves in the body as carbon dioxide (CO2), mostly exhaled through the lungs, the rest (about 1.6 kg) is converted into water (H2O), which is removed via urine, sweat, feces, tears, or exhaled breath.

In other words: about 84% of the lost fat leaves the body through the lungs as CO2, and 16% as water.

So, if you were wondering where the fat has gone, the answer is: for the most part, into the air when you breathe.
What this means and why it’s important

Interestingly, most people, even some doctors and dietitians, believe that fat turns into muscle or that it burns off as heat. Researchers point out that this is not the case, as this would violate the law of conservation of mass, meaning mass cannot just disappear without a trace.

So, when the body breaks down fat, it’s not that it just burns away and nothing is left. CO2 and H2O remain, and these must leave the body.

So, if you want to successfully lose fat, it's not just about eating less or moving more. It's true that these two things are the foundation, but it's also important to understand that you need to create conditions where the body can break down and remove that fat.

What this looks like in practice – concrete data

If you lose 10 kg of fat, you first need to inhale about 29 kg of oxygen (O2) for the metabolism to take place. The result is 28 kg of CO2 and 11 kg of H2O.

If you think about it: an average person weighing about 70 kg can exhale around 200 g of carbon (as part of CO2) through the lungs in one day during normal activity.

If you change your lifestyle, for example by standing up from the chair, walking, cleaning, you can already significantly increase the excretion of CO2 and thus speed up fat breakdown.

Home and inexpensive solutions – steps

Based on this understanding, we suggest a practical strategy that does not require expensive supplements or miracle diets but is simple, smart, and evidence-based. Create an appropriate energy balance, meaning you consume fewer calories than your body uses in one day. There is no need for extreme depletion. Even a slight reduction and regular activity makes a difference. Therefore, moderately increase your physical activity. Every movement counts. Climbing stairs, walking, even housework... All this increases oxygen intake and CO2 exhalation.

It is also good to get enough sleep, because sleep deprivation affects hormones and increases the likelihood that the body will store more fat. Furthermore, research from 2023 shows that certain biomarkers (features of gut bacteria, body proteins, the CO2 exhalation ratio) can predict in advance who will lose weight more successfully, which means we are not all identical and that optimizing lifestyle can make a difference.

Since part of the fat becomes water during breakdown, it is important to stay properly hydrated. Water helps remove metabolic byproducts.

Regular breathing and deep exercise

Although deep breathing alone will not remove fat, because you first need to break down the fat, exercise that increases oxygen consumption (for example, brisk walking, running, cycling) helps the body to perform this process better.

Australian researcher Ruben Meerman, based on his own experience of losing about 15 kg, began thinking about where this mass had gone. When he did not get a clear answer from doctors, he did the calculations and the surprise was great: most of it went out through the lungs!

This proves that even experts did not always have a clear understanding, so it is important that you at least get well informed.

What this means for you

If you’re wondering, does this mean that if I breathe faster, I’ll lose fat faster?, the answer is no! Deep or fast breathing without muscle activation and fat breakdown does not work. This is called hyperventilation (excessive and rapid breathing) and can cause dizziness or even fainting.

So: it’s important to activate your body and create the conditions for metabolism, then breathing can help with fat elimination.

For you this means: if you really want to lose fat in a lasting and healthy way, you do not need to buy miracle supplements or follow fad diets. It’s important to understand what’s happening in your body and then act with simple, everyday steps.