Where does body fat go when we lose weight? The answer is stranger than you expect

When body weight falls, some of the fat stored by the body disappears – but where does that mass actually go?

Two researchers from the University of Birmingham explain in an article published in The Conversation that fat loss is linked to a process that begins when the body needs more energy than it gets from food, so it turns to its fat reserves to meet that need.

The fat we eat reaches the digestive system, where it is broken down into fatty acids.

The body uses part of these fatty acids for essential functions such as building cells, regulating hormones and protecting organs, while the surplus is converted into triglycerides that are stored inside fat cells to be used later as an energy source.

The body mainly stores fat in two ways: subcutaneous fat found just under the skin, and visceral fat that builds up deeper inside the body around the organs.

When the body enters a state of calorie deficit, hormonal and neural signals start to stimulate fat cells to break down stored fat, in a process known as lipolysis.

Fat is converted into energy

After triglycerides are broken down, fatty acids travel to the mitochondria inside cells, known as the body's "power stations", where they undergo beta-oxidation.

During this process, fatty acids are broken down into compounds the body uses to produce energy, and these reactions generate carbon dioxide and water as the final by-products of metabolism.

And here lies the surprise, according to the article: most of the fat the body loses does not leave through sweat, but through the lungs while we breathe.

Based on the researchers' calculations, around 84% of lost fat is turned into carbon dioxide and leaves the body when we exhale, while about 16% becomes water that is eliminated through sweat and other bodily fluids.

The researchers note that these percentages remain estimates, as it is difficult to measure all of the body's outputs precisely during fat loss.

What does this mean for exercise?

When people exercise or cut their calorie intake, the body's need for energy rises.

If the energy supplied by food is not enough to cover this demand, the body starts using its fat stores.

But this does not mean that breathing rapidly causes fat loss. Exhaling is the route through which the products of fat burning leave the body, not the process that triggers fat burning itself. Maintaining a calorie deficit, through diet and physical activity, remains the key factor that pushes the body to tap into its stored energy.