Scientists explain where body fat actually goes when you lose weight
University of Birmingham researchers reveal that most lost body fat leaves the body through exhaled breath rather than sweat, via a metabolic process that converts fat into carbon dioxide and water.
When a person loses weight, some of the fat stored in the body disappears — but exactly where that mass goes is less obvious than it seems. Two researchers from the University of Birmingham, writing in The Conversation, explain that fat loss begins when the body needs more energy than it receives from food, prompting it to draw on its fat reserves.
Dietary fat is broken down into fatty acids during digestion. Part of these fatty acids is used for essential functions such as building cells, regulating hormones and protecting organs, while the surplus is converted into triglycerides stored in fat cells as a future energy source. The body stores fat mainly in two forms: subcutaneous fat just beneath the skin, and visceral fat that accumulates deeper around internal organs.
When the body enters a calorie deficit, hormonal and neural signals trigger fat cells to break down stored fat, a process called lipolysis.
Fat converts into energy
Once triglycerides are broken down, fatty acids travel to the mitochondria within cells — often called the body's "power stations" — where they undergo beta-oxidation. This process breaks fatty acids into compounds the body uses for energy production, with carbon dioxide and water generated as the final metabolic by-products.
This is where the surprise lies: according to the researchers' calculations, around 84% of lost fat is converted into carbon dioxide and leaves the body through exhaled breath, while about 16% becomes water eliminated through sweat and other bodily fluids. The researchers note these figures remain estimates, since precisely measuring all of the body's outputs during fat loss is difficult.
What this means for exercise
When people exercise or reduce their calorie intake, the body's energy demand increases. If the energy supplied by food isn't sufficient to meet that demand, the body starts drawing on its fat stores. However, this does not mean that breathing faster itself causes fat loss — exhaling is simply the route through which fat-burning by-products leave the body, not the trigger for 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.
