Most fat lost during weight reduction leaves the body as carbon dioxide

Most of the fat lost when a person reduces body weight leaves the body as carbon dioxide through breathing, according to an article by two University of Birmingham researchers published in The Conversation. The process begins when the body needs more energy than it receives from food. It then draws on stored fat, which is held mainly as subcutaneous fat beneath the skin and visceral fat around internal organs.

Hormonal and neural signals stimulate fat cells to break down triglycerides in a process called lipolysis. The resulting fatty acids travel to mitochondria inside cells, where they undergo beta-oxidation and are used in reactions that produce energy. Those reactions create carbon dioxide and water as final by-products.

The researchers’ calculations estimate that about 84% of the mass of lost fat becomes carbon dioxide and is exhaled. The remaining 16% becomes water, which leaves through sweat and other bodily fluids. The figures are estimates because it is difficult to measure every output from the body precisely during weight loss.

The explanation also challenges the common idea that fat is primarily lost through sweat. Exercise and reducing food intake can increase the body’s demand for energy. When food does not supply enough energy, stored fat can be used to meet the shortfall.

However, the article cautions that breathing rapidly does not itself cause fat loss. Breathing provides the route by which products of fat metabolism leave the body; a calorie deficit is what prompts the body to use stored energy.