How Does the Body Excrete Fat?

Although losing weight is sometimes described as "melting" or "shedding" pounds, what you're really doing is exhaling them away. Fat loss happens when your body senses an energy shortage between the calories going in versus calories going out.

When your body is in this state of caloric deficit, it uses your fat stores for energy in a complex chemical process that eventually leads you to excrete most of the excess fat via your lungs.

Boston Medical Center reports that 45 million people, on average, go on a "diet" each year to lose weight. Given that more than two-thirds of adults have overweight or obesity, according to the 2010 National Health and Nutrition Examination Survey, it makes sense that losing fat is a priority for so many people.

Having overweight, which is defined as having a body mass index of 25 or greater, or obesity, a body mass index of 30 or greater, carries significant health risks. For example, your risk for heart disease, metabolic dysfunction, some cancers and osteoarthritis increase. Losing weight helps improve your health and diminishes your risk for developing these conditions.

A weight-loss plan that involves moving more and eating less works because you use up stored energy in the form of fat. Where that fat goes when used for energy, though, is somewhat mysterious to most people.

Many misconceptions exist about the fate of fat during weight loss; even medical professionals and scientists are sometimes unclear as to the chemical process.

Researcher Ruben Meerman from the University of New South Wales interviewed doctors and biochemistry students prior to publishing research about how fat leaves the body in a issue of the 2014 British Medical Journal; many people he talked to believed that the body turned excess fat into heat that radiates into the atmosphere.

Other inaccurate myths suggest that you urinate excess fat out of your body or that it just disappears. Fat doesn't turn into muscle, either. Meerman's evidence, which tracked every atom as it left the body, conclusively determined that oxidized fat leaves primarily through the lungs.

Your body stores fat in adipocytes, or fat cells, in a form known as triglycerides. This form can't be used directly for energy. When your body senses a calorie deficit, it breaks them down into glycerol and fatty acids that get released into the bloodstream.

As a result, the fat cells shrink, but never disappear. The glycerol and fatty acids are then used to create fuel for energy -- to support basic body functions, chores and exercise.

Fat consists of three elements: carbon, hydrogen and oxygen. When the triglycerides break down, it unlocks the carbon stored in the fat cells in a process that results in the creation of carbon dioxide and water.

The chemical reaction creates heat as a byproduct, but that isn't how the fat leaves your body. The fat actually excretes as about 85 percent carbon dioxide through the lungs and 15 percent water through your urine, feces, sweat and tears.

To lose fat, create a calorie deficit so your body must reach into its fat stores for energy. A pound of fat is equal to 3,500 calories. If you eat 250 to 500 calories fewer per day and exercise off an additional 250 to 500 calories per day, you'll create a large enough deficit to lose 1 to 2 pounds per week.

Exercise helps you lose fat more quickly because it increases your need for fuel and creates a larger calorie deficit. When you consume more calories than you use, however, your body goes back to filling up the deflated fat cells, and you gain weight.