The surprising final step of fat loss happens in the lungs, where most burned fat appears to leave as carbon dioxide.
In order to lose weight, most of us know it’s a matter of eating less and being more physically active. By putting our body into a calorie deficit, we “burn” more calories than we’re taking in—and so we lose weight.
But while most of us are familiar with how to make weight loss happen, we rarely think about where the weight actually goes when we lose it—other than off the scale and away from our waistlines.
If you’re not sure where the weight goes, you’re not alone—there’s even confusion among dietitians and doctors. But if you want a hint at the answer, you’re going to want to take a deep breath and exhale.
How the Body Stores Fat
When we lose weight, we primarily lose body fat. This fat has been taken from the foods we eat and stored by the body.
Any fat we consume is broken down by our digestive system into its smallest components, called fatty acids. These fatty acids are then transported around the body to help maintain its normal functions, including supporting cell structure, regulating hormones, protecting our organs, and acting as a vital energy store.
The body stores any remaining fatty acids in the form of triglycerides. These triglycerides are then stored within our fat cells (adipocytes) for later use, such as when we need extra energy or when there’s limited food available.
From Fat Stores to Cellular Fuel
We have two main forms of stored fat: subcutaneous fat, which sits under our skin (such as on our tummy and legs), and visceral fat, which sits deeper in our body (including around our organs). Most of the fat we lose is subcutaneous fat, as this is where most of our body fat is stored.
When the body is under metabolic stress (such as during a calorie deficit), it begins relying on its energy stores. The fat cells, which have stored this energy, receive signals from specific neurotransmitters and hormones in the body to trigger the process of lipolysis.
During lipolysis, the triglycerides stored in our fat cells are broken back down into fatty acids. These fatty acids are then taken up into our cells’ mitochondria (their energy production center), where they are used to provide energy via the fat-burning process of beta-oxidation.
How Lost Fat Leaves the Body
The beta-oxidation process essentially cuts carbon dioxide molecules from the fatty acids, leaving a compound called acetyl-CoA. This acetyl-CoA is then converted into adenosine triphosphate (ATP), an extremely important molecule that provides the energy needed for almost every process that happens in the body.
As a consequence of the beta-oxidation process—and the subsequent production of carbon dioxide that occurs—we actually end up breathing out the majority of the fat we lose during weight loss. A lesser proportion of the weight we lose is water, which is expelled in our sweat.
Some researchers have suggested that around 84% of the fat we lose is exhaled as carbon dioxide, and 16% is excreted as water via sweat. But the exact proportions are difficult to estimate directly due to the logistical challenge of measuring possible human outputs over the amount of time required to observe fat loss.
Turning Fat Burning Into Weight Loss
So overall, metabolic stress from diet (calorie intake), exercise (calorie expenditure), or a combination of the two can increase beta-oxidation (fat burning) to provide energy. When we lose fat, that fat is lost from our body either in the air we breathe out or in our sweat.
So if you want to trigger the fat-burning process and lose weight, the simple age-old message remains. Eat a little less and prioritize healthy foods. Exercise can also aid in fat loss as well as improve muscle mass.
Adapted from an article originally published in The Conversation. 
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