How Does Fat Leave the Body?

We live in a society obsessed with fad diets and weight loss. You may even be trying to shed some body fat. Have you ever wondered where fat goes when you lose it?


There are misconceptions about fat loss among doctors, dietitians, and fitness professionals according to physicist Ruben Meerman. His fat metabolism research was published in the British Medical Journal in 2014 and suggests that most health professionals don’t know how weight loss works at the molecular level.

Energy and Fat Metabolism

Most of us believe that fat turns into energy and is burned off during exercise or when calories are reduced. Meerman suggests that this belief violates the law of conservation of mass. He believes that the “energy in/energy out” theory stems from university science courses focusing only on energy production. What has been overlooked, according to Meerman, is the respiratory component necessary to completely metabolize human fat.

There may be misconceptions regarding the byproducts (CO2 and H20) of energy production and how they leave the body, according to Tony Maloney, an ACSM-certified exercise physiologist. Meerman is clarifying that the majority is exhaled through the lungs and explains how fat actually leaves the body, says Maloney.

The research sheds new light on common myths about fat metabolism. It makes sense that fat doesn’t magically turn into muscle or exit your body when you poop. Also, what you have believed about energy and fat loss is not entirely wrong considering fat is your secondary energy source.1

The point of the research is not to argue that fat is used or converted into energy, but more how it actually leaves the body, suggests Maloney. There is more to the fat loss equation than just “energy in/energy out.” Understanding fat metabolism at the cellular level will clarify how and where fat goes once it is burned. 

What Is Fat?

The clinical term for body fat is adipose tissue. There are two different types in the human body. The white adipose tissue is primarily responsible for energy storage and releasing fatty acids when fuel is low. Your body contains mostly this type of fat. It is stored beneath the skin and surrounding organs. This is the kind of fat that most of us are trying to lose. 

Brown adipose tissue is considered good fat that helps regulate body temperature. It’s derived from muscle tissue and burns calories to keep you warm. Brown fat also contains more capillaries than white fat and shuttles valuable nutrients and oxygen throughout the body.

Fat is made up of individual cells called adipocytes (cells that contain fat). The human body contains billions of fat cells ranging in different sizes. White fat cells are filled with one large fat droplet surrounded by water, salts, and protein. The fat droplet is comprised mostly of triglycerides (glycerol and three fatty acids). High levels of triglycerides in the bloodstream have been shown to increase your risk of heart disease and stroke.

Brown fat cells contain multiple fat droplets and considerably more water, salt, and protein. These cells are also filled with lots of mitochondria responsible for the chemical energy that burns calories to produce heat in your body. 

What Does Fat Do?

Fat is made up of cells in your body that are used primarily for stored energy and protection, according to Maloney. The body uses this stored energy for working muscles as well as a host of other metabolic pathways and enzymatic breakdowns.

When you consume more calories than your body needs, it will store the rest within your fat cells or adipocytes. The storage form of energy is known as triglycerides, a type of fat or lipid collected within individual fat cells. Besides providing energy, stored fat also helps insulate the body and protect vital organs. 

During Fat Burning

Before explaining what happens during the fat burning process, it will help to understand where all the weight within the fat cell comes from. 

Meerman indicates the average American breathes in about 1.5 pounds of oxygen daily. This is in addition to what you eat and drink every day. According to the latest government figures, the average person consumes approximately five to 7.8 pounds of food and beverages daily. What you eat and what you breathe needs to exit your body somehow if you want to lose weight. 

During the fat burning process, the body converts fat into usable energy causing the fat cell to shrink, according to Maloney. This metabolic energy conversion also generates heat which helps to control body temperature. At the same time, oxygen is also converted into byproducts.

Many enzymes and biochemical steps are involved to completely break down a single triglyceride molecule, according to Meerman. Some of the fat is available for usable energy, but carbon dioxide (CO2) and water are also released from the fat cell during the process. In fact, a large percentage of carbon dioxide (CO2) is created and expelled from the body when you burn fat. 

Where Fat Goes

Most of us really don’t think about where fat goes when we lose it. We’re just happy the scale says it’s gone. You may be curious to know fat doesn’t magically disappear after going through the fat burning process.

The research calculations show when fat is lost, 84 percent is exhaled as carbon dioxide. The remaining 16 percent is excreted as water.1 During the conversion of energy, carbon dioxide and water are byproducts or waste, according to Maloney. They are excreted via urine, perspiration, and exhalation.

The research study also reveals the lungs as the primary organ used to remove fat from your body.

Ways to Improve Fat Loss

Since fat leaves the body by exhaling carbon dioxide, you may be wondering if breathing faster will help you lose weight. Unfortunately, this isn't an effective method. You will only cause hyperventilation, feel dizzy, and possibly faint. 

There are healthy ways to increase oxygen intake and improve weight loss. Working toward improving your metabolic rate would be a great start. This includes being more active in general and participating in regular exercise.

Meerman suggests you can increase carbon dioxide (CO2) exhalation by performing physical activities that double the metabolic rate.1 For example, swapping out one hour of rest with exercise like jogging removes more CO2 from the body and improves your ability to lose fat.

Other basic suggestions to increase your metabolic rate and rid your body of CO2 include the following:

Take the stairs instead of the elevator/escalator.Park your car far away and walk more.Engage in active play time with your kids.Stand at your computer vs. sitting.Take walk and stretch breaks at work.Stay active over the weekend and avoid being a couch potato.

Your body is also at work removing CO2 while you sleep. In fact, you exhale approximately seven ounces of carbon dioxide which is 25 percent of the daily amount you need to get rid of. This means you are waking up starting your day ahead of the game.  

A Final Word

The lungs are the primary excretory organ for fat, according to Meerman. He suggests the key to weight loss is unlocking the carbon stored in fat cells. What is recommended for successful fat loss is to eat less and move more. This means reducing caloric intake to cause an energy deficit, but also exercising regularly. Exercise will naturally increase the rate oxygen is used and help remove more carbon dioxide from your body. 

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