How Does Fat Leave the Body When You Lose Weight?

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Most people who want to lose weight are generally interested in reducing their body fat. But many people don't really understand how fat loss works at the molecular level. For instance, a common belief is that fat turns into energy and is burned off during exercise or when calories are reduced. While this may be partially accurate, there is a more precise explanation for what happens to fat when it leaves the body.

According to Ruben Meerman, an Australian physicist and the author of "Big Fat Myths," this common belief violates the law of conservation of mass, which states that mass, like energy, is neither created nor destroyed. Meerman, who is renowned for his fat metabolism research, proposed in his 2014 paper in the British Medical Journal that the "energy in/energy out" theory focuses solely on energy production.

When you lose weight, most of it is excreted through the exhalation of carbon dioxide. Therefore, your lungs are the primary excretory organ for weight loss.

If fat, therefore, cannot be destroyed, then where does it go? Learn how your body gets rid of fat on a cellular level and where it goes when you lose weight, plus tips to enhance the fat-burning process.

What Is Fat?

There are two different types of body fat, or adipose tissue, in the human body. White adipose tissue is primarily responsible for energy storage and releasing fatty acids when fuel is low. Your body mostly contains this type of fat, which is stored beneath the skin and around the organs. Visceral fat accumulates around the organs and causes weight gain in the midsection, which most people associate with belly fat.

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

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

White adipose tissue is the fat measured during a body fat assessment. Unless you are overweight, white fat generally makes up approximately 20% of total body weight in men and 25% in women.

Stored Energy

Fat cells are used primarily for energy to work the muscles and move the body. In addition, energy stored as fat also helps insulate the body and protect its vital organs. 

When you consume more calories than your body needs, however, the excess energy remains stored. The stored energy (triglycerides) is collected as fat (lipid) within the individual fat cells, which slowly accumulate over time and result in weight gain.

How Does Fat Leave the Body?

There is a key respiratory component to fat metabolism. Contrary to what some people may believe, the lungs are the primary organ used to remove fat from your body. Body fat does not turn into muscle or exit the body through the colon. During the energy conversion process, fat leaves the body either as CO2 during exhalations or as H20 in the form of urine, sweat, and exhalations.

Fat-Burning Process

During the fat-burning process, the body converts fat into usable energy, which causes the fat cell to shrink in size. The stored fatty acids are broken down, which releases energy and converts them into carbon dioxide (CO2) and water (H2O). This metabolic energy conversion also generates heat, which helps to control the body's temperature.

According to Meerman's research, there are a number of enzymes and biochemical steps involved to completely break down a single triglyceride molecule. Some of the fat is available for usable energy. Carbon dioxide and water are essentially waste products in the fat-burning process, and most of the fat is expelled from the body as CO2.

When fat leaves the body, 84% is exhaled as CO2 and the remaining 16% is excreted as water. So most of the fat we burn is literally released into the air.

Tips to Promote Fat Loss

When combined with a healthy, balanced diet filled with nutrient-dense foods, physical activity is probably the best thing you can do to get your heart rate up and increase your oxygen intake, which can help promote fat loss. Keep in mind that you'll still need to burn more calories than you're consuming to create a calorie deficit in order to lose weight.

Exercise increases your metabolism or the rate your body uses energy. Meerman suggests that you can use up more stored fat by performing physical activities that double your metabolic rate—for example, swapping out one hour of rest with exercises like brisk walking or light jogging.

The American College of Sports Medicine (ACSM) and the Centers for Disease Control and Prevention (CDC) recommend at least 150 minutes of moderate-intensity exercise per week, plus at least two days of strength training to promote both weight loss and weight management.

Other basic suggestions to increase your metabolic rate and reduce fat from your body include:

  • Engage in active playtime with your kids.
  • Park your car further away and walk more.
  • Stand at your computer vs. sitting.
  • Stay active over the weekend and avoid being a couch potato.
  • Take the stairs instead of the elevator or escalator.
  • Take short walks and stretch during your breaks at work.
  • Get a good night's sleep.

Your body is also hard at work removing CO2 while you sleep, and research shows that poor sleep quality is a common cause of weight gain. Not getting enough sleep can also affect your ability to lose weight.

Other Considerations

Keep in mind that everyone experiences fat loss differently, and some people, depending on factors such as age, sex, or genetics, may have a more difficult time losing weight than others. In addition, where you experience fat loss on your body varies from person to person.

While many people hope to target belly fat with diet and exercise, research shows that spot-reducing fat in one area of the body is not possible. Instead, weight loss usually causes a gradual reduction in body fat all over the body.

To successfully lose weight, a reduction of 500 calories per day is typically recommended, though this number can also vary based on certain factors like age, sex, weight, height, and level of physical activity. This calculator can give you an estimate for your daily calorie needs to meet your goals.

A Word From Verywell

It sounds simple to burn fat and make it disappear into the air, but anyone who has tried to lose weight knows it's anything but easy. There's no magic shortcut to do it: Counting calories and staying within your recommended daily range combined with a consistent exercise regimen are the best ways to promote a healthy, sustainable rate of weight loss.

If you're looking to lose weight but are unsure of the best weight loss plan for you, talk to your doctor. They can refer you to a registered dietitian or nutritionist and may suggest you work with a personal trainer to help you meet your weight loss and fitness goals.

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