How Does Fat Leave The Body When You Lose Weight 2024?

Have you ever wondered how fat leaves the body when you lose weight? The answer may surprise you. 

An Australian physicist named Ruben Meerman asked this question while on his personal weight loss journey. His initial research^[1] published in the British Medical Journal in 2014 uncovered confusion even among medical professionals regarding exactly how fat leaves the body. A survey of doctors, dietitians, and personal trainers found the most popular answer was that fat is converted to energy or heat. Other responses included an exit through feces, sweat, urine, or even conversion to muscle.

Fat burners are a tempting option to get rid of problem areas, but fat is naturally lost through your lungs as carbon dioxide and water. Fat loss is the result of waste products in fat metabolism leaving the body. 

We’ll dive deep into the science of how your body burns fat and what it means for weight loss. 

How Does Body Fat Leave The Body?

The resulting carbon and hydrogen leftover from stored fat breakdown will form carbon dioxide (CO2) and water (H2O) molecules. 

The carbon dioxide will exit through the lungs as part of normal breathing. Of course, this isn’t something you can visually see unless you are outside on a cold day. But both carbon dioxide and water have a weight that is lost when they leave the body. 

Of course, we are breathing out carbon dioxide and water droplets throughout the day. It stands to reason that we potentially have a fat-burning process all the time.

To some degree, you are. But a very complex cascade of chemical reactions occurs to signal a need to release fat from adipocytes, break them down, and expel the by-products. The carbon dioxide and water you are breathing out are from other bodily processes as well. 

There must be an energy deficit to mobilize fat stores, release carbon, and have that mass leave the body on a molecular level.

What Happens Inside Your Body When You Burn Fat?

To understand what happens inside your body when you lose fat, it is important first to understand how fat is stored in the human body. 

Excess energy is stored as triglycerides (a type of fat) in fat cells called adipocytes. Excess carbohydrates, proteins, and fats are converted to triglycerides, circulating fat molecules for storage. Therefore, any excess energy is stored as triglycerides in fat cells.

There are three different types of fat cells – brown, beige, and white fat cells. 

Brown fat^[2] is located primarily around internal organs. Its purpose is to burn glucose and other lipids to maintain body temperature and protect internal organs. This type of fat is not what we want to “burn” or mobilize in times of an energy deficit.

Beige^[3] fat cells are white cells that have turned brown fat. This process usually happens in response to cold and can be reversed. 

White fat cells contain a large droplet of fat surrounded by water and salts. They are the predominant mode of fat storage in the body. The size of white fat cells can expand when excess energy is present to be stored, or decrease in size with weight loss. 

If there is a need for available energy, triglycerides in white fat cells are released from the adipocytes to be made available for usable energy. 

Hormone-sensitive enzymes known as lipases break down the triglycerides into free fatty acids. These fatty acids then enter other metabolic pathways to produce energy. However, this process alone (known as lipolysis)^[4] is not what causes actual fat loss. 

The leftover carbon and hydrogen by-products of lipolysis are excreted and thus result in weight loss.

Effective Tips To Promote Weight Loss

It may be easy to conclude from this research that exercise and a higher respiratory rate to expel more fat is the key to fat loss. While exercise is certainly important, Meerman also points out that poor lifestyle choices and excess calorie intake can quickly spoil weight loss efforts.

If you’ve ever heard the phrase “you can’t out-exercise a bad diet,” Meerman’s research would support that theory. The excess carbon lost during a high respiratory activity is not significant enough to negate high-calorie choices. 

Muscles first burn glycogen (stored carbohydrate) and then stored fat. Using stored carbohydrates for fuel is a more efficient process than breaking down stored fat.

Therefore, it is the preferred method, especially in high-intensity exercises. Once glycogen stores are depleted, fat breakdown will then occur.

However, it doesn’t matter which type of fuel you are utilizing during exercise when it comes to weight loss. What’s more important is the overall calorie deficit you create. 

With that foundation of understanding, consider these tips to promote fat loss:

• Eat less. This can be easier said than done! But a slight calorie deficit compared to your energy needs will help mobilize stored fat and result in a loss of fat. Consult with a doctor or dietitian to help determine the best approach for you to modify eating habits to create that deficit. 
• Get more movement into your day however possible. Moving more will help facilitate the energy gap needed for fat loss.
• Exercising in the fasted state (typically after waking up in the morning and before eating), has been shown to increase the mobilization of fat stores^[5].
• Strength training^[6] may also play a role in fat loss. While it may be tempting to try and target fat loss in certain areas of your body composition, research has not supported^[7] the feasibility of targeting specific areas alone. There may be some improvement^[8] when targeted strength training in combination with aerobic or endurance exercise, however. 

Precautions

While the biochemical process of how fat leaves the body sounds straightforward, it is not necessarily simple. There is significant role of genetics and many other factors at play in obesity and weight loss strategies. Everyone’s body reacts differently to energy restriction. 

Research has indicated there is a biological component in regard to weight loss.

For example, a review published in 2015^[9] showed fat cells present after weight loss create an appetite increase above biological energy needs. There is essentially a biological drive to restore fat storage amounts and promote weight regain. 

Subsequent research^[10] has similarly investigated the role of adipose (i.e. fat) tissue in maintaining weight loss. The stress on fat cells after shrinking with weight loss could be a focus of future research and interventions to combat weight gain again. 

Conclusion

The most accurate description of how fat leaves the body is through your lungs! Smaller amounts leave as water through sweat, urine, and other bodily fluids. The key to losing fat mass is mobilizing stored triglycerides in fat cells. An energy deficit needs to be present to signal fat mobilization. It’s important to consult with a nutrition expert such as a registered dietitian to determine the best diet and exercise plan for you to achieve that deficit.

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