How Does Fat Leave The Body? Facts You Should Know 2023

How Does Fat Leave The Body Facts You Should Know

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Obesity and the regulation of fat metabolism are not well understood by the general population. I have come across so many baseless ideas. For example, I was once told that fat is excreted through the stool. This could not be further from the truth unless you experience malabsorption. 

When we consume an excess of calories from food, instead of being wasted, the calories are stored in the fat cells as fatty acids, which creates a bulky appearance around the body.  Even excess protein can be converted into fat when the body is taking in too many calories overall.

Body fat is not just there to give you an uneven shape, it is actually supposed to play a crucial role in life as mentioned below. 

Fat is a reserve of energy to be used in times of famine, this makes sense from an evolutionary perspective where our ancestors went days without food. 

The amount of body fat that we can store on the frame varies from person to person, as everyone has a different number of fat cells. After fat stores are drawn for energy, they leave the body in the form of sweat and carbon dioxide. We have covered this phenomenon in much more detail in the following sections of this short article. 

How Does Fat Leave the Body?

As humans, we breathe in the air for oxygen (good air). Then breathe out the leftover carbon dioxide (bad air). As we have learned, the body uses fat for energy in times of need. Fat then leaves the body through carbon dioxide and water (sweat or urine). Do not take fat burning out of context and assume that the faster you breathe out the more fat you will lose. 

What Is Body Fat?

How Does Fat Leave The Body

There are 3 types of fat within the human body, which include:

  • Brown fat
  • White fat
  • Beige fat

Brown Fat

Brown fat regulates body temperature by shivering in cold environments. For this reason, brown fat is the most thermoactive fat tissue. If your brown fat to white fat ratio is higher, you will naturally present a leaner body composition. With age the ratio of white to brown fat increases, which is one of the reasons it may be easier to gain weight. 

White Fat

White fat cells are the less active tissue that makes up most of the human body. All of the excess calories that we eat are transported into white adipose tissue as fatty acids. This fat is drawn for energy when we go through periods of not eating enough calories to maintain our current weight i.e., the weight loss phase. 

Beige Fat

Beige fat is in between brown and white fat cells. They function similarly to brown fat cells and become browned when one is exposed to cold temperatures for long periods of time. 

How Does Fat Loss Work?

There are billions of fat cells in the human body. These fat cells bundle together to make visible adipose tissue. This is known as the stored fat, which is located in various areas of the body like the abdomen, arms, legs, buttocks, etc. 

Fat cells are saturated with fatty acids, which chain into triglycerides (3 fatty acid chains). Without going into too much detail, during the fat-burning process, fatty acids are transported from the adipose tissue into the energy-producing cells called mitochondria. 

The fatty acids are then broken down into a molecule called Acetyl-coA. This molecule is then further broken down to make ATP, formally known as energy. To keep it simple, a single fatty acid chain can make up to 107 ATP energy molecules. 

If fatty acid burning is higher than fatty acid storing, weight loss can be achieved. The caloric intake from food and activity levels are what will determine the reduction in the size of fat tissue.   

What Happens to Fat When You Lose Weight?

Typically, when we lose weight from fat, the stored fatty acids within fat cells are used, which shrinks the size of fat tissue, hence why the results are noticeable during a weight loss journey. 

Even though the size of a fat cell can shrink, the fat cell itself can not be removed from the body. However, excessive weight gain can cause fat cells to multiply as a mechanism to store more fatty acids as energy reserves. 

6 Tips to Promote Weight Loss

How Does Fat Leave The Body

We have discussed 6 ways to tap into those fat cells to lose weight. These include:

Reduce Food Intake 

Sensible eating is one of many components that can lower the total number of calories that go into your body every day. If we take in fewer calories than our body needs for “normal living”, fat reserves can be tapped into for energy over time. This is termed as a caloric deficit. 

For example, let’s say that a male needs to eat 2500 calories per day to maintain weight, but cuts back to 2000 calories. This would put him into a daily deficit of 500 calories. One pound of fat holds around 3500 calories, which can be achieved in a week with such a deficit (500 x 7 = 3500). 

I urge all our readers not to take this information out of content. Although you would eat less food, you must make sure that you eat enough micronutrients and macronutrients through good food choices like fruit, vegetables, lean proteins, grains, etc. 

You can look through resources online like the NHS better health, to put together more personalized diets that focus on both calories and good food choices.  

Food Selection 

Some nutrients burn more calories to digest and absorb than others. Protein is unique and burns 30% of its own calories for digestion, absorption, and metabolism. For example, let’s say that you consumed 200g of protein per day. This 200g protein would have 800 calories (4 cals/gram), 30% of these 800 calories would be burnt to use the protein, which leaves us with 560 calories. 

On the other hand, not only does dietary fat have an additional 5 calories per gram compared to protein, but also has a much lower calorie-burning effect of around 3%. So eating the same 800 calories (88.8 grams of fat), would only burn an extra 24 calories, leaving you with 788 calories. 

The idea of food selection during your fat loss journey could be to favor food groups that are higher in protein and lower in fat.  This strategy may help lower daily calories. Generally, I recommend lowering dietary fats and carbohydrates during a weight-loss phase, while keeping protein high. 

A bonus tip for you is isolated caffeine intake. Black coffee has practically 0 calories, yet has a reasonable calorie-burning effect. For every 300 mg of caffeine that you take in, approximately 50 extra calories are burnt while breaking it down in the body. This caloric expenditure does not seem like much but does add up over the weeks. Not to mention, caffeine stimulates the central nervous system, therefore making you more active to burn extra calories through either exercise or none exercise-related movements. 

Please note that most countries have an upper daily limit of caffeine. The upper daily limit in the USA is 400 mg per day, which could look like 3-5 cups of coffee. 

Observe Exercise

Exercise is another component that should be considered in a weight loss program. The ACSM recommends 30 minutes of aerobic exercise, 5 times per week. Another option is vigorous aerobic exercise 3 times per week. Cardiovascular exercise burns lots of calories and in the short term helps with fat burning. On average, we are looking at 400-500 calories being burned during an hour of exercise. 

Aerobic exercise has also been shown to help burn visceral fat. Visceral fat is the “unhealthy” fat surrounding the organs like the: intestines, heart, liver, and stomach. 

Resistance training burns fewer calories compared to cardiovascular exercise. However, it may help put on lean muscle over time. This is beneficial as muscle is the most thermoactive tissue in the human body. For each pound of muscle that we hold on our skeletal frame, we can burn an additional 6 calories. In other words, not only does muscle improve body composition, but it can boost metabolism. 

For those with underlying health conditions and the elderly, visiting your licensed healthcare professional before starting an exercise program is suggested. 

Some examples of exercise could include:

  • Running 
  • Cycling
  • Brisk walking 
  • Rowing 
  • Cross trainer
  • Bench press
  • Barbell back squat 
  • Barbell back rows 

Consider Non-Exercise Related Activity  

Generally, people tend to value exercise too much when it comes to weight loss.  We should also consider the effects of movements that are not directly exercised. The calories from the light activities that one performs outside of an exercise routine will build up throughout the day to burn fat. 

These activities could consist of

  • Gentle walking
  • Hoovering the house
  • Mowing the lawn
  • Hand washing the dishes 
  • Hand washing your car
  • Doing the laundry 
  • Staking the stairs instead of the elevator
  • Walking to the grocery shop instead of taking the car  

Nonexercise activities have been shown to burn hundreds of extra calories per day, which can help lose fat. Personally, I like to keep track of my steps through a mobile phone tracker. On a normal day, my aim is towards 3000 steps, but I tend to build up my step count by 5000-7000 during a weight loss phase. My step count alone can help me burn over 400 calories per day. 

Manage Your Appetite

Consuming food types that are more filling than others can help cut down on overall daily calories. Protein and fiber can keep you fuller for longer so that you do not have the urge to eat larger meals or extra snacks throughout the day. In turn, this can limit the amount of calories that you eat throughout the day. 

For example, let’s say that you need 2000 calories to lose weight, but the foods you eat are not very filling. Hunger strikes encourage you to snack on peanuts, chips, candy bars, and sodas which all add up in calories. 

A study documented in the American Journal of Clinical Nutrition found that people eating a 33g fiber cereal, have less appetite compared to others who ate white bread and low-fiber cereal. All food groups had similar amounts of calories, so this “feel the full effect” was from the fiber content. 

As you can see, this reflected the average daily calories (High fiber cereal: 937 ± 86, White bread: 970 ± 65, 1109 ± 90, Low fiber cereal and water: 1224 ± 89 kcal). 

Sleep Well

Sleep does not seem to affect weight loss that much, but it does affect the bodily tissue that you lose weight from.

A classic 2010 study by Nedechelva and colleagues looked at the effect of sleep on humans who were using a weight loss diet. The groups that the researchers were testing, either slept for 5.5 hrs or 8.5 hrs per night. Both groups experienced weight loss of ~6 lbs. The groups that slept less experienced a 40% loss in fat mass, whereas the well-slept group lost 55%. This suggests that not sleeping well does not help fat loss and may draw lean mass loss. 

Based on this theory, it would be ideal to allow yourself a reasonable sleep opportunity for at least 7 hrs. 


We have learned that there are 3 types of fat present in the human body. These include brown, white, and beige. White fat is what we see stored in the body as an energy reserve. White fat is not very thermoactive compared to brown and beige variants. 

The stored fatty acids within fat cells are burnt for energy, and then leave the body through carbon dioxide and water. 

When we lose fat mass, the fat cells only shrink in size and not in number, which is different in everyone. However, fat cells may multiply if we continue to eat calories in excess. 

To lose fat you could; lower calories through a sensible diet, choose foods that burn more calories to digest, exercise, be more active during the day, and sleep well.

Frequently Asked Questions

Can you get fat by eating more carbs?

Yes! If you overeat on calories, your body would likely change them into fatty acids and store them in fat cells.

How does fat leave the body?

It is burnt for energy and the bi-product is carbon dioxide and water.

+ 8 Sources

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  1. Wang W, Seale P. Control of brown and beige fat development. Nature Reviews Molecular Cell Biology. 2016;17(11):691-702. doi:
  2. Schrauwen P, van Aggel-Leijssen DPC, Hul G, et al. The effect of a 3-month low-intensity endurance training program on fat oxidation and acetyl-CoA carboxylase-2 expression. Diabetes. 2002;51(7):2220-2226. doi:
  3. Fat Tissue Growth and Development in Humans. Accessed March 31, 2023.
  4. NHS. Lose Weight – Better Health. Published September 17, 2020.
  5. Commissioner O of the. Spilling the Beans: How Much Caffeine is Too Much? FDA. Published online December 2, 2021.
  6. ACSM’s General Exercise Guidelines. East Tennessee State University.
  7. Samra RA, Anderson GH. Insoluble cereal fiber reduces appetite and short-term food intake and glycemic response to food consumed 75 min later by healthy men. The American Journal of Clinical Nutrition. 2007;86(4):972-979. doi:
  8. Nedeltcheva AV, Kilkus JM, Imperial J, Schoeller DA, Penev PD. Insufficient Sleep Undermines Dietary Efforts to Reduce Adiposity. Annals of Internal Medicine. 2010;153(7):435. doi:


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