The incredible shrinking fat cell, what really happens to fat when it's burned? Tom Venuto explains how the fat burning process works in detail.

Earlier this week someone in our discussion forum wrote, "I haven't "LOST" any fat... I know EXACTLY where it went! I got a chuckle out of that because I "got" the joke, but truth is, most people really don't know how fat cells work, how the fat burning process takes place or where the fat goes when it's burned. It's actually quite a complex biochemical process, but I'll explain it as simply as possible, so by the end of this article, you'll be a "fat burning" expert!

When you "lose" body fat, the fat cell (also called an adipocyte) does not go anywhere or "move into the muscle cell to be burned. The fat cell itself, (unfortunately) stays right where it was - under the skin in your thighs, stomach, hips, arms, etc., and on top of the muscles - which is why you can't see muscle "definition" when your body fat is high.

The fat is not burned right there in the fat cell,
it must be liberated from the fat cell.

Fat is stored inside the fat cell in the form of triaglycerol. The fat is not burned right there in the fat cell, it must be liberated from the fat cell through somewhat complex hormonal/enzymatic pathways. When stimulated to do so, the fat cell simply releases its contents (triaglycerol) into the bloodstream as free fatty acids (FFA's), and they are transported through the blood to the tissues where the energy is needed.

A typical young male adult stores about 60,000 to 100,000 calories of energy in body fat cells. What triggers the release of all these stored fatty acids from the fat cell? Simple: When your body needs energy because you're consuming fewer calories than you are burning (an energy deficit), then your body releases hormones and enzymes that signal your fat cells to release your fat reserves instead of keeping them in storage.

For stored fat to be liberated from the fat cell, hydrolysis (lipolysis or fat breakdown), splits the molecule of triaglycerol into glycerol and three fatty acids. An important enzyme called hormone sensitive lipase (HSL) is the catalyst for this reaction. The stored fat (energy) gets released into the bloodstream as FFA's and they are shuttled off to the muscles where the energy is needed. As blood flow increases to the active muscles, more FFA's are delivered to the muscles that need them.

An important enzyme called lipoprotein lipase (LPL), then helps the FFA's get inside the mitochondria of the muscle cell, where the FFA's can be burned for energy. If you've ever taken a biology class, then you've probably heard of the mitochondria. This is the "cellular powerhouse" where energy production takes place and this is where the FFA's go to be burned for energy.

When the FFA's are released from the fat cell, the fat cell shrinks and that's why you look leaner when you lose body fat - because the fat cell is now smaller. A small or "empty" fat cell is what you're after if you want the lean, defined look.

It was once believed that the number of fat cells could not increase after adulthood, only the size of the fat cells could increase (or decrease). We now know that fat cells can indeed increase both in size (hypertrophy) and in number (hyperplasia) and that they are more likely to increase in number at certain times and under certain circumstances, such as:

  1. During late childhood and early puberty
  2. During pregnancy
  3. During adulthood when extreme amounts of weight are gained.

Some people are genetically predisposed to have more fat cells than others and women have more fat cells than men. (learn about your body type) An infant usually has about 5 - 6 billion fat cells. This number increases during early childhood and puberty, and a healthy adult with normal body composition has about 25 to 30 billion fat cells. A typical overweight adult has around 75 billion fat cells. But in the case of severe obesity, this number can be as high as 250 to 300 billion!

The average size (weight) of an adult fat cell is about 0.6 micrograms, but they can vary in size from 0.2 micograms to 0.9 micrograms. An overweight person's fat cells can be up to three times larger than a person with ideal body composition.

Remember, body fat is basically just a reserve source of energy and fat cells are the like the storage tanks. Unlike a gas tank in your car which is fixed in size, however, fat cells can expand or shrink in size depending on how "filled" they are.

Picture a balloon that is not inflated: It's tiny when not filled with air - maybe the size of your thumb. When you blow it up with air, it can expand 10 or 12 times it's normal size, because it simply fills up. That's what happens to fat cells: They start as nearly empty fat storage "tanks" (when you are lean), and when energy intake exceeds your needs, your fat cells "fill up" and "stretch out" like balloons filling up with jelly (not a pretty picture, is it?)

So you don't actually "lose" fat cells, you "empty out" fat cells.

Take-home lessons:

  1. Calories count! The signal that triggers your body to release adipose from fat cells is an energy deficit... you have to burn more than you eat. More info: burning fat without burning muscle article.
  2. Cut calories conservatively. Starving yourself may cause quick weightloss at first, but never works long term because it actually decreases the activity of fat burning enzymes that release fat from the cells. to avoid this "starvation mode" use exercise to BURN THE FAT, not very low calorie crash diets.
  3. Get control of your weight now. If you are gaining weight, and especially if your weight is climbing upwards out of control, make a decision to STOP RIGHT NOW. Your fat cells might be multiplying, making it more difficult to burn fat in the future. NOW is the time!
  4. If you've already lost weight, you must be forever diligent. Your fat cells are not gone, they have merely "shrunk" or "emptied out." Fitness is not a 12 week program, its a lifestyle. To stay lean you have to eat clean and stay active
  5. Genetics are only a minor factor. You may not have control over how many fat cells you were born with, but you do control the major factors that determine how much fat you store: lifestyle, exercise, nutrition, mental attitude.

Genetics are not an excuse. The past is not an excuse. Your present condition is not an excuse. You can either make excuses or get results, but you can't do both.

So keep educating yourself about the science, read these newsletters, take action every day and go out there and make it happen!

Posted on: Mon, 08/08/2016 - 17:15

Doesn't liposuction actually reduce the number of fat cells?

Posted on: Wed, 01/21/2015 - 20:30

Good article Tom, thanks for sharing and breaking it down.

Monifa Maat
Posted on: Wed, 12/17/2014 - 17:48

Thank you for this article. You clearly know what you are talking about! Unlike some fitness professionals that just try to prove how smart they are and really have superficial knowledge of subject matter and thus cannot explain it as clearly as you have in this article

Posted on: Mon, 11/17/2014 - 23:31

Dear author:
triacylglycerol, not triaglycerol.

rajni dound
Posted on: Wed, 07/30/2014 - 14:04

i already reduced from 63kg to 56 kg.n still i want to reduce till 50kg.i am little bit confused that now is it fat cell that will shrink or will need to reduce muscles for that .

rajni dound
Posted on: Wed, 07/30/2014 - 14:04

i already reduced from 63kg to 56 kg.n still i want to reduce till 50kg.i am little bit confused that now is it fat cell that will shrink or will need to reduce muscles for that .

rajni dound
Posted on: Wed, 07/30/2014 - 13:40

I like the way u explained thank you.and would like to know on calories and fats.Difference between them

rajni dound
Posted on: Wed, 07/30/2014 - 13:40

I like the way u explained thank you.and would like to know on calories and fats.Difference between them

Desi Lund
Posted on: Tue, 06/24/2014 - 05:00

good article ,

Ethan Chong
Posted on: Sat, 08/24/2013 - 09:47

Hey, I think I have all the answers that you might be looking for here at

Posted on: Sun, 08/18/2013 - 18:06

TRIGLYCERIDES not triaglycerol. Triaglycerol is not a real word. Triglycerides are what you are referring to and they are esters that are composed of three fatty acids with a glycerol backbone. Do not write an article with made up words please.

zeren gursoy
Posted on: Mon, 07/22/2013 - 17:35

excellent information!

Posted on: Thu, 07/11/2013 - 14:26

I have lost 60lbs over the course of a year not fad dieting but been been really healthy. However whenever I have been very diligent for a week or two and I am at the point of the fat actually breaking down I feel really run down and lousy. They day I loose a pound I feel really lousy can this process make you feel a bit week. I am not eating too few calories. Then a day or so later I feel fine.

Posted on: Mon, 04/29/2013 - 03:04

Very informative, thanks a lot!

Posted on: Sun, 12/30/2012 - 23:05

question: I have recurrent shingles each and every time I start to lose weight... I have heard that fat cell can store long time use of medicine... is this or could this be true for me getting shingles each time i do start to loose weight? can our fat cell store other than just fat or combo of fat and medicine and other type of stuff? or is my body getting stress out when i start to loose weight and it's to much for my immune system?

Posted on: Tue, 12/25/2012 - 22:23

I lost 175 calories

Posted on: Thu, 10/18/2012 - 11:19

My firend told me, if i go to the gym, I will just be putting weight on, She said that my fat stores lots of bad chemicals, and when i go to the gym its released. My body then reacts to the bad chemicals by storing them in more fat? This is making me unmotivated. Please let me know.

Posted on: Fri, 12/14/2012 - 09:05

Your friend is misleading you with completely false information. Going to the gym is an excellent way of working out and staying healthy. It sounds like your "friend" is holding you back from achieving your weight loss goals.

Posted on: Sat, 09/29/2012 - 03:43

It's really a gud article....i was lean and thin in my childhood days.....but in my school n college days i became fat.....will i be able to become lean again????

sridhar Kocharlakota
Posted on: Mon, 07/29/2013 - 09:19

yeap. cut the intake ( remove oily , sugary , salty food) and run a few miles every day

Posted on: Thu, 08/30/2012 - 16:00

If an obese person loses weight, does the number of fat cells remain the same, or does it revert to the normal number for a person of normal size?

Deontey Banks
Posted on: Thu, 06/05/2014 - 08:53

I think the number of cells remain the same. The cells may shrink but the fat cells that are produced can not be reduced in any way. I think thats why individuals who were at one point obese but lose a lot of weight have the loose skin after weight loss occurs. I feel that loose skin represents the fat cells that have been produced.

Posted on: Mon, 08/06/2012 - 19:42

i have a doubt. when do u break fat down and when do u break muscle down. i thought cardio is good for fatloss. but i herd people saying. too much cardio would burn my muscle down. am confused on when the fat is burned and when the muscle is burnt

Posted on: Sun, 05/06/2012 - 02:06

I am having a debate. A friend believes that at least some of the fat burned, is excerted in the urine and feces. I dont believe it. But apparently many people do.
Any comments with supported facts would be appreciated

Charita 24
Posted on: Sat, 04/21/2012 - 19:27

I have taken every diet pill under the sun, and until now the Dr Max Powers Burn supplement was the only one that worked for me consistently. But I was in desperate search for a pill that was more on the safe side and with Dr Max Powers Burn soon as i stopped taking them it seemed like my metabolism just shut off! I would gain lbs a day basically! When I started using these I got a boost of energy, and since I have taken them I don't get that hunger feeling at all. No kidding that grumble in the tummy is gone. It will be a month I've taken them in a couple of days and I have lost 8lbs and a lot of inches(don't know the exact number of inches) but i can tell you my pants are no longer tight around my belly area which is always my problem spot. I would probably have lost a lot more by now but I am recently out of work and just sit around some days and just snack out of boredom lol. But when i get up and workout just a little most of the time i do the wii dance games or zumba and my weighted hula hoop(which is a miracle for my problem tummy!) I would definitely recommend the Dr Max Powers Burn.

Posted on: Mon, 12/12/2011 - 04:52

Very good and informative article.

At the adipocyte, when adrenaline helps to unlock the molecule perilipin to allow the fat into the bloodstream, fat in the bloodstream doesn't get burned or used up by cells. It needs to go through the liver first to be turned into "usable energy," right?

M&S Team Badge
Posted on: Tue, 03/16/2010 - 11:27

Steve, here's an news article on the topic:

Steve B
Posted on: Tue, 03/16/2010 - 11:09 if u gain weight u have the potential to gain more fat cells right? But if u then lose weight, will u always have that extra amount of fat cells? If so they would just be empty fat cells?

Posted on: Sat, 10/29/2011 - 03:10

the answer is right in the atricle. it will never lose a fat cell that has been produced. it stays with you forever...just shrinked or empty. they have yet to discover how to remove a fat cell. of course another option would be lipsuction where the fat cells in a certain area is removed forever. that is why when liposuction is done and too much fat is taken out..the person looks deformed if they gain weight. so for instance...the mid section...they gain weight after and have this indent because the fat cells have not multiplied but the ones that did remain have gotten bigger...but still not big enough or in quantity of fat cells as the other areas..and i think this whole idea of fat cells never leaving ( and just shrinking) explains why some people will never look as lean as others. if you were obese the amount of empty fat add you can be very lean but still be a bit..and i mean a bit..thicker than someone who has always been slender lol i dont know if i am making sense. just sucks that i have higher amount of fat cells than thin people...means i have to be on top of my weight constantly!

sridhar Kocharlakota
Posted on: Mon, 07/29/2013 - 09:13

Yes! those empty structures are there to stay; as long as they are not refilled , you should be ok.

Posted on: Mon, 02/01/2010 - 21:14

That was very informative! thanks