The fat burning process is a complex biochemical process.

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.

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? 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, and 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. 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 micrograms 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 a reserve source of energy and fat cells operate like reserve 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 “shrink” or “empty out” fat cells.

FAT is not merely an inert storage tank for surplus calories. Fat cells don’t just store fat, the fat tissue is actually very much part of the body. Fat is an active organ than sends chemical signals to other parts of the body. The adipose tissue (FAT), occupies the niche of an endocrine organ, along with the pancreas, thyroid and adrenal gland.

The more traditional organs typically produce one or two hormones. However, fat cells, spit out a huge array of chemicals - at least 80 different substances. Among these hormones is leptin, which controls appetite, and adiponectin, which makes the body more sensitive to insulin and controls blood sugar levels. However, little is known about most of the proteins produced by the billions of fat cells in the adult body. Scientists have identified 80 different proteins produced by fat cells. These include new proteins and 20 proteins that have not been previously detected in human fat cells.

Many of the fat chemicals are not unique to fat cells, but a handful, such as leptin and adiponectin, are only made by fat cells. Scientists are trying to figure out what all these chemicals do, but many of them are involved in inflammation. The official term for all these chemicals being produced by fat cells (adipocytes) is adipokines.

Important Lessons:

Calories count. The signal that triggers your body to release adipose from fat cells is an energy deficit… you have to take in less energy from eating or drinking food than the amount of energy that your body uses ("burns") to sustain all of its ongoing activities.  Most of the body's activities are involuntary (such as breathing, cell repair, pumping blood etc.) which would continue even in a coma.  Voluntary activity, such as normal physical movement - which includes additional exercise/fitness activities - normally accounts for only about 16% to 20% (sedentary) to 25% (physically active) of all of the energy used by the body.

Eating food and fat burning are inter-related. You cannot lose weight without cutting calories, but when you cut calories, your body decreases many of its automatic functions, including the activity of fat burning enzymes that release fat from the cells.

After weight-loss, you must be forever diligent. Your fat cells are not gone, they have merely “shrunk” or “emptied out.” Weight-loss maintenance requires continuing a lower-calorie lifestyle forever.

Genetics and Life-History are factors. You can’t control the number of fat cells you currently have, but you CAN achieve control over SOME of the factors that determine how much fat you store. Reducing and limiting your food intake, through portion controlled eating, is the most important helpful factor in weight-control.

"Approximately two-thirds of people who lose weight will regain it within 1 year, and almost all of them will regain it within 5 years.

Although dieting (ie, caloric restriction) to lose weight is a difficult task, the maintenance of lost weight requires the patient to deploy even greater efforts.

Rather than a simple lack of willpower, the relapse of most individuals to their previous weight after otherwise successful weight loss is largely driven by the coordinated actions of metabolic, neuroendocrine, autonomic, and behavioural changes that oppose the maintenance of reduced body weight.

The few individuals successful at maintaining weight loss (at least 13.6 kg (30 lbs) for at least 1 year) generally have common behaviour and strategies that include consuming low-energy, low-fat diets; engaging in high levels of physical activity; consistent self-monitoring of body weight and food intake; eating breakfast regularly; and demonstrating a high level of dietary restraint.

It is highly unlikely that some of this behaviour can be emulated by most of the population with excess weight.

There is also concern that unhealthy weight control methods (eg, fasting, meal skipping, laxatives, diuretics, stimulants) might ultimately lead to a larger weight regain and pose a risk to both mental and physical health.

Thus, although sustained weight loss with diet alone can be possible for some individuals, agreeing on realistic weight-loss expectations and sustainable behavioural changes is critical to avoid disappointment and nonadherence.

Weight regain (relapse) should not be framed as failure but as an expected consequence of dealing with a chronic and complex condition like obesity."

This has been true for me personally.  See my previous article:  Running DOWN the UP Escalator.

The above article involves a paper about the Widespread Misconceptions About Obesity published in Canadian Family Medicine in November 2014 written by obesity experts - Dr. Sharma et al, and it was originally posted originally posted at his website - Dr. Sharma’s Obesity Notes.