About half of the adults in the United States are overweight. That's 97 million people who have too much fat. Many Americans exercise and go on diets to reduce their amount of body fat.

But have you ever wondered what fat is? When a person "gets fat" -- gains weight -- what is actually happening inside the person's body? What are "fat cells" and how do they work?

In this edition of How Stuff Works, we will look at the world of the fat cell. We will examine where fat cells are located, how they store fat and how they get rid of it!

Where's the Fat?
Fat, or adipose tissue, is found in several places in your body. Generally, fat is found underneath your skin (subcutaneous fat). There's also some on top of each of your kidneys. Other locations depend upon whether you are a man or woman:

  • An adult man tends to carry body fat in his chest, abdomen and buttocks, producing an "apple" shape.
  • An adult woman tends to carry fat in her breasts, hips, waist and buttocks, creating a "pear" shape.
The difference in fat location comes from the sex hormones estrogen and testosterone.


Figure 1. Cross-section view of your skin. The fat is in the subcutaneous layer, which is richly supplied with blood vessels.

Your body contains two types of fat tissue:

  • White fat - important in energy metabolism, heat insulation and mechanical cushioning.
  • Brown fat - found mostly in newborn babies, between the shoulders; important for thermogenesis (making heat).
Fat tissue is made up of fat cells. Fat cells are a unique type of cell. You can think of a fat cell as a tiny plastic bag that holds a drop of fat:
  • White fat cells are large cells that have very little cytoplasm, only 15 percent cell volume, a small nucleus and one large fat droplet that makes up 85 percent of cell volume.
  • brown fat cells are somewhat smaller, are loaded with mitochondria and are composed of several smaller fat droplets. The mitochondria are able to generate heat.
Fat cells are formed in the developing fetus during the third trimester of pregnancy, and later at the onset of puberty, when the sex hormones "kick in." It is during puberty that the differences in fat distribution between men and women begin to take form. One amazing fact is that fat cells do not multiply after peuberty -- as your body stores more fat, the number of fat cells remains the same. Each fat cell simply gets bigger!

In addition to fat tissue, some fat is stored in the liver, and an even smaller amount in muscle.

How Fat Enters Your Body
When you eat food that contains fat, mostly triglycerides, it goes through your stomach and intestines. In the intestines, the following happens:

  1. Large fat droplets get mixed with bile salts from the gall bladder in a process called emulsification. The mixture breaks up the large droplets into several smaller droplets called micelles, increasing the fat's surface area.
    Emulsification in Your Kitchen
    When you add water to a greasy skillet, the grease forms a layer on top of the water. If you squeeze one drop of dishwashing liquid into the center of the skillet, you'll see the large grease layer immediately break up into small droplets.
  2. The pancreas secretes enzymes called lipases that attack the surface of each micelle and break the fats down into their parts, glycerol and fatty acids.
  3. These parts get absorbed into the cells lining the intestine.
  4. In the intestinal cell, the parts are reassembled into packages of fat molecules (triglycerides) with a protein coating called chylomicrons. The protein coating makes the fat dissolve more easily in water.
  5. The chylomicrons are released into the lymphatic system -- they do not go directly into the bloodstream because they are too big to pass through the wall of the capillary.
  6. The lymphatic system eventually merges with the veins, at which point the chylomicrons pass into the bloodstream.
You might be wondering why fat molecules get broken down into glycerol and fatty acids if they're just going to be rebuilt. This is because fat molecules are too big to easily cross cell membranes. So when passing from the intestine through the intestinal cells into the lymph, or when crossing any cell barrier, the fats must be broken down. But, when fats are being transported in the lymph or blood, it is better to have a few, large fat molecules than many smaller fatty acids, because the larger fats do not "attract" as many excess water molecules by osmosis as many smaller molecules would.

How Fat is Stored in Your Body
Chylomicrons do not last long in the bloodstream -- only about eight minutes -- because enzymes called lipoprotein lipases break the fats into fatty acids. Lipoprotein lipases are found in the walls of blood vessels in fat tissue, muscle tissue and heart muscle. The activity of lipoprotein lipases depends upon the levels of insulin in the body. If insulin is high, then the lipases are highly active; if insulin is low, the lipases are inactive.


Figure 2. How a fat cell stores fat, and converts glucose and amino acids into fat.

Insulin
When you eat a candy bar or a meal, the presence of glucose, amino acids or fatty acids in the intestine stimulates the pancreas to secrete a hormone called insulin. Insulin acts on many cells in your body, especially those in the liver, muscle and fat tissue. Insulin tells the cells to do the following:
  • Absorb glucose, fatty acids and amino acids
  • Stop breaking down:
    • glucose, fatty acids and amino acids
    • glycogen into glucose
    • fats into fatty acids and glycerol
    • proteins into amino acids
  • Start building:
    • glycogen from glucose
    • fats (triglycerides) from glycerol and fatty acids
    • proteins from amino acids
The fatty acids are then absorbed from the blood into fat cells, muscle cells and liver cells. In these cells, under stimulation by insulin, fatty acids are made into fat molecules and stored as fat droplets.

It is also possible for fat cells to take up glucose and amino acids, which have been absorbed into the bloodstream after a meal, and convert those into fat molecules. The conversion of carbohydrates or protein into fat is 10 times less efficient than simply storing fat in a fat cell, but the body can do it. If you have 100 extra calories in fat (about 11 grams) floating in your bloodstream, fat cells can store it using only 2.5 calories of energy. On the other hand, if you have 100 extra calories in glucose (about 25 grams) floating in your bloodstream, it takes 23 calories of energy to convert the glucose into fat and then store it. Given a choice, a fat cell will grab the fat and store it rather than the carbohydrates because fat is so much easier to store.

It is important to note that as your body stores more fat, the number of fat cells remains the same; each fat cell simply gets bigger.

Hormones That Act Opposite to Insulin
When you are not eating, your body is not absorbing food. If your body is not absorbing food, there is little insulin in the blood. However, your body is always using energy; and if you're not absorbing food, this energy must come from internal stores of complex carbohydrates, fats and proteins. Under these conditions, various organs in your body secrete hormones:

  • pancreas - glucagon
  • pituitary gland - growth hormone
  • pituitary gland - ACTH (adrenocorticotropic hormone)
  • adrenal gland - epinephrine (adrenaline)
  • thyroid gland - thyroid hormone
These hormones act on cells of the liver, muscle and fat tissue, and have the opposite effects of insulin.

How Your Body Breaks Down Fat
When you are not eating, or you are exercising, your body must draw on its internal energy stores of complex carbohydrates, fats and proteins. Your body's prime source of energy is glucose. In fact, some cells in your body, such as brain cells, can get energy only from glucose.

The first line of defense in maintaining energy is to break down carbohydrates, or glycogen, into simple glucose molecules -- this process is called glycogenolysis. Next, your body breaks down fats into glycerol and fatty acids in the process of lipolysis. The fatty acids can then be broken down directly to get energy, or can be used to make glucose through a multi-step process called gluconeogenesis. In gluconeogenesis, amino acids can also be used to make glucose.


Figure 3. How a fat cell breaks down fat.

In the fat cell, other types of lipases work to break down fats into fatty acids and glycerol. These lipases are activated by various hormones, such as glucagon, epinephrine and growth hormone. The resulting glycerol and fatty acids are released into the blood, and travel to the liver through the bloodstream. Once in the liver, the glycerol and fatty acids can be either further broken down or used to make glucose.

Brown Fat: Making Heat
When you are first born, your body does not have much white fat to help insulate and retain body heat; although there are white fat cells, there is not much fat stored in them. A newborn baby produces heat (thermogenesis) primarily by breaking down fat molecules into fatty acids in brown fat cells. Instead of those fatty acids leaving the brown fat cell, as happens in white fat cells, they get further broken down in the mitochondria and their energy is released directly as heat. This same process occurs in hibernating animals, which have more brown fat than humans. Once the newborn baby starts eating more, developing layers of white fat, the brown fat goes away. Adult humans have little or no brown fat.

Losing Weight and Losing Fat
Your weight is determined by the rate at which you store energy from the food that you eat, and the rate at which you use that energy. Most experts agree that the way to maintain a healthy weight is:

  • Eat a balanced diet - appropriate amounts glucose, fat and protein
  • Do not eat excessively - for most people, a diet of 1,500 to 2,000 calories a day is sufficient to maintain a healthy weight
  • Exercise regularly

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