How fat cells work -- article one --

[Jann]

I know not everyone wants to know how things work, it's enough to know that they do, but I found this series to be facinating!

Jann

http://health.howstuffworks.com/fat-cell.htm

How Fat Cells Work

by Craig Freudenrich, Ph.D.

Freudenrich, Ph.D., Craig. "How Fat Cells Work." 27 October 2000. HowStuffWorks.com. <http://health.howstuffworks.com/fat-cell.htm> 06 October 2008.

Inside this Article

Introduction to How Fat Cells Work

Body Fat Basics

Fat Storage

Breaking Down Fat

Lots More Information

See all Cells & Tissues articles

What Are the Odds? Obesity

A little more than half of the adults in the United States are overweight. Statistics show that an incredible 65.2 percent of the U.S. population is considered to be "overweight" or "obese." According to the Centers for Disease Control and Prevention (CDC), obesity and overweight status is determined in adults by finding a person's "Body Mass Index" or BMI.

BMI is a calculation that takes into consideration both a person's body weight and height to determine whether they are underweight, overweight or at a healthy weight. An adult who is considered "overweight" has a BMI somewhere between 25 and 29.9. An adult with a BMI of at least 30 is considered "obese." This measurement is used because it's typically a good indicator of body fat.

Whether due to concern for related health risks (high blood pressure, heart disease, diabetes, sleep apnea, respiratory problems, etc.), or just for sheer aesthetics, many Americans worry about fat. In fact, at this very moment, thousands of Americans are exercising or dieting 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 article, 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. In addition to fat tissue, some fat is stored in the liver, and an even smaller amount in muscle.

Where fat is concentrated in your body depends upon whet her 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. 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 generally do not generate after puberty -- as your body stores more fat, the number of fat cells remains the same. Each fat cell simply gets bigger! (There are two exceptions: the body might produce more fat cells if an adult gains a significant amount of weight or has liposuction performed.)

Body Fat Basics

The human body contains two types of fat tissue:

White fat is important in energy metabolism, heat insulation and mechanical cushioning.

Brown fat is found mostly in newborn babies, between the shoulders, and is important for thermogenesis (making heat). Since adult humans have little to no brown fat, we'll concentrate on white fat in this article. See the bottom of this page for more on brown fat.

Fat tissue is made up of fat cells, which 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.

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:

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.

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.

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. These parts get absorbed into the cells lining the intestine. 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. 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. 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.

In the next section, we'll look at how fat is stored in your body.

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. Brown fat cells are somewhat smaller than white, are composed of several smaller fat droplets and are loaded with mitochondria, which can generate heat. A newborn baby produces heat (a process called 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.

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[Willi333]

Great article - thanks Jann !!!