How Fat Cells Work

[Guest guest]

How Fat Cells Work

JoAnn Guest

Oct 27, 2006 09:56 PDT

---

How Fat Cells Work

by Craig C. Freudenrich, Ph.D.

 

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

 

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. For an adult to be

considered " overweight, " he has a BMI somewhere between 25 and 29.9.

For an adult to be considered " obese, " he has a BMI of at least 30.

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. 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 puberty -- 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: 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 dish washing 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.

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.

 

JoAnn Guest

mrsjo-

www.geocities.com/mrsjoguest/Diets