Where Is Body Fat Stored?

Fat (triglyceride) is an energy source for many of our cells (in particular muscle and liver) and is our primary means of storing the excessive energy

from the foods we eat. Although some fat can be found in several cell types in our body (such as skeletal and cardiac muscle cells), by and large most of the fat stored in our body is housed in fat cells. Collections of fat cells or adipocytes are commonly referred to as fat tissue or adipose tissue. Because a larger percentage of the fatty acids stored in adipose tissue are monounsaturated and saturated, the fat tissue is more semisolid than liquid. This can contribute to the dimpling appearance in the layer of fat found beneath our skin (subcutaneous fat) that is often referred to as cellulite.

What Information Can We Derive from a Blood Cholesterol Test?

When a health professional refers to our blood cholesterol level it is usually total cholesterol. Total cholesterol is the sum of the cholesterol in all of the lipoproteins circulating in our blood at the time of the blood draw. Since chylomicrons will circulate only for a couple of hours after a meal, they should be absent from blood drawn after an overnight fast. If there are chylomicrons in a fasting blood sample it could indicate a medical condition whereby chylomicrons are not rapidly and efficiently processed.

The fractions of total cholesterol are the amount of cholesterol found in each type or class of lipoproteins. Thus LDL-cholesterol is the cholesterol only found in LDL. And likewise HDL-cholesterol is the cholesterol found only in HDL. With regard to heart attacks and strokes, having a total cholesterol level greater than 200 milligrams per 100 milliliters of blood and elevated LDL- and low HDL-cholesterol levels increase the risk (Table 5.6 has a sample lipid profile).

A total cholesterol level is the sum of all the cholesterol in lipoproteins primarily LDLs, HDLs and VLDLs.

Where Do High Density Lipoproteins Come From and What Do They Do?

The last type of lipoprotein is HDL. HDL is made in our liver and to a lesser extent in our intestines. It is HDL’s job to circulate and pick up excess cholesterol from tissues throughout our body and return it to the liver. The whole process is very interesting because in order for circulating HDL to return the cholesterol to our liver, some of the cholesterol is first passed to circulating LDLs. The LDL is then subject to removal from our circulation by the liver and broken down. HDL delivers the rest of its cholesterol directly to the liver. In regard to heart disease, if LDL wears the villain’s black hat, as higher levels are linked to increased risk of a heart attack and stroke, then HDL wears the hero’s white hat, as higher levels are linked to lower risk. We will spend more time talking about blood lipids and cardiovascular disease in Chapter 13.

What Are Low Density Lipoproteins and How Do They Function?

As mentioned earlier, not only will the liver receive cholesterol and some fat from chylomicrons, but it is also a primary cholesterol- and triglyceride-producing organ in the body. Fat and cholesterol in excess of the liver’s needs are packaged up into VLDLs and released into our circulation. As VLDLs circulate throughout our body, they unload a lot of their fat, mostly in fat cells. As a result their lipid to protein ratio decreases, which renders them denser, and they become LDLs (Figure 5.10). Therefore, LDL is derived from circulating VLDL.

LDL has two fates. One fate is to continue to circulate throughout the body and deposit cholesterol in various tissues. The second fate is to be recognized by tissue, removed from the blood, and broken down. Many tissues throughout our body can do this, but the liver handles more than half of the task. The longer LDLs circulate, the more opportunity there is for cholesterol to be deposited throughout our body.

LDLs contain mostly cholesterol and serve to deliver it throughout the body.

What Is the General Activity of Chylomicrons?

As summarized in Table 5.5, chylomicrons are made by the cells lining our small intestine and transport diet-derived lipids throughout the body. Chylomicron composition reflects our dietary lipid intake; therefore, they contain mostly fat. As chylomicrons circulate they unload most of their fat in fat tissue and other tissues such as muscle, as described previously. Once most of the fat has been removed the chylomicron is much smaller and is recognized and removed from the blood by the liver where it is broken down. Any cholesterol and leftover fat becomes the property of the liver.

How Are Lipids Shuttled Around in Our Blood?

Not only will our liver make a fair amount of cholesterol and fat on a daily basis, but it will also receive these nutrients from diet-derived chylomicrons. Like fat, most cholesterol is housed in the liver for only a short period of time as it is destined for other tissues throughout the body. Once cholesterol reaches other tissues, it can be used to make some of the substances listed previously or to become part of cell membranes. Some of the cholesterol in our liver is also used to make bile salts, a key component of bile.

Whether they are coming from the digestive tract or the liver special transportation vehicles or lipoproteins are needed to circulate lipids. Generally speaking, lipoproteins are a protein-containing shell encasing the lipid substances in need of transportation (Figure 5.9). Lipoproteins can be divided into four general classes based upon their densities (see Figure 5.9). In order of increasing density lipoproteins are chylomicrons, very low density lipoproteins (VLDLs), low density lipoproteins (LDLs), and high density lipoproteins (HDLs). Looking at the composition of these lipoproteins in Figure 5.9, we see that the greater the lipid to protein ratio, the lower the density. This makes perfect sense because lipids are less dense than proteins.

The proteins that help make up the lipoprotein shell are called apoproteins. Not only do they make the lipoprotein more soluble in water, but they will also function in helping the lipoprotein be recognized by specific tissues throughout our body. This allows a lipoprotein either to unload some of its lipid cargo or to be removed from the blood and broken down. For instance, the receptor for LDLs is located in the liver tissue and also in other tissue throughout the body. When a specific apoprotein on an LDL docks on the LDL receptor, this allows the LDL to be removed from the blood.

How and When Do We Remove Fat from Our Fat Cells?

The fat stored in fat cells is available to us when food energy is not being absorbed (fasting) and when we exercise. Just as the hormone insulin promoted the storage of fat when energy was coming into our body, the process of mobilizing fat from fat cells is promoted by the hormones released into our blood when we are fasting and/or exercising (Figure 5.8). These hormones are glucagon, epinephrine, and cortisol, and all promote the release of fat from fat stores.

In order for fat to be released from fat cells, fat is first broken down to fatty acids and glycerol, which then enter our blood and circulate. However, because of their general water insolubility, the fatty acids will hitch a ride aboard a protein in the blood called albumin. On the contrary, glycerol is fairly water soluble and can dissolve into blood. In fact, researchers will measure the level of glycerol in the blood to estimate how much fat is being broken down.

Body fat is broken down to serve as energy in-between meals and during exercise.

Circulating fatty acids are removed by cells, especially skeletal muscle and our heart, liver, and other organs and then used by those tissues primarily for energy. However, keep in mind that cells of the brain and red blood cells (RBC) cannot use fatty acids for energy and will continue to use glucose. Conveniently the glycerol released from fat tissue can be used to make glucose in the liver and released into circulation to help maintain a desirable level of circulating glucose during prolonged exercise and fasting.