FAT AND CHOLESTEROL: Fuel, Function, Facts and Fiction
How are lipids shuttled around in our blood?
Lipids are shuttled around in our body within lipoproteins. Generally, lipoproteins are a protein-containing shell encasing the lipid substances in need of transportation. We have already discussed chylomicrons, a class of lipoprotein responsible for transporting absorbed lipids from our diet throughout our body. Lipoproteins can be divided into four general classes based upon their densities. Because they differ in this regard, we can quantify the different lipoprotein classes in our blood using a machine (centrifuge) designed to separate substances based upon their density. In order of increasing density our lipoproteins are Chylomicrons, Very Low Density Lipoproteins (VLDL), Low Density Lipoproteins (LDL), and High Density Lipoproteins (HDL). Looking at the composition of these lipoproteins below, it demonstrates that the greater the lipid to protein ratio the lower the density. This makes absolute sense as lipids are less dense than proteins.
What do the proteins in the lipoprotein shell do?
The proteins that help make up the lipoprotein shell are called apolipoproteins. Not only do they make the lipoprotein more soluble in water, but they also function in helping the lipoprotein be recognized by specific tissue throughout our body. This allows a lipoprotein to either unload or load some of its lipid cargo or to be removed from our blood and degraded.
What is the general activity of the lipoproteins?
The activity of lipoproteins is complex, fairly well understood, and extremely fascinating. Chylomicrons are made by the cells lining our small intestine and transport diet derived lipid throughout our body. Chylomicron composition reflects our dietary lipid intake, and therefore they contain mostly fat. As chylomicrons travel throughout our circulation they unload most of their fat in fat cells and skeletal muscle cells. Once most of the fat has been removed the remaining chylomicron-remnant is recognized and removed from our blood by our liver, and broken down. Any cholesterol and left-over fat becomes the property of our liver.
Not only will our liver receive cholesterol and some fat from chylomicrons, but it is also the primary cholesterol and triglyceride making organ in our body. Contrary to these functions is the limited ability of our liver to store either of these substances. Fat and cholesterol in excess of the liver's needs are packaged up into VLDL and released into our circulation. As VLDL circulate throughout our body, they unload some of their fat, on fat cells mostly. As a result their lipid to protein ratio decreases. This renders them a little denser and they become LDL. So you see, LDL are a derivative of circulating VLDL. he circulating fatty
LDL has two fates. One fate is to continue to circulate throughout our body and deposit cholesterol in various tissue. The second fate is to be recognized by tissue, removed from the blood, and be broken down. Many tissue throughout our body can do this, however, the liver handles more than half of the task. The longer LDL circulate, the more opportunity there is for cholesterol to be deposited throughout our body. The last type of lipoprotein is HDL. In regard to heart disease, if LDL wears the villain's black hat then HDL wears the hero's white hat. HDL are made in our liver and to a lesser extent in our intestines. It is the job of HDL to circulate and pick up excess cholesterol from cells 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 LDL. The LDL is then subject to removal from our circulation and broken down. HDL will also deliver cholesterol to the liver as well.
What information can we derive from a blood cholesterol test?
It is important to remember that when health professionals refer to our "blood cholesterol" levels, they are describing the total cholesterol content of our blood. This is the sum of the cholesterol in all of the lipoproteins circulating in our blood at the time it was drawn. Typically, our blood is drawn after an overnight fast so there should not be chylomicrons in our blood. Chylomicrons will only circulate for a couple hours after a meal. The fractions of total cholesterol (VLDL, HDL & LDL) 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. In regard to atherosclerotic heart disease the blood cholesterol-related risk factors include a total cholesterol level greater than 200 milligrams per 100 milliliters of blood, a higher LDL-cholesterol, a low HDL-cholesterol level, and an LDL to HDL ratio greater than 4:1.