LDL vs HDL

What is lipoproteins ?

A lipoprotein is born as a chylomicron, a particle that your intestinal cells assemble from the proteins and fats you eat. Chylomicrons are very, very low-density particles.

Why are some lipoproteins called low-density and others high-density?

• The term density refers to a lipoprotein’s weight.
• Protein weighs more than fat.
• Lipoproteins containing proportionately less protein than fat are low density lipoproteins, also known as LDLs. LDLs are the “bad” particles that carry cholesterol into your arteries.
• Lipoproteins containing proportionately more protein than fat are high density lipoproteins, also known as HDLs. HDLs are the “good” particles that ferry cholesterol out of your body.

Now, back to chylomicrons. These lipoproteins start out with very little proteinand a lot of light and fluffy fat and cholesterol. But as they flow through your bloodstream from your intestines on their way to your liver (your body’s lipoprotein factory), the chylomicrons release their fats, known astriglycerides, into your blood.

The stripped-down chylomicron, also known as a chylomicron remnant, still has its cholesterol and protein. Now, the remnant slides into your liver, and fat comes back into the picture

LDL vs HDL

When the chylomicron hits the liver, it picks up fat particles and mutates into the largest kind of lipoprotein, a fluffy particle called a very low-density lipoprotein (VLDL).

Then your liver sends the VLDL out into the wide world — your body.

As the VLDL travels far and wide, it drops globs of fat, picks up globs of cholesterol, and changes into a slightly smaller, heavier particle called an intermediate low-density lipoprotein (IDL), and then a slightly smaller, heavier low-density lipoprotein (LDL).

The last step in the transformation of the lipoprotein (the chylomicron) occurs when an LDL has dropped so much fat and cholesterol into body tissue that it’s mostly protein. Now, you’re looking at a high-density lipoprotein (HDL).

The primary proteins in VLDLs, IDLs, and LDLs belong to a class of apolipoproteins called apoB. The primary proteins in HDLs belong to a class of apolipoproteins called apoA. Other less prominent apolipoproteins found in lipoproteins are apoC and apoE.

You may have heard about a blood test for apoA; this test is interesting because a high level of apoA indicates a high level of protective HDLs (the “good” particles that haul cholesterol out of your body).

What is HDL?

HDLs truly deserve the name “good cholesterol.” These particles don’t carry cholesterol into your arteries for the simple reason that they’re so compact and dense that they can’t squeeze through the spaces in the walls of your arteries.

As a result, HDLs — and their cholesterol — travel away from your arteries and out of your body with the rest of your, um, solid waste.

What a neat set of facts to park in the back of your brain for the next time you’re at a party and someone asks you to explain the differences between VLDLs, IDLs, LDLs, and HDLs. “Well,” you can say, “it’s all a question of density, which, as you know, means. . . .” Don’t you just love being the smartest kid in class?

You can think of HDLs as scavenger molecules that remove cholesterol from the arteries. Having a lot of HDLs reduces your risk of heart attack regardless of your total cholesterol levels.

In fact, X-ray studies have shown that people who raise their HDLs by exercising, stopping smoking, or taking medication not only reduce the cholesterol in the arteries but also remove the plaque — thus opening the arteries.

Having read that paragraph carefully, you may assume that all LDLs are bad guys, right? Wrong.

What is LDL?

For years, everyone — that is, all the experts evaluating your cholesterol — conversely believed that a person with a lot of light and mushy LDLs (which can squeeze through your artery walls) inevitably had a higher risk of heart attack. The fact that some people with high levels of LDLs sailed happily into old age without experiencing heart problems was dismissed as plain good luck.

Maybe not, says a group of researchers at Albert Einstein College of Medicine in New York City. In 2003, looking for clues to longevity, the team, which included members from the University of Maryland School of Medicine, Tufts University, Boston University School of Medicine, and Roche Molecular Systems, ran various tests, including cholesterol tests, on 213 senior citizens, plus 216 of their children and grandchildren. For comparison, they ran the same tests on a control group of non-blood relatives, such as the children’s husbands and wives.

The tests showed something really surprising: The long-lived oldsters were three times more likely than other people to have a mutation in a gene that regulates cholesteryl ester transfer protein (CETP), an enzyme that affects the size of lipoproteins. As a result, compared with other people, including those non-related husbands and wives, even the oldsters who had high levels of LDLs had relatively larger low-density lipoproteins. (Their HDLs were also relatively bigger.)

According to the Einstein team, led by Dr. Nir Barzilai, the level of LDLs doesn’t predict heart disease; it’s the size of the LDLs in the mix. In other words, having many small LDLs may raise the risk of heart attack even if your overall cholesterol level is low.