November 4, 2012
Believe It or Not, You Need Cholesterol
Your healthy body needs cholesterol, but I haven’t told you the reasons why.
Let me list them now:
- Cholesterol directs the development of some cells in the growing fetus.
- Cholesterol is part of the membrane that surrounds and protects each cell in your body.
- Cholesterol comprises a major portion of your brain, which is composed of mostly fatty tissue.
- Cholesterol contributes to the construction of synapses, structures through which nerve cells transmit messages.
- Cholesterol is a building block for hormones, including the male sex hormone testosterone and the vital adrenal hormone cortisone.
- Cholesterol is an ingredient in digestive juices, such as bile.
- Cholesterol is used as a building block for vitamin D, which is made when sunlight hits the fatty tissue just under your skin.
- And, oh yes, cholesterol is part of body fat.
Is that an impressive list or what? I think it’s impressive as all get out, so I’m going to take some time to explain exactly how cholesterol performs each of these incredibly important jobs.
Pinning a blue ribbon on good lipoproteins
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?
The good news about HDLs
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.
With LDLs, size may make all the difference
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. Definitely more to come on this one.
Moving through the fat factory
As anyone who has ever read a nutrient chart knows, liver (as a food) is very high in fat and cholesterol. In fact, your liver is a veritable fat and cholesterol factory that collects fat fragments from your blood and uses them to make cholesterol and new fats that your body can use to build tissue and perform other physiological functions.
The next few sections explain exactly how lipoproteins are made.
Putting the fats in lipoproteins (and taking them out again)
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 baby 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).
Naming the proteins in lipoproteins
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).
November 3, 2012
Bringing up baby 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 protein and 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 as triglycerides, 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.
Shaking Hands with Cholesterol
Cholesterol is the Dr. Jekyll and Mr. Hyde of the nutrition world. This fat-like substance is both essential for your healthy body and potentially hazardous to your heart.
Making the most of cholesterol’s Jekyll-like good characteristics while counteracting its Hyde-like bad impulses can be a delicate but not impossible balancing act. The task begins with understanding how and where cholesterol does its good work and how and where it can cause problems. Begin your mission, in the true scientific spirit, at the beginning.
Where cholesterol comes from
Yes, you get some cholesterol from food, but the curious fact is that most of the cholesterol in your blood and body tissues is produced right in your very own liver. Your liver uses the proteins, fats, and carbohydrates in food to manufacture and churn out about 1 gram (1,000 milligrams) of cholesterol a day.
How cholesterol travels around your body
Whether your cholesterol comes from food or your liver, it travels through your bloodstream in particles called lipoproteins, a name derived from lipos (the Greek word for “fat”) and protos (Greek for “first” or “most important”).
The fatty substances in lipoproteins include cholesterol and triglycerides, the most common fatty substance in the human body (more about triglycerides in the section “Focusing on Other Blood Baddies”). The proteins that combine with fats to produce lipoproteins are called apolipoproteins, often abbreviated as apo.
Lipoproteins develop through five distinct phases as they mature into the particles that carry cholesterol around your body:
- Phase 1: Chylomicrons
- Phase 2: Very low-density lipoproteins (VLDLs)
- Phase 3: Intermediate-density lipoproteins (IDLs)
- Phase 4: Low-density lipoproteins (LDLs)
- Phase 5: High-density lipoproteins (HDLs)
How does a chylomicron become a VLDL, then an IDL, then an LDL, and finally, maybe, an HDL? The following roadmap marks the route.
