What is Cholesterol? What are triglycerides?

Cholesterol and triglycerides are two forms of lipid, or fat. Both cholesterol and triglycerides are necessary for life itself. Cholesterol is necessary, among other things, for building cell membranes and for making several essential hormones. Triglycerides, which are chains of high-energy fatty acids, provide much of the energy needed for cells to function.

Where do cholesterol and triglycerides come from?

There are two sources for these lipids: dietary sources, and endogenous sources (i.e., manufactured within the body).

Dietary cholesterol and triglycerides mainly come from eating animal products and saturated fat. These dietary lipids are absorbed through the gut, assembled there into special packets called chylomicrons, and then are delivered through the bloodstream to the liver, where they are processed.

One of the main jobs of the liver is to make sure all the tissues of the body receive the cholesterol and triglycerides they need to function. Whenever possible (i.e., for about 8 hours after a meal), the liver takes up dietary cholesterol and triglycerides from the chylomicrons produced in the intestines. During times when dietary lipids are not available, the liver produces cholesterol and triglycerides itself.

The liver then packages the cholesterol and triglycerides, along with special proteins, into tiny spheres called lipoproteins. The lipoproteins are released into the circulation, and are delivered to the cells of the body. The cells remove the needed cholesterol and triglycerides from the lipoproteins, as they are needed.

What are LDL, HDL, and VLDL?

LDL, HDL and VLDL are the names of the three major varieties of lipoproteins. LDL stands for “low density lipoprotein;” HDL for “high density lipoprotein;” and VLDL for “very low density lipoprotein.”

Why are high cholesterol levels bad?

Why is some cholesterol called “good cholesterol”?

Much evidence has now accumulated that increased HDL cholesterol levels are associated with a lower risk of heart disease, and that low HDL cholesterol levels are associated with an increased risk of heart disease. Thus, HDL cholesterol appears to be “good.”

Why is HDL cholesterol protective? Nobody knows for sure, but it appears that it’s not the cholesterol itself that is good, it’s the "vehicle." There is some evidence that the HDL molecule “scours” the walls of blood vessels, and cleans out excess cholesterol. If this is the case, the cholesterol being carried by HDL (that is, the “good” HDL cholesterol) is actually “bad” cholesterol that has just been removed from blood vessels, and is being transported back to the liver for further processing. Apparently, unlike some bad humans, bad cholesterol can be rehabilitated.

Why are triglycerides the Rodney Dangerfield of lipids?

Triglycerides get little respect. Their measurement is part of a standard blood lipid profile, but for the most part doctors don’t know what to do when triglyceride levels are modestly elevated.

Why is that? While high triglyceride levels have been associated with heart disease, no study has yet proven that high triglyceride levels are an independent risk factor for heart disease. So doctors don’t have the evidence they need to recommend aggressive triglyceride-lowering therapy.

The problem is, patients with elevated triglyceride levels almost invariably have other major risk factors for heart disease (mainly obesity, diabetes, and/or high blood pressure), and so far it has not been possible to sort out whether the triglycerides themselves pose an independent risk.

The most difficult-to-sort-out association is that between triglycerides and HDL cholesterol. It turns out that whenever triglycerides are increased, HDL cholesterol decreases. So is the increased risk seen with high triglycerides due to the triglycerides themselves, or to the associated reduction in “good” cholesterol? So far, nobody can say for sure.  

What can cause increased cholesterol?

Elevated cholesterol levels can be caused by several factors, including:

   - Increased cholesterol levels can be hereditary.
   - A diet high in saturated fat and cholesterol can increase cholesterol levels.
   - Being overweight increases LDL cholesterol and decreases HDL cholesterol.
   - Being sedentary increases LDL cholesterol and decreases HDL cholesterol.
   - Age: cholesterol levels increase with age, beginning at about age 20.
   - Gender: females prior to menopause have cholesterol levels lower than men at the same age, but when menopause occurs their LDL cholesterol levels increase, as does the risk of heart disease.

Of these causative factors, heredity, age and gender cannot be controlled. The other causative factors can.

"Secondary" elevation of cholesterol

Some people have elevated cholesterol levels as a result of specific diseases or medical conditions. These people are said to have "secondary lipid disorders." In these individuals, treating the underlying medical problem often results in an improvement in cholesterol levels. Conversely, if the underlying medical condition is "missed," successfully reducing cholesterol is difficult if not impossible. Thus, any patient whose cholesterol levels are elevated should be screened for one of these causes of secondary lipid disorders. These causes are: diabetes, hypothyroidism (low thyroid,) obstructive liver disease, chronic renal (kidney) failure, and drugs (anabolic steroids, progesterone drugs, and corticosteroids.)

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